ASTM E2981-21

Standard Guide for Nondestructive Examination of Composite Overwraps in Filament Wound Pressure Vessels Used in Aerospace Applications

SIGNIFICANCE AND USE
4.1 The COPVs covered in this guide consist of a metallic liner overwrapped with high-strength fibers embedded in polymeric matrix resin (typically a thermoset) (Fig. 1). Metallic liners may be spun-formed from a deep drawn/extruded monolithic blank or may be fabricated by welding formed components. Designers often seek to minimize the liner thickness in the interest of weight reduction. COPV liner materials used can be aluminum alloys, titanium alloys, nickel-chromium alloys, and stainless steels, impermeable polymer liner such as high density polyethylene, or integrated composite materials. Fiber materials can be carbon, aramid, glass, PBO, metals, or hybrids (two or more types of fibers). Matrix resins include epoxies, cyanate esters, polyurethanes, phenolic resins, polyimides (including bismaleimides), polyamides, and other high performance polymers. Common bond line adhesives are FM-73, urethane, West 105, and Epon 862 with thicknesses ranging from 0.13 mm (0.005 in.) to 0.38 mm (0.015 in.). Metallic liner and composite overwrap materials requirements are found in ANSI/AIAA S-080 and ANSI/AIAA S-081, respectively.  
Note 6: When carbon fiber is used, galvanic protection should be provided for the metallic liner using a physical barrier such as glass cloth in a resin matrix, or similarly, a bond line adhesive.
Note 7: Per the discretion of the cognizant engineering organization, composite materials not developed and qualified in accordance with the guidelines in MIL-HDBK-17, Volumes 1 and 3 should have an approved material usage agreement.
FIG. 1 Typical Carbon Fiber Reinforced COPVs (NASA)  
4.2 The as-wound COPV is then cured and an autofrettage/proof cycle is performed to evaluate performance and increase fatigue characteristics.  
4.3 The strong drive to reduce weight and spatial needs in aerospace applications has pushed designers to adopt COPVs constructed with high modulus carbon fibers embedded in an epoxy matrix. Unfortunately, high modulus fiber...
SCOPE
1.1 This guide discusses current and potential nondestructive testing (NDT) procedures for finding indications of discontinuities and accumulated damage in the composite overwrap of filament wound pressure vessels, also known as composite overwrapped pressure vessels (COPVs). In general, these vessels have metallic liner thicknesses less than 2.3 mm (0.090 in.), and fiber loadings in the composite overwrap greater than 60 % by weight. In COPVs, the composite overwrap thickness will be of the order of 2.0 mm (0.080 in.) for smaller vessels and up to 20 mm (0.80 in.) for larger ones.  
1.2 This guide focuses on COPVs with nonload-sharing metallic liners used at ambient temperature, which most closely represents a Compressed Gas Association (CGA) Type III metal-lined composite tank. However, it also has relevance to (1) monolithic metallic pressure vessels (PVs) (CGA Type I), (2) metal-lined hoop-wrapped COPVs (CGA Type II), (3) plastic-lined composite pressure vessels (CPVs) with a nonload-sharing liner (CGA Type IV), and (4) an all-composite, linerless COPV (undefined Type). This guide also has relevance to COPVs used at cryogenic temperatures.  
1.3 The vessels covered by this guide are used in aerospace applications; therefore, the inspection requirements for discontinuities and inspection points will in general be different and more stringent than for vessels used in non aerospace applications.  
1.4 This guide applies to (1) low pressure COPVs used for storing aerospace media at maximum allowable working pressures (MAWPs) up to 3.5 MPa (500 psia) and volumes up to 2 L (70 ft3), and (2) high pressure COPVs used for storing compressed gases at MAWPs up to 70 MPa (10 000 psia) and volumes down to 8 L (500 in.3). Internal vacuum storage or exposure is not considered appropriate for any vessel size.
Note 1: Some vessels are evacuated during filling operations, requiring the tank to withstand external (atmospheric) pre...

General Information

• Status: Published
• Publication Date: 31-Jan-2021
• ICS: 23.020.30 - Pressure vessels, gas cylinders
• ICS: 49.025.01 - Materials for aerospace construction in general
• Technical Committee: E07 - Nondestructive Testing
• Drafting Committee: E07.10 - Specialized NDT Methods

Relations

• Refers: ASTM E2580-24 - Standard Practice for Ultrasonic Testing of Flat Panel Composites and Sandwich Core Materials Used in Aerospace Applications
• Effective Date: 01-Feb-2024
• Refers: ASTM E1316-24 - Standard Terminology for <?Pub Dtl?>Nondestructive Examinations
• Effective Date: 01-Feb-2024
• Refers: ASTM E1742/E1742M-23 - Standard Practice for Radiographic Examination
• Effective Date: 15-Dec-2023
• Refers: ASTM E2581-14(2023) - Standard Practice for Shearography of Polymer Matrix Composites and Sandwich Core Materials in Aerospace Applications
• Effective Date: 01-Dec-2023
• Refers: ASTM E750-15(2020) - Standard Practice for Characterizing Acoustic Emission Instrumentation
• Effective Date: 01-Jun-2020
• Refers: ASTM E2581-14(2019) - Standard Practice for Shearography of Polymer Matrix Composites and Sandwich Core Materials in Aerospace Applications
• Effective Date: 01-Dec-2019
• Refers: ASTM E1316-19b - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Dec-2019
• Refers: ASTM D3878-19a - Standard Terminology for Composite Materials
• Effective Date: 15-Oct-2019
• Refers: ASTM E1781/E1781M-19 - Standard Practice for Secondary Calibration of Acoustic Emission Sensors
• Effective Date: 01-Jul-2019
• Refers: ASTM D3878-19 - Standard Terminology for Composite Materials
• Effective Date: 15-Apr-2019
• Refers: ASTM E1316-19 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Mar-2019
• Refers: ASTM D3878-18 - Standard Terminology for Composite Materials
• Effective Date: 01-Apr-2018
• Refers: ASTM E1742/E1742M-18 - Standard Practice for Radiographic Examination
• Effective Date: 01-Mar-2018
• Refers: ASTM E2698-18 - Standard Practice for Radiographic Examination Using Digital Detector Arrays
• Effective Date: 01-Feb-2018
• Refers: ASTM E1316-18 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jan-2018

Overview

ASTM E2981-21 provides guidance for the nondestructive examination (NDE) of composite overwraps in filament wound pressure vessels, commonly known as composite overwrapped pressure vessels (COPVs), used in aerospace applications. These specialized vessels typically consist of a thin metallic or nonmetallic liner overwrapped with high-strength fibers embedded in a polymeric matrix, delivering high pressure containment with significant weight savings. Aerospace demands for lighter and stronger hardware have driven the adoption of these advanced COPVs, but robust inspection and maintenance methods are crucial to assure safety and reliable performance.

This standard focuses on current and emerging nondestructive testing (NDT) methods for detecting discontinuities and accumulated damage in composite overwraps, supporting the high inspection standards required in the aerospace sector.

Key Topics

• Scope of COPVs

  • Typically feature metallic liners (aluminum, titanium, nickel-chrome alloys, stainless steel, or impermeable polymers)
  • Overwraps use high-strength fibers (carbon, aramid, glass, PBO, or hybrids) in high-performance resins (epoxies, polyimides, etc.)
  • Designed for ambient or cryogenic aerospace environments

• NDT Methods Covered

  • Acoustic Emission (AE)
  • Eddy Current Testing (ET)
  • Laser Shearography
  • Radiographic Testing (RT)
  • Thermographic Testing (TT) / Infrared Thermography (IRT)
  • Ultrasonic Testing (UT)
  • Visual Testing (VT)

• Discontinuity and Damage Detection

  • Emphasis on methods capable of identifying subsurface damage not visible externally
  • Concern for critical defects: fiber breakage, delamination, voids, inclusions, matrix cracking, and bond line failures

• Performance and Safety Considerations

  • Addresses severe consequences of “burst before leak” failure modes
  • Emphasizes stringent inspection regimes typical of aerospace versus commercial practice

• Role of Engineering Oversight

  • Recommends involvement of certified NDT specialists in COPV design, quality assurance, and in-service maintenance
  • Specific accept/reject criteria are to be provided by the project or OEM where applicable

Applications

• Aerospace Storage Vessels

  • COPVs are used to store pressurized gases and fluids critical to propulsion and life-support systems in space vehicles, satellites, and aircraft
  • The guide applies to both low-pressure (up to 3.5 MPa/500 psia) and high-pressure (up to 70 MPa/10,000 psia) vessels of varying sizes

• Safety and Compliance

  • Ensures structural integrity through detection of in-service or manufacturing-induced damage that could lead to catastrophic failure
  • Supports compliance with NASA, DoD, and commercial aerospace performance standards

• Lifecycle Management

  • Guidance applies to new manufacture, periodic maintenance, and post-event (e.g., impact or stress) inspection of COPVs
  • NDT methods assist in implementing proactive damage control plans and maintaining safe operations for critical aerospace missions

Related Standards

• Composite and Pressure Vessel Standards

  • ANSI/AIAA S-080: Requirements for Metallic Pressure Vessels
  • ANSI/AIAA S-081: Composite Overwrapped Pressure Vessel criteria
  • MIL-HDBK-17: Composite Materials Handbook
  • ASTM E2982: NDT of Thin-Walled Metallic Liners in Filament Wound Pressure Vessels

• NDT Methods and Personnel Qualifications

  • ASTM E114, E164, E569, E750, E1001 (various NDT techniques)
  • ASNT SNT-TC-1A and CP-189: Qualification and certification for NDT personnel
  • EN 4179: Aerospace NDT personnel standards
  • ISO 9712: International NDT certification

• Safety and Quality Guidelines

  • NASA-STD-(I)-5019: Fracture Control Requirements
  • MIL-HDBK-340: Test Requirements for Space Vehicles
  • ASME Boiler and Pressure Vessel Code, Section V and X

