ASTM D7249/D7249M-06
(Test Method)Standard Test Method for Facing Properties of Sandwich Constructions by Long Beam Flexure
Standard Test Method for Facing Properties of Sandwich Constructions by Long Beam Flexure
SCOPE
1.1 This test method covers determination of facing properties of flat sandwich constructions subjected to flexure in such a manner that the applied moments produce curvature of the sandwich facing planes and result in compressive and tensile forces in the facings. 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. Within the text the inch-pound units are shown in brackets. The values stated in each system are not exact equivalents; therefore, each system must be used independently of the other. Combining values from the two systems may result in nonconformance with the standard.
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.
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 D 5467.
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Standards Content (Sample)
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Designation: D7249/D7249M − 06
StandardTest Method for
Facing 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 erties and Equilibrium Conditioning of Polymer Matrix
Composite Materials
1.1 This test method covers determination of facing prop-
D5467/D5467MTest Method for Compressive Properties of
erties of flat sandwich constructions subjected to flexure in
Unidirectional Polymer Matrix Composite Materials Us-
such a manner that the applied moments produce curvature of
ing a Sandwich Beam
the sandwich facing planes and result in compressive and
D7250/D7250MPractice for Determining Sandwich Beam
tensile forces in the facings. Permissible core material forms
Flexural and Shear Stiffness
include those with continuous bonding surfaces (such as balsa
E6Terminology Relating to Methods of Mechanical Testing
wood and foams) as well as those with discontinuous bonding
E122PracticeforCalculatingSampleSizetoEstimate,With
surfaces (such as honeycomb).
Specified Precision, the Average for a Characteristic of a
1.2 The values stated in either SI units or inch-pound units
Lot or Process
are to be regarded separately as standard. Within the text the
E177Practice for Use of the Terms Precision and Bias in
inch-pound units are shown in brackets. The values stated in
ASTM Test Methods
each system are not exact equivalents; therefore, each system
E251Test Methods for Performance Characteristics of Me-
must be used independently of the other. Combining values
tallic Bonded Resistance Strain Gauges
from the two systems may result in nonconformance with the
E456Terminology Relating to Quality and Statistics
standard.
E1309 Guide for Identification of Fiber-Reinforced
1.3 This standard does not purport to address all of the Polymer-Matrix Composite Materials in Databases
safety concerns, if any, associated with its use. It is the
E1434Guide for Recording Mechanical Test Data of Fiber-
responsibility of the user of this standard to establish appro- Reinforced Composite Materials in Databases
priate safety and health practices and determine the applica-
3. Terminology
bility of regulatory limitations prior to use.
3.1 Definitions—Terminology D3878 defines terms relating
NOTE 1—Alternate procedures for determining the compressive
to high-modulus fibers and their composites. Terminology
strength of unidirectional polymer matrix composites materials in a
sandwich beam configuration may be found in Test Method D5467/ C274 defines terms relating to structural sandwich construc-
D5467M.
tions. Terminology C393 defines terms relating to plastics.
Terminology E6 defines terms relating to mechanical testing.
2. Referenced Documents
Terminology E456 and Practice E177 define terms relating to
2.1 ASTM Standards:
statistics.Intheeventofaconflictbetweenterms,Terminology
C274Terminology of Structural Sandwich Constructions
D3878 shall have precedence over the other terminologies.
C393Test Method for Flexural Properties of Sandwich
3.2 Symbols:
Constructions
D3878Terminology for Composite Materials
b = specimen width
D5229/D5229MTestMethodforMoistureAbsorptionProp-
c = core thickness
CV = coefficient of variation statistic of a sample popu-
lation for a given property (in percent)
This test method is under the jurisdiction of ASTM Committee D30 on
d = sandwich total thickness
Composite Materials and is the direct responsibility of Subcommittee D30.09 on F,nom
D = effective sandwich flexural stiffness
Sandwich Construction.
f
E = effective facing chord modulus
Current edition approved Sept. 1, 2006. Published October 2006. DOI: 10.1520/
ε = measuring strain in facing
D7249_D7249M-06.
u
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
F = facing ultimate strength (tensile or compressive)
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
F = core shear allowable strength
s
Standards volume information, refer to the standard’s Document Summary page on
F = core compression allowable strength
c
the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7249/D7249M − 06
5.5 Factors that influence the facing strength and shall
k = core shear strength factor to ensure facing failure
therefore be reported include the following: facing material,
l = length of loading span
core material, adhesive material, methods of material
L = length of support span
l = length of loading pad fabrication, facing stacking sequence and overall thickness,
pad
n = number of specimens
core geometry (cell size), core density, adhesive thickness,
P = applied force
specimen geometry, specimen preparation, specimen
P = maximum force carried by test specimen before
max conditioning, environment of testing, specimen alignment,
failure
loading procedure, speed of testing, facing void content,
S = standard deviation statistic of a sample population
n–1
adhesive void content, and facing volume percent reinforce-
for a given property
ment. Further, facing strength may be different between
σ = facing stress
precured/bonded and co-cured facesheets of the same material.
t = facing thickness
NOTE 2—Concentrated forces on beams with thin facings and low
x = test result for an individual specimen from the
density cores can produce results that are difficult to interpret, especially
sample population for a given property
close to the failure point. Wider loading blocks and rubber pressure pads
x¯ = mean or average (estimate of mean) of a sample
may assist in distributing the forces.
population for a given property
NOTE 3—To ensure that simple sandwich beam theory is valid, a good
rule of thumb for the four-point bending test is the span length divided by
the sandwich thickness should be greater than 20 (L/d > 20) with the ratio
4. Summary of Test Method
of facing 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
specimenisrequiredasfacingfailureoccurringinproximityto
or other core discontinuities, out-of-plane curvature, and sur-
the loading points can be caused by local through-thickness
face roughness.
compressionorshearfailureofthecorethatprecedesfailureof
6.2 Geometry—Specific geometric factors that affect sand-
the facing.
wich facing strength include facing thickness, core cell
geometry, and facing surface flatness (toolside or bagside
5. Significance and Use
surface in compression).
