ASTM D7249/D7249M-20

Standard Test Method for Facesheet Properties of Sandwich Constructions by Long Beam Flexure

SIGNIFICANCE AND USE
5.1 Flexure tests on flat sandwich construction may be conducted to determine the sandwich flexural stiffness, the core shear strength, and shear modulus, or the facesheets’ compressive and tensile strengths. Tests to evaluate core shear strength may also be used to evaluate core-to-facesheet bonds.  
5.2 This test method is limited to obtaining the strength and stiffness of the sandwich panel facesheets, and to obtaining load-deflection data for use in calculating sandwich beam flexural and shear stiffness using Practice D7250/D7250M. Due to the curvature of the flexural test specimen when loaded, facesheet compression strength from this test may not be equivalent to the facesheet compression strength of sandwich structures subjected to pure edgewise (in-plane) compression.  
5.3 Core shear strength and shear modulus are best determined in accordance with Test Method C273/C273M provided bare core material is available. Test Method C393/C393M may also be used to determine core shear strength. Practice D7250/D7250M may be used to calculate the flexural and shear stiffness of sandwich beams.  
5.4 This test method can be used to produce facesheet strength data for structural design allowables, material specifications, and research and development applications; it may also be used as a quality control test for bonded sandwich panels.  
5.5 Factors that influence the facesheet strength and shall therefore be reported include the following: facesheet material, core material, adhesive material, methods of material fabrication, facesheet stacking sequence and overall thickness, core geometry (cell size), core density, adhesive thickness, specimen geometry, specimen preparation, specimen conditioning, environment of testing, specimen alignment, loading procedure, speed of testing, facesheet void content, adhesive void content, and facesheet volume percent reinforcement. Further, facesheet strength may be different between precured/bonded and co-cured facesheets of the s...
SCOPE
1.1 This test method covers determination of facesheet properties of flat sandwich constructions subjected to flexure in such a manner that the applied moments produce curvature of the sandwich facesheet planes and result in compressive and tensile forces in the facesheets. 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 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.  
1.2.1 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
Note 1: Alternate procedures for determining the compressive strength of unidirectional polymer matrix composites materials in a sandwich beam configuration may be found in Test Method D5467/D5467M.  
1.4 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: 31-Jan-2020
• ICS: 79.060.01 - Wood-based panels in general
• Technical Committee: D30 - Composite Materials
• Drafting Committee: D30.09 - Sandwich Construction

Relations

• Replaces: ASTM D7249/D7249M-18 - Standard Test Method for Facesheet Properties of Sandwich Constructions by Long Beam Flexure
• Effective Date: 01-Feb-2020
• Refers: ASTM E456-13a(2022)e1 - Standard Terminology Relating to Quality and Statistics
• Effective Date: 01-Apr-2022
• Refers: ASTM E1237-20 - Standard Guide for Installing Bonded Resistance Strain Gages
• Effective Date: 15-Aug-2020
• Refers: ASTM E251-20a - Standard Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gages
• Effective Date: 01-Jun-2020
• Refers: ASTM E251-20 - Standard Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gages
• Effective Date: 01-May-2020
• 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 C273/C273M-20 - Standard Test Method for Shear Properties of Sandwich Core Materials
• Effective Date: 15-Feb-2020
• Refers: ASTM D3878-19a - Standard Terminology for Composite Materials
• Effective Date: 15-Oct-2019
• Refers: ASTM C273/C273M-19 - Standard Test Method for Shear Properties of Sandwich Core Materials
• Effective Date: 01-Sep-2019
• Refers: ASTM D3878-19 - Standard Terminology for Composite Materials
• Effective Date: 15-Apr-2019
• Refers: ASTM C273/C273M-18 - Standard Test Method for Shear Properties of Sandwich Core Materials
• Effective Date: 01-Nov-2018
• Refers: ASTM D3878-18 - Standard Terminology for Composite Materials
• Effective Date: 01-Apr-2018
• Refers: ASTM E456-13A(2017)e1 - Standard Terminology Relating to Quality and Statistics
• Effective Date: 01-Oct-2017
• Refers: ASTM E456-13A(2017)e3 - Standard Terminology Relating to Quality and Statistics
• Effective Date: 01-Oct-2017
• Refers: ASTM D5467/D5467M-97(2017) - Standard Test Method for Compressive Properties of Unidirectional Polymer Matrix Composite Materials Using a Sandwich Beam
• Effective Date: 01-Jan-2017

Overview

ASTM D7249/D7249M-20, titled Standard Test Method for Facesheet Properties of Sandwich Constructions by Long Beam Flexure, is a globally recognized ASTM International standard for evaluating the mechanical properties of sandwich panel facesheets using long beam flexure. This test method is essential in the composite materials and sandwich panel industry to assess facesheet tensile and compressive strength, flexural stiffness, and related structural characteristics under load. The standard plays a key role in quality control, material characterization, and research and development involving advanced composites and bonded sandwich constructions.

Key Topics

• Scope of the Test Method

  • Determines facesheet properties of flat sandwich constructions subjected to flexure.
  • Applicable to structures with either continuous (e.g., foams, balsa) or discontinuous (e.g., honeycomb) core materials.
  • Both SI and inch-pound unit systems supported.

