ASTM C297/C297M-16

Standard Test Method for Flatwise Tensile Strength of Sandwich Constructions

SIGNIFICANCE AND USE
5.1 In a sandwich panel, core-to-facing bond integrity is necessary to maintain facing stability and permit load transfer between the facings and core. This test method can be used to provide information on the strength and quality of core-to-facing bonds. It can also be used to produce flatwise tensile strength data for the core material. While it is primarily used as a quality control test for bonded sandwich panels, it can also be used to produce flatwise tensile strength data for structural design properties, material specifications, and research and development applications.  
5.2 Factors that influence the flatwise tensile strength and shall therefore be reported include the following: facing material, core material, adhesive material, methods of material fabrication, facing stacking sequence and overall thickness, core geometry (cell size, cell wall thickness), core density, adhesive thickness, specimen geometry, specimen preparation, specimen conditioning, environment of testing, specimen alignment, loading procedure, speed of testing, facing void content, adhesive void content, and facing volume percent reinforcement. Properties that may be derived from this test method include flatwise tensile strength.
SCOPE
1.1 This test method determines the flatwise tensile strength of the core, the core-to-facing bond, or the facing of an assembled sandwich panel. Permissible core material forms include those with continuous bonding surfaces (such as balsa wood and foams) as well as those with discontinuous bonding surfaces (such as honeycomb).  
1.2 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 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.  
1.2.1 Within the text the inch-pound units are shown in brackets.  
1.3 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 and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Published
• Publication Date: 31-Mar-2016
• ICS: 83.120 - Reinforced plastics
• Technical Committee: D30 - Composite Materials
• Drafting Committee: D30.09 - Sandwich Construction

Relations

• Replaces: ASTM C297/C297M-15 - Standard Test Method for Flatwise Tensile Strength of Sandwich Constructions
• Effective Date: 01-Apr-2016
• 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 D883-19a - Standard Terminology Relating to Plastics
• Effective Date: 15-Apr-2019
• Refers: ASTM D3878-19 - Standard Terminology for Composite Materials
• 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

Overview

ASTM C297/C297M-16: Standard Test Method for Flatwise Tensile Strength of Sandwich Constructions is an internationally recognized standard issued by ASTM International. This test method is designed to determine the flatwise tensile strength of sandwich panel assemblies, focusing on the core material, the core-to-facing bond, or the facing material itself. Sandwich constructions, commonly used in aerospace, marine, automotive, and construction industries, rely on strong bonding between the core and facings to maintain structural integrity and facilitate load transfer.

This method is crucial for quality control and for developing structural design data for sandwich panels with a variety of core materials, including those with continuous (e.g., balsa wood, foams) and discontinuous (e.g., honeycomb) bonding surfaces.

Key Topics

• Flatwise Tensile Strength: Evaluation of panel’s ability to resist tension perpendicular to the plane of the sandwich.
• Core-to-Facing Bond Integrity: Assessment of the bond quality between the core and outer facings to ensure reliable load transfer.
• Specimen Preparation: Requirements for accurate specimen geometry, surface preparation, and bonding to loading blocks.
• Testing Conditions: Importance of reporting all parameters that affect tensile strength, such as facing and core material, adhesive, fabrication methods, specimen conditioning, test environment, and testing speed.
• Acceptable Failure Modes: Only internal failures within the sandwich construction (core break, adhesive layer break, or facing failure) are considered valid. Failures at the loading block interface are excluded.
• Statistical Analysis: Calculation and reporting of average values, standard deviation, and coefficient of variation for tensile strength measurements.

Applications

The ASTM C297/C297M-16 standard is widely used in multiple industrial sectors where sandwich construction panels are prevalent. Typical applications include:

• Quality Control: Routine verification of sandwich panel manufacturing consistency and reliable bonding performance.
• Structural Design: Generation of mechanical property data essential for engineers and designers during the development and validation of new products.
• Material Specifications: Comparison of various core and facing materials, adhesives, and fabrication techniques.
• Research and Development: Investigation of new sandwich constructions, adhesives, or core materials for improved performance.
• Regulatory Compliance: Used by manufacturers and testing laboratories to meet contractual or regulatory specifications, including those set by defense or aerospace agencies.

