ASTM D8131/D8131M-23

Standard Practice for Tensile Properties of Tapered and Stepped Joints of Polymer Matrix Composite Laminates

SIGNIFICANCE AND USE
5.1 This test practice is designed to produce tensile property data for material specifications, research and development, quality assurance, and structural design and analysis. Factors that influence the tensile response and should therefore be reported include the following: materials (laminates and adhesive), methods of material preparation including surface preparation prior to bonding, lay-ups, specimen stacking sequence, joint taper ratio or step length, ply overlap length, material relative thicknesses and stiffness of the parent and repair laminates, adhesive bond stiffness, specimen preparation, specimen conditioning, environment of testing, specimen alignment and gripping, speed of testing, time at temperature, void content, and volume percent reinforcement. Properties in the test direction, which may be obtained from this test practice, include the following:  
5.1.1 Ultimate tensile strength (based on the nominal parent material thickness), (Fptu).  
5.1.2 Ultimate tensile strength (based on the nominal repair material thickness), (Frtu).  
5.1.3 Ultimate running force per repair ply, (Nj).
SCOPE
1.1 This test practice defines the procedure for determination of the tensile strength of a tapered or stepped joint of polymer matrix composite materials. It is applicable to secondary bonded or co-bonded laminates with either unidirectional plies or woven fabric reinforcements. The materials to be bonded may be different material systems. In the bondline, a separate adhesive material may or may not be used (example: adhesives may be used with a prepreg system or may not be used with a wet lay-up repair system). The range of acceptable test laminates and thicknesses is described in 8.2.1.  
1.2 This practice supplements Test Method D3039/D3039M for tensile loading. Several important test specimen parameters (for example, joint length, ply overlaps, step depth, and taper ratio) are not mandated by this practice, however, these parameters are required to be specified and reported to support repeatable results.  
1.3 Unidirectional (0° ply orientation) tape composites, textile composites, as well as multidirectional composite laminates, can be tested.  
1.4 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.  
1.4.1 Within the text the inch-pound units are shown in brackets.  
1.5 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.6 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.

General Information

• Status: Published
• Publication Date: 30-Apr-2023
• ICS: 83.140.20 - Laminated sheets
• Technical Committee: D30 - Composite Materials
• Drafting Committee: D30.05 - Structural Test Methods

Relations

• Refers: ASTM D883-24 - Standard Terminology Relating to Plastics
• Effective Date: 01-Feb-2024
• Refers: ASTM D883-23 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2023
• Refers: ASTM D2734-23 - Standard Test Methods for Void Content of Reinforced Plastics
• Effective Date: 01-Oct-2023
• Refers: ASTM E456-13a(2022)e1 - Standard Terminology Relating to Quality and Statistics
• Effective Date: 01-Apr-2022
• Refers: ASTM D5229/D5229M-20 - Standard Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite Materials
• Effective Date: 01-Mar-2020
• Refers: ASTM D883-20 - Standard Terminology Relating to Plastics
• Effective Date: 01-Jan-2020
• Refers: ASTM D3878-19a - Standard Terminology for Composite Materials
• Effective Date: 15-Oct-2019
• Refers: ASTM D883-19c - Standard Terminology Relating to Plastics
• Effective Date: 01-Aug-2019
• Refers: ASTM D3878-19 - Standard Terminology for Composite Materials
• Effective Date: 15-Apr-2019
• Refers: ASTM D883-19a - Standard Terminology Relating to Plastics
• Effective Date: 15-Apr-2019
• Refers: ASTM D883-19 - Standard Terminology Relating to Plastics
• Effective Date: 01-Feb-2019
• Refers: ASTM D883-18a - Standard Terminology Relating to Plastics
• Effective Date: 01-Dec-2018
• Refers: ASTM D883-18 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2018
• Refers: ASTM D2584-18 - Standard Test Method for Ignition Loss of Cured Reinforced Resins
• Effective Date: 15-Sep-2018
• Refers: ASTM D3878-18 - Standard Terminology for Composite Materials
• Effective Date: 01-Apr-2018

Overview

ASTM D8131/D8131M-23 is a widely recognized standard practice for determining the tensile properties of tapered and stepped joints in polymer matrix composite laminates. Developed by ASTM International, this standard plays a critical role in evaluating the performance and integrity of secondary bonded and co-bonded joints in composite materials with unidirectional plies or woven fabric reinforcements. The test practice is applicable to a wide range of laminate types and joint configurations, making it indispensable for research and development, quality control, and structural design in industries that utilize polymer matrix composites.