• Significance of Flexure Testing

  • Measures sandwich flexural stiffness and facesheet tensile/compressive strengths.
  • Provides data for calculating flexural and shear stiffness in conjunction with ASTM D7250/D7250M.
  • Assesses quality of core-to-facesheet bonding and consistency in manufacturing.

• Factors Impacting Results

  • Material selection (facesheet, core, adhesives).
  • Specimen and test geometry.
  • Preparation and conditioning procedures.
  • Environmental influences during testing (e.g., humidity, temperature).
  • Test speed and loading protocols.

• Reporting and Measurement

  • Comprehensive data collection and reporting on load-deflection behavior.
  • Requirement to document failure modes, key specimen details, and environmental conditions.
  • Calibration and accuracy of measuring equipment are mandatory for reliable results.

Applications

The ASTM D7249/D7249M-20 test method provides substantial value across multiple sectors that rely on composite sandwich panels:

• Aerospace and Aviation: Ensures that lightweight composite panels in aircraft exhibit required facesheet strength and structural reliability under operational loads.
• Automotive and Transportation: Supports material selection for vehicle body components, enhancing safety and weight reduction.
• Marine and Wind Energy: Assesses panels and blades made from sandwich composites for durability and in-service performance.
• Construction and Infrastructure: Validates the integrity of architectural panels, doors, and prefabricated structural elements exposed to mechanical stresses.
• Research, Development, and Quality Control: Guides development of new composite material systems and ensures batch-to-batch consistency in production.

This ASTM test method is also commonly referenced in procurement specifications and regulatory compliance documentation to demonstrate conformity with industry-accepted mechanical testing protocols.

Related Standards

ASTM D7249/D7249M-20 is interconnected with several other key international standards, especially for composite and sandwich construction testing:

• ASTM D7250/D7250M: Practice for Determining Sandwich Beam Flexural and Shear Stiffness.
• ASTM C273/C273M: Test Method for Shear Properties of Sandwich Core Materials.
• ASTM C393/C393M: Test Method for Core Shear Properties of Sandwich Constructions by Beam Flexure.
• ASTM D5467/D5467M: Test Method for Compressive Properties of Unidirectional Polymer Matrix Composite Materials Using a Sandwich Beam.
• ASTM D5229/D5229M: Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composites.

These related standards enable comprehensive mechanical characterization of all key sandwich panel constituents-facesheets, core materials, and adhesive bonds-offering a complete framework for sandwich structure evaluation.

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Keywords: ASTM D7249, sandwich panel testing, facesheet properties, flexural strength, composite materials, long beam flexure, structural panels, test method, quality control, bonded sandwich constructions, mechanical testing standards.