Related Standards

Understanding ASTM C297/C297M-16 is enhanced by familiarity with related test methods and terminology standards:

• ASTM C271/C271M: Test Method for Density of Sandwich Core Materials
• ASTM D792: Test Methods for Density and Specific Gravity of Plastics
• ASTM D2584: Test Method for Ignition Loss of Cured Reinforced Resins
• ASTM D2734: Methods for Void Content of Reinforced Plastics
• ASTM D3039/D3039M: Test Method for Tensile Properties of Polymer Matrix Composite Materials
• ASTM D3171: Test Methods for Constituent Content of Composite Materials
• ASTM D5229/D5229M: Test Method for MoistureAbsorption Properties and Conditioning of Polymer Matrix Composites
• ASTM E4: Practices for Force Verification of Testing Machines
• ASTM E122: Practice for Calculating Sample Size for Estimating Characteristics of a Lot or Process

Practical Value

Adhering to ASTM C297/C297M-16 helps manufacturers, engineers, and quality auditors ensure that sandwich panels meet rigorous performance requirements. Reliable flatwise tensile strength testing provides confidence in product durability, reduces the risk of structural failures, and supports the continuous improvement of advanced composite materials and bonding processes. By following the standardized procedures and reporting guidelines, stakeholders can make informed decisions on material selection and manufacturing methods while ensuring compliance with international and industry-specific standards.

Keywords: ASTM C297, sandwich construction, flatwise tensile strength, core-to-facing bond, composite materials, mechanical testing, quality control, structural panels.