Key Topics

• Scope of Application: The standard applies to joint configurations in polymer matrix composites, including both secondary bonded and co-bonded laminates. Test specimens can be unidirectional, textile, or multidirectional fiber composites.
• Tensile Property Data: The primary goal is to produce data on ultimate tensile strength (UTS) based on both the parent and repair laminate thickness, as well as the ultimate running force per repair ply. This data is crucial for establishing material specifications and ensuring structural reliability.
• Test Methodology: The standard supplements ASTM D3039/D3039M, offering additional guidance on specimen preparation, specimen geometry (tapered or stepped joints), layup details, stacking sequence, ply overlap, taper ratio, and joint design parameters. While it allows flexibility in these parameters, all must be meticulously documented for repeatable and comparable results.
• Influential Factors: Factors such as laminate and adhesive selection, surface preparation, specimen conditioning, environmental conditions, specimen alignment and gripping, and testing speed significantly influence tensile results and must be reported.
• Quality and Validity: Accuracy depends on precise specimen preparation, appropriate environmental conditioning, and correct alignment during testing. Failure modes and locations are also to be classified and documented.
• Reporting Requirements: The standard details the specific information that must be included in test reports to ensure clarity and compliance.

Applications

ASTM D8131/D8131M-23 is highly valuable in several practical contexts, including:

• Material Specifications: Used by manufacturers and engineers to qualify new composite materials and adhesive systems for use in critical bonded or co-bonded joints.
• Structural Design & Analysis: Supports engineers in designing safe, high-performance repairs or joints in aerospace, automotive, marine, and civil infrastructure applications by providing reliable tensile data.
• Quality Assurance: Ensures that production and repair processes meet strict mechanical performance criteria by verifying the integrity of joint configurations under tensile loads.
• Research & Development: Facilitates the comparison of different joint designs, surface preparations, adhesive types, and laminate stacking sequences to optimize the strength and durability of composite joints.
• Composite Repair Validation: Helps in validating repair techniques by simulating the mechanical loading and failure modes encountered in service.

Related Standards

For broader context and comprehensive evaluation, the following ASTM standards are frequently referenced alongside ASTM D8131/D8131M-23:

• ASTM D3039/D3039M: Tensile Properties of Polymer Matrix Composite Materials
• ASTM D792: Test Methods for Density and Specific Gravity of Plastics by Displacement
• ASTM D883: Terminology Relating to Plastics
• ASTM D2584: Ignition Loss of Cured Reinforced Resins
• ASTM D2734: Void Content of Reinforced Plastics
• ASTM D3171: Constituent Content of Composite Materials
• ASTM D3878: Terminology for Composite Materials
• ASTM D5229/D5229M: Moisture Absorption Properties and Equilibrium Conditioning
• ASTM E4: Practices for Force Calibration and Verification of Testing Machines
• ASTM E2533: Guide for Nondestructive Examination of Polymer Matrix Composites

These related standards help ensure comprehensive testing, reliable results, and consistent terminology across the field of polymer matrix composites.

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Keywords: ASTM D8131/D8131M-23, tensile properties, polymer matrix composites, stepped joints, tapered joints, composite laminate testing, bonded joints, co-bonded laminates, adhesive joints, quality assurance, structural design, composite repair testing.