By referencing these key standards and NDT protocols, ASTM E2981-21 plays a critical role in ensuring the safe, reliable, and compliant operation of advanced composite pressure vessels in aerospace environments.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E2981 − 21
Standard Guide for
Nondestructive Examination of Composite Overwraps in
Filament Wound Pressure Vessels Used in Aerospace
Applications
This standard is issued under the fixed designation E2981; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
NOTE 1—Some vessels are evacuated during filling operations, requir-
1. Scope
ing the tank to withstand external (atmospheric) pressure, while other
1.1 This guide discusses current and potential nondestruc-
vessels may either contain or be immersed in cryogenic fluids, or both,
tive testing (NDT) procedures for finding indications of dis- requiring the tanks to withstand any potentially deleterious effects of
differential thermal contraction.
continuities and accumulated damage in the composite over-
wrap of filament wound pressure vessels, also known as
1.5 The composite overwraps under consideration include,
composite overwrapped pressure vessels (COPVs). In general,
but are not limited to, ones made from various polymer matrix
these vessels have metallic liner thicknesses less than 2.3 mm
resins (for example, epoxies, cyanate esters, polyurethanes,
(0.090 in.), and fiber loadings in the composite overwrap
phenolic resins, polyimides (including bismaleimides), and
greater than 60 % by weight. In COPVs, the composite
polyamides)withcontinuousfiberreinforcement(forexample,
overwrap thickness will be of the order of 2.0 mm (0.080 in.)
carbon, aramid, glass, or poly-(phenylenebenzobisoxazole)
for smaller vessels and up to 20 mm (0.80 in.) for larger ones.
(PBO)). The metallic liners under consideration include, but
are not limited to, aluminum alloys, titanium alloys, nickel-
1.2 This guide focuses on COPVs with nonload-sharing
chromium alloys, and stainless steels.
metallic liners used at ambient temperature, which most
closely represents a Compressed GasAssociation (CGA) Type
1.6 ThisguidedescribestheapplicationofestablishedNDT
III metal-lined composite tank. However, it also has relevance
methods; namely, Acoustic Emission (AE, Section 7), Eddy
to (1) monolithic metallic pressure vessels (PVs) (CGA Type
Current Testing (ET, Section 8), Laser Shearography (Section
I), (2) metal-lined hoop-wrapped COPVs (CGA Type II), (3)
9), Radiographic Testing (RT, Section 10), Infrared Thermog-
plastic-lined composite pressure vessels (CPVs) with a
raphy (IRT, Section 11), Ultrasonic Testing (UT, Section 12),
nonload-sharing liner (CGA Type IV), and (4) an all-
and Visual Testing (VT, Section 13). These methods can be
composite, linerless COPV (undefined Type). This guide also
used by cognizant engineering organizations for detecting and
has relevance to COPVs used at cryogenic temperatures.
evaluating flaws, defects, and accumulated damage in the
1.3 The vessels covered by this guide are used in aerospace composite overwrap of new and in-service COPVs.
applications; therefore, the inspection requirements for discon-
NOTE 2—Although visual testing is discussed and required by current
tinuities and inspection points will in general be different and
range standards, emphasis is placed on complementary NDT procedures
more stringent than for vessels used in non aerospace applica-
that are sensitive to detecting flaws, defects, and damage that leave no
tions.
visible indication on the COPV surface.
NOTE 3—In aerospace applications, a high priority is placed on light
1.4 This guide applies to (1) low pressure COPVs used for
weight material, while in commercial applications, weight is typically
storing aerospace media at maximum allowable working pres-
sacrificed to obtain increased robustness. Accordingly, the need to detect
sures (MAWPs) up to 3.5 MPa (500 psia) and volumes up to
damagebelowthevisualdamagethresholdismoreimportantinaerospace
vessels.
2L (70 ft ), and (2) high pressure COPVs used for storing
NOTE 4—Currently, no determination of residual strength can be made
compressed gases at MAWPs up to 70 MPa (10000 psia) and
by any NDT method.
volumes down to 8L (500 in. ). Internal vacuum storage or
exposure is not considered appropriate for any vessel size.
1.7 All methods discussed in this guide (AE, ET,
shearography, RT, IRT, UT, and VT) are performed on the
composite overwrap after overwrapping and structural cure.
For NDT procedures for detecting discontinuities in thin-
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-
tive Testing and is the direct responsibility of Subcommittee E07.10 on Specialized
walledmetalliclinersinfilamentwoundpressurevessels,orin
NDT Methods.
the bare metallic liner before overwrapping; namely, AE, ET,
Current edition approved Feb. 1, 2021. Published February 2021. Originally
ε1
laserprofilometry,leaktesting(LT),penetranttesting(PT),and
approved in 2015. Last previous edition approved in 2015 as E2981–15 . DOI:
10.1520/E2981-21. RT; consult Guide E2982.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2981 − 21
1.8 In the case of COPVs which are impact damage sensi- E114 Practice for Ultrasonic Pulse-Echo Straight-Beam
tive and require implementation of a damage control plan, Contact Testing
emphasis is placed on NDT methods that are sensitive to
E164Practice for Contact Ultrasonic Testing of Weldments
detecting damage in the composite overwrap caused by im-
E317PracticeforEvaluatingPerformanceCharacteristicsof
pacts at energy levels and which may or may not leave any
Ultrasonic Pulse-Echo Testing Instruments and Systems
visible indication on the COPV composite surface.
without the Use of Electronic Measurement Instruments
E543Specification forAgencies Performing Nondestructive
1.9 Thisguidedoesnotspecifyaccept-rejectcriteria(4.9)to
Testing
be used in procurement or used as a means for approving
E569Practice for Acoustic Emission Monitoring of Struc-
filament wound pressure vessels for service. Any acceptance
tures During Controlled Stimulation
criteria specified are given solely for purposes of refinement
and further elaboration of the procedures described in this E650/E650MGuide for Mounting Piezoelectric Acoustic
guide. Project or original equipment manufacturer (OEM) Emission Sensors
specific accept/reject criteria should be used when available
E750Practice for Characterizing Acoustic Emission Instru-
and take precedence over any acceptance criteria contained in
mentation
this document. If no accept/reject criteria are available, any
E976GuideforDeterminingtheReproducibilityofAcoustic
NDT method discussed in this guide that identifies broken
Emission Sensor Response
fibers should require disposition by the cognizant engineering
E1001PracticeforDetectionandEvaluationofDiscontinui-
organization.
ties by the Immersed Pulse-Echo Ultrasonic Method
Using Longitudinal Waves
1.10 This guide references both establishedASTM methods
thathaveafoundationofexperienceandthatyieldanumerical E1065/E1065MPractice for Evaluating Characteristics of
result, and newer procedures that have yet to be validated and Ultrasonic Search Units
are better categorized as qualitative guidelines and practices.
E1067PracticeforAcousticEmissionExaminationofFiber-
The latter are included to promote research and later elabora-
glass Reinforced Plastic Resin (FRP) Tanks/Vessels
tion in this guide as methods of the former type.
E1106Test Method for Primary Calibration of Acoustic
Emission Sensors
1.11 To ensure proper use of the referenced standard
E1118Practice forAcoustic Emission Examination of Rein-
documents, there are recognized NDT specialists that are
forced Thermosetting Resin Pipe (RTRP)
certified according to industry and company NDT specifica-
E1316Terminology for Nondestructive Examinations
tions.ItisrecommendedthatanNDTspecialistbeapartofany
E1416Practice for Radioscopic Examination of Weldments
composite component design, quality assurance, in-service
maintenance, or damage examination. E1742/E1742MPractice for Radiographic Examination
E1781/E1781M Practice for Secondary Calibration of
1.12 Units—The values stated in SI units are to be regarded
Acoustic Emission Sensors
as standard. The English units given in parentheses are
E1815Test Method for Classification of Film Systems for
provided for information only.
Industrial Radiography
1.13 This standard does not purport to address all of the
E2104Practice for Radiographic Examination of Advanced
safety concerns, if any, associated with its use. It is the
Aero and Turbine Materials and Components
responsibility of the user of this standard to establish appro-
E2191Practice for Examination of Gas-Filled Filament-
priate safety, health, and environmental practices and deter-
WoundCompositePressureVesselsUsingAcousticEmis-
mine the applicability of regulatory limitations prior to use.
sion
Some specific hazards statements are given in Section 7 on
E2033Practice for Radiographic Examination Using Com-
Hazards.
puted Radiography (Photostimulable Luminescence
1.14 This international standard was developed in accor-
Method)
dance with internationally recognized principles on standard-
E2338Practice for Characterization of Coatings Using Con-
ization established in the Decision on Principles for the
formable Eddy Current Sensors without Coating Refer-
Development of International Standards, Guides and Recom-
ence Standards
mendations issued by the World Trade Organization Technical
E2375Practice for Ultrasonic Testing of Wrought Products
Barriers to Trade (TBT) Committee.
E2533Guide for Nondestructive Testing of Polymer Matrix
2. Referenced Documents
Composites Used in Aerospace Applications
E2580PracticeforUltrasonicTestingofFlatPanelCompos-
2.1 ASTM Standards:
ites and Sandwich Core Materials Used in Aerospace
D3878Terminology for Composite Materials
Applications
D5687Guide for Preparation of Flat Composite Panels with
E2581Practice for Shearography of Polymer Matrix Com-
Processing Guidelines for Specimen Preparation
posites and Sandwich Core Materials inAerospaceAppli-
cations
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
E2582Practice for Infrared Flash Thermography of Com-
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
posite Panels and Repair Patches Used in Aerospace
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. Applications
E2981 − 21
E2661/E2661MPracticeforAcousticEmissionExamination 2.8 Federal Standards:
of Plate-like and Flat Panel Composite Structures Used in 21 CFR 1040.10 Laser Products
Aerospace Applications 21 CFR 1040.11Specific Purpose Laser Products
E2662Practice for Radiographic Examination of Flat Panel
2.9 ISO Document:
Composites and Sandwich Core Materials Used in Aero-
ISO 9712 Non-destructive Testing—Qualification and Cer-
space Applications
tification of NDT Personnel
E2698Practice for Radiographic Examination Using Digital
2.10 LIA Document:
Detector Arrays
ANSI Z136.1-2000 Safe Use of Lasers
E2884Guide for Eddy Current Testing of Electrically Con-
2.11 MIL Documents:
ducting Materials Using Conformable Sensor Arrays
MIL-HDBK-17 Composite Materials Handbook, Guide-
E2982Guide for Nondestructive Testing of Thin-Walled
lines for Characterization of Structural Materials
MetallicLinersinFilament-WoundPressureVesselsUsed
MIL-HDBK-6870 Inspection Program Requirements, Non-
in Aerospace Applications
destructive for Aircraft and Missile Materials and Parts
2.2 AIA Standard:
MIL-HDBK-340 Test Requirements for Launch, Upper-
NAS 410NAS Certification and Qualification of Nonde-
Stage, and Space Vehicles, Vol. I: Baselines
structive Test Personnel
MIL-HDBK-787 Nondestructive Testing Methods of Com-
2.3 ANSI/AIAA Standards:
posite Materials—Ultrasonics
ANSI/AIAA S-080 Space Systems—Metallic Pressure
MIL-HDBK-1823 Nondestructive Evaluation System Reli-
Vessels, Pressurized Structures, and Pressure Components ability Assessment
ANSI/AIAA S-081 Space Systems—Composite Over-
2.12 NASA Documents:
wrapped Pressure Vessels (COPVs)
KNPR 8715.3(Kennedy NASA Procedural Requirements)
ANSI NGV2-2007American National Standard for Natural
Chapter 13: NASAKSC Requirements for Ground-Based
Gas Vehicle Containers
Vessels and Pressurized Systems (PV/S), Rev. G.
2.4 ASME Standards:
NASA/TM-2012-21737 Elements of Nondestructive Ex-
ASME Boiler and Pressure Vessel Code, Section V, Non- amination for the Visual Inspection of Composite Struc-