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: D7249/D7249M − 20
Standard Test Method for
Facesheet Properties of Sandwich Constructions by Long
Beam Flexure
This standard is issued under the fixed designation D7249/D7249M; 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 2. Referenced Documents
2.1 ASTM Standards:
1.1 This test method covers determination of facesheet
C273/C273MTestMethodforShearPropertiesofSandwich
propertiesofflatsandwichconstructionssubjectedtoflexurein
Core Materials
such a manner that the applied moments produce curvature of
C393/C393MTest Method for Core Shear Properties of
the sandwich facesheet planes and result in compressive and
Sandwich Constructions by Beam Flexure
tensileforcesinthefacesheets.Permissiblecorematerialforms
D3410/D3410MTest Method for Compressive Properties of
include those with continuous bonding surfaces (such as balsa
Polymer Matrix Composite Materials with Unsupported
wood and foams) as well as those with discontinuous bonding
Gage Section by Shear Loading
surfaces (such as honeycomb).
D3878Terminology for Composite Materials
D5229/D5229MTestMethodforMoistureAbsorptionProp-
1.2 Units—The values stated in either SI units or inch-
erties and Equilibrium Conditioning of Polymer Matrix
pound units are to be regarded separately as standard. The
Composite Materials
values stated in each system are not necessarily exact equiva-
D5467/D5467MTest Method for Compressive Properties of
lents; therefore, to ensure conformance with the standard, each
Unidirectional Polymer Matrix Composite Materials Us-
system shall be used independently of the other, and values
ing a Sandwich Beam
from the two systems shall not be combined.
D7250/D7250MPractice for Determining Sandwich Beam
1.2.1 Within the text, the inch-pound units are shown in
Flexural and Shear Stiffness
brackets.
E4Practices for Force Verification of Testing Machines
E6Terminology Relating to Methods of Mechanical Testing
1.3 This standard does not purport to address all of the
E122PracticeforCalculatingSampleSizetoEstimate,With
safety concerns, if any, associated with its use. It is the
Specified Precision, the Average for a Characteristic of a
responsibility of the user of this standard to establish appro-
Lot or Process
priate safety, health, and environmental practices and deter-
E177Practice for Use of the Terms Precision and Bias in
mine the applicability of regulatory limitations prior to use.
ASTM Test Methods
NOTE 1—Alternate procedures for determining the compressive E251Test Methods for Performance Characteristics of Me-
strength of unidirectional polymer matrix composites materials in a tallic Bonded Resistance Strain Gages
sandwich beam configuration may be found in Test Method D5467/
E456Terminology Relating to Quality and Statistics
D5467M.
E1237Guide for Installing Bonded Resistance Strain Gages
1.4 This international standard was developed in accor-
3. Terminology
dance with internationally recognized principles on standard-
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. Terminology E6
mendations issued by the World Trade Organization Technical
definestermsrelatingtomechanicaltesting.TerminologyE456
Barriers to Trade (TBT) Committee. 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.
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. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Feb. 1, 2020. Published February 2020. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2006. Last previous edition approved in 2018 as D7249/D7249M–18. Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/D7249_D7249M-20. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7249/D7249M − 20
3.2 Symbols: 5.3 Core shear strength and shear modulus are best deter-
mined in accordance withTest Method C273/C273M provided
b = specimen width
barecorematerialisavailable.TestMethodC393/C393Mmay
c = core thickness
also be used to determine core shear strength. Practice D7250/
CV = coefficient of variation statistic of a sample popu-
D7250M may be used to calculate the flexural and shear
lation for a given property (in percent)
stiffness of sandwich beams.
d = sandwich total thickness
F,nom
5.4 This test method can be used to produce facesheet
D = effective sandwich flexural stiffness
f
E = effective facesheet chord modulus strength data for structural design allowables, material
ε = measuring strain in facesheet specifications, and research and development applications; it
u
F = facesheetultimatestrength(tensileorcompressive)
may also be used as a quality control test for bonded sandwich
F = core shear allowable strength
s panels.
F = core compression allowable strength
c
5.5 Factors that influence the facesheet strength and shall
k = core shear strength factor to ensure facesheet fail-
thereforebereportedincludethefollowing:facesheetmaterial,
ure
core material, adhesive material, methods of material
L = length of loading span
fabrication, facesheet stacking sequence and overall thickness,
l = length of loading pad
pad
core geometry (cell size), core density, adhesive thickness,
n = number of specimens
specimen geometry, specimen preparation, specimen
P = applied force
conditioning, environment of testing, specimen alignment,
P = maximum force carried by test specimen before
max
failure loading procedure, speed of testing, facesheet void content,
S = length of support span adhesivevoidcontent,andfacesheetvolumepercentreinforce-
S = standard deviation statistic of a sample population
ment. Further, facesheet strength may be different between
n–1
for a given property
precured/bonded and co-cured facesheets of the same material.
σ = facesheet stress
NOTE 2—Concentrated forces on beams with thin facesheets and low
t = facesheet thickness
density cores can produce results that are difficult to interpret, especially
x = test result for an individual specimen from the
close to the failure point. Wider loading blocks and rubber pressure pads
sample population for a given property
may assist in distributing the forces.
x¯ = mean or average (estimate of mean) of a sample
NOTE 3—To ensure that simple sandwich beam theory is valid, a good
rule of thumb for the four-point bending test is the support span length
population for a given property
divided by the sandwich thickness should be greater than 20 (S/d > 20)
4. Summary of Test Method with the ratio of facesheet thickness to core thickness less than 0.1 (t/c <
0.1).
4.1 This test method consists of subjecting a long beam of
sandwich construction to a bending moment normal to the
6. Interferences