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: D8131/D8131M − 23
Standard Practice for
Tensile Properties of Tapered and Stepped Joints of
Polymer Matrix Composite Laminates
This standard is issued under the fixed designation D8131/D8131M; 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.
1. Scope ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
1.1 This test practice defines the procedure for determina-
mendations issued by the World Trade Organization Technical
tion of the tensile strength of a tapered or stepped joint of
Barriers to Trade (TBT) Committee.
polymer matrix composite materials. It is applicable to second-
ary bonded or co-bonded laminates with either unidirectional
2. Referenced Documents
plies or woven fabric reinforcements. The materials to be
bonded may be different material systems. In the bondline, a 2
2.1 ASTM Standards:
separate adhesive material may or may not be used (example:
D792 Test Methods for Density and Specific Gravity (Rela-
adhesives may be used with a prepreg system or may not be
tive Density) of Plastics by Displacement
used with a wet lay-up repair system). The range of acceptable
D883 Terminology Relating to Plastics
test laminates and thicknesses is described in 8.2.1.
D2584 Test Method for Ignition Loss of Cured Reinforced
1.2 This practice supplements Test Method D3039/D3039M
Resins
for tensile loading. Several important test specimen parameters
D2734 Test Methods for Void Content of Reinforced Plastics
(for example, joint length, ply overlaps, step depth, and taper
D3039/D3039M Test Method for Tensile Properties of Poly-
ratio) are not mandated by this practice, however, these
mer Matrix Composite Materials
parameters are required to be specified and reported to support
D3171 Test Methods for Constituent Content of Composite
repeatable results.
Materials
D3878 Terminology for Composite Materials
1.3 Unidirectional (0° ply orientation) tape composites,
D5229/D5229M Test Method for Moisture Absorption Prop-
textile composites, as well as multidirectional composite
laminates, can be tested. erties and Equilibrium Conditioning of Polymer Matrix
Composite Materials
1.4 Units—The values stated in either SI units or inch-
E4 Practices for Force Calibration and Verification of Test-
pound units are to be regarded separately as standard. The
ing Machines
values stated in each system are not necessarily exact equiva-
E6 Terminology Relating to Methods of Mechanical Testing
lents; therefore, to ensure conformance with the standard, each
E122 Practice for Calculating Sample Size to Estimate, With
system shall be used independently of the other, and values
Specified Precision, the Average for a Characteristic of a
from the two systems shall not be combined.
Lot or Process
1.4.1 Within the text the inch-pound units are shown in
E177 Practice for Use of the Terms Precision and Bias in
brackets.
ASTM Test Methods
1.5 This standard does not purport to address all of the
E456 Terminology Relating to Quality and Statistics
safety concerns, if any, associated with its use. It is the
E2533 Guide for Nondestructive Examination of Polymer
responsibility of the user of this standard to establish appro-
Matrix Composites Used in Aerospace Applications
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use. 2.2 Other Documents:
CMH-17 Composite Materials Handbook-17 – Volume I
1.6 This international standard was developed in accor-
dance with internationally recognized principles on standard-
1 2
This practice is under the jurisdiction of ASTM Committee D30 on Composite For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Materials and is the direct responsibility of Subcommittee D30.05 on Structural Test contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Methods. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved May 1, 2023. Published June 2023. Originally the ASTM website.
approved in 2017. Last previous edition approved in 2017 as D8131/ Available for purchase in print and e-book format from SAE International,
ɛ1
D8131M – 17 . DOI: 10.1520/D8131_D8131M-23. http://books.sae.org/r-422/.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D8131/D8131M − 23
3. Terminology 3.3.13 x¯—mean or average (estimate of mean) of a sample
population for a given property
3.1 Terminology D3878 defines terms relating to high-
modulus fibers and their composites. Terminology D883 de-
4. Summary of Practice
fines terms relating to plastics. Terminology E6 defines terms
relating to mechanical testing. Terminology E456 and Practice 4.1 Tapered or Stepped Joint Tensile Strength—In accor-
E177 define terms relating to statistics. In the event of a dance with Test Method D3039/D3039M, but using a tapered
conflict between terms, Terminology D3878 shall have prece- or stepped joint configured specimen (Fig. 1 or Fig. 2), perform
dence over the other terminology standards. a uniaxial tension test of the composite laminate joint speci-
men.
NOTE 1—If the term represents a physical quantity, its analytical
dimensions are stated immediately following the term (or letter symbol) in
5. Significance and Use
fundamental dimension form, using the following ASTM standard sym-
bology for fundamental dimensions, shown within square brackets: [M]
5.1 This test practice is designed to produce tensile property
for mass, [L] for length, [T] for time, [θ] for thermodynamic temperature,
data for material specifications, research and development,
and [nd] for nondimensional quantities. Use of these symbols is restricted
quality assurance, and structural design and analysis. Factors
to analytical dimensions when used with square brackets, as the symbols
may have other definitions when used without the brackets.
that influence the tensile response and should therefore be
reported include the following: materials (laminates and
3.2 Definitions of Terms Specific to This Standard:
adhesive), methods of material preparation including surface
3.2.1 co-bonded (repair) laminate, n—the co-bonded lami-
preparation prior to bonding, lay-ups, specimen stacking
nate is the laminate that is bonded to the parent pre-cured
