ASTM D6416/D6416M-01(2012)
(Test Method)Standard Test Method for Two-Dimensional Flexural Properties of Simply Supported Sandwich Composite Plates Subjected to a Distributed Load
Standard Test Method for Two-Dimensional Flexural Properties of Simply Supported Sandwich Composite Plates Subjected to a Distributed Load
SIGNIFICANCE AND USE
This test method simulates the hydrostatic loading conditions which are often present in actual sandwich structures, such as marine hulls. This test method can be used to compare the two-dimensional flexural stiffness of a sandwich composite made with different combinations of materials or with different fabrication processes. Since it is based on distributed loading rather than concentrated loading, it may also provide more realistic information on the failure mechanisms of sandwich structures loaded in a similar manner. Test data should be useful for design and engineering, material specification, quality assurance, and process development. In addition, data from this test method would be useful in refining predictive mathematical models or computer code for use as structural design tools. Properties that may be obtained from this test method include:
Panel surface deflection at load,
Panel face-sheet strain at load,
Panel bending stiffness,
Panel shear stiffness,
Panel strength, and
Panel failure modes.
SCOPE
1.1 This test method determines the two-dimensional flexural properties of sandwich composite plates subjected to a distributed load. The test fixture uses a relatively large square panel sample which is simply supported all around and has the distributed load provided by a water-filled bladder. This type of loading differs from the procedure of Test Method C393, where concentrated loads induce one-dimensional, simple bending in beam specimens.
1.2 This test method is applicable to composite structures of the sandwich type which involve a relatively thick layer of core material bonded on both faces with an adhesive to thin-face sheets composed of a denser, higher-modulus material, typically, a polymer matrix reinforced with high-modulus fibers.
1.3 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.
1.4 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.
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Standards Content (Sample)
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Designation: D6416/D6416M − 01(Reapproved 2012)
Standard Test Method for
Two-Dimensional Flexural Properties of Simply Supported
Sandwich Composite Plates Subjected to a Distributed
Load
This standard is issued under the fixed designation D6416/D6416M; 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 C365/C365M Test Method for Flatwise Compressive Prop-
erties of Sandwich Cores
1.1 This test method determines the two-dimensional flex-
C393 Test Method for Flexural Properties of Sandwich
ural properties of sandwich composite plates subjected to a
Constructions
distributed load. The test fixture uses a relatively large square
D792 Test Methods for Density and Specific Gravity (Rela-
panel sample which is simply supported all around and has the
tive Density) of Plastics by Displacement
distributedloadprovidedbyawater-filledbladder.Thistypeof
D2584 Test Method for Ignition Loss of Cured Reinforced
loadingdiffersfromtheprocedureofTestMethodC393,where
Resins
concentrated loads induce one-dimensional, simple bending in
D2734 TestMethodsforVoidContentofReinforcedPlastics
beam specimens.
D3171 Test Methods for Constituent Content of Composite
1.2 This test method is applicable to composite structures of
Materials
thesandwichtypewhichinvolvearelativelythicklayerofcore
D3878 Terminology for Composite Materials
material bonded on both faces with an adhesive to thin-face
E4 Practices for Force Verification of Testing Machines
sheets composed of a denser, higher-modulus material,
E6 Terminology Relating to Methods of Mechanical Testing
typically, a polymer matrix reinforced with high-modulus
E251 Test Methods for Performance Characteristics of Me-
fibers.
tallic Bonded Resistance Strain Gages
1.3 The values stated in either SI units or inch-pound units E1237 Guide for Installing Bonded Resistance Strain Gages
2.2 ASTM Adjunct:
are to be regarded separately as standard. Within the text the
inch-pound units are shown in brackets. The values stated in Sandwich Plate Test Fixture and Hydromat Pressure Blad-
each system are not exact equivalents; therefore, each system der, ASTM D6416/D6416M
must be used independently of the other. Combining values
3. Terminology
from the two systems may result in nonconformance with the
3.1 Terminology D3878 defines terms relating to high-
standard.
modulus fibers and their composites. Terminology C274 de-
1.4 This standard does not purport to address all of the
fines terms relating to structural sandwich constructions. Ter-
safety concerns, if any, associated with its use. It is the
minology E6 defines terms relating to mechanical testing. In
responsibility of the user of this standard to establish appro-
theeventofaconflictbetweenterms,TerminologyD3878shall
priate safety and health practices and determine the applica-
have precedence over the other terminology standards.
bility of regulatory limitations prior to use.
3.2 Definitions of Terms Specific to This Standard:
2. Referenced Documents
3.2.1 bending stiffness, n—the sandwich property which
2.1 ASTM Standards: resists bending deflections.
C274 Terminology of Structural Sandwich Constructions
3.2.2 core, n—a centrally located layer of a sandwich
construction, usually low density, which separates and stabi-
This test method is under the jurisdiction of ASTM Committee D30 on
lizes the facings and transmits shear between the facings and
Composite Materials and is the direct responsibility of Subcommittee D30.09 on
provides most of the shear rigidity of the construction.