destructive Examination, Article 11, Acoustic Emission
tures
Examination of Fiber-Reinforced Plastic Vessels NASA-STD-(I)-5019 Fracture Control Requirements for
ASME Boiler and Pressure Vessel Code, Section X, Man-
Spaceflight Hardware
datory Appendix 8, Class III Vessels With Liners for MSFC-RQMT-3479 Fracture Control Requirements for
Gaseous Hydrogen in Stationary Service, Subsection
Composite and Bonded Vehicle and Payload Structures
8-600EXAMINATION,8-600.2.7AcousticEmissionEx-
2.13 Air Force Documents:
amination
AFSPCMAN 91-710 v3Range Safety User Requirements
2.5 ASNT Standards: Manual Volume 3 - Launch Vehicles, Payloads, and
ASNT CP-189 Standard for Qualification and Certification Ground Support Systems Requirements
of Nondestructive Testing Personnel AFSPCMAN 91-710 v6Range Safety User Requirements
SNT-TC-1A Recommended Practice for Nondestructive Manual Volume 6 - Ground and Launch Personnel,
Testing Personnel Qualification and Certification Equipment, Systems, and Material Operations Safety
Requirements
2.6 BSI Document:
EN4179 AerospaceSeries—QualificationandApprovalof
2.14 ECSS Document:
Personnel for Non-Destructive Testing ECSS-E-30-01ASpace Engineering Fracture Control
2.7 CGA Standards:
3. Terminology
CGA Pamphlet C-6.2Standard for Visual Inspection and
Requalification of Fiber Reinforced High Pressure Cylin- 3.1 Abbreviations—The following abbreviations are ad-
ders
opted in this guide: acoustic emission (AE), eddy current
CGAPamphletC-6.4MethodsforVisualInspectionofAGA testing(ET),radiographictesting(RT),ultrasonictesting(UT),
NGV2 Containers
and visual testing (VT).
3 9
Available from Aerospace Industries Association (AIA), 1000 Wilson Blvd., Available from U.S. Food and Drug Administration (FDA), 10903 New
Suite 1700, Arlington, VA 22209-3928, http://www.aia-aerospace.org. Hampshire Ave., Silver Spring, MD 20993, http://www.fda.gov.
4 10
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St., Available from International Organization for Standardization (ISO), 1, ch. de
4th Floor, New York, NY 10036, http://www.ansi.org. la Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http://www.iso.org.
5 11
Available from American Society of Mechanical Engineers (ASME), ASME Available from the Laser Institute of America, 13501 Ingenuity Drive, Suite
International Headquarters, Two Park Ave., New York, NY 10016-5990, http:// 128, Orlando, FL 32826.
www.asme.org. AvailablefromStandardizationDocumentsOrderDesk,Bldg4SectionD,700
AvailablefromAmericanSocietyforNondestructiveTesting(ASNT),P.O.Box Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org. Available from National Aeronautics and Space Administration, Technical
Available from British Standards Institution (BSI), 389 Chiswick High Rd., Standards Program, 300 E. Street SW, Suite 5R30, Washington, D. C. 20546.
London W4 4AL, U.K., http://www.bsigroup.com. https://standards. nasa.gov/documents/nasa.
8 14
Available from Compressed Gas Association (CGA), 14501 George Carter Available from ESA Publications Division, ESTEC, P.O. Box 299, 2200 AG
Way, Suite 103, Chantilly, VA 20151, http://www.cganet.com. Noordwijk, The Netherlands.
E2981 − 21
3.2 Definitions:TerminologyinaccordancewithTerminolo- 3.3.1 active thermography, n—active thermography refers
gies E1316 and D3878 shall be used where applicable. to the examination of an object upon intentional application of
an external energy source.
3.2.1 active source, n—see Test Method E569, Section 3,
3.3.1.1 Discussion—The energy source (active or passive)
Terminology.
may be a source of heat, mechanical energy (vibration or
3.2.2 AE activity, n—see Test Method E569, Section 3,
fatigue testing), electrical current, or any other form of energy.
Terminology.
3.3.2 aspect ratio, n—the diameter to depth ratio of a flaw.
3.2.3 AE counts (N), n—the number of times the acoustic
3.3.2.1 Discussion—For irregularly shaped flaws, diameter
emission signal exceeds a preset threshold during any selected
refers to the minor axis of an equivalent rectangle that
portion of a test.
approximates the flaw shape and area.
3.2.4 AE source, n—a region of impact damage, fiber/
3.3.3 burst-before-leak (BBL), n—an insidious failure
bundle breakage, delamination growth, etc., in the composite
mechanism exhibited by composite materials usually associ-
overwraporgrowingcrackinthemetalliclinerofaCOPVthat
ated with broken fibers caused by mechanical damage, or with
canbeclassifiedasactive,criticallyactive,intense,orcritically
stress rupture at an applied constant load (pressure), whereby
intense.
the minimum time during which the composite maintains
structural integrity considering the combined effects of stress
3.2.5 AE source intensity, n—seeTestMethodE569,Section
3, Terminology. level(s), time at stress level(s), and associated environment is
exceeded, resulting in a sudden, catastrophic event.
3.2.6 AE test pressure, n—see Test Method E2191, Section
3.3.4 coherent light source, n—a monochromatic beam of
3, Terminology.
light having uniform phase over a minimum specified length
3.2.7 cognizant engineering organization, n—the company,
known as the coherent length.
government agency, or other authority responsible for the
3.3.5 composite overwrapped pressure vessel (COPV),
design or end use of the system or component for which NDT
n—an inner shell overwrapped with multiple plies of polymer
is required.
matrix impregnated reinforcing fiber wound at different wrap
3.2.7.1 Discussion—This, in addition to the design
angles that form a composite shell.
personnel, may include personnel from engineering, materials
3.3.5.1 Discussion—The inner shell or liner may consist of
and process engineering, stress analysis, NDT, or quality
an impervious metallic or nonmetallic material. The vessel
groups and other, as appropriate.
may be cylindrical or spherical and be manufactured with a
3.2.8 critically active source, n—see Test Method E569,
minimum of one interface port for pressure fitting or valve
Section 3, Terminology.
attachment (synonymous with filament wound pressure
vessel), or both.
3.2.9 critically intense source, n—see Test Method E569,
Section 3, Terminology.
3.3.6 critical Felicity ratio, n—the lower threshold of the
Felicity ratio at which rupture has been previously observed,
3.2.10 defect, n—see Terminology E1316.
regardless of what the current applied load or pressure is.
3.2.11 discontinuity, n—see Terminology E1316.
3.3.7 damage control plan (DCP)—a control document that
3.2.12 flaw, n—see Terminology E1316.
captures the credible damage threats to a COPV during
manufacturing, transportation and handling, and integration
3.2.13 Felicity effect, n—the presence of acoustic emission,
into a space system up to the time of launch/re-launch, reentry
detectable at a fixed, predetermined sensitivity level at stress
and landing, as applicable, and the steps taken to mitigate the
levels below those previously applied. E1106
possibility of damage due to these threats, as well as delinea-
3.2.14 Felicity ratio, n—the ratio of the stress at which the
tion of NDT performed (for example, visual testing) through-
Felicity effect occurs to the previously applied maximum
out the life cycle of the COPV.
stress. E1106, E1118
3.3.7.1 Discussion—The DPC shall be provided by the
designagencyandmadeavailableforreviewbytheapplicable
NOTE 5—The fixed sensitivity level will usually be the same as was
safety/range organization per ANSI/AIAA S-081, KNPR
used for the previous loading or test (Practice E1118).
8715.3, and AFSPCMAN 91-710.
3.2.15 high-amplitude threshold, n—a threshold for large
amplitudeAE events. (SeeA2.3 ofAnnexA2, Practice E1106) 3.3.8 de-correlation, n—loss of shearography phase data
causedbytestpartdeformationexceedingtheresolutionofthe
3.2.16 intense source, n—see Test Method E569, Section 3,
shearing interferometer sensor or motion between the test
Terminology.
object and shearing interferometer during data acquisition.
3.2.17 low-amplitude threshold, n—the threshold above
3.3.9 discrete discontinuity, n—a thermal discontinuity
which AE counts (N) are measured. (See A2.2 of Annex A2,
whoseprojectionontotheinspectionsurfaceissmallerthanthe
Practice E1106).
field of view of the inspection apparatus.
3.2.18 operating pressure, n—alternatively known as the
3.3.10 emissivity (ɛ), n—the ratio of the radiance of a body
service pressure; see Practice E1067, Section 3, Terminology.
at a given temperature to the corresponding radiance of a
3.3 Definitions of Terms Specific to This Standard: blackbody at the same temperature.
E2981 − 21
3.3.11 extended discontinuity, n—a thermal discontinuity collectively they shall be two-fault tolerant from causing the
whose projection onto the inspection surface completely fills pressure to exceed the MDP of the system.
the field of view of the inspection apparatus.
3.3.22 miss, n—an existing discontinuity that is missed
3.3.12 field of view (FOV), n—theshapeandangulardimen-
during a POD examination.
sions of the cone or the pyramid that defines the object space
3.3.23 non-relevant or false indications, n—defined as ther-
imaged by the system; for example, rectangular 4° wide by 3°
mography system signals whose source or sources are from
high.
conditionsnotassociatedwithdefects,degradations,ordiscon-
3.3.13 hit, n—(in reference to probability of detection
tinuities of interest to the inspection process.
(POD), not AE) an existing discontinuity that is identified as a
3.3.24 probability of detection (POD), n—the fraction of
find during a POD demonstration examination.
nominal discontinuity sizes expected to be found given their
3.3.14 indication, n—the response or evidence from a non-
existence.
destructive examination; an indication is determined by inter-
3.3.25 shearogram, n—is the resulting image from the
pretation to be relevant, non-relevant, or false.
complex arithmetic combination of interferograms made with
3.3.15 inspection surface, n—the surface of the specimen
an image shearing interferometer showing target surface out-
that is exposed to the FT apparatus.
of-plane deformation derivatives and presented for interpreta-
3.3.16 Kaiser effect, n—the absence of detectable acoustic
tioninvariousimageprocessingalgorithms,includingstaticor
emission at a fixed sensitivity level, until previously applied
real-time wrapped phase maps, unwrapped phase maps, inte-
stress levels are exceeded.
grated images, or Doppler shift map.
3.3.17 leak-before-burst (LBB), n—a design approach in
3.3.26 shearography camera, shear camera, n—an image
which, at and below MAWP, potentially pre-existing flaws in
shearinginterferometercapableofimagingthetestpartsurface
themetallicliner,shouldtheygrow,willgrowthroughtheliner
for out-of-plane deformation derivatives when the test part is
and result in more gradual pressure-relieving leakage rather
subjected to a change in stress, used for shearography nonde-
than a more abrupt Burst-Before-Leak (BBL) rupture.
structive testing, usually including features for adjustment of
image focus, iris, shear vector adjustment and for the projec-
3.3.18 Level I indication, n—adefect/discontinuity/flawthat
tion of coherent light onto the test object area to be examined.
does not involve broken tow(s) or known reductions in
component residual burst pressure.
3.3.27 shear vector, n—in shearography, the separation
3.3.18.1 Discussion—ALevelIindicationdoesnotrequirea
vector between two identical images of the target in the output
problem report (PR) or discrepancy report (DR) and resulting
of an image shearing interferometer.
Material Review Board disposition.
3.3.27.1 Discussion—The shear vector is expressed in de-
3.3.19 Level II indication, n—a defect/discontinuity/flaw
grees of angle from the X axis, with a maximum of 90°, with
that does involve broken tow(s) or known reductions in
+ being in the positive Y direction and – in the negative Y
component residual burst pressure.
direction and the shear distance between identical points in the
3.3.19.1 Discussion—ALevel II indication requires a prob-
two sheared images expressed in inches or mm. (See Fig. 15,
lem report (PR) or discrepancy report (DR) and resulting
Shear Vector Convention.)
Material Review Board disposition.
3.3.28 soak period, n—the time during which a thermal
3.3.20 maximum allowable working pressure (MAWP),
image is acquired, beginning with the introduction of a gas or
n—the maximum operating pressure, to which operational
liquid into the COPV.
personnel may be exposed, for a pressure vessel.