sequence, joint taper ratio or step length, ply overlap length,
laminate and cured in a second cure cycle.
material relative thicknesses and stiffness of the parent and
3.2.2 joint tensile strength, n—ultimate tensile force expe-
repair laminates, adhesive bond stiffness, specimen
rienced by the test specimen divided by the initial width of the
preparation, specimen conditioning, environment of testing,
joint area and the nominal thickness of the parent laminate.
specimen alignment and gripping, speed of testing, time at
3.2.3 nominal value, n—a value, existing in name only,
temperature, void content, and volume percent reinforcement.
assigned to a measureable property for the purpose of conve-
Properties in the test direction, which may be obtained from
nient designation. Tolerances may be applied to a nominal
this test practice, include the following:
value to define and acceptable range for the property.
5.1.1 Ultimate tensile strength (based on the nominal parent
tu
material thickness), (F ).
3.2.4 parent laminate, n—the parent laminate is the laminate
p
5.1.2 Ultimate tensile strength (based on the nominal repair
that is cured during the first cure cycle.
tu
material thickness), (F ).
r
3.2.5 secondary bonded (repair) laminate, n—the secondary
5.1.3 Ultimate running force per repair ply, (N ).
j
bonded laminate is a pre-cured laminate that is bonded to the
parent pre-cured laminate using a separate adhesive material
6. Interferences
(sometimes referred to as a pre-cured patch repair).
6.1 Material and Specimen Preparation—Poor material fab-
3.3 Symbols:
rication practices, lack of control of fiber alignment, and
3.3.1 CV—sample coefficient of variation, in percent
damage induced by improper coupon machining are known
tu
3.3.2 F —ultimate tensile strength, based on parent lami-
p
causes of high material data scatter in composites. For the
nate thickness
bonded joint specimens, the quality of the co-cured laminate
tu
3.3.3 F —ultimate tensile strength, based on repair lami-
(ply positioning, lengths, impregnation for wet lay-up material
r
nate thickness systems), lack of orientation control of the parent laminate or
pre-cured patch, and quality of the bond between the parent
3.3.4 h —specimen nominal parent laminate thickness as
p
and pre-cured or co-cured bonded laminates will have signifi-
specified by the test requestor (nominal ply thickness may be
cant effects on the test results.
available from the relevant material specification)
6.2 Gripping—A high percentage of grip-induced failures,
3.3.5 h —specimen nominal repair laminate thickness as
r
especially when combined with high material data scatter, is an
specified by the test requestor (nominal ply thickness may be
indicator of specimen gripping problems. Specimen gripping
available from the relevant material specification)
methods are discussed further in Test Method D3039/D3039M.
3.3.6 N —ultimate joint running force per ply
j
Specimen shall not be gripped within 25 mm [1.0 in.] of the
3.3.7 n—number of specimens
bondline (joint) area.
3.3.8 n —number of parent laminate plies
p 6.3 System Alignment—Excessive bending will cause pre-
mature failure, as well as highly inappropriate load-versus-
3.3.9 P —maximum force carried by test specimen at failure
f
deflection response. Every effort should be made to eliminate
3.3.10 S —standard deviation statistic of a sample popu-
n–1
excess bending from the test system. Bending may occur as a
lation for a given property
result of misaligned grips or from specimens themselves if
3.3.11 w—specimen width
improperly installed in the grips or out-of-tolerance caused by
3.3.12 x —test result for an individual coupon from the poor specimen preparation. If there are differences in material
i
sample population for a given property thickness and stiffness of the parent versus repair laminates, it
D8131/D8131M − 23
Notes:
1) Interpret drawing in accordance with ANSI Y14.5M-1982, subject to the following:
2) All dimensions in millimeters with decimal tolerances as follows:
No Decimal .X .XX
±3 ±1 ±.3
3) All angles have a tolerance of 65°.
4) Ply orientation direction tolerance relative to -A- within 6.5°.
5) Finish on machine edges not to exceed 1.6= (symbology in accordance with ASA B46.1, with roughness height in micrometers).
6) Values to be provided for the following, subject to any ranges shown in the field of drawing: material, lay-up, ply orientation reference relative to -A-, overall length,
gage length, coupon thickness, tab material, tab thickness, tab length, tab bevel angle, tab adhesive.
7) Repair specimen length design should not exceed scarf joint length by more than 20 %.
FIG. 1 Tensile Tapered and Stepped Joint Specimen – Geometry (SI)
may be necessary to intentionally induce distortion and mis- exceed the stiffness of the parent laminate, and ideally the
alignment upon gripping or to employ the use of a shim(s) on
repair material type (fabric or tape) and the ply thicknesses are
the thinner part of the specimen and accept the induced
the same as for the parent material. When they are different,
moment. If there is any doubt as to the alignment inherent in a
repair design compromises are necessary to obtain sufficient
given test machine, then the alignment should be checked as
repair stiffness and strength while not making the repair thicker
discussed in Test Method D3039/D3039M.
than the parent laminate. Details of repair design are beyond
the scope of this test practice. When filler plies or extra repair
6.4 Specimen Design—The bonded joint test specimen in-
plies are used, or when the repair material thicknesses do not
volves a parent and a repair laminate. These two laminates
match the parent material thicknesses, a difference in joint test
typically do not use the same material system. There are a
number of variables and factors which influence the selection specimen thickness on the two ends will occur, leading to some
induced bending, even if shims are applied. Also, any unbal-
of the repair laminate lay-up relative to the parent laminate.
Generally the repair lay-up is designed to match or slightly ance or asymmetry in the repair lay-up from filler plies, the