Sandwich Construction.
Current edition approved Aug. 1, 2012. Published December 2012. Originally 3.2.3 face sheet, n—the outermost layer or composite com-
approved in 1999. Last previous edition approved in 2007 as D6416/D6416M – 01
ponent of a sandwich construction, generally thin and of high
(2007). DOI: 10.1520/D6416_D6416M-01R12.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Detailed drawings for the fabrication of the 500–mm test fixture and pressure
Standards volume information, refer to the standard’s Document Summary page on bladder shown in Fig. 3 and Fig. 4 are available from ASTM Headquarters. Order
the ASTM website. Adjunct No. ADJD6416.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6416/D6416M − 01 (2012)
density, which resists most of the edgewise loads and flatwise 4.2 Panel deflection at load is monitored by a centrally
bending moments: synonymous with face, skin, and facing. positioned LVDT which contacts the tension-side surface.
3.2.4 footprint, n—the enclosed area of the face sheet 4.3 Load is monitored by both a crosshead-mounted load
surface of a sandwich panel in contact with the pressure
cell, in series with the test fixture, and a pressure transducer in
bladder during loading. the pressure bladder itself. Since the pressure bladder is also at
all times in series with the load cell and test fixture, the
3.2.5 hydromat, n—a pressure bladder with a square perim-
effective contact area of the pressure field is continuously
eter fabricated from two square pieces of industrial belting
monitored as the load/pressure quotient.
which are superposed and clamped at the edges with through-
bolted, mild steel bar stock.
4.4 Strain can be monitored with strategically placed strain
gage rosettes bonded to the tension-side face-sheet surface. A
3.2.6 isotropicmaterial,n—amaterialhavingessentiallythe
typical arrangement has four rosettes equally spaced along one
same properties in any direction.
of the axes of symmetry of the plate.
3.2.7 orthotropic material, n—a material in which a prop-
erty of interest, at a given point, possesses three mutually 5. Significance and Use
perpendicular planes of symmetry, which taken together define
5.1 This test method simulates the hydrostatic loading
the principal material coordinate system.
conditions which are often present in actual sandwich
3.2.8 pressure bladder, n—a durable, yet pliable closed
structures, such as marine hulls. This test method can be used
container filled with water, or other incompressible fluid, to compare the two-dimensional flexural stiffness of a sand-
capable of conforming to the contour of a normally loaded test
wich composite made with different combinations of materials
panel when compressed against its face sheet surface by a test or with different fabrication processes. Since it is based on
machine.
distributed loading rather than concentrated loading, it may
also provide more realistic information on the failure mecha-
3.2.9 shear stiffness, n—thesandwichpropertywhichresists
nisms of sandwich structures loaded in a similar manner. Test
shear distortions: synonymous with shear rigidity.
data should be useful for design and engineering, material
3.2.10 test panel, n—asquarecouponofsandwichconstruc-
specification, quality assurance, and process development. In
tion fabricated for two-dimensional flexural testing: synony-
addition, data from this test method would be useful in refining
mous with sandwich panel, sandwich composite plate, sand-
predictive mathematical models or computer code for use as
wich composite panel, and panel test specimen.
structural design tools. Properties that may be obtained from
3.3 Symbols:
this test method include:
3.3.1 a = support span of the test fixture or the length and 5.1.1 Panel surface deflection at load,
5.1.2 Panel face-sheet strain at load,
width of the test panel structure between supports.
5.1.3 Panel bending stiffness,
3.3.2 A = effective contact area of the pressure bladder
eff
5.1.4 Panel shear stiffness,
when compressed against the test panel.
5.1.5 Panel strength, and
3.3.3 B = test panel bending stiffness.
5.1.6 Panel failure modes.
3.3.4 c = core thickness.
3.3.5 ε = normal face sheet strain, x component.
x
6. Interferences
3.3.6 ε = normal face sheet strain, y component.
y
6.1 Material and Specimen Preparation—Poormaterialfab-
3.3.7 f = face sheet thickness.
rication practices, lack of control of fiber alignment, and
3.3.8 F = total normal force applied to a test panel as
m
damage induced by improper coupon machining are known
measured by the test machine load cell.
causes of high material data scatter in composites in general.
3.3.9 h = average overall thickness of the test panel.
Specific material factors that affect sandwich composites in-
3.3.10 N = the number of included terms of the series.
clude variability in core density and degree of cure of resin in
3.3.11 P = experimentally measured bladder pressure.
m
both face sheet matrix material and core bonding adhesive.