3.3.29 stressing method, n—the application of a measured
3.3.20.1 Discussion—This pressure is synonymous with
and repeatable stress to the test object during a shearography
Maximum Expected Operating Pressure (MEOP), as used and
examination is selected for a particular defect type.
defined in ANSI/AIAA S-080 or ANSI/AIAA S-081.
3.3.29.1 Discussion—The applied stress changes may be in
3.3.21 maximum design pressure (MDP), n—the highest
the form of a partial or full vacuum, pressure, heat, vibration,
pressure defined by maximum relief pressure, maximum regu-
magnetic field, electric field, microwave, or mechanical load,
lator pressure, or maximum temperature.
and are timed with respect to the shear camera image acquisi-
3.3.21.1 Discussion—Transient pressures shall be consid-
tion in order to obtain the highest probability for defect
ered.WhendeterminingMDP,themaximumtemperaturetobe
detection. The applied stress method is engineered to develop
experienced during a launch abort to a site without cooling
a surface differential strain at the site of an anomaly. Also
facilities shall also be considered. In designing, analyzing, or
referred to as the “excitation method.”
testing pressurized hardware, loads other than pressure that are
3.3.30 thermal conductivity, n—the time rate of steady heat
present shall be considered and added to the MDP loads as
flowthroughthethicknessofaninfiniteslabofahomogeneous
appropriate. MDP in this standard is to be interpreted as
materialperpendiculartothesurface,inducedbyunittempera-
including the effects of these combined loads when the
ture difference.
non-pressure loads are significant. Where pressure regulators,
relief devices, or a thermal control system (for example, 3.3.30.1 Discussion—Thepropertymustbeidentifiedwitha
heaters),oracombinationthereof,areusedtocontrolpressure, specific mean temperature, since it varies with temperature.
E2981 − 21
3.3.31 thermal diffusivity, n—the ratio of thermal conduc- resins include epoxies, cyanate esters, polyurethanes, phenolic
tivity to the product of density and specific heat; a measure of resins, polyimides (including bismaleimides), polyamides, and
the rate at which heat propagates in a material; units [length / other high performance polymers. Common bond line adhe-
time]. sives are FM-73, urethane, West 105, and Epon 862 with
thicknesses ranging from 0.13 mm (0.005 in.) to 0.38 mm
3.3.32 thermal discontinuity, n—a change in the thermo-
(0.015 in.). Metallic liner and composite overwrap materials
physical properties of a specimen that disrupts the diffusion of
requirementsarefoundinANSI/AIAAS-080andANSI/AIAA
heat.
S-081, respectively.
3.4 Symbols:
NOTE 6—When carbon fiber is used, galvanic protection should be
3.4.1 a—the physical dimension of a discontinuity, flaw or
provided for the metallic liner using a physical barrier such as glass cloth
target—can be its depth, surface length, or diameter of a
in a resin matrix, or similarly, a bond line adhesive.
circulardiscontinuity,orradiusofsemi-circularorcornercrack
NOTE 7—Per the discretion of the cognizant engineering organization,
having the same cross-sectional area.
composite materials not developed and qualified in accordance with the
guidelines in MIL-HDBK-17, Volumes 1 and 3 should have an approved
3.4.2 a —the size of an initial, severe, worst case
material usage agreement.
discontinuity, also known as a rogue flaw.
4.2 The as-wound COPV is then cured and an autofrettage/
3.4.3 a —the size of a severe discontinuity that causes
crit
proof cycle is performed to evaluate performance and increase
LBB or BBL failure, often caused by a growing rogue flaw.
fatigue characteristics.
3.4.4 a —the discontinuity size that can be detected with
p
4.3 The strong drive to reduce weight and spatial needs in
probability p.
aerospace applications has pushed designers to adopt COPVs
3.4.5 a —the discontinuity size that can be detected with
pc
constructed with high modulus carbon fibers embedded in an
probability p with a statistical confidence level of c.
epoxy matrix. Unfortunately, high modulus fibers are weak in
3.4.6 â—(pronounced a-hat) the measured response of an
shear and therefore highly susceptible to fracture caused by
NDT system, to a target of size a. Units depend on testing
mechanical damage. Mechanical damage to the overwrap can
apparatus, and can be scale divisions, counts, number of
leave no visible indication on the composite surface, yet
contiguous illuminated pixels, millivolts, etc.
produce subsurface damage.
4. Significance and Use NOTE 8—The impact damage tolerance of the composite overwrap will
depend on the size and shape of the vessel, composite thickness (number
4.1 The COPVs covered in this guide consist of a metallic
of plies), and thickness of the composite overwrap relative to that of the
liner overwrapped with high-strength fibers embedded in
liner.
polymeric matrix resin (typically a thermoset) (Fig. 1). Metal-
4.4 Per MIL-HDBK-340 and ANSI/AIAA S-081, the pri-
lic liners may be spun-formed from a deep drawn/extruded
mary intended function of COPVs as discussed in this guide
monolithic blank or may be fabricated by welding formed
willbetostorepressurizedgasesandfluidswhereoneormore
components. Designers often seek to minimize the liner thick-
of the following apply:
ness in the interest of weight reduction. COPV liner materials
4.4.1 Contains stored energy of 19 310 J (14 240 ft-lbf) or
used can be aluminum alloys, titanium alloys, nickel-
greater based on adiabatic expansion of a perfect gas.
chromium alloys, and stainless steels, impermeable polymer
4.4.2 Contains a gas or liquid that would endanger person-
liner such as high density polyethylene, or integrated compos-
nel or equipment or create a mishap (accident) if released.
ite materials. Fiber materials can be carbon, aramid, glass,
4.4.3 Experiences a design limit pressure greater than 690
PBO, metals, or hybrids (two or more types of fibers). Matrix
kPa (100 psi).
4.5 According to NASA-STD-(I)-5019, COPVs shall com-
ply with the latest revision ofANSI/AIAAS-081. The follow-
ing requirements also apply when implementing S-081:
4.5.1 MaximumDesignPressure(MDP)shallbesubstituted
for all references to Maximum Expected Operating Pressure
(MEOP) in S-081.
4.5.2 COPVs shall have a minimum of 0.999 probability of
no stress rupture failure of the composite shell during the
service life.
NOTE 9—For other aerospace applications, the cognizant engineering
organization should select the appropriate probability of survival, for
example, 0.99, 0.999, 0.9999, etc., depending on the anticipated failure
mode, damage tolerance, safety factor, or consequence of failure, or a
combinationthereof.Forexample,aprobabilityofsurvivalof0.99means
that on average, 1 in 100 COPVs will fail. COPVs exhibiting catastrophic
failuremodes(BBLcompositeshellstressruptureversusLBBlinerleak),
lower damage tolerance (cylindrical versus spherical vessels), lower
safety factor, and high consequence of failure will be subject to more
FIG. 1 Typical Carbon Fiber Reinforced COPVs (NASA) rigorous NDT.
E2981 − 21
4.6 Application of the NDT procedures discussed in this The suitability of various NDT methods for detecting com-
guide is intended to reduce the likelihood of composite monly occurring composite flaw types is given in Table 1 in
overwrap failure, commonly denoted “burst before leak” Guide E2533.
(BBL), characterized by catastrophic rupture of the overwrap
4.9 Acceptance Criteria—Determination about whether a
and significant energy release, thus mitigating or eliminating
COPV meets acceptance criteria and is suitable for aerospace
the attendant risks associated with loss of pressurized
service should be made by the cognizant engineering organi-
commodity, and possibly ground support personnel, crew, or
zation. When examinations are performed in accordance with
mission.
this guide, the engineering drawing, specification, purchase
4.6.1 NDT is done on fracture-critical parts such as COPVs
order, or contract should indicate the acceptance criteria.
to establish that a low probability of preexisting flaws is
4.9.1 Accept/reject criteria should consist of a listing of the
present in the hardware.
expected kinds of imperfections and the rejection level for
4.6.2 Following the discretion of the cognizant engineering
each.
organization, NDT for fracture control of COPVs should
4.9.2 The classification of the articles under test into zones
follow additional general and detailed guidance described in
for various accept/reject criteria should be determined from
MIL-HDBK-6870, NASA-STD-(I)-5019, MSFC-RQMT-
contractual documents.
3479, or ECSS-E-30-01A, or a combination thereof, not
4.9.3 Rejection of COPVs—If the type, size, or quantities of
covered in this guide.
defectsarefoundtobeoutsidetheallowablelimitsspecifiedby
4.6.3 Hardware that is proof tested as part of its acceptance
the drawing, purchase order, or contract, the composite article
(that is, not screening for specific flaws) should receive
should be separated from acceptable articles, appropriately
post-proof NDT at critical welds and other critical locations.
identified as discrepant, and submitted for material review by
the cognizant engineering organization, and given one of the
4.7 Discontinuity Types—Specific discontinuity types are
following dispositions: (1) acceptable as is, (2) subject to
associated with the particular processing, fabrication, and
further rework or repair to make the materials or component
service history of the COPV. Metallic liners can have cracks,
acceptable,or (3)scrapped(madepermanentlyunusable)when
buckles, leaks, and a variety of weld discontinuities (see 4.6 in
required by contractual documents.
Guide E2982). Non-bonding flaws (voids) between the liner
4.9.4 Acceptance criteria and interpretation of results
and composite overwrap can also occur. Similarly, the com-
shouldbedefinedinrequirementsdocumentspriortoperform-
posite overwrap can have preexisting manufacturing flaws
ing the examination. Advance agreement should be reached
introduced during fabrication, and damage caused by autofret-
betweenthepurchaserandsupplierregardingtheinterpretation
tage or proof testing before being placed into service. Once in
of the results of the examinations. All discontinuities having
service, additional damage can be incurred due to low velocity
signalsthatexceedtherejectionlevelasdefinedbytheprocess
or micrometeorite orbital debris impacts, cuts/scratches/
requirements documents should be rejected unless it is deter-
abrasion, fire, exposure to aerospace media, loading stresses,
mined from the part drawing that the rejectable discontinuities
thermal cycling, physical aging, oxidative degradation,
will not remain in the finished part.
weathering,andspaceenvironmenteffects(exposuretoatomic
oxygen and ionizing radiation). These factors will lead to
4.10 Certification of COPVs—ANSI/AIAA S-081 defines
complex damage states in the overwrap that can be visible or
the approach for design, analysis, and certification of COPVs.
invisible, macroscopic or microscopic. These damage states
Morespecifically,theCOPVshouldexhibitaleakbeforeburst
can be characterized by the presence of porosity, depressions,
(LBB) failure mode or should possess adequate damage
blisters, wrinkling, erosion, chemical modification, foreign
tolerance life (safe-life), or both, depending on criticality and
objectdebris(inclusions),towterminationerrors,towslippage,
whether the application is for a hazardous or nonhazardous
misaligned tows, distorted tows, matrix crazing, matrix
fluid. Consequently, the NDT method should detect any dis-
cracking, matrix-rich regions, under and over-cure of the
continuitythatcancauseburstatexpectedoperatingconditions
matrix, fiber-rich regions, fiber-matrix debonding, fiber pull-
during the life of the COPV. The Damage-Tolerance Life
out, fiber splitting, fiber breakage, bridging, liner/overwrap
requiresthatanydiscontinuitypresentinthelinerwillnotgrow
debonding, and delamination. Often these discontinuities can
to failure during the expected life of the COPV. Fracture
placed into four major categories: (1) manufacturing; (2)
mechanics assessments of flaw growth are the typical method
scratch/scuff/abrasion; (3) mechanical damage; and (4) discol-
of setting limits on the sizes of discontinuities that can safely
oration.
exist. This establishes the defect criteria: all discontinuities