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: C297/C297M − 16
Standard Test Method for
Flatwise Tensile Strength of Sandwich Constructions
This standard is issued under the fixed designation C297/C297M; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope tive Density) of Plastics by Displacement
D883 Terminology Relating to Plastics
1.1 This test method determines the flatwise tensile strength
D2584 Test Method for Ignition Loss of Cured Reinforced
of the core, the core-to-facing bond, or the facing of an
Resins
assembled sandwich panel. Permissible core material forms
D2734 TestMethodsforVoidContentofReinforcedPlastics
include those with continuous bonding surfaces (such as balsa
D3039/D3039M Test Method for Tensile Properties of Poly-
wood and foams) as well as those with discontinuous bonding
mer Matrix Composite Materials
surfaces (such as honeycomb).
D3171 Test Methods for Constituent Content of Composite
1.2 The values stated in either SI units or inch-pound units
Materials
are to be regarded separately as standard. The values stated in
D3878 Terminology for Composite Materials
each system are not exact equivalents; therefore, each system
D5229/D5229M Test Method for MoistureAbsorption Prop-
must be used independently of the other. Combining values
erties and Equilibrium Conditioning of Polymer Matrix
from the two systems may result in nonconformance with the
Composite Materials
standard.
E4 Practices for Force Verification of Testing Machines
1.2.1 Within the text the inch-pound units are shown in
E6 Terminology Relating to Methods of Mechanical Testing
brackets.
E122 Practice for Calculating Sample Size to Estimate,With
1.3 This standard does not purport to address all of the
Specified Precision, the Average for a Characteristic of a
safety concerns, if any, associated with its use. It is the
Lot or Process
responsibility of the user of this standard to establish appro-
E177 Practice for Use of the Terms Precision and Bias in
priate safety and health practices and determine the applica-
ASTM Test Methods
bility of regulatory limitations prior to use.
E456 Terminology Relating to Quality and Statistics
1.4 This international standard was developed in accor-
dance with internationally recognized principles on standard-
3. Terminology
ization established in the Decision on Principles for the
3.1 Definitions—Terminology D3878 defines terms relating
Development of International Standards, Guides and Recom-
to high-modulus fibers and their composites, as well as terms
mendations issued by the World Trade Organization Technical
relating to sandwich constructions. Terminology D883 defines
Barriers to Trade (TBT) Committee.
terms relating to plastics. Terminology E6 defines terms
2. Referenced Documents relating to mechanical testing. Terminology E456 and Practice
2 E177 define terms relating to statistics. In the event of a
2.1 ASTM Standards:
conflict between terms, Terminology D3878 shall have prece-
C271/C271M Test Method for Density of Sandwich Core
dence over the other terminologies.
Materials
3.2 Symbols:
D792 Test Methods for Density and Specific Gravity (Rela-
3.2.1 A—cross-sectional area of a test specimen
3.2.2 CV—coefficient of variation statistic of a sample
This test method is under the jurisdiction of ASTM Committee D30 on
Composite Materials and is the direct responsibility of Subcommittee D30.09 on population for a given property (in percent)
Sandwich Construction.
3.2.3 e—edge tolerance between specimen and loading
Current edition approved April 1, 2016. Published April 2016. Originally
approved in 1952. Last previous edition approved in 2015 as C297/C297M – 15).
blocks
DOI: 10.1520/C0297_C0297M-16.
ftu
3.2.4 F —ultimate flatwise tensile strength
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
z
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
3.2.5 P —maximumforcecarriedbytestspecimenbefore
Standards volume information, refer to the standard’s Document Summary page on max
the ASTM website. failure
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C297/C297M − 16
3.2.6 S — standard deviation statistic of a sample popula- bending from the test system. Bending may occur as a result of
n-1
tion for a given property misaligned grips, poor specimen preparation, or poor align-
ment of the loading blocks and loading fixture. If there is any
3.2.7 x—test result for an individual specimen from the
i
doubt as to the alignment inherent in a given test machine, then
sample population for a given property
the alignment should be checked as discussed in Test Method
3.2.8 x¯—mean or average (estimate of mean) of a sample
D3039/D3039M.