3.3.12 φ = width of the unloaded border area of a test panel
Important aspects of sandwich panel specimen preparation that
betweentheedgesupportsandtheeffectivefootprintboundary.
contribute to data scatter are incomplete wetout of face sheet
3.3.13 S = test panel shear stiffness.
fabric, incomplete or nonuniform core bonding of face sheets,
3.3.14 ω = experimentally determined deflection at center
the non-squareness of adjacent panel edges, the misalignment
e
of test panel.
of core and face sheet elements, the existence of joints or other
core and face sheet discontinuities, out-of-plane curvature, and
4. Summary of Test Method
surface roughness.
4.1 Asquaretestpanelissimplysupportedonallfouredges 6.2 Test Fixture Characteristics—Configurationofthepanel
and uniformly loaded over a portion of its surface by a edge-constraint structure can have a significant effect on test
water-filled bladder. Pressure on the panel is increased by results. Correct interpretation of test data depends on the
moving the platens of the test frame. The test measures the fixture supporting the test panel in such a manner that the
two-dimensional flexural response of a sandwich composite boundary conditions consistent with simple support can be
plate in terms of deflections and strains when subjected to a assumed to apply. Panel edge support journals must be copla-
well-defined distributed load. nar and perpendicular to the loading axis. Given the fixture
D6416/D6416M − 01 (2012)
itself has sufficient rigidity, erroneous conclusions about panel
strength and stiffness might be drawn if insufficient torque has
been applied to the fasteners securing the lower panel edge
support frame. In general, panels with more flexural rigidity
and shear rigidity require more bolt torque to approach simple
support.
6.3 Pressure Bladder Characteristics—When a pressure
bladderisusedtointroducenormalloadtoaplate,theresponse
of the plate is dependent on the resulting pressure distribution.
The true function of the pressure bladder is to convert the
absolute load applied by the test machine into a pressure field
that can be specified by a relatively simple mathematical
model. With the hydromat-style bladder, two simplifying
assumptions are permitted: (1) the shape of the contact area is
a readily definable geometric shape (or combination of shapes)
and (2) the pressure is constant within the boundaries of the
contact area. The pressure distribution is then characterized
merely by the magnitude of the pressure and the size of the
footprint. Obviously, the size and shape of the pressure bladder
have a significant effect on test results in terms of the observed
strains and deflections. Some errors in data interpretation are
possible insofar as the actual pressure distribution differs from
the simple mathematical model used in calculations.
NOTE1—Theerrorinthehydromatmodelhasmainlytodowithdetails
of the footprint shape, since the effective contact area can be calculated at
FIG. 1 Elements of the Two-Dimensional Sandwich Plate Flexural
any time by dividing the absolute applied load by the bladder pressure.A
Test
secondary error arises from the non-zero bending stiffness of the fiber-
reinforcedindustrialbeltingfabricthatresultsinanarrowbandofvarying
pressure at the very edge of the footprint. Calibration tests using a steel
7.1.1 Testing Machine—The testing machine shall be in
plate equipped with strain gages are recommended for each bladder unit
conformance with Practices E4 and shall satisfy the following
to verify that the errors in the pressure distribution model are negligible
(see Section 9). requirements:
7.1.1.1 Testing Machine Heads—The testing machine shall
6.4 Tolerances—Test panels need to meet the dimensional
have both an essentially stationary head and a movable head.
and squareness tolerances specified in 8.2 to ensure proper
7.1.1.2 Drive Mechanism—The testing machine drive
edge support and constraint.
mechanism shall be capable of imparting to the movable head
6.5 SystemAlignment—Errorscanresultifthepanelsupport
a controlled velocity with respect to the stationary head. The
structure is not centered with respect to the actuator of the test
velocity of the movable head shall be capable of being
machine, or if the plane defined by the panel edge-bearing
regulated in accordance with 11.3.
surfaces is not perpendicular to the loading axis of the test
7.1.1.3 Load Indicator—The testing machine load-sensing
machine. Errors can also result if the pressure bladder is not
device shall be capable of indicating the total load being
centered properly with respect to fixture and actuator or if the
carried by the test specimen. This device shall be essentially
edges of the bladder clamping bars are not parallel to the panel free from inertia-lag at the specified rate of testing and shall
edge-support journals.
indicate the load with an accuracy over the load range(s) of
interest of within 61 % of the indicated value. The load
6.6 Other System Characteristics—When attempting to
range(s) of interest may be fairly low for bending and shear
measure panel surface deflection, an error results which is an
modulus evaluation or much higher for strength evaluation, or
artifact of the test. It arises as normal load is applied, to the
both, as required.
extent that the edges of the sandwich specimen are compressed
7.1.2 Loading Fixture—As illustrated in the schematic dia-
from the reactive line loads generated by the upper and lower
gram of Fig. 1, the loading fixture has two parts, a rigid,
panel support structure. This direct rigid-body addition affects
overhead upper panel support structure, which is attached to
any LVDTpositioned to contact the tension-side panel surface.
the load cell on the load frame crosshead, and a rigid lower
To minimize the error, the edges of soft-c
...
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