equal to or larger than the minimum size or have J-integral or
4.8 Effect of Defect—The effect of a given composite flaw
other applicable fracture mechanics based criteria that will
type or size (“effect of defect”) is difficult to determine unless
result in failure of the vessel within the expected service life
test specimens or articles with known types and sizes of flaws
are classified as defects and should be addressed by the
are tested to failure. Given this potential uncertainty, detection
cognizant engineering organization.
of a flaw is not necessarily grounds for rejection (that is, a
defect)unlesstheeffectofdefecthasbeendemonstrated.Even 4.10.1 Design Requirements—COPV design requirements
the detection of a given flaw type and size can be in doubt related to the composite overwrap are given in ANSI/AIAA
unless physical reference specimens with known flaw types S-081. The key requirement is the stipulation that the COPV
andsizesundergoevaluationusingtheNDTmethodofchoice. shall exhibit a LBB failure mode or shall possess adequate
E2981 − 21
damage tolerance life (safe-life), or both, depending on criti- 4.11.3.2 Those which also provide some quantitative mea-
cality and application. The overwrap design shall be such that, sure of the size of the target (for example, flaw or crack), that
if the liner develops a leak, the composite will allow the is, â versus a data.
leaking fluid (liquid or gas) to pass through it so that there will 4.11.3.3 Those which produce visual images of the target
benoriskofcompositerupture.However,underuseconditions and its surroundings.
of prolonged, elevated stress, assurance should be made that
5. Basis of Application
the COPVoverwrap will also not fail by stress (creep) rupture,
5.1 Personnel Certification—NDT personnel should be cer-
as verified by theoretical analysis of experimental data (deter-
minationofriskreliabilityfactors)orbytest(couponsorflight tified in accordance with a nationally or internationally recog-
nized practice or standard, such asANSI/ASNT-CP-189, SNT-
hardware).
TC-1A, NAS 410, ISO 9712, or a similar document. The
4.11 Probability of Detection (POD)—Detailed instruction
practiceorstandardusedanditsapplicablerevisionsshouldbe
for assessing the reliability of NDT data using POD of a
specified in any contractual agreement between the using
complex structure such as a COPV is beyond the scope of this
parties.
guide. Therefore, only general guidance is provided. More
detailed instruction for assessing the capability of an NDT 5.2 Personnel Qualification—NDT personnel should be
method in terms of the POD as a function of flaw size, a, can qualified by accepted training programs, applicable on-the-job
be found in MIL-HDBK-1823. The statistical precision of the trainingunderacompetentmentororcomponentmanufacturer.
estimated POD(a) function (Fig. 2) depends on the number of Cognizant engineering organization and manufacturer qualifi-
cation will only be applied to the components under direct
inspection sites with targets, the size of the targets at the
inspection sites, and the basic nature of the examination result training experience or production.
(hit/miss or magnitude of signal response).
5.3 Qualification of Nondestructive Test Agencies—Ifspeci-
4.11.1 Given that a has become a de facto design
90/95
fied in the contractual agreement, NDT agencies should be
criterion,itismoreimportanttoestimatethe90thpercentileof
qualifiedandevaluatedasdescribedinSpecificationE543.The
the POD(a) function more precisely than lower parts of the
applicable edition of Specification E543 should be specified in
curve.Thiscanbeaccomplishedbyplacingmoretargetsinthe
the contractual agreement.
region of the a value but with a range of sizes so the entire
5.4 Selection of NDT—ChoiceoftheproperNDTprocedure
curve can still be estimated.
(outside of those required per ANSI/AIAA S-081, KNPR
NOTE 10—a for a composite overwrap and generation of a POD(a)
90/95
8715.3, and AFSPCMAN 91-710) is based on the following
function is predicated on the assumption that effect of defect has been
considerations: (a)theflawtobedetectedandthesensitivityof
demonstrated and is known for a specific composite flaw type and size,
theNDTmethodforthatgivenflaw, (b)anyspecialequipment
and that detection of a flaw of that same type and size is grounds for
or facilities requirements, or both, (c) cost of examination, and
rejection, that is, the flaw is a rejectable defect.
(d) personnel and facilities qualification.
4.11.2 To provide reasonable precision in the estimates of
5.4.1 The desired NDT output should be clearly separated
the POD(a) function, experience suggests that the specimen
from responses from surrounding material and configurations
test set contain at least 60 targeted sites if the system provides
and should be applicable to the general material conditions,
only a binary, hit/miss response and at least 40 targeted sites if
environment and operational restraints.
the system provides a quantitative target response, â. These
numbers are minimums. 5.5 Life Cycle Considerations—NDT has been shown to be
4.11.3 For purposes of POD studies, the NDT method usefulduring: (a)productandprocessdesignandoptimization,
should be classified into one of three categories: (b) on-line process control, (c) after manufacture examination,
4.11.3.1 Those which produce only qualitative information (d) in service examination (including re-certification), and (e)
as to the presence or absence of a flaw, that is, hit/miss data. healthmonitoring.AftertheCOPVhasbeeninstalled(stages d
NOTE 1—POD(a), showing the location of the smallest detectable flaw and a (left). POD(a) with confidence bounds added and showing the location
of a (right).
90/95
FIG. 2 Probability of Detection as a Function of Flaw Size
E2981 − 21
and e), NDT measurements should be made on a “remove and thresholdofNDTcapability(denoted a ;see4.6)tohaveone
p/c
inspect” or “in-situ” basis depending on the processing area or more opportunities in this usage interval to detect the defect
controls, pressure system accessibility, and the procedure and and repair/replace the COPV before failure (Fig. 2 in Guide
equipment used. E2982).
5.5.1 Visual testing between stages a through e through
5.7 COPV Mapping Convention—All NDT techniques cov-
decommissioning, during which the partially assembled or
ered in this guide require establishment of a coordinate
completed COPV is handled should also be considered and is
convention allowing the location of indications detected to be
required prior to flight perANSI/AIAAS-081, KNPR 8715.3,
located on the outside surface
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
´1
Designation: E2981 − 15 E2981 − 21
Standard Guide for
Nondestructive TestingExamination of the Composite
Overwraps in Filament Wound Pressure Vessels Used in
Aerospace Applications
This standard is issued under the fixed designation E2981; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Fig. 2 and the definition in 3.4.3 were updated editorially in April 2019.
1. Scope
1.1 This guide discusses current and potential nondestructive testing (NDT) procedures for finding indications of discontinuities
and accumulated damage in the composite overwrap of filament wound pressure vessels, also known as composite overwrapped
pressure vessels (COPVs). In general, these vessels have metallic liner thicknesses less than 2.3 mm (0.090 in.), and fiber loadings
in the composite overwrap greater than 60 percent% by weight. In COPVs, the composite overwrap thickness will be of the order
of 2.0 mm (0.080 in.) for smaller vessels and up to 20 mm (0.80 in.) for larger ones.
1.2 This guide focuses on COPVs with nonload-sharing metallic liners used at ambient temperature, which most closely represents
a Compressed Gas Association (CGA) Type III metal-lined composite tank. However, it also has relevance to 1)(1) monolithic
metallic pressure vessels (PVs) (CGA Type I), 2)(2) metal-lined hoop-wrapped COPVs (CGA Type II), 3)(3) plastic-lined
composite pressure vessels (CPVs) with a nonload-sharing liner (CGA Type IV), and 4)(4) an all-composite, linerless COPV
(undefined Type). This guide also has relevance to COPVs used at cryogenic temperatures.
1.3 The vessels covered by this guide are used in aerospace applications; therefore, the inspection requirements for discontinuities
and inspection points will in general be different and more stringent than for vessels used in non aerospace applications.
1.4 This guide applies to 1)(1) low pressure COPVs used for storing aerospace media at maximum allowable working pressures
3 3
(MAWPs) up to 3.5 MPa (500 psia) and volumes up to 2 m2 L (70 ft ), and 2)(2) high pressure COPVs used for storing
3 3
compressed gases at MAWPs up to 70 MPa (10,000(10 000 psia) and volumes down to 8000 cm8 L (500 in. ). Internal vacuum
storage or exposure is not considered appropriate for any vessel size.
NOTE 1—Some vessels are evacuated during filling operations, requiring the tank to withstand external (atmospheric) pressure, while other vessels may
either contain or be immersed in cryogenic fluids, or both, requiring the tanks to withstand any potentially deleterious effects of differential thermal
contraction.
1.5 The composite overwraps under consideration include, but are not limited to, ones made from various polymer matrix resins
(for example, epoxies, cyanate esters, polyurethanes, phenolic resins, polyimides (including bismaleimides), and polyamides) with
This test method guide is under the jurisdiction of ASTM Committee E07 on Nondestructive Testing and is the direct responsibility of Subcommittee E07.10 on
Specialized NDT Methods.
Current edition approved July 1, 2015Feb. 1, 2021. Published September 2015February 2021. Originally approved in 2015. Last previous edition approved in 2015 as
ε1
E2981 – 15 . DOI: 10.1520/E2981-15E01.10.1520/E2981-21.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2981 − 21
continuous fiber reinforcement (for example, carbon, aramid, glass, or poly-(phenylenebenzobisoxazole) (PBO)). The metallic
liners under consideration include, but are not limited to, aluminum alloys, titanium alloys, nickel-chromium alloys, and stainless
steels.
1.6 This guide describes the application of established NDT methods; namely, Acoustic Emission (AE, Section 7), Eddy Current
Testing (ECT,(ET, Section 8), Laser Shearography (Section 9), RadiologicRadiographic Testing (RT, Section 10), Thermographic
Testing (TT,Infrared Thermography (IRT, Section 11), Ultrasonic Testing (UT, Section 12), and Visual Testing (VT, Section 13).
These methods can be used by cognizant engineering organizations for detecting and evaluating flaws, defects, and accumulated
damage in the composite overwrap of new and in-service COPVs.
NOTE 2—Although visual testing is discussed and required by current range standards, emphasis is placed on complementary NDT procedures that are
sensitive to detecting flaws, defects, and damage that leave no visible indication on the COPV surface.
NOTE 3—In aerospace applications, a high priority is placed on light weight material, while in commercial applications;applications, weight is typically
sacrificed to obtain increased robustness. Accordingly, the need to detect damage below the visual damage threshold is more important in aerospace
vessels.
NOTE 4—Currently, no determination of residual strength can be made by any NDT method.
1.7 All methods discussed in this guide (AE, ET, shearography, RT, TT,IRT, UT, and VT) are performed on the composite
overwrap after overwrapping and structural cure. For NDT procedures for detecting discontinuities in thin-walled metallic liners
in filament wound pressure vessels, or in the bare metallic liner before overwrapping; namely, AE, ET, laser profilometry, leak
testing (LT), penetrant testing (PT), and RT; consult Guide E2982.
1.8 In the case of COPVs which are impact damage sensitive and require implementation of a damage control plan, emphasis is
placed on NDT methods that are sensitive to detecting damage in the composite overwrap caused by impacts at energy levels and
which may or may not leave any visible indication on the COPV composite surface.
1.9 This guide does not specify accept-reject criteria (subsection (4.9) to be used in procurement or used as a means for approving
filament wound pressure vessels for service. Any acceptance criteria specified are given solely for purposes of refinement and