population for a given property
6.3 Geometry—Specific geometric factors that affect sand-
4. Summary of Test Method
wich flatwise tensile strength include core cell geometry, core
thickness, specimen shape (square or circular), adhesive
4.1 This test method consists of subjecting a sandwich
thickness, facing thickness, and facing per-ply thickness.
construction to a uniaxial tensile force normal to the plane of
the sandwich. The force is transmitted to the sandwich through
6.4 Environment—Resultsareaffectedbytheenvironmental
thickloadingblocks,whicharebondedtothesandwichfacings
conditions under which the tests are conducted. Specimens
or directly to the core.
tested in various environments can exhibit significant differ-
4.2 The only acceptable failure modes for flatwise tensile ences in both strength behavior and failure mode. Critical
strength are those which are internal to the sandwich construc- environments must be assessed independently for each facing,
tion. Failure of the loading block-to-sandwich bond is not an adhesive and core material tested.
acceptable failure mode.
6.5 Conditioning—Asitisinappropriatetobondamoisture-
conditioned specimen to the loading blocks, it is necessary to
5. Significance and Use
perform the bonding operation prior to such conditioning. The
5.1 In a sandwich panel, core-to-facing bond integrity is
presence of the loading blocks will affect the degree of
necessary to maintain facing stability and permit load transfer
moisture intake into the specimen, in comparison to a non-
between the facings and core. This test method can be used to
bonded sample.
provide information on the strength and quality of core-to-
facing bonds. It can also be used to produce flatwise tensile
7. Apparatus
strength data for the core material.While it is primarily used as
7.1 Micrometers and Calipers—A micrometer witha4to
aqualitycontroltestforbondedsandwichpanels,itcanalsobe
7 mm[0.16to0.28in.]nominaldiameterball-interfaceoraflat
used to produce flatwise tensile strength data for structural
anvil interface shall be used to measure the specimen thick-
design properties, material specifications, and research and
ness. A ball interface is recommended for thickness measure-
development applications.
ments when facings are bonded to the core and at least one
5.2 Factors that influence the flatwise tensile strength and
surface is irregular (e.g., the bag-side of a thin facing laminate
shall therefore be reported include the following: facing
that is neither smooth nor flat).Amicrometer or caliper with a
material, core material, adhesive material, methods of material
flat anvil interface is recommended for thickness measure-
fabrication, facing stacking sequence and overall thickness,
ments when facings are bonded to the core and both surfaces
core geometry (cell size, cell wall thickness), core density,
aresmooth(e.g.,tooledsurfaces).Amicrometerorcaliperwith
adhesive thickness, specimen geometry, specimen preparation,
a flat anvil interface shall be used for measuring length and
specimen conditioning, environment of testing, specimen
width, as well as the specimen thickness when no facings are
alignment, loading procedure, speed of testing, facing void
present. The use of alternative measurement devices is permit-
content, adhesive void content, and facing volume percent
ted if specified (or agreed to) by the test requestor and reported
reinforcement. Properties that may be derived from this test
by the testing laboratory. The accuracy of the instruments shall
method include flatwise tensile strength.
be suitable for reading to within1%ofthe sample dimensions.
For typical specimen geometries, an instrument with an accu-
6. Interferences
racy of 60.025 mm [60.001 in.] is adequate for the length,
6.1 Material and Specimen Preparation—Poormaterialfab-
width, and thickness measurements.
rication practices, lack of control of fiber alignment, and
7.2 Loading Fixtures—The loading fixtures shall be self-
damage induced by improper specimen machining are known
aligning and shall not apply eccentric loads.Asatisfactory type
causes of high data scatter in composites in general. Specific
of apparatus is shown in Fig. 1. A dual-pin connection or
material factors that affect sandwich composites include vari-
universal joint shall be attached to each loading block to
abilityincoredensityanddegreeofcureofresininbothfacing
minimize any moments imparted to the test specimen. The
matrix material and core bonding adhesive. Important aspects
loadingblocksshallbesufficientlystifftokeepthebondedcore
of sandwich panel specimen preparation that contribute to data
or facings essentially flat under load. Loading blocks 40 to