plane of the sandwich, using a 4-point loading fixture. Deflec-
6.1 Material and Specimen Preparation—Poormaterialfab-
tion and strain versus force measurements are recorded.
rication practices and damage induced by improper specimen
4.2 The only acceptable failure modes for sandwich
machiningareknowncausesofhighdatascatterincomposites
facesheet strength are those which are internal to one of the
and sandwich structures in general. A specific material factor
facesheets. Failure of the sandwich core or the core-to-
that affects sandwich cores is variability in core density.
facesheetbondprecedingfailureofoneofthefacesheetsisnot
Important aspects of sandwich core specimen preparation that
an acceptable failure mode. Careful post-test inspection of the
contribute to data scatter include the existence of joints, voids
specimen is required as facesheet failure occurring in proxim-
or other core discontinuities, out-of-plane curvature, and sur-
ity to the loading points can be caused by local through-
face roughness.
thicknesscompressionorshearfailureofthecorethatprecedes
6.2 Geometry—Specific geometric factors that affect sand-
failure of the facesheet.
wich facesheet strength include facesheet thickness, core cell
5. Significance and Use geometry, and facesheet surface flatness (toolside or bagside
surface in compression).
5.1 Flexure tests on flat sandwich construction may be
conductedtodeterminethesandwichflexuralstiffness,thecore 6.3 Environment—Resultsareaffectedbytheenvironmental
shear strength, and shear modulus, or the facesheets’compres- conditions under which specimens are conditioned, as well as
sive and tensile strengths. Tests to evaluate core shear strength the conditions under which the tests are conducted. Specimens
may also be used to evaluate core-to-facesheet bonds. tested in various environments can exhibit significant differ-
ences in both strength behavior and failure mode. Critical
5.2 This test method is limited to obtaining the strength and
environmentsmustbeassessedindependentlyforeachspecific
stiffness of the sandwich panel facesheets, and to obtaining
combination of core material, facesheet material, and core-to-
load-deflection data for use in calculating sandwich beam
facesheet interfacial adhesive (if used) that is tested.
flexural and shear stiffness using Practice D7250/D7250M.
Duetothecurvatureoftheflexuraltestspecimenwhenloaded, 6.4 Core Material—If the core material has insufficient
facesheet compression strength from this test may not be shear or compressive strength, it is possible that the core may
equivalent to the facesheet compression strength of sandwich locally crush at or near the loading points thereby resulting in
structures subjected to pure edgewise (in-plane) compression. facesheet failure due to local stresses. In other cases, facesheet
D7249/D7249M − 20
failure can cause local core crushing. When there is both
facesheet and core failure in the vicinity of one of the loading
points, it can be difficult to determine the failure sequence in a
post-mortem inspection of the specimen as the failed speci-
mens look very similar for both sequences.
7. Apparatus
7.1 Micrometers and Calipers—A micrometer having a flat
anvil interface, or a caliper of suitable size, shall be used. The
instrument(s) shall have an accuracy of 625 µm [60.001 in.]
for thickness measurement, and an accuracy of 6250 µm
Configuration Support Span (S) Load Span (L)
[60.010 in.] for length and width measurement.
Standard 4-Point 560 mm [22.0 in.] 100 mm [4.0 in.]
NOTE 4—The accuracies given above are based on achieving measure-
Non-Standard 3-Point (Mid- S 0.0
ments that are within 1% of the sample length, width, and thickness.
span)
4-Point (Quarter-SS/2
7.2 Loading Fixtures
Span)
7.2.1 Standard Configuration—The standard loading fixture
4-Point (Third- SS/3
shall consist of a 4-point loading configuration with two
Span)
support bars that span the specimen width located below the
FIG. 2 Loading Configurations
specimen, and two loading bars that span the specimen width
located on the top of the specimen (Fig. 1). The force shall be
applied vertically through the loading bars, with the support
bars fixed in place in the test machine. The standard loading
fixture shall have the centerlines of the support bars separated
by a distance of 560 mm [22.0 in.] and the centerlines of the
loading bars separated by a distance of 100 mm [4.0 in.].
7.2.2 Non-Standard Configurations—All other loading fix-
ture configurations (see Fig. 2) are considered non-standard
and details of the fixture geometry shall be documented in the
testreport.Figs.3-5showtypicaltestfixtures.Non-standard3-
and 4-point loading configurations have been retained within
this standard (a) for historical continuity with previous ver-
sions of Test Method C393/C393M, (b) because some sand-
wich panel designs require the use of non-standard loading
configurations to achieve facesheet failure modes, and (c)
load-deflection data from non-standard configurations may be
used with Practice D7250/D7250M to obtain sandwich beam
FIG. 3 Standard 4-Point Loading Configuration
flexural and shear stiffnesses.
7.2.3 Support and Loading Bars—The bars shall be de-
signedtoallowfreerotationofthespecimenattheloadingand
support points. The bars shall have sufficient stiffness to avoid
of 3 mm [0.12 in.].TheV-groove in the loading pad shall have
significant deflection of the bars under load; any obvious
a radius larger than the loading bar tip and the angular opening
bowing of the bars or any gaps occurring between the bars and
ofthegrooveshallbesuchthatthesidesoftheloadingbarsdo
thetestspecimenduringloadingshallbeconsideredsignificant
not contact the sides of the V-groove during the test. Loading
deflection. The recommended configuration has a 25mm
barsconsistingof25mm[1.0in.]diametersteelcylindersmay
[1.0in.] wide flat steel loading block to contact the specimen
also be used, but there is a greater risk of local specimen
(through rubber pressure pads) and is loaded via either a
crushing with cylindrical bars.Also, the load and support span
cylindrical pivot (see Fig. 3) or a V-shaped bar riding in a
lengths tend to increase as the specimen deflects when cylin-
V-groove in the top of the flat-bottomed steel loading pad.The
drical loading bars without V-grooved loading pads are used
tips of the V-shaped loading bars shall have a minimum radius
(for example, rolling supports).
7.2.4 Pressure Pads—Rubber pressure pads having a Shore
A durometer of 60, a width of 25 mm [1.0 in.], a nominal
thickness of 3 mm [0.125 in.], and spanning the full width of