further elaboration of the procedures described in this guide. Project or original equipment manufacturer (OEM) specific
accept/reject criteria shallshould be used when available and take precedence over any acceptance criteria contained in this
document. If no accept/reject criteria are available, any NDT method discussed in this guide that identifies broken fibers
shallshould require disposition by the cognizant engineering organization.
1.10 This guide references both established ASTM methods that have a foundation of experience and that yield a numerical result,
and newer procedures that have yet to be validated and are better categorized as qualitative guidelines and practices. The latter are
included to promote research and later elaboration in this guide as methods of the former type.
1.11 To ensure proper use of the referenced standard documents, there are recognized NDT specialists that are certified according
to industry and company NDT specifications. It is recommended that an NDT specialist be a part of any composite component
design, quality assurance, in-service maintenance, or damage examination.
1.12 Units—The values stated in SI units are to be regarded as standard. The English units given in parentheses are provided for
information only.
1.13 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use. Some specific hazards statements are given in Section 77 on Hazards.
1.14 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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2. Referenced Documents
2.1 ASTM Standards:
D3878 Terminology for Composite Materials
D5687 Guide for Preparation of Flat Composite Panels with Processing Guidelines for Specimen Preparation
E114 Practice for Ultrasonic Pulse-Echo Straight-Beam Contact Testing
E164 Practice for Contact Ultrasonic Testing of Weldments
E317 Practice for Evaluating Performance Characteristics of Ultrasonic Pulse-Echo Testing Instruments and Systems without the
Use of Electronic Measurement Instruments
E543 Specification for Agencies Performing Nondestructive Testing
E569 Practice for Acoustic Emission Monitoring of Structures During Controlled Stimulation
E650E650/E650M Guide for Mounting Piezoelectric Acoustic Emission Sensors
E750 Practice for Characterizing Acoustic Emission Instrumentation
E976 Guide for Determining the Reproducibility of Acoustic Emission Sensor Response
E1001 Practice for Detection and Evaluation of Discontinuities by the Immersed Pulse-Echo Ultrasonic Method Using
Longitudinal Waves
E1065E1065/E1065M Practice for Evaluating Characteristics of Ultrasonic Search Units
E1067 Practice for Acoustic Emission Examination of Fiberglass Reinforced Plastic Resin (FRP) Tanks/Vessels
E1106 Test Method for Primary Calibration of Acoustic Emission Sensors
E1118 Practice for Acoustic Emission Examination of Reinforced Thermosetting Resin Pipe (RTRP)
E1316 Terminology for Nondestructive Examinations
E1416 Practice for Radioscopic Examination of Weldments
E1742/E1742M Practice for Radiographic Examination
E1781/E1781M Practice for Secondary Calibration of Acoustic Emission Sensors
E1815 Test Method for Classification of Film Systems for Industrial Radiography
E2104 Practice for Radiographic Examination of Advanced Aero and Turbine Materials and Components
E2191 Practice for Examination of Gas-Filled Filament-Wound Composite Pressure Vessels Using Acoustic Emission
E2033 Practice for Radiographic Examination Using Computed Radiography (Photostimulable Luminescence Method)
E2338 Practice for Characterization of Coatings Using Conformable Eddy Current Sensors without Coating Reference
Standards
E2375 Practice for Ultrasonic Testing of Wrought Products
E2533 Guide for Nondestructive Testing of Polymer Matrix Composites Used in Aerospace Applications
E2580 Practice for Ultrasonic Testing of Flat Panel Composites and Sandwich Core Materials Used in Aerospace Applications
E2581 Practice for Shearography of Polymer Matrix Composites and Sandwich Core Materials in Aerospace Applications
E2582 Practice for Infrared Flash Thermography of Composite Panels and Repair Patches Used in Aerospace Applications
E2661/E2661M Practice for Acoustic Emission Examination of Plate-like and Flat Panel Composite Structures Used in
Aerospace Applications
E2662 Practice for Radiographic Examination of Flat Panel Composites and Sandwich Core Materials Used in Aerospace
Applications
E2698 Practice for Radiographic Examination Using Digital Detector Arrays
E2884 Guide for Eddy Current Testing of Electrically Conducting Materials Using Conformable Sensor Arrays
E2982 Guide for Nondestructive Testing of Thin-Walled Metallic Liners in Filament-Wound Pressure Vessels Used in Aerospace
Applications
2.2 AIA Standard:
NAS 410 NAS Certification and Qualification of Nondestructive Test Personnel
2.3 ANSI/AIAA Standards:
ANSI/AIAA S-080 Space Systems—Metallic Pressure Vessels, Pressurized Structures, and Pressure Components
ANSI/AIAA S-0801S-081 Space Systems—Composite Overwrapped Pressure Vessels (COPVs)
ANSI NGV2-2007 American National Standard for Natural Gas Vehicle Containers
2.4 ASME Standard:Standards:
ASME Boiler and Pressure Vessel CodeCode, Section V, Nondestructive Examination, Article 11, Acoustic Emission
Examination of Fiber-Reinforced Plastic Vessels
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from Aerospace Industries Association (AIA), 1000 Wilson Blvd., Suite 1700, Arlington, VA 22209-3928, http://www.aia-aerospace.org.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
Available from American Society of Mechanical Engineers (ASME), ASME International Headquarters, Two Park Ave., New York, NY 10016-5990, http://
www.asme.org.
E2981 − 21
ASME Boiler and Pressure Vessel Code, Section X, Mandatory Appendix 8, Class III Vessels With Liners for Gaseous
Hydrogen in Stationary Service, Subsection 8-600 EXAMINATION, 8-600.2.7 Acoustic Emission Examination
2.5 ASNT Standards:
ASNT CP-189 Standard for Qualification and Certification of Nondestructive Testing Personnel
SNT-TC-1A Recommended Practice for Nondestructive Testing Personnel Qualification and Certification
2.6 BSI Documents:Document:
EN 4179 Aerospace Series — Qualification and Approval of Personnel for Non-Destructive Testing
2.7 Compressed Gas Association CGA Standards:
CGA Pamphlet C-6.2 Standard for Visual Inspection and Requalification of Fiber Reinforced High Pressure Cylinders
CGA Pamphlet C-6.4 Methods for Visual Inspection of AGA NGV2 Containers
2.8 Federal Standards:
21 CFR 1040.10 Laser Products
21 CFR 1040.11 Specific Purpose Laser Products
2.9 ISO Document:
ISO 9712 Non-destructive Testing—Qualification and Certification of NDT Personnel
2.10 LIA Document:
ANSI,ANSI Z136.1-2000 Safe Use of Lasers
2.11 MIL Documents:
MIL-HDBK-17 Composite Materials Handbook, Guidelines for Characterization of Structural Materials
MIL-HDBK-6870 Inspection Program Requirements, Nondestructive for Aircraft and Missile Materials and Parts
MIL-HDBK-340 Test Requirements for Launch, Upper-Stage, and Space Vehicles, Vol. I: Baselines
MIL-HDBK-787 Nondestructive Testing Methods of Composite Materials—Ultrasonics
MIL-HDBK-1823 Nondestructive Evaluation System Reliability Assessment
2.12 NASA Documents:
KNPR 8715.3 (Kennedy NASA Procedural Requirements) Chapter 13: NASA KSC Requirements for Ground-Based Vessels
and Pressurized Systems (PV/S), Rev. G.
NASA/TM-2012-21737 Elements of Nondestructive Examination for the Visual Inspection of Composite Structures
NASA-STD-(I)-5019 Fracture Control Requirements for Spaceflight Hardware
MSFC-RQMT-3479 Fracture Control Requirements for Composite and Bonded Vehicle and Payload Structures
2.13 Air Force Documents:
AFSPCMAN 91-710 v3 Range Safety User Requirements Manual Volume 3 - Launch Vehicles, Payloads, and Ground Support
Systems Requirements
AFSPCMAN 91-710 v6 Range Safety User Requirements Manual Volume 6 - Ground and Launch Personnel, Equipment,
Systems, and Material Operations Safety Requirements
2.14 ECSS Document:
ECSS-E-30-01A Space Engineering Fracture Control
3. Terminology
3.1 Abbreviations—The following abbreviations are adopted in this guide: acoustic emission (AE), eddy current testing (ET),
radiologicradiographic testing (RT), ultrasonic testing (UT), and visual testing (VT).
3.2 Definitions: Terminology in accordance with Terminologies E1316 and D3878 shall be used where applicable.
3.2.1 active source—source, n—see Test Method E569, Section 3, Terminology.
3.2.2 AE activity—activity, n—see Test Method E569, Section 3, Terminology.
Available from American Society for Nondestructive Testing (ASNT), P.O. Box 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org.
Available from British Standards Institution (BSI), 389 Chiswick High Rd., London W4 4AL, U.K., http://www.bsigroup.com.
Available from Compressed Gas Association (CGA), 14501 George Carter Way, Suite 103, Chantilly, VA 20151, http://www.cganet.com.
Available from U.S. Food and Drug Administration (FDA), 10903 New Hampshire Ave., Silver Spring, MD 20993, http://www.fda.gov.
Available from International Organization for Standardization (ISO), 1, ch. de la Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http://www.iso.org.
Available from the Laser Institute of America, 13501 Ingenuity Drive, Suite 128, Orlando, FL 32826.
Available from Standardization Documents Order Desk, Bldg 4 Section D, 700 Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Available from National Aeronautics and Space Administration, Technical Standards Program, 300 E. Street SW, Suite 5R30, Washington, D. C. 20546. https://standards.
nasa.gov/documents/nasa.
Available from ESA Publications Division, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands.
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3.2.3 AE counts (N)—(N), n—the number of times the acoustic emission signal exceeds a preset threshold during any selected
portion of a test.
3.2.4 AE source—source, n—a region of impact damage damage, fiber/bundle breakage, delamination growth, etc., in the
composite overwrap or growing crack in the metallic liner of a COPV that can be classified as active, critically active, intense, or
critically intense.
3.2.5 AE source intensity—intensity, n—see Test Method E569, Section 3, Terminology.
3.2.6 AE test pressure—pressure, n—see Test Method E2191, Section 3, TerminologyTerminology.
3.2.7 cognizant engineering organization—organization, n—the company, government agency, or other authority responsible for
the design or end use of the system or component for which NDT is required. This, in addition to the design personnel, may include
personnel from engineering, materials and process engineering, stress analysis, NDT, or quality groups and other, as appropriate.
3.2.7.1 Discussion—
This, in addition to the design personnel, may include personnel from engineering, materials and process engineering, stress
analysis, NDT, or quality groups and other, as appropriate.
3.2.8 critically active source—source, n—see Test Method E569, Section 3, Terminology.
3.2.9 critically intense source—source, n—see Test Method E569, Section 3, Terminology.
3.2.10 defect—defect, n—see Terminology E1316.
3.2.11 discontinuity—discontinuity, n—see Terminology E1316.
3.2.12 flaw—flaw, n—see Terminology E1316.
3.2.13 Felicity effect—effect, n—the presence of acoustic emission, detectable at a fixed, predetermined sensitivity level at stress
levels below those previously applied. E1106
3.2.14 Felicity ratio—ratio, n—the ratio of the stress at which the Felicity effect occurs to the previously applied maximum stress.
E1106, E1118
NOTE 5—The fixed sensitivity level will usually be the same as was used for the previous loading or test ((Practice E1118).
3.2.15 high-amplitude threshold—threshold, n—a threshold for large amplitude AE events. (See A2.3 of Annex A2, Practice
E1106)
3.2.16 intense source—source, n—see Test Method E569, Section 3, Terminology.
3.2.17 low-amplitude threshold—threshold, n—the threshold above which AE counts (N) are measured. (See A2.2 of Annex A2,
Practice E1106).
3.2.18 operating pressure—pressure, n—alternatively known as the service pressure,pressure; see Practice E1067, Section 3,
Terminology.
3.3 Definitions of Terms Specific to This Standard:
3.3.1 active thermography—thermography, n—active thermography refers to the examination of an object upon intentional
application of an external energy source. The energy source (active or passive) may be a source of heat, mechanical energy