5.1 Flexure tests on flat sandwich construction may be
6.3 Environment—Resultsareaffectedbytheenvironmental
conductedtodeterminethesandwichflexuralstiffness,thecore
conditions under which specimens are conditioned, as well as
shear strength, and shear modulus, or the facings’compressive
the conditions under which the tests are conducted. Specimens
and tensile strengths.Tests to evaluate core shear strength may
tested in various environments can exhibit significant differ-
also be used to evaluate core-to-facing bonds.
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 facings, and to obtaining
combination of core material, facing material, and core-to-
load-deflection data for use in calculating sandwich beam
facing interfacial adhesive (if used) that is tested.
flexural and shear stiffness using Standard Practice D7250/
6.4 Core Material—If the core material has insufficient
D7250M. Due to the curvature of the flexural test specimen
shear or compressive strength, it is possible that the core may
when loaded, facesheet compression strength from this test
locally crush at or near the loading points thereby resulting in
maynotbeequivalenttothefacesheetcompressionstrengthof
facesheet failure due to local stresses. In other cases, facing
sandwich structures subjected to pure edgewise (in-plane)
failurecancauselocalcorecrushing.Whenthereisbothfacing
compression.
and core failure in the vicinity of one of the loading points it
5.3 Core shear strength and shear modulus are best deter-
can be difficult to determine the failure sequence in a post-
mined in accordance with Test Method C273 provided bare mortem inspection of the specimen as the failed specimens
core material is available.Test Method C393 may also be used
look very similar for both sequences.
to determine core shear strength. Standard Practice D7250/
D7250M may be used to calculate the flexural and shear
7. Apparatus
stiffness of sandwich beams.
7.1 Micrometers and Calipers—A micrometer having a flat
5.4 This test method can be used to produce facing strength anvil interface, or a caliper of suitable size, shall be used. The
data for structural design allowables, material specifications, instrument(s) shall have an accuracy of 625 mm [60.001 in.]
andresearchanddevelopmentapplications;itmayalsobeused for thickness measurement, and an accuracy of 6250 mm
as a quality control test for bonded sandwich panels. [60.010 in.] for length and width measurement.
D7249/D7249M − 06
NOTE 4—The accuracies given above are based on achieving measure-
ments 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
Configuration Support Span (S) Load Span (L)
by a distance of 560 mm [22.0 in.] and the centerlines of the
Standard 4-Point 560 mm [22.0 in.] 100 mm [4.0 in.]
loading bars separated by a distance of 100 mm [4.0 in.].
Non-Standard 3-Point (Mid- S 0.0
7.2.2 Non-Standard Configurations—All other loading fix-
span)
ture configurations (see Fig. 2) are considered non-standard
4-Point (Quarter-SS/2
and details of the fixture geometry shall be documented in the
Span)
4-Point (Third- SS/3
testreport.Figs.3-5showtypicaltestfixtures.Non-standard3-
Span)
and 4-point loading configurations have been retained within
this standard a) for historical continuity with previous versions
FIG. 2 Loading Configurations
ofTestMethodC393,b)becausesomesandwichpaneldesigns
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 Standard
PracticeD7250/D7250Mtoobtainsandwichbeamflexuraland
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
significant deflection of the bars under load; any obvious
bowing of the bars or any gaps occurring between the bars and
thetestspecimenduringloadingshallbeconsideredsignificant
deflection. The recommended configuration has a 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
FIG. 3 Standard 4-Point Loading Configuration
of 3 mm [0.12 in.].TheV-groove in the loading pad shall have
a radius larger than the loading bar tip and the angular opening
thicknessof3mm[0.125in.]andspanningthefullwidthofthe
ofthegrooveshallbesuchthatthesidesoftheloadingbarsdo
specimen shall be used between the loading bars and specimen
not contact the sides of the V-groove during the test. Loading
to prevent local damage to the facings.
barsconsistingof25mm[1.0in.]diametersteelcylindersmay
7.3 Testing Machine—The testing machine shall be in ac-
also be used, but there is a greater risk of local specimen
cordance with Practices E4 and shall satisfy the following
crushing with cylindrical bars.Also, the load and support span
requirements:
lengths tend to increase as the specimen deflects when cylin-
7.3.1 Testing Machine Configuration—The testing machine
drical loading bars without V-grooved loading pads are used
shall have both an essentially stationary head and a movable
(e.g., rolling supports).
head.
7.2.4 Pressure Pads—Rubber pressure pads having a Shore
7.3.2 Drive Mechanism—The testing machine drive mecha-
A durometer of 60, a width of 25 mm [1.0 in.], a nominal
nism 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
FIG. 1 Test Specimen and Fixture interest of within 6 1% of the indicated value.
D7249/D7249M − 06
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
c
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