the specimen shall be used between the loading bars and
specimen to prevent local damage to the facesheets.
7.3 Testing Machine—The testing machine shall be in ac-
cordance with Practices E4 and shall satisfy the following
FIG. 1 Test Specimen and Fixture requirements:
D7249/D7249M − 20
the use of strain gages on composite materials follow. A
general reference on the subject is Tuttle and Brinson.
7.5.1.1 Surface preparation of fiber-reinforced composites
in accordance with Guide E1237 can penetrate the matrix
material and cause damage to the reinforcing fibers, resulting
in improper coupon failures. Reinforcing fibers should not be
exposed or damaged during the surface preparation process.
The strain gage manufacturer should be consulted regarding
surface preparation guidelines and recommended bonding
agents for composites, pending the development of a set of
standard practices for strain gage installation surface prepara-
tion of fiber-reinforced composite materials.
7.5.1.2 Consideration should be given to the selection of
gageshavinglargerresistancestoreduceheatingeffectsonlow
conductivity materials. Resistances of 350 Ω or higher are
preferred. Additional consideration should be given to the use
FIG. 4 3-Point Mid-Span Loading Configuration (Non-Standard)
of the minimum possible gage excitation voltage consistent
with the desired accuracy (1 to 2Vis recommended) to reduce
thepowerconsumedbythegage.Heatingofthecouponbythe
7.3.1 Testing Machine Configuration—The testing machine
gage may affect the performance of the material directly or it
shall have both an essentially stationary head and a movable
may affect the indicated strain as a result of a difference
head.
between the gage temperature compensation factor and the
7.3.2 Drive Mechanism—The testing machine drive mecha-
coefficient of thermal expansion of the coupon material.
nism shall be capable of imparting to the movable head a
7.5.1.3 Considerationofsomeformoftemperaturecompen-
controlled velocity with respect to the stationary head. The
sation is recommended, even when testing at standard labora-
velocity of the movable head shall be capable of being
tory atmosphere. Temperature compensation may be required
regulated in accordance with 11.4.
when testing in non-ambient temperature environments.
7.3.3 Force Indicator—The testing machine force-sensing
7.5.1.4 Consideration should be given to the transverse
device shall be capable of indicating the total force being
sensitivity of the selected strain gage. The strain gage manu-
carried by the test specimen. This device shall be essentially
facturer should be consulted for recommendations on trans-
free from inertia-lag at the specified rate of testing and shall
verse sensitivity corrections and effects on composites.
indicate the force with an accuracy over the force range(s) of
interest of within 61% of the indicated value. 7.6 Conditioning Chamber—When conditioning materials
at non-laboratory environments, a temperature/vapor-level
7.4 Deflectometer (LVDT)—The deflection of the specimen
controlledenvironmentalconditioningchamberisrequiredthat
shall be measured in the center of the support span by a
shall be capable of maintaining the required temperature to
properly calibrated device having an accuracy of 61% or
within 63°C[65°F] and the required relative humidity level
better.
to within 63%. Chamber conditions shall be monitored either
NOTE 5—The use of crosshead or actuator displacement for the beam
on an automated continuous basis or on a manual basis at
mid-span deflection produces inaccurate results; the direct measurement
regular intervals.
of the deflection of the mid-span of the beam must be made by a suitable
instrument.
7.7 Environmental Test Chamber—An environmental test
chamber is required for test environments other than ambient
7.5 Strain-Indicating Device—Strain data, when required,
testing laboratory conditions.This chamber shall be capable of
shall be determined by means of bonded resistance strain
maintaining the gage section of the test specimen at the
gages. One axial gage element shall be located on each face at
required test environment during the mechanical test.
the center of the specimen, with the gage aligned with the
specimen length axis. Strain gages cannot be used on the
8. Sampling and Test Specimen
non-standard 3-point loading configuration due to interference
with the center loading bar.
8.1 Sampling—Test at least five specimens per test condi-
7.5.1 Bonded Resistance Strain Gage Selection—Strain
tionunlessvalidresultscanbegainedthroughtheuseoffewer
gage selection is based on the type of material to be tested.An
specimens, as in the case of a designed experiment. For
active gage length of 1.5 mm [0.062 in.] is recommended for
statistically significant data, consult the procedures outlined in
composite laminates fabricated from unidirectional layers.
Practice E122. Report the method of sampling.
Larger strain gage sizes may be more suitable for some textile
8.2 Geometry—Thestandardspecimenconfigurationshould
fabrics. Gage calibration certification shall comply with Test
be used whenever the specimen design equations in 8.2.3
Methods E251. Strain gages with a minimum normal strain
range of approximately 3% are recommended. When testing
textile fabric laminates, gage selection should consider the use
Tuttle, M. E., and Brinson, H. F., “Resistance-Foil Strain-Gage Technology as
of an active gage length that is at least as great as the
Applied to Composite Materials,” Experimental Mechanics, Vol 24, No. 1, March
characteristic repeating unit of the fabric. Some guidelines on 1984, pp. 54–65; errata noted in Vol 26, No. 2, June 1986, pp. 153–154.
D7249/D7249M − 20
FIG. 5 4-Point Quarter-Point Loading Configuration (Non-
Standard)
indicate that the specimen will produce the desired facesheet 2σt
or, thecoreshearstrengthshallsatisfy:F $ (2)
s
k S 2 L
failure mode. In cases where the standard specimen configu- ~ !
ration will not produce a facesheet failure, a non-standard
2 c1t σt
~ !
Thecorecompressionstrengthshallsatisfy:F $ (3)
specimen shall be designed to produce a facesheet failure
c
~S 2 L!L
pad
mode.
where:
8.2.1 Standard Configuration—The standard test specimen
S = support span length, mm [in.],
shall be rectangular in cross section, with a width of 75 mm
L = loading span length, mm [in.] (L = 0 for 3-point
[3.0 in.] and a length of 600 mm [24.0 in.]. The depth of the
loading),
specimen shall be equal to the thickness of the sandwich
σ = expected facesheet ultimate strength, MPa [psi],
construction.
t = facesheet thickness, mm [in.],