(vibration or fatigue testing), electrical current, or any other form of energy.
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3.3.1.1 Discussion—
The energy source (active or passive) may be a source of heat, mechanical energy (vibration or fatigue testing), electrical current,
or any other form of energy.
3.3.2 aspect ratio, —n—the diameter to depth ratio of a flaw. For irregularly shaped flaws, diameter refers to the minor axis of
an equivalent rectangle that approximates the flaw shape and area.
3.3.2.1 Discussion—
For irregularly shaped flaws, diameter refers to the minor axis of an equivalent rectangle that approximates the flaw shape and area.
3.3.3 burst-before-leak (BBL)(BBL), —n—an insidious failure mechanism exhibited by composite materials usually associated
with broken fibers caused by mechanical damage, or with stress rupture at an applied constant load (pressure), whereby the
minimum time during which the composite maintains structural integrity considering the combined effects of stress level(s), time
at stress level(s), and associated environment is exceeded, resulting in a sudden, catastrophic event.
3.3.4 coherent light source—source, n—a monochromatic beam of light having uniform phase over a minimum specified length
known as the coherent length.
3.3.5 composite overwrapped pressure vessel (COPV)—(COPV), n—an inner shell overwrapped with multiple plies of polymer
matrix impregnated reinforcing fiber wound at different wrap angles that form a composite shell. The inner shell or liner may
consist of an impervious metallic or nonmetallic material. The vessel may be cylindrical or spherical and be manufactured with
a minimum of one interface port for pressure fitting or valve attachment (synonymous with filament wound pressure vessel), or
both.
3.3.5.1 Discussion—
The inner shell or liner may consist of an impervious metallic or nonmetallic material. The vessel may be cylindrical or spherical
and be manufactured with a minimum of one interface port for pressure fitting or valve attachment (synonymous with filament
wound pressure vessel), or both.
3.3.6 critical Felicity ratio—ratio, n—the lower threshold of the Felicity ratio at which rupture has been previously observed,
regardless of what the current applied load or pressure is.
3.3.7 damage control plan (DCP)—a control document that captures the credible damage threats to a COPV during manufacturing,
transportation and handling, and integration into a space system up to the time of launch/re-launch, reentry and landing, as
applicable, and the steps taken to mitigate the possibility of damage due to these threats, as well as delineation of NDT performed
(for example, visual testing) throughout the life cycle of the COPV. The DPC shall be provided by the design agency and made
available for review by the applicable safety/range organization per AIAA S 081, KNPR 8715.3, and AFSPCMAN 91 710.
3.3.7.1 Discussion—
The DPC shall be provided by the design agency and made available for review by the applicable safety/range organization per
ANSI/AIAA S-081, KNPR 8715.3, and AFSPCMAN 91-710.
3.3.8 de-correlation—de-correlation, n—loss of shearography phase data caused by test part deformation exceeding the resolution
of the shearing interferometer sensor or motion between the test object and shearing interferometer during data acquisition.
3.3.9 discrete discontinuity—discontinuity, n—a thermal discontinuity whose projection onto the inspection surface is smaller than
the field of view of the inspection apparatus.
3.3.10 emissivity (ɛ)—(ɛ), n—the ratio of the radiance of a body at a given temperature to the corresponding radiance of a
blackbody at the same temperature.
3.3.11 extended discontinuity—discontinuity, n—a thermal discontinuity whose projection onto the inspection surface completely
fills the field of view of the inspection apparatus.
3.3.12 field of view (FOV)—(FOV), n—Thethe shape and angular dimensions of the cone or the pyramid that defines the object
space imaged by the system; for example, rectangular 4 degrees 4° wide by 3 degrees 3° high.
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3.3.13 hit—hit, n—(in reference to probability of detection (POD), not AE) an existing discontinuity that is identified as a find
during a POD demonstration examination.
3.3.14 indication, —n—Thethe response or evidence from a nondestructive examination. Anexamination; an indication is
determined by interpretation to be relevant, non-relevant, or false.
3.3.15 inspection surface—surface, n—the surface of the specimen that is exposed to the FT apparatus.
3.3.16 Kaiser effect—effect, n—the absence of detectable acoustic emission at a fixed sensitivity level, until previously applied
stress levels are exceeded.
3.3.17 leak-before-burst (LBB)—(LBB), n—a design approach in which, at and below MAWP, potentially pre-existing flaws in
the metallic liner, should they grow, will grow through the liner and result in more gradual pressure-relieving leakage rather than
a more abrupt Burst-Before-Leak (BBL) rupture.
3.3.18 Level I indication—indication, n—a defect/discontinuity/flaw that doesn’t does not involve broken tow(s) or known
reductions in component residual burst pressure. A Level I indication does not require a problem report (PR) or discrepancy report
(DR) and resulting Material Review Board disposition.
3.3.18.1 Discussion—
A Level I indication does not require a problem report (PR) or discrepancy report (DR) and resulting Material Review Board
disposition.
3.3.19 Level II indication—indication, n—a defect/discontinuity/flaw that does involve broken tow(s) or known reductions in
component residual burst pressure. A Level II indication requires a problem report (PR) or discrepancy report (DR) and resulting
Material Review Board disposition.
3.3.19.1 Discussion—
A Level II indication requires a problem report (PR) or discrepancy report (DR) and resulting Material Review Board disposition.
3.3.20 maximum allowable working pressure (MAWP)—(MAWP), n—Thethe maximum operating pressure, to which operational
personnel may be exposed, for a pressure vessel. This pressure is synonymous with Maximum Expected Operating Pressure
(MEOP), as used and defined in ANSI/AIAA S-080 or ANSI/AIAA S-081.
3.3.20.1 Discussion—
This pressure is synonymous with Maximum Expected Operating Pressure (MEOP), as used and defined in ANSI/AIAA S-080 or
ANSI/AIAA S-081.
3.3.21 maximum design pressure (MDP)—(MDP), n—Thethe highest pressure defined by maximum relief pressure, maximum
regulator pressure, or maximum temperature. Transient pressures shall be considered. When determining MDP, the maximum
temperature to be experienced during a launch abort to a site without cooling facilities shall also be considered. In designing,
analyzing, or testing pressurized hardware, loads other than pressure that are present shall be considered and added to the MDP
loads as appropriate. MDP in this standard is to be interpreted as including the effects of these combined loads when the
non-pressure loads are significant. Where pressure regulators, relief devices, or a thermal control system (e.g., heaters), or a
combination thereof, are used to control pressure, collectively they shall be two-fault tolerant from causing the pressure to exceed
the MDP of the system.
3.3.21.1 Discussion—
Transient pressures shall be considered. When determining MDP, the maximum temperature to be experienced during a launch
abort to a site without cooling facilities shall also be considered. In designing, analyzing, or testing pressurized hardware, loads
other than pressure that are present shall be considered and added to the MDP loads as appropriate. MDP in this standard is to be
interpreted as including the effects of these combined loads when the non-pressure loads are significant. Where pressure regulators,
relief devices, or a thermal control system (for example, heaters), or a combination thereof, are used to control pressure,
collectively they shall be two-fault tolerant from causing the pressure to exceed the MDP of the system.
3.3.22 miss—miss, n—an existing discontinuity that is missed during a POD examination.
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3.3.23 non-relevant or false indications—indications, n—defined as thermography system signals whose source or sources are
from conditions not associated with defects, degradations, or discontinuities of interest to the inspection process.
3.3.24 probability of detection (POD)—(POD), n—the fraction of nominal discontinuity sizes expected to be found given their
existence.
3.3.25 shearogram—shearogram, n—is the resulting image from the complex arithmetic combination of interferograms made
with an image shearing interferometer showing target surface out-of-plane deformation derivatives and presented for interpretation
in various image processing algorithms, including static or real-time wrapped phase maps, unwrapped phase maps, integrated
images, or Doppler shift map.
3.3.26 shearography camera, shear camera—camera, n—an image shearing interferometer capable of imaging the test part
surface for out-of-plane deformation derivatives when the test part is subjected to a change in stress, used for shearography
nondestructive testing, usually including features for adjustment of image focus, iris, shear vector adjustment and for the projection
of coherent light onto the test object area to be examined.
3.3.27 shear vector—vector, n—in Shearography,shearography, the separation vector between two identical images of the target
in the output of an image shearing interferometer. The shear vector is expressed in degrees of angle from the X axis, with a
maximum of 90°, with + being in the positive Y direction and – in the negative Y direction and the shear distance between identical
points in the two sheared images expressed in inches or mm. (See Figure 15, Shear Vector Convention.)
3.3.27.1 Discussion—
The shear vector is expressed in degrees of angle from the X axis, with a maximum of 90°, with + being in the positive Y direction
and – in the negative Y direction and the shear distance between identical points in the two sheared images expressed in inches
or mm. (See Fig. 15, Shear Vector Convention.)
3.3.28 soak period, —n—the time during which a thermal image is acquired, beginning with the introduction of a gas or liquid
into the COPV.
3.3.29 stressing method—method, n—the application of a measured and repeatable stress to the test object during a shearography
examination,examination is selected for a particular defect type. The applied stress changes may be in the form of a partial or full
vacuum, pressure, heat, vibration, magnetic field, electric field, microwave, or mechanical load, and is timed with respect to the
shear camera image acquisition in order to obtain the highest probability for defect detection. The applied stress method is
engineered to develop a surface differential strain at the site of an anomaly. Also referred to as the “excitation method.”
3.3.29.1 Discussion—
The applied stress changes may be in the form of a partial or full vacuum, pressure, heat, vibration, magnetic field, electric field,
microwave, or mechanical load, and are timed with respect to the shear camera image acquisition in order to obtain the highest
probability for defect detection. The applied stress method is engineered to develop a surface differential strain at the site of an
anomaly. Also referred to as the “excitation method.”
3.3.30 thermal conductivity—conductivity, n—Thethe time rate of steady heat flow through the thickness of an infinite slab of a
homogeneous material perpendicular to the surface, induced by unit temperature difference. The property must be identified with
a specific mean temperature, since it varies with temperature.
3.3.30.1 Discussion—
The property must be identified with a specific mean temperature, since it varies with temperature.
3.3.31 thermal diffusivity—diffusivity, n—the ratio of thermal conductivity to the product of density and specific heat; a measure
of the rate at which heat propagates in a material; units [length /time].
3.3.32 thermal discontinuity, —n—a change in the thermophysical properties of a specimen that disrupts the diffusion of heat.
3.4 Symbols:
3.4.1 a—the physical dimension of a discontinuity, flaw or target—can be its depth, surface length, or diameter of a circular
discontinuity, or radius of semi-circular or corner crack having the same cross-sectional area.