scatter are incomplete or nonuniform core bonding to facings,
50 mm [1.5 to 2.0 in.] thick have been found to perform
misalignment of core and facing elements, the existence of
satisfactorily.Recommendeddimensionsandtolerancesforthe
joints, voids or other core and facing discontinuities, out-of-
loading blocks are provided in Fig. 2.
plane curvature, facing thickness variation, and surface rough-
ness.
7.3 Testing Machine—The testing machine shall be in ac-
6.2 System Alignment—Excessive bending will cause pre- cordance with Practices E4 and shall satisfy the following
mature failure. Every effort should be made to eliminate excess requirements:
C297/C297M − 16
testing laboratory conditions. This chamber shall be capable of
maintaining the gage section of the test specimen at the
required test environment during the mechanical test.
8. Sampling and Test Specimens
8.1 Sampling—Test at least five specimens per test condi-
tion unless valid results can be gained through the use of fewer
specimens, as in the case of a designed experiment. For
statistically significant data, consult the procedures outlined in
Practice E122. Report the method of sampling.
8.2 Geometry—Test specimens shall have a square or cir-
cular cross section, and shall be equal in thickness to the
sandwich panel thickness. Recommended dimensions and
tolerances for the core specimens are provided in Fig. 3.
Minimum specimen facing areas for various types of core
materials are as follows:
8.2.1 Continuous Bonding Surfaces (for example, balsa
wood, foams)—Theminimumfacingareaofthespecimenshall
2 2
be 625 mm [1.0 in. ], and the minimum width or diameter of
the loading blocks shall be 25 mm [1.0 in.]. Recommended
tolerances for the core specimens are provided in Fig. 3.
8.2.2 Discontinuous Cellular Bonding Surfaces (for
example, honeycomb)—The required facing area of the speci-
men is dependent upon the cell size, to ensure a minimum
number of cells are tested. Minimum facing areas and speci-
men dimensions are recommended in Table 1 for the more
common cell sizes. These are intended to provide approxi-
mately 60 cells minimum in the test specimen. The largest
2 2
facing area listed in the table (5625 mm [9.0 in. ]) is a
practical maximum for this test method. Cores with cell sizes
larger than 9 mm [0.375 in.] may require a smaller number of
FIG. 1 Flatwise Tension Test Setup
cells to be tested in the specimen.
8.3 Specimen Preparation and Machining—Specimen
7.3.1 Testing Machine Configuration—The testing machine preparation is extremely important for this test method. Take
precautions when cutting specimens from large panels to avoid
shall have both an essentially stationary head and a movable
head. notches, undercuts, rough or uneven surfaces, or delaminations
due to inappropriate machining methods. Obtain final dimen-
7.3.2 Drive Mechanism—The testing machine drive mecha-
nism shall be capable of imparting to the movable head a sions by water-lubricated precision sawing, milling, or grind-
ing. The use of diamond tooling has been found to be
controlled velocity with respect to the stationary head. The
velocity of the movable head shall be capable of being extremely effective for many material systems. Edges should
be flat and parallel within the specified tolerances. Record and
regulated in accordance with 11.6.
7.3.3 Force Indicator—The testing machine force-sensing report the specimen cutting preparation method.
device shall be capable of indicating the total force being
8.4 Labeling—Label the test specimens so that they will be
carried by the test specimen. This device shall be essentially
distinct from each other and traceable back to the panel of
free from inertia lag at the specified rate of testing and shall
origin, and will neither influence the test nor be affected by it.
indicate the force with an accuracy over the force range(s) of
8.5 Loading Fixture Bonding—The loading blocks shall be
interest of within 61 % of the indicated value.
bonded to the core or facings of the test specimen using a
7.4 Conditioning Chamber—When conditioning materials
suitable adhesive. To minimize thermal exposure effects upon
at non-laboratory environments, a temperature/vapor-level
the existing core-to-facing bonds, it is recommended that the
controlledenvironmentalconditioningchamberisrequiredthat
assembly bonding temperature be at room temperature, or at
shall be capable of maintaining the required temperature to
least 28 °C [50 °F] lower than that at which the sandwich was
within 63°C[65 °F] and the required relative humidity level
originally bonded. Similarly, the assembly bonding pressure
to within 63
...