D8131/D8131M − 23
Notes:
1) Interpret drawing in accordance with ANSI Y14.5M-1982, subject to the following:
2) All dimensions in inches with decimal tolerances as follows:
No Decimal .X .XX
±.1 ±.03 ±.01
3) All angles have a tolerance of 65°.
4) Ply orientation direction tolerance relative to -A- within 6.5°.
5) Finish on machine edges not to exceed 64= (symbology in accordance with ASA B46.1, with roughness height in microinches).
6) Values to be provided for the following, subject to any ranges shown in the field of drawing: material, lay-up, ply orientation reference relative to -A-, overall length,
gage length, coupon thickness, tab material, tab thickness, tab length, tab bevel angle, tab adhesive.
7) Repair specimen length design should not exceed scarf joint length by more than 20 %.
FIG. 2 Tensile Tapered and Stepped Joint Specimen – Geometry (Inch-Pound)
repair laminate, or from extra repair plies may also lead to uses a +45 ply as the filler ply; this may induce some bending
some induced bending. The results from this test practice are
or twisting in the specimen under load. If an equivalent
reported as simple stress and force-per-width values; the joint
thickness fabric repair material is available, that could be used
specimen design will influence the validity of these reported
for the filler ply to provide a balanced lay-up.
results for use with bonded repair analysis methods.
6.5 Edge Effects in Angle Ply Laminates—Premature failure
6.4.1 Step Joint Filler Plies—The stepped joint specimen
can occur as a result of edge softening in laminates containing
should have a filler ply to avoid waviness in the repair plies.
off-axis plies. Because of this, the strength for angle ply
The filler ply should ideally be the same thickness as the
bottom ply in the parent laminate. The standard test specimen
configuration in this practice for unidirectional tape materials
D8131/D8131M − 23
laminates can be lower than expected. For multidirectional section of the test specimen within 63 °C [65 °F] of the
laminates containing significant axial fibers, the effect is not as required test temperature during the mechanical test. In
significant. addition, the chamber may have to be capable of maintaining
environmental conditions such as fluid exposure or relative
6.6 Environment—Results are affected by the environmental
humidity during the test.
conditions under which the tests are conducted. Laminates
tested in various environments can exhibit significant differ- 7.5 Data Acquisition Equipment—Equipment capable of
ences in both failure force and failure mode. Experience has continuous recording of force or displacement data, or both, is
demonstrated that cold-temperature environments can be criti- required.
cal for laminate tensile strength, however, elevated-
8. Sampling and Test Specimens
temperature, humid environments can also be critical. The
critical environmental condition for bonded joint specimens is
8.1 Sampling—Test at least five specimens per test condi-
complicated by having multiple materials in the test specimens.
tion unless valid results can be gained through the use of fewer
The failure modes may change between laminate tensile
specimens, such as in the case of a designed experiment. For
modes, bondline modes or laminate interfacial modes as the
statistically significant data, the procedures outlined in Practice
environmental condition changes. Critical environments must
E122 should be consulted. The method of sampling shall be
be assessed independently for each material system
reported.
combination, joint configuration, and stacking sequence tested.
8.2 Geometry—The test specimen is a rectangular strip of
6.7 Failure Mode—For a valid test, final failure of the
the composite to be tested with either a tapered or stepped
specimen must occur within the gage section. Which failure
joint, as shown in Figs. 1-3 (tapered) or Fig. 1, Fig. 2, and Fig.
modes are deemed acceptable will be governed by the particu-
4 (stepped). The standard specimen joint geometries are listed
lar material, configuration, and application (see 11.5 and Fig.
in the following subsections. Non-standard specimen configu-
5).
rations may be tested.
8.2.1 Tapered Joint Standard Specimen—The standard ta-
7. Apparatus
pered specimen is defined in Table 1. The specimen joint length
7.1 General Apparatus—General apparatus shall be in ac- is equal to the taper ratio times the parent laminate thickness.
cordance with Test Method D3039/D3039M.
For additional description, dimensions, and tolerances see Figs.
1-3.
7.2 Testing Machine—The testing machine shall be in con-
8.2.2 Tapered Joint Standard Specimen Length—The rec-
formance with Practices E4 and shall satisfy all requirements
ommended standard taper joint specimen length is defined as
defined in Test Method D3039/D3039M.
the repair length plus a minimum of 25 mm [1.0 in.] on both
7.3 Conditioning Chamber—When conditioning materials
ends between the end of the repair and the tabs or grips plus the
in other than ambient laboratory environments, a temperature-
length of the tabbed or gripped area. The recommended tab or
/moisture-level controlled environmental conditioning cham-
grip length is 50 mm [2.0 in.].
ber is required that shall be capable of maintaining the required
8.2.3 Tapered Joint Non-Standard Specimen—A number of
relative temperature to within 63 °C [65 °F] and the required
specimen parameters may be modified by the test requestor
relative vapor level to within 63 %. Chamber conditions shall
provided they are fully documented in the test report. These
be monitored either on an automated continuous basis or on a
include the parent and repair material and lay-ups, taper ratio,
manual basis at regular intervals.
and repair ply overlap length. Taper ratios for bonded joints are
7.4 Environmental Chamber—A chamber capable of enclos- typically between 30:1 and 50:1, with the repair ply overlap
ing the test fixture and specimen while they are mounted in the length typically rounded up from the value equal to the repair
testing machine, and capable of achieving the specified pl
...