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3.4.2 a —the size of an initial, severe, worst case discontinuity, also known as a rogue flaw.
3.4.3 a —the size of a severe discontinuity that causes LBB or BBL failure, often caused by a growing rogue flaw.
crit
3.4.4 a —the discontinuity size that can be detected with probability p.
p
3.4.5 a —the discontinuity size that can be detected with probability p with a statistical confidence level of c.
pc
3.4.6 â—(pronounced a-hat) the measured response of an NDT system, to a target of size a. Units depend on testing apparatus,
and can be scale divisions, counts, number of contiguous illuminated pixels, millivolts, etc.
4. Significance and Use
4.1 The COPVs covered in this guide consist of a metallic liner overwrapped with high-strength fibers embedded in polymeric
matrix resin (typically a thermoset) (Fig. 1). Metallic liners may be spun-formed from a deep drawn/extruded monolithic blank
or may be fabricated by welding formed components. Designers often seek to minimize the liner thickness in the interest of weight
reduction. COPV liner materials used can be aluminum alloys, titanium alloys, nickel-chromium alloys, and stainless steels,
impermeable polymer liner such as high density polyethylene, or integrated composite materials. Fiber materials can be carbon,
aramid, glass, PBO, metals, or hybrids (two or more types of fibers). Matrix resins include epoxies, cyanate esters, polyurethanes,
phenolic resins, polyimides (including bismaleimides), polyamides, and other high performance polymers. Common bond line
adhesives are FM-73, urethane, West 105, and Epon 862 with thicknesses ranging from 0.13 mm (0.005 in.) to 0.38 mm (0.015
in.). Metallic liner and composite overwrap materials requirements are found in ANSI/AIAA S-080 and ANSI/AIAA S-081,
respectively.
NOTE 6—When carbon fiber is used, galvanic protection mustshould be provided for the metallic liner using a physical barrier such as glass cloth in a
resin matrix, or similarly, a bond line adhesive.
NOTE 7—Per the discretion of the cognizant engineering organization, composite materials not developed and qualified in accordance with the guidelines
in MIL-HDBK-17, Volumes 1 and 3 shallshould have an approved material usage agreement.
4.2 The as-wound COPV is then cured and an autofrettage/proof cycle is performed to evaluate performance and increase fatigue
characteristics.
4.3 The strong drive to reduce weight and spatial needs in aerospace applications has pushed designers to adopt COPVs
constructed with high modulus carbon fibers embedded in an epoxy matrix. Unfortunately, high modulus fibers are weak in shear
and therefore highly susceptible to fracture caused by mechanical damage. Mechanical damage to the overwrap can leave no
visible indication on the composite surface, yet produce subsurface damage.
FIG. 1 Typical Carbon Fiber Reinforced COPVs (NASA)
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NOTE 8—The impact damage tolerance of the composite overwrap will depend on the size and shape of the vessel, composite thickness (number of plies),
and thickness of the composite overwrap relative to that of the liner.
4.4 Per MIL-HDBK-340 and ANSI/AIAA S-081, the primary intended function of COPVs as discussed in this guide will be to
store pressurized gases and fluids where one or more of the following apply:
4.4.1 Contains stored energy of 19 310 J (14 240 ft-lbf) or greater based on adiabatic expansion of a perfect gas.
4.4.2 Contains a gas or liquid that would endanger personnel or equipment or create a mishap (accident) if released.
4.4.3 Experiences a design limit pressure greater than 690 kPa (100 psi).
4.5 According to NASA-STD-(I)-5019, COPVs shall comply with the latest revision of ANSI/AIAA Standard S-081. The
following requirements also apply when implementing S-081:
4.5.1 Maximum Design Pressure (MDP) shall be substituted for all references to Maximum Expected Operating Pressure (MEOP)
in S-081.
4.5.2 COPVs shall have a minimum of 0.999 probability of no stress rupture failure of the composite shell during the service life.
NOTE 9—For other aerospace applications, the cognizant engineering organization should select the appropriate probability of survival, for example, 0.99,
0.999, 0.9999, etc., depending on the anticipated failure mode, damage tolerance, safety factor, or consequence of failure, or a combination thereof. For
example, a probability of survival of 0.99 means that on average, 1 in 100 COPVs will fail. COPVs exhibiting catastrophic failure modes (BBL composite
shell stress rupture versus LBB liner leak), lower damage tolerance (cylindrical versus spherical vessels), lower safety factor, and high consequence of
failure will be subject to more rigorous NDT.
4.6 Application of the NDT procedures discussed in this guide is intended to reduce the likelihood of composite overwrap failure,
commonly denoted “burst before leak” (BBL), characterized by catastrophic rupture of the overwrap and significant energy release,
thus mitigating or eliminating the attendant risks associated with loss of pressurized commodity, and possibly ground support
personnel, crew, or mission.
4.6.1 NDT is done on fracture-critical parts such as COPVs to establish that a low probability of preexisting flaws is present in
the hardware.
4.6.2 Following the discretion of the cognizant engineering organization, NDT for fracture control of COPVs shallshould follow
additional general and detailed guidance described in MIL-HDBK-6870 MIL-HDBK-6870, NASA-STD-(I)-5019, MSFC-RQMT-
3479, or ECSS-E-30-01A, or a combination thereof, not covered in this guide.
4.6.3 Hardware that is proof tested as part of its acceptance (i.e., (that is, not screening for specific flaws) shallshould receive
post-proof NDT at critical welds and other critical locations.
4.7 Discontinuity Types—Specific discontinuity types are associated with the particular processing, fabrication, and service history
of the COPV. Metallic liners can have cracks, buckles, leaks, and a variety of weld discontinuities (see Section 4.5 4.6 in Guide
E2982). Non-bonding flaws (voids) between the liner and composite overwrap can also occur. Similarly, the composite overwrap
can have preexisting manufacturing flaws introduced during fabrication, and damage caused by autofrettage or proof testing before
being placed into service. Once in service, additional damage can be incurred due to low velocity or micrometeorite orbital debris
impacts, cuts/scratches/abrasion, fire, exposure to aerospace media, loading stresses, thermal cycling, physical aging, oxidative
degradation, weathering, and space environment effects (exposure to atomic oxygen and ionizing radiation). These factors will lead
to complex damage states in the overwrap that can be visible or invisible, macroscopic or microscopic. These damage states can
be characterized by the presence of porosity, depressions, blisters, wrinkling, erosion, chemical modification, foreign object debris
(inclusions), tow termination errors, tow slippage, misaligned tows, distorted tows, matrix crazing, matrix cracking, matrix-rich
regions, under and over-cure of the matrix, fiber-rich regions, fiber-matrix debonding, fiber pull-out, fiber splitting, fiber breakage,
bridging, liner/overwrap debonding, and delamination. Often these discontinuities can placed into four major categories: 1)(1)
manufacturing; 2)(2) scratch/scuff/abrasion; 3)(3) mechanical damage; and 4)(4) discoloration.
4.8 Effect of Defect—The effect of a given composite flaw type or size (“effect of defect”) is difficult to determine unless test
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specimens or articles with known types and sizes of flaws are tested to failure. Given this potential uncertainty, detection of a flaw
is not necessarily grounds for rejection (i.e., (that is, a defect) unless the effect of defect has been demonstrated. Even the detection
of a given flaw type and size can be in doubt unless physical reference specimens with known flaw types and sizes undergo
evaluation using the NDT method of choice. The suitability of various NDT methods for detecting commonly occurring composite
flaw types is given in Table 1 in Guide E2533.
4.9 Acceptance Criteria—Determination about whether a COPV meets acceptance criteria and is suitable for aerospace service
mustshould be made by the cognizant engineering organization. When examinations are performed in accordance with this guide,
the engineering drawing, specification, purchase order, or contract shallshould indicate the acceptance criteria.
4.9.1 Accept/reject criteria shallshould consist of a listing of the expected kinds of imperfections and the rejection level for each.
4.9.2 The classification of the articles under test into zones for various accept/reject criteria shallshould be determined from
contractual documents.
4.9.3 Rejection of COPVs—If the type, size, or quantities of defects are found to be outside the allowable limits specified by the
drawing, purchase order, or contract, the composite article shallshould be separated from acceptable articles, appropriately
identified as discrepant, and submitted for material review by the cognizant engineering organization, and given one of the
following dispositions: 1)(1) acceptable as is, 2)(2) subject to further rework or repair to make the materials or component
acceptable, or 3)(3) scrapped (made permanently unusable) when required by contractual documents.
4.9.4 Acceptance criteria and interpretation of result shallresults should be defined in requirements documents prior to performing
the examination. Advance agreement should be reached between the purchaser and supplier regarding the interpretation of the
results of the examinations. All discontinuities having signals that exceed the rejection level as defined by the process requirements
documents shallshould be rejected unless it is determined from the part drawing that the rejectable discontinuities will not remain
in the finished part.
4.10 Certification of COPVs—ANSI/AIAA S-081 defines the approach for design, analysis, and certification of COPVs. More
specifically, the COPV shallshould exhibit a leak before burst (LBB) failure mode or shallshould possess adequate damage
tolerance life (safe-life), or both, depending on criticality and whether the application is for a hazardous or nonhazardous fluid.
Consequently, the NDT method mustshould detect any discontinuity that can cause burst at expected operating conditions during
the life of the COPV. The Damage-Tolerance Life requires that any discontinuity present in the liner will not grow to failure during
the expected life of the COPV. Fracture mechanics assessments of flaw growth are the typical method of setting limits on the sizes
of discontinuities that can safely exist. This establishes the defect criteria: all discontinuities equal to or larger than the minimum
size or have J-integral or other applicable fracture mechanics based criteria that will result in failure of the vessel within the
expected service life are classified as defects and mustshould be addressed by the cognizant engineering organization.
4.10.1 Design Requirements—COPV design requirements related to the composite overwrap are given in ANSI/AIAA S-081. The
key requirement is the stipulation that the COPV shall exhibit a LBB failure mode or shall possess adequate damage tolerance life
(safe-life), or both, depending on criticality and application. The overwrap design shall be such that, if the liner develops a leak,
the composite will allow the leaking fluid (liquid or gas) to pass through it so that there will be no risk of composite rupture.
However, under use conditions of prolonged, elevated stress, assurance mustshould be made that the COPV overwrap will also not
fail by stress (creep) rupture, as verified by theoretical analysis of experimental data (determination of risk reliability factors) or
by test (coupons or flight hardware).
4.11 Probability of Detection (POD)—Detailed instruction for assessing the reliability of NDT data using POD of a complex
structure such as a COPV is beyond the scope of this guide. Therefore, only general guidance is provided. More detailed instruction
for assessing the capability of an NDT method in terms of the POD as a function of flaw size, a, can be found in MIL-HDBK-1823.
Th
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