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.
Designation: C297/C297M − 15 C297/C297M − 16
Standard Test Method for
Flatwise Tensile Strength of Sandwich Constructions
This standard is issued under the fixed designation C297/C297M; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope
1.1 This test method determines the flatwise tensile strength of the core, the core-to-facing bond, or the facing of an assembled
sandwich panel. Permissible core material forms include those with continuous bonding surfaces (such as balsa wood and foams)
as well as those with discontinuous bonding surfaces (such as honeycomb).
1.2 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 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.
1.2.1 Within the text the inch-pound units are shown in brackets.
1.3 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 and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
C271/C271M Test Method for Density of Sandwich Core Materials
C274 Terminology of Structural Sandwich Constructions (Withdrawn 2016)
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 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
E1309 Guide for Identification of Fiber-Reinforced Polymer-Matrix Composite Materials in Databases (Withdrawn 2015)
E1434 Guide for Recording Mechanical Test Data of Fiber-Reinforced Composite Materials in Databases (Withdrawn 2015)
E1471 Guide for Identification of Fibers, Fillers, and Core Materials in Computerized Material Property Databases (Withdrawn
2015)
This test method is under the jurisdiction of ASTM Committee D30 on Composite Materials and is the direct responsibility of Subcommittee D30.09 on Sandwich
Construction.
Current edition approved April 1, 2015April 1, 2016. Published April 2015April 2016. Originally approved in 1952. Last previous edition approved in 20102015 as
C297/C297M – 04C297/C297M – 15(2010).). DOI: 10.1520/C0297_C0297M-15.10.1520/C0297_C0297M-16.
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.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C297/C297M − 16
3. Terminology
3.1 Definitions—Terminology D3878 defines terms relating to high-modulus fibers and their composites.
Terminologycomposites, as C274 defineswell as terms relating to structural sandwich constructions. 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
terminologies.
3.2 Symbols:
3.2.1 A—cross-sectional area of a test specimen
3.2.2 CV—coefficient of variation statistic of a sample population for a given property (in percent)
3.2.3 e—edge tolerance between specimen and loading blocks
ftu
3.2.4 F —ultimate flatwise tensile strength
z
3.2.5 P —maximum force carried by test specimen before failure
max
3.2.6 S — standard deviation statistic of a sample population for a given property
n-1
3.2.7 x —test result for an individual specimen from the sample population for a given property
i
3.2.8 x¯—mean or average (estimate of mean) of a sample population for a given property
4. Summary of Test Method
4.1 This test method consists of subjecting a sandwich construction to a uniaxial tensile force normal to the plane of the
sandwich. The force is transmitted to the sandwich through thick loading blocks, which are bonded to the sandwich facings or
directly to the core.
4.2 The only acceptable failure modes for flatwise tensile strength are those which are internal to the sandwich construction.
Failure of the loading block-to-sandwich bond is not an acceptable failure mode.
5. Significance and Use
5.1 In a sandwich panel, core-to-facing bond integrity is necessary to maintain facing stability and permit load transfer between
the facings and core. This test method can be used to provide information on the strength and quality of core-to-facing bonds. It
can also be used to produce flatwise tensile strength data for the core material. While it is primarily used as a quality control test
for bonded sandwich panels, it can also be used to produce flatwise tensile strength data for structural design properties, material
specifications, and research and development applications.
5.2 Factors that influence the flatwise tensile strength and shall therefore be reported include the following: facing material, core
material, adhesive material, methods of material fabrication, facing stacking sequence and overall thickness, core geometry (cell
size, cell wall thickness), core density, adhesive thickness, specimen geometry, specimen preparation, specimen conditioning,
environment of testing, specimen alignment, loading procedure, speed of testing, facing void content, adhesive void content, and
facing volume percent reinforcement. Properties that may be derived from this test method include flatwise tensile strength.
6. Interferences
6.1 Material and Specimen Preparation—Poor material fabrication practices, lack of control of fiber alignment, and damage
induced by improper specimen machining are known causes of high data scatter in composites in general. Specific material factors
that affect sandwich composites include variability in core density and degree of cure of resin in both facing matrix material and
core bonding adhesive. Important aspects of sandwich panel specimen preparation that contribute to data scatter are incomplete
or nonuniform core bonding to facings, misalignment of core and facing elements, the existence of joints, voids or other core and
facing discontinuities, out-of-plane curvature, facing thickness variation, and surface roughness.
6.2 System Alignment—Excessive bending will cause premature failure. Every effort should be made to eliminate excess
bending from the test system. Bending may occur as a result of misaligned grips, poor specimen preparation, or poor alignment
of the loading blocks and loading fixture. 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.
6.3 Geometry—Specific geometric factors that affect sandwich flatwise tensile strength include core cell geometry, core
thickness, specimen shape (square or circular), adhesive thickness, facing thickness, and facing per-ply thickness.
6.4 Environment—Results are affected by the environmental conditions under which the tests are conducted. Specimens tested