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: D8131/D8131M − 17 D8131/D8131M − 23
Standard Practice for
Tensile Properties of Tapered and Stepped Joints of
Polymer Matrix Composite Laminates
This standard is issued under the fixed designation D8131/D8131M; 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—Editorial changes made in September 2017.
1. Scope
1.1 This test practice defines the procedure for determination of the tensile strength of a tapered or stepped joint of polymer matrix
composite materials. It is applicable to secondary bonded or co-bonded laminates with either unidirectional plies or woven fabric
reinforcements. The materials to be bonded may be different material systems. In the bondline, a separate adhesive material may
or may not be used (example: adhesives may be used with a prepreg system or may not be used with a wet lay-up repair system).
The range of acceptable test laminates and thicknesses is described in 8.2.1.
1.2 This practice supplements Test Method D3039/D3039M for tensile loading. Several important test specimen parameters (for
example, joint length, ply overlaps, step depth, and taper ratio) are not mandated by this practice, however, these parameters are
required to be specified and reported to support repeatable results.
1.3 Unidirectional (0° ply orientation) tape composites, textile composites, as well as multidirectional composite laminates, can
be tested.
1.4 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in
each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used
independently of the other, and values from the two systems shall not be combined.
1.4.1 Within the text the inch-pound units are shown in brackets. The values stated in each system are not exact equivalents;
therefore, each system must be used independently of the other. Combining values from the two systems may result in
nonconformance with the standard
1.5 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.6 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.
This practice is under the jurisdiction of ASTM Committee D30 on Composite Materials and is the direct responsibility of Subcommittee D30.05 on Structural Test
Methods.
Current edition approved Aug. 1, 2017May 1, 2023. Published August 2017June 2023. Originally approved in 2017. Last previous edition approved in 2017 as
ɛ1
D8131/D8131M – 17 . DOI: 10.1520/D8131_D8131M-17E01.10.1520/D8131_D8131M-23.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D8131/D8131M − 23
2. Referenced Documents
2.1 ASTM Standards:
D792 Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement
D883 Terminology Relating to Plastics
D2584 Test Method for Ignition Loss of Cured Reinforced Resins
D2734 Test Methods for Void Content of Reinforced Plastics
D3039/D3039M Test Method for Tensile Properties of Polymer Matrix Composite Materials
D3171 Test Methods for Constituent Content of Composite Materials
D3878 Terminology for Composite Materials
D5229/D5229M Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite
Materials
E4 Practices for Force Calibration and Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing
E122 Practice for Calculating Sample Size to Estimate, With Specified Precision, the Average for a Characteristic of a Lot or
Process
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E456 Terminology Relating to Quality and Statistics
E2533 Guide for Nondestructive Examination of Polymer Matrix Composites Used in Aerospace Applications
2.2 Other Documents:
CMH-17 Composite Materials Handbook-17 – Volume I
3. Terminology
3.1 Terminology D3878 defines terms relating to high-modulus fibers and their composites. Terminology D883 defines terms
relating to plastics. Terminology E6 defines terms relating to mechanical testing. Terminology E456 and Practice E177 define terms
relating to statistics. In the event of a conflict between terms, Terminology D3878 shall have precedence over the other terminology
standards.
NOTE 1—If the term represents a physical quantity, its analytical dimensions are stated immediately following the term (or letter symbol) in fundamental
dimension form, using the following ASTM standard symbology for fundamental dimensions, shown within square brackets: [M] for mass, [L] for length,
[T] for time, [θ] for thermodynamic temperature, and [nd] for nondimensional quantities. Use of these symbols is restricted to analytical dimensions when
used with square brackets, as the symbols may have other definitions when used without the brackets.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 co-bonded (repair) laminate—laminate, n—the co-bonded laminate is the laminate that is bonded to the parent pre-cured
laminate and cured in a second cure cycle.
3.2.2 joint tensile strength—strength, n—ultimate tensile force experienced by the test specimen divided by the initial width of the
joint area and the nominal thickness of the parent laminate.
3.2.3 nominal value—value, n—a value, existing in name only, assigned to a measureable property for the purpose of convenient
designation. Tolerances may be applied to a nominal value to define and acceptable range for the property.
3.2.4 parent laminate—laminate, n—the parent laminate is the laminate that is cured during the first cure cycle.
3.2.5 secondary bonded (repair) laminate—laminate, n—the secondary bonded laminate is a pre-cured laminate that is bonded to
the parent pre-cured laminate using a separate adhesive material (sometimes referred to as a pre-cured patch repair).
3.3 Symbols:
3.3.1 CV—sample coefficient of variation, in percent
tu
3.3.2 F —ultimate tensile strength, based on parent laminate thickness
p
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 for purchase in print and e-book format from SAE International, http://books.sae.org/r-422/.
D8131/D8131M − 23
tu
3.3.3 F —ultimate tensile strength, based on repair laminate thickness
r
3.3.4 h —specimen nominal parent laminate thickness as specified by the test requestor (nominal ply thickness may be available
p
from the relevant material specification)