8.2.2 Non-Standard Configurations—For non-standard
c = core thickness,
specimen geometries, the width shall be not less than twice the
F = core shear allowable strength, MPa [psi],
s
total thickness nor more than six times the total thickness, not
k = core shear strength factor to ensure facesheet failure
less than three times the dimension of a core cell, nor greater
(recommend k = 0.75),
than one quarter the span length. The specimen length shall be
L = dimension of loading pad in specimen lengthwise
pad
equal to the support span length plus 50 mm [2 in.] or plus one
direction, mm [in.], and
half the sandwich thickness, whichever is the greater. Limita-
F = core compression allowable strength, Mpa [psi].
c
tions on the maximum specimen width are intended to allow
8.3 Facesheets
for the use of simplified sandwich beam calculations; plate
8.3.1 Compression Side Facesheet—Unless otherwise
flexureeffectsmustbeconsideredforspecimensthatarewider
specified by the test requestor, the bag-side facesheet of a
than the restrictions specified above.
co-cured composite sandwich panel shall be placed as the
8.2.3 Specimen Design—Proper design of the sandwich
upper, compression loaded facesheet during test, as facesheet
flexure test specimen for determining compressive or tensile
compressionstrengthismoresensitivetoimperfecti
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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: D7249/D7249M − 18 D7249/D7249M − 20
Standard Test Method for
Facesheet Properties of Sandwich Constructions by Long
Beam Flexure
This standard is issued under the fixed designation D7249/D7249M; 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
1.1 This test method covers determination of facesheet properties of flat sandwich constructions subjected to flexure in such a
manner that the applied moments produce curvature of the sandwich facesheet planes and result in compressive and tensile forces
in the facesheets. 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 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.
1.2.1 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
NOTE 1—Alternate procedures for determining the compressive strength of unidirectional polymer matrix composites materials in a sandwich beam
configuration may be found in Test Method D5467/D5467M.
1.4 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.
2. Referenced Documents
2.1 ASTM Standards:
C273/C273M Test Method for Shear Properties of Sandwich Core Materials
C393/C393M Test Method for Core Shear Properties of Sandwich Constructions by Beam Flexure
D3410/D3410M Test Method for Compressive Properties of Polymer Matrix Composite Materials with Unsupported Gage
Section by Shear Loading
D3878 Terminology for Composite Materials
D5229/D5229M Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite
Materials
D5467/D5467M Test Method for Compressive Properties of Unidirectional Polymer Matrix Composite Materials Using a
Sandwich Beam
D7250/D7250M Practice for Determining Sandwich Beam Flexural and Shear Stiffness
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
E251 Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gages
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, 2018Feb. 1, 2020. Published April 2018February 2020. Originally approved in 2006. Last previous edition approved in 20162018 as
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D7249/D7249M – 16D7249/D7249M – 18. . DOI: 10.1520/D7249_D7249M-18.10.1520/D7249_D7249M-20.
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
D7249/D7249M − 20
E456 Terminology Relating to Quality and Statistics
E1237 Guide for Installing Bonded Resistance Strain Gages
3. Terminology
3.1 Definitions—Terminology D3878 defines terms relating to high-modulus fibers and their composites. 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:
b = specimen width
c = core thickness
CV = coefficient of variation statistic of a sample population for a given property (in percent)
d = sandwich total thickness
F,nom
D = effective sandwich flexural stiffness
f
E = effective facesheet chord modulus
ε = measuring strain in facesheet
u
F = facesheet ultimate strength (tensile or compressive)
F = core shear allowable strength
s
F = core compression allowable strength
c
k = core shear strength factor to ensure facesheet failure
l = length of loading span
L = length of loading span
L = length of support span
l = length of loading pad
pad
n = number of specimens
P = applied force
P = maximum force carried by test specimen before failure
max
S = length of support span
S = standard deviation statistic of a sample population for a given property
n–1
σ = facesheet stress
t = facesheet thickness
x = test result for an individual specimen from the sample population for a given property
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 long beam of sandwich construction to a bending moment normal to the plane of
the sandwich, using a 4-point loading fixture. Deflection and strain versus force measurements are recorded.
4.2 The only acceptable failure modes for sandwich facesheet strength are those which are internal to one of the facesheets.
Failure of the sandwich core or the core-to-facesheet bond preceding failure of one of the facesheets is not an acceptable failure
mode. Careful post-test inspection of the specimen is required as facesheet failure occurring in proximity to the loading points can
be caused by local through-thickness compression or shear failure of the core that precedes failure of the facesheet.
5. Significance and Use
5.1 Flexure tests on flat sandwich construction may be conducted to determine the sandwich flexural stiffness, the core shear
strength, and shear modulus, or the facesheets’ compressive and tensile strengths. Tests to evaluate core shear strength may also
be used to evaluate core-to-facesheet bonds.
5.2 This test method is limited to obtaining the strength and stiffness of the sandwich panel facesheets, and to obtaining
load-deflection data for use in calculating sandwich beam flexural and shear stiffness using Standard Practice D7250/D7250M. Due
to the curvature of the flexural test specimen when loaded, facesheet compression strength from this test may not be equivalent
to the facesheet compression strength of sandwich structures subjected to pure edgewise (in-plane) compression.
5.3 Core shear strength and shear modulus are best determined in accordance with Test Method C273/C273M provided bare
core material is available. Test Method C393/C393M may also be used to determine core shear strength. Standard Practice