in various environments can exhibit significant differences in both strength behavior and failure mode. Critical environments must
be assessed independently for each facing, adhesive and core material tested.
6.5 Conditioning—As it is inappropriate to bond a moisture-conditioned specimen to the loading blocks, it is necessary to
perform the bonding operation prior to such conditioning. The presence of the loading blocks will affect the degree of moisture
intake into the specimen, in comparison to a non-bonded sample.
C297/C297M − 16
7. Apparatus
7.1 Micrometers and Calipers—A micrometer with a 4 to 7 mm [0.16 to 0.28 in.] nominal diameter ball-interface or a flat anvil
interface shall be used to measure the specimen thickness. A ball interface is recommended for thickness measurements when
facings are bonded to the core and at least one surface is irregular (e.g.(e.g., the bag-side of a thin facing laminate that is neither
smooth nor flat). A micrometer or caliper with a flat anvil interface is recommended for thickness measurements when facings are
bonded to the core and both surfaces are smooth (e.g.(e.g., tooled surfaces). A micrometer or caliper with a flat anvil interface shall
be used for measuring length and width, as well as the specimen thickness when no facings are present. The use of alternative
measurement devices is permitted if specified (or agreed to) by the test requestor and reported by the testing laboratory. The
accuracy of the instruments shall be suitable for reading to within 1 % of the sample dimensions. For typical specimen geometries,
an instrument with an accuracy of 60.025 mm [60.001 in.] is adequate for the length, width, and thickness measurements.
7.2 Loading Fixtures—The loading fixtures shall be self-aligning and shall not apply eccentric loads. A satisfactory type of
apparatus is shown in Fig. 1. A dual-pin connection or universal joint shall be attached to each loading block to minimize any
moments imparted to the test specimen. The loading blocks shall be sufficiently stiff to keep the bonded core or facings essentially
flat under load. Loading blocks 40 to 50 mm [1.5 to 2.0 in.] thick have been found to perform satisfactorily. Recommended
dimensions and tolerances for the loading blocks are provided in Fig. 2.
7.3 Testing Machine—The testing machine shall be in accordance with Practices E4 and shall satisfy the following
requirements:
7.3.1 Testing Machine Configuration—The testing machine shall have both an essentially stationary head and a movable head.
7.3.2 Drive Mechanism—The testing machine drive mechanism shall be capable of imparting to the movable head a controlled
velocity with respect to the stationary head. The velocity of the movable head shall be capable of being regulated in accordance
with 11.6.
7.3.3 Force Indicator—The testing machine force-sensing device shall be capable of indicating the total force being carried by
the test specimen. This device shall be essentially free from inertia lag at the specified rate of testing and shall indicate the force
with an accuracy over the force range(s) of interest of within 61 % of the indicated value.
FIG. 1 Flatwise Tension Test Setup
C297/C297M − 16
FIG. 2 Loading Block Dimensions and Tolerances
7.4 Conditioning Chamber—When conditioning materials at non-laboratory environments, a temperature/vapor-level controlled
environmental conditioning chamber is required that shall be capable of maintaining the required temperature to within 63 °C
[65 °F] and the required relative humidity level to within 63 %. Chamber conditions shall be monitored either on an automated
continuous basis or on a manual basis at regular intervals.
7.5 Environmental Test Chamber—An environmental test chamber is required for test environments other than ambient testing
laboratory conditions. This chamber shall be capable of maintaining the gage section of the test specimen at the required test
environment during the mechanical test.
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, as in the case of a designed experiment. For statistically significant data, consult the procedures outlined in Practice
E122. Report the method of sampling.
8.2 Geometry—Test specimens shall have a square or circular cross section, and shall be equal in thickness to the sandwich
panel thickness. Recommended dimensions and tolerances for the core specimens are provided in Fig. 3. Minimum specimen
facing areas for various types of core materials are as follows:
8.2.1 Continuous Bonding Surfaces (for example, balsa wood, foams)—The minimum facing area of the specimen shall be 625
2 2
mm [1.0 in. ], and the minimum width or diameter of the loading blocks shall be 25 mm [1.0 in.]. Recommended tolerances for
the core specimens are provided in Fig. 3.
8.2.2 Discontinuous Cellular Bonding Surfaces (for example, honeycomb)—The required facing area of the specimen is
dependent upon the cell size, to ensure a minimum number of cells are tested. Minimum facing areas and specimen dimensions
are recommended in Table 1 for the more common cell sizes. These are intended to provide approximately 60 cells minimum in
2 2
the test specimen. The largest facing area listed in the table (5625 mm [9.0 in. ]) is a practical maximum for this test method.
Cores with cell sizes larger than 9 mm [0.375 in.] may require a smaller number of cells to be tested in the specimen.
8.3 Specimen Preparation and Machining—Specimen preparation is extremely important for this test method. Take precautions
when cutting specimens from large panels to avoid notches, undercuts, rough or uneven surfaces, or delaminations due to
inappropriate machining methods. Obtain final dimensions by water-lubricated precision sawing, milling, or grinding. The use of
diamond tooling has been found to be extremely effective for many material systems. Edges should be flat and parallel within the
specified tolerances. Record and report the specimen cutting preparation method.
8.4 Labeling—Label the test specimens so that they will be distinct from each other and traceable back to the panel of origin,
and will neither influence the test nor be affected by it.
8.5 Loading Fixture Bonding—The loading blocks shall be bonded to
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