3.3.5 h —specimen nominal repair laminate thickness as specified by the test requestor (nominal ply thickness may be available
r
from the relevant material specification)
3.3.6 N —ultimate joint running force per ply
j
3.3.7 n—number of specimens
3.3.8 n —number of parent laminate plies
p
3.3.9 P —maximum force carried by test specimen at failure
f
3.3.10 S —standard deviation statistic of a sample population for a given property
n–1
3.3.11 w—specimen width
3.3.12 x —test result for an individual coupon from the sample population for a given property
i
3.3.13 x¯—mean or average (estimate of mean) of a sample population for a given property
4. Summary of Practice
4.1 Tapered or Stepped Joint Tensile Strength—In accordance with Test Method D3039/D3039M, but using a tapered or stepped
joint configured specimen (Fig. 1 or Fig. 2), perform a uniaxial tension test of the composite laminate joint specimen.
5. Significance and Use
5.1 This test practice is designed to produce tensile property data for material specifications, research and development, quality
assurance, and structural design and analysis. Factors that influence the tensile response and should therefore be reported include
the following: materials (laminates and adhesive), methods of material preparation including surface preparation prior to bonding,
lay-ups, specimen stacking sequence, joint taper ratio or step length, ply overlap length, material relative thicknesses and stiffness
of the parent and repair laminates, adhesive bond stiffness, specimen preparation, specimen conditioning, environment of testing,
specimen alignment and gripping, speed of testing, time at temperature, void content, and volume percent reinforcement. Properties
in the test direction, which may be obtained from this test practice, include the following:
tu
5.1.1 Ultimate tensile strength (based on the nominal parent material thickness), (F ).
p
tu
5.1.2 Ultimate tensile strength (based on the nominal repair material thickness), (F ).
r
5.1.3 Ultimate running force per repair ply, (N ).
j
6. Interferences
6.1 Material and Specimen Preparation—Poor material fabrication practices, lack of control of fiber alignment, and damage
induced by improper coupon machining are known causes of high material data scatter in composites. For the bonded joint
specimens, the quality of the co-cured laminate (ply positioning, lengths, impregnation for wet lay-up material systems), lack of
orientation control of the parent laminate or pre-cured patch, and quality of the bond between the parent and pre-cured or co-cured
bonded laminates will have significant effects on the test results.
6.2 Gripping—A high percentage of grip-induced failures, especially when combined with high material data scatter, is an
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Notes:
1) Interpret drawing in accordance with ANSI Y14.5M-1982, subject to the following:
2) All dimensions in millimeters with decimal tolerances as follows:
No Decimal .X .XX
±3 ±1 ±.3
3) All angles have a tolerance of 65°.
4) Ply orientation direction tolerance relative to -A- within 6.5°.
5) Finish on machine edges not to exceed 1.6= (symbology in accordance with ASA B46.1, with roughness height in micrometers).
6) Values to be provided for the following, subject to any ranges shown in the field of drawing: material, lay-up, ply orientation reference relative to -A-, overall length,
gage length, coupon thickness, tab material, tab thickness, tab length, tab bevel angle, tab adhesive.
7) Repair specimen length design should not exceed scarf joint length by more than 20 %.
FIG. 1 Tensile Tapered and Stepped Joint Specimen – Geometry (SI)
indicator of specimen gripping problems. Specimen gripping methods are discussed further in Test Method D3039/D3039M.
Specimen shall not be gripped within 25 mm [1.0 in.] of the bondline (joint) area.
6.3 System Alignment—Excessive bending will cause premature failure, as well as highly inappropriate load-versus-deflection
response. Every effort should be made to eliminate excess bending from the test system. Bending may occur as a result of
misaligned grips or from specimens themselves if improperly installed in the grips or out-of-tolerance caused by poor specimen
preparation. If there are differences in material thickness and stiffness of the parent versus repair laminates, it may be necessary
to intentionally induce distortion and misalignment upon gripping or to employ the use of a shim(s) on the thinner part of the
specimen and accept the induced moment. If there is any doubt as to the alignment inherent in a given test machine, then the
alignment should be checked as discussed in Test Method D3039/D3039M.
6.4 Specimen Design—The bonded joint test specimen involves a parent and a repair laminate. These two laminates typically do
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Notes:
1) Interpret drawing in accordance with ANSI Y14.5M-1982, subject to the following:
2) All dimensions in inches with decimal tolerances as follows:
No Decimal .X .XX
±.1 ±.03 ±.01
3) All angles have a tolerance of 65°.
4) Ply orientation direction tolerance relative to -A- within 6.5°.
5) Finish on machine edges not to exceed 64= (symbology in accordance with ASA B46.1, with roughness height in microinches).
6) Values to be provided for the following, subject to any ranges shown in the field of drawing: material, lay-up, ply orientation reference relative to -A-, overall length,
gage length, coupon thickness, tab material, tab thickness, tab length, tab bevel angle, tab adhesive.
7) Repair specimen length design should not exceed scarf joint length by more than 20 %.
FIG. 2 Tensile Tapered and Stepped Joint Specimen – Geometry (Inch-Pound)
not use the same material system. There are a number of variables and factors which influence the selection of the repair laminate
lay-up relative to the parent laminate. Generally the repair lay-up is designed to match or slightly exceed the stiffness of the parent
laminate, and ideally the repair material type (fabric or tape) and the ply thicknesses are the same as for the parent material. When