D7250/D7250M may be used to calculate the flexural and shear stiffness of sandwich beams.
5.4 This test method can be used to produce facesheet strength data for structural design allowables, material specifications, and
research and development applications; it may also be used as a quality control test for bonded sandwich panels.
5.5 Factors that influence the facesheet strength and shall therefore be reported include the following: facesheet material, core
material, adhesive material, methods of material fabrication, facesheet stacking sequence and overall thickness, core geometry (cell
size), core density, adhesive thickness, specimen geometry, specimen preparation, specimen conditioning, environment of testing,
D7249/D7249M − 20
specimen alignment, loading procedure, speed of testing, facesheet void content, adhesive void content, and facesheet volume
percent reinforcement. Further, facesheet strength may be different between precured/bonded and co-cured facesheets of the same
material.
NOTE 2—Concentrated forces on beams with thin facesheets and low density cores can produce results that are difficult to interpret, especially close
to the failure point. Wider loading blocks and rubber pressure pads may assist in distributing the forces.
NOTE 3—To ensure that simple sandwich beam theory is valid, a good rule of thumb for the four-point bending test is the support span length divided
by the sandwich thickness should be greater than 20 (L/dS/d > 20) with the ratio of facesheet thickness to core thickness less than 0.1 (t/c < 0.1).
6. Interferences
6.1 Material and Specimen Preparation—Poor material fabrication practices and damage induced by improper specimen
machining are known causes of high data scatter in composites and sandwich structures in general. A specific material factor that
affects sandwich cores is variability in core density. Important aspects of sandwich core specimen preparation that contribute to
data scatter include the existence of joints, voids or other core discontinuities, out-of-plane curvature, and surface roughness.
6.2 Geometry—Specific geometric factors that affect sandwich facesheet strength include facesheet thickness, core cell
geometry, and facesheet surface flatness (toolside or bagside surface in compression).
6.3 Environment—Results are affected by the environmental conditions under which specimens are conditioned, as well as the
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 specific combination of
core material, facesheet material, and core-to-facesheet interfacial adhesive (if used) that is tested.
6.4 Core Material—If the core material has insufficient shear or compressive strength, it is possible that the core may locally
crush at or near the loading points thereby resulting in facesheet failure due to local stresses. In other cases, facesheet failure can
cause local core crushing. When there is both facesheet and core failure in the vicinity of one of the loading points, it can be
difficult to determine the failure sequence in a post-mortem inspection of the specimen as the failed specimens look very similar
for both sequences.
7. Apparatus
7.1 Micrometers and Calipers—A micrometer having a flat anvil interface, or a caliper of suitable size, shall be used. The
instrument(s) shall have an accuracy of 625 μm [60.001 in.] for thickness measurement, and an accuracy of 6250 μm [60.010
in.] for length and width measurement.
NOTE 4—The accuracies given above are based on achieving measurements that are within 1 % of the sample length, width, and thickness.
7.2 Loading Fixtures
7.2.1 Standard Configuration—The standard loading fixture shall consist of a 4-point loading configuration with two support
bars that span the specimen width located below the specimen, and two loading bars that span the specimen width located on the
top of the specimen (Fig. 1),). The force shall be applied vertically through the loading bars, with the support bars fixed in place
in the test machine. The standard loading fixture shall have the centerlines of the support bars separated by a distance of 560 mm
[22.0 in.] and the centerlines of the loading bars separated by a distance of 100 mm [4.0 in.].
7.2.2 Non-Standard Configurations—All other loading fixture configurations (see Fig. 2) are considered non-standard and
details of the fixture geometry shall be documented in the test report. Figs. 3-5 show typical test fixtures. Non-standard 3- and
4-point loading configurations have been retained within this standard a)(a) for historical continuity with previous versions of Test
Method C393/C393M, b)(b) because some sandwich panel designs require the use of non-standard loading configurations to
achieve facesheet failure modes, and c)(c) load-deflection data from non-standard configurations may be used with Standard
Practice D7250/D7250M to obtain sandwich beam flexural and shear stiffnesses.
7.2.3 Support and Loading Bars—The bars shall be designed to allow free rotation of the specimen at the loading and support
points. The bars shall have sufficient stiffness to avoid significant deflection of the bars under load; any obvious bowing of the bars
or any gaps occurring between the bars and the test specimen during loading shall be considered significant deflection. The
recommended configuration has a 25 mm [1.0 in.] 25 mm [1.0 in.] wide flat steel loading block to contact the specimen (through
rubber pressure pads) and is loaded via either a cylindrical pivot (see Fig. 3) or a V-shaped bar riding in a V-groove in the top of
the flat-bottomed steel loading pad. The tips of the V-shaped loading bars shall have a minimum radius of 3 mm [0.12 in.]. The
FIG. 1 Test Specimen and Fixture
D7249/D7249M − 20
Configuration Support Span (S) Load Span (L)
Standard 4-Point 560 mm [22.0 in.] 100 mm [4.0 in.]
Non-Standard 3-Point (Mid- S 0.0
span)
4-Point (Quarter- S S/2
Span)
4-Point (Third- S S/3
Span)
FIG. 2 Loading Configurations
FIG. 3 Standard 4-Point Loading Configuration
FIG. 4 3-Point Mid-Span Loading Configuration (Non-Standard)
V-groove in the loading pad shall have a radius larger than the loading bar tip and the angular opening of the groove shall be such
that the sides of the loading bars do not contact the sides of the V-groove during the test. Loading bars consisting of 25 mm [1.0
D7249/D7249M − 20
FIG. 5 4-Point Quarter-Point Loading Configuration (Non-
Standard)
in.] diameter steel cylinders may also be used, but there is a greater risk of local specimen crushing with cylindrical bars. Also,