they are different, repair design compromises are necessary to obtain sufficient repair stiffness and strength while not making the
repair thicker than the parent laminate. Details of repair design are beyond the scope of this test practice. When filler plies or extra
repair plies are used, or when the repair material thicknesses do not match the parent material thicknesses, a difference in joint test
specimen thickness on the two ends will occur, leading to some induced bending, even if shims are applied. Also, any unbalance
or asymmetry in the repair lay-up from filler plies, the repair laminate, or from extra repair plies may also lead to some induced
bending. The results from this test practice are reported as simple stress and force-per-width values; the joint specimen design will
influence the validity of these reported results for use with bonded repair analysis methods.
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6.4.1 Step Joint Filler Plies—The stepped joint specimen should have a filler ply to avoid waviness in the repair plies. The filler
ply should ideally be the same thickness as the bottom ply in the parent laminate. The standard test specimen configuration in this
practice for unidirectional tape materials uses a +45 ply as the filler ply; this may induce some bending or twisting in the specimen
under load. If an equivalent thickness fabric repair material is available, that could be used for the filler ply to provide a balanced
lay-up.
6.5 Edge Effects in Angle Ply Laminates—Premature failure can occur as a result of edge softening in laminates containing off-axis
plies. Because of this, the strength for angle ply laminates can be lower than expected. For multidirectional laminates containing
significant axial fibers, the effect is not as significant.
6.6 Environment—Results are affected by the environmental conditions under which the tests are conducted. Laminates tested in
various environments can exhibit significant differences in both failure force and failure mode. Experience has demonstrated that
cold-temperature environments can be critical for laminate tensile strength, however, elevated-temperature, humid environments
can also be critical. The critical environmental condition for bonded joint specimens is complicated by having multiple materials
in the test specimens. The failure modes may change between laminate tensile modes, bondline modes or laminate interfacial
modes as the environmental condition changes. Critical environments must be assessed independently for each material system
combination, joint configuration, and stacking sequence tested.
6.7 Failure Mode—For a valid test, final failure of the specimen must occur within the gage section. Which failure modes are
deemed acceptable will be governed by the particular material, configuration, and application (see 11.5 and Fig. 5).
7. Apparatus
7.1 General Apparatus—General apparatus shall be in accordance with Test Method D3039/D3039M.
7.2 Testing Machine—The testing machine shall be in conformance with Practices E4 and shall satisfy all requirements defined
in Test Method D3039/D3039M.
7.3 Conditioning Chamber—When conditioning materials in other than ambient laboratory environments, a temperature-
/moisture-level controlled environmental conditioning chamber is required that shall be capable of maintaining the required
relative temperature to within 63 °C [65 °F] and the required relative vapor level to within 63 %. Chamber conditions shall be
monitored either on an automated continuous basis or on a manual basis at regular intervals.
7.4 Environmental Chamber—A chamber capable of enclosing the test fixture and specimen while they are mounted in the testing
machine, and capable of achieving the specified heating/cooling rates, test temperatures, and environments, shall be used when
nonambient conditions are required during testing. This chamber shall be capable of maintaining the gage section of the test
specimen within 63 °C [65 °F] of the required test temperature during the mechanical test. In addition, the chamber may have
to be capable of maintaining environmental conditions such as fluid exposure or relative humidity during the test.
7.5 Data Acquisition Equipment—Equipment capable of continuous recording of force or displacement data, or both, is required.
8. Sampling and Test Specimens
8.1 Sampling—Test at least five specimens per test condition unless valid results can be gained through the use of fewer specimens,
such as in the case of a designed experiment. For statistically significant data, the procedures outlined in Practice E122 should be
consulted. The method of sampling shall be reported.
8.2 Geometry—The test specimen is a rectangular strip of the composite to be tested with either a tapered or stepped joint, as
shown in Figs. 1-3 (tapered) or Fig. 1, Fig. 2, and Fig. 4 (stepped). The standard specimen joint geometries are listed in the
following subsections. Non-standard specimen configurations may be tested.
8.2.1 Tapered Joint Standard Specimen—The standard tapered specimen is defined in Table 1. The specimen joint length is equal
to the taper ratio times the parent laminate thickness. For additional description, dimensions, and tolerances see Figs. 1-3.
8.2.2 Tapered Joint Standard Specimen Length—The recommended standard taper joint specimen length is defined as the repair
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FIG. 3 Close-Up of Tensile Tapered Joint Specimen – Geometry
FIG. 4 Close-Up of Tensile Stepped Joint Specimen – Geometry
TABLE 1 Tensile Tapered Joint Standard Specimen Geometry Requirements
Specimen Repair Ply Overlap
B
Specimen Lay-Up – Parent and Repair Taper Ratio Specimen Width Repair Length
A
Type Length
[+45°/90°/-45°/0°]s (Tape) or
10 ± 2 mm 25 ± 3 mm 80 ± 3 mm
Tapered [(0°/90°) (+45°/–45°) (90°/0°) (–45°/+45°)]s 50:1
[0.4 ± 0.08 in.] [1.0 ± 0.1 in.] [3.0 ± 0.1 in.]
(Fabric)
[+45°/90°/-45°/0°]s (Tape) or
10 mm ± 2 mm 25 mm ± 3 mm 80 mm ± 3 mm
Tapered [(0°/90°) (+45°/–45°) (90°/0°) (–45°/+45°)
...