the load and support span lengths tend to increase as the specimen deflects when cylindrical loading bars without V-grooved
loading pads are used (e.g., (for example, rolling supports).
7.2.4 Pressure Pads—Rubber pressure pads having a Shore A durometer of 60, a width of 25 mm [1.0 in.], a nominal thickness
of 3 mm [0.125 in.]in.], and spanning the full width of the specimen shall be used between the loading bars and specimen to prevent
local damage to the facesheets.
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 11.4.
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.
7.4 Deflectometer (LVDT)—The deflection of the specimen shall be measured in the center of the support span by a properly
calibrated device having an accuracy of 61 % or better.
NOTE 5—The use of crosshead or actuator displacement for the beam mid-span deflection produces inaccurate results; the direct measurement of the
deflection of the mid-span of the beam must be made by a suitable instrument.
7.5 Strain-Indicating Device—Strain data, when required, shall be determined by means of bonded resistance strain gages. One
axial gage element shall be located on each face at the center of the specimen, with the gage aligned with the specimen length axis.
Strain gages cannot be used on the non-standard 3-point loading configuration due to interference with the center loading bar.
7.5.1 Bonded Resistance Strain Gage Selection—Strain gage selection is based on the type of material to be tested. An active
gage length of 1.5 mm [0.062 in.] is recommended for composite laminates fabricated from unidirectional layers. Larger strain
gage sizes may be more suitable for some textile fabrics. Gage calibration certification shall comply with Test MethodMethods
E251. Strain gages with a minimum normal strain range of approximately 3 % are recommended. When testing textile fabric
laminates, gage selection should consider the use of an active gage length that is at least as great as the characteristic repeating
unit of the fabric. Some guidelines on the use of strain gages on composite materials follow. A general reference on the subject
is Tuttle and Brinson.
7.5.1.1 Surface preparation of fiber-reinforced composites in accordance with Guide E1237 can penetrate the matrix material
and cause damage to the reinforcing fibers, resulting in improper coupon failures. Reinforcing fibers should not be exposed or
damaged during the surface preparation process. The strain gage manufacturer should be consulted regarding surface preparation
Tuttle, M. E., and Brinson, H. F., “Resistance-Foil Strain-Gage Technology as Applied to Composite Materials,” Experimental Mechanics, Vol 24, No. 1, March 1984,
pp. 54–65; errata noted in Vol 26, No. 2, June 1986, pp. 153–154.
D7249/D7249M − 20
guidelines and recommended bonding agents for composites, pending the development of a set of standard practices for strain gage
installation surface preparation of fiber-reinforced composite materials.
7.5.1.2 Consideration should be given to the selection of gages having larger resistances to reduce heating effects on low
conductivity materials. Resistances of 350 Ω or higher are preferred. Additional consideration should be given to the use of the
minimum possible gage excitation voltage consistent with the desired accuracy (1 to 2 V is recommended) to reduce the power
consumed by the gage. Heating of the coupon by the gage may affect the performance of the material directly or it may affect the
indicated strain as a result of a difference between the gage temperature compensation factor and the coefficient of thermal
expansion of the coupon material.
7.5.1.3 Consideration of some form of temperature compensation is recommended, even when testing at standard laboratory
atmosphere. Temperature compensation may be required when testing in non-ambient temperature environments.
7.5.1.4 Consideration should be given to the transverse sensitivity of the selected strain gage. The strain gage manufacturer
should be consulted for recommendations on transverse sensitivity corrections and effects on composites.
7.6 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]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.
7.7 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 Specimen
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—The standard specimen configuration should be used whenever the specimen design equations in section 8.2.3
indicate that the specimen will produce the desired facesheet failure mode. In cases where the standard specimen configuration will
not produce a facesheet failure, a non-standard specimen shall be designed to produce a facesheet failure mode.
8.2.1 Standard Configuration—The standard test specimen shall be rectangular in cross section, with a width of 75 mm [3.0 in.]
and a length of 600 mm [24.0 in.]. The depth of the specimen shall be equal to the thickness of the sandwich construction.
8.2.2 Non-Standard Configurations—For non-standard specimen geometries, the width shall be not less than twice the total
thickness nor more than six times the total thickness, not less than three times the dimension of a core cell, nor greater than one
quarter the span length. The specimen length shall be equal to the support span length plus 50 mm [2 in.] or plus one half the
sandwich thickness, whichever is the greater. Limitations on the maximum specimen width are intended to allow for the use of
simplified sandwich beam calculations; plate flexure effects must be considered for specimens that are wider than the restrictions
specified above.
8.2.3 Specimen Design—Proper design of the sandwich flexure test specimen for determining compressive or tensile strength
of the facesheets is required to avoid core crushing, core shear, or core-to-facesheet failures. The facesheets must be sufficiently
thin and the support span sufficiently long such that moments are produced at applied forces low enough so that the allowable core
shear stress will not be exceeded. The core must be sufficiently thick to avoid excessive deflection. The following equations can
be used to size the test specimen (these equations assume that both facesheets have the same thickness and modulus, and that the
facesheet thickness is small relative to the core thickness [t/c £ ≤ ~0.10] ):
2σt
The support span length shall satisfy:S $ 1L (1)
kF
s
2σt
or, the core shear strength shall satisfy:F $ (2)
s
k S 2 L
~ !
2 c1t σt
~ !
The core compression strength shall satisfy:F $ (3)
c
~S 2L!L
pad
where:
S = support span length, mm [in.],
L = loading
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