ASTM D6416/D6416M-99
(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
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 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.
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.
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Designation: D 6416/D 6416M – 99
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 D 6416/D 6416M; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope D 2584 Test Method for Ignition Loss of Cured Reinforced
Resins
1.1 This test method determines the two-dimensional flex-
D 2734 Test Method for Void Content of Reinforced Plas-
ural properties of sandwich composite plates subjected to a
tics
distributed load. The test fixture uses a relatively large square
D 3171 Test Method for Fiber Content of Resin-Matrix
panel sample which is simply supported all around and has the
Composites by Matrix Digestion
distributed load provided by a water-filled bladder. This type of
D 3878 Terminology of High-Modulus Reinforcing Fibers
loading differs from the procedure of Test Method C 393,
and Their Composites
where concentrated loads induce one-dimensional, simple
E 4 Practices for Force Verification of Testing Machines
bending in beam specimens.
E 6 Terminology Relating to Methods of Mechanical Test-
1.2 This test method is applicable to composite structures of
ing
the sandwich type which involve a relatively thick layer of core
E 251 Test Methods for Performance Characteristics of
material bonded on both faces with an adhesive to thin-face
Metallic Bonded Resistance Strain Gages
sheets composed of a denser, higher-modulus material, typi-
E 1237 Guide for Installing Bonded Resistance Strain
cally, a polymer matrix reinforced with high-modulus fibers.
Gages
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3. Terminology
responsibility of the user of this standard to establish appro-
3.1 Terminology D 3878 defines terms relating to high-
priate safety and health practices and determine the applica-
modulus fibers and their composites. Terminology C 274
bility of regulatory limitations prior to use.
defines terms relating to structural sandwich constructions.
1.4 The values stated in either SI units or inch-pound units
Terminology E 6 defines terms relating to mechanical testing.
are to be regarded separately as standard. Within the text the
In the event of a conflict between terms, Terminology D 3878
inch-pound units are shown in brackets. The values stated in
shall have precedence over the other terminology standards.
each system are not exact equivalents; therefore, each system
3.2 Definitions of Terms Specific to This Standard:
must be used independently of the other. Combining values
3.2.1 bending stiffness, n—the sandwich property which
from the two systems may result in nonconformance with the
resists bending deflections.
standard.
3.2.2 core, n—a centrally located layer of a sandwich
2. Referenced Documents construction, usually low density, which separates and stabi-
lizes the facings and transmits shear between the facings and
2.1 ASTM Standards:
2 provides most of the shear rigidity of the construction.
C 274 Terminology of Structural Sandwich Constructions
3.2.3 face sheet, n—the outermost layer or composite com-
C 365 Test Methods for Flatwise Compressive Strength of
2 ponent of a sandwich construction, generally thin and of high
Sandwich Cores
density, which resists most of the edgewise loads and flatwise
C 393 Test Method for Flexural Properties of Flat Sandwich
2 bending moments: synonymous with face, skin, and facing.
Constructions
3.2.4 footprint, n—the enclosed area of the face sheet
D 792 Test Method for Density and Specific Gravity (Rela-
3 surface of a sandwich panel in contact with the pressure
tive Density) and Density of Plastics by Displacement
bladder during loading.
3.2.5 hydromat, n—a pressure bladder with a square perim-
eter fabricated from two square pieces of industrial belting
This test method is under the jurisdiction of ASTM Committee D-30 on
Composite Materials and is the direct responsibility of Subcommittee D30.05 on
Structural Test Methods.
Current edition approved May 10, 1999. Published July 1999.
2 4
Annual Book of ASTM Standards, Vol 15.03. Annual Book of ASTM Standards, Vol 08.02.
3 5
Annual Book of ASTM Standards, Vol 08.01. Annual Book of ASTM Standards, Vol 03.01.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 6416/D 6416M
which are superposed and clamped at the edges with through- of the axes of symmetry of the plate.
bolted, mild steel bar stock.
5. Significance and Use
3.2.6 isotropic material, n—a material having essentially
5.1 This test method simulates the hydrostatic loading
the same properties in any direction.
3.2.7 orthotropic material, n—a material in which a prop- conditions which are often present in actual sandwich struc-
tures, such as marine hulls. This test method can be used to
erty of interest, at a given point, possesses three mutually
perpendicular planes of symmetry, which taken together define compare the two-dimensional flexural stiffness of a sandwich
composite made with different combinations of materials or
the principal material coordinate system.
with different fabrication processes. Since it is based on
3.2.8 pressure bladder, n—a durable, yet pliable closed
distributed loading rather than concentrated loading, it may
container filled with water, or other incompressible fluid,
also provide more realistic information on the failure mecha-
capable of conforming to the contour of a normally loaded test
nisms of sandwich structures loaded in a similar manner. Test
panel when compressed against its face sheet surface by a test
data should be useful for design and engineering, material
machine.
specification, quality assurance, and process development. In
3.2.9 shear stiffness, n—the sandwich property which re-
addition, data from this test method would be useful in refining
sists shear distortions: synonymous with shear rigidity.
predictive mathematical models or computer code for use as
3.2.10 test panel, n—a square coupon of sandwich construc-
structural design tools. Properties that may be obtained from
tion fabricated for two-dimensional flexural testing: synony-
this test method include:
mous with sandwich panel, sandwich composite plate, sand-
5.1.1 Panel surface deflection at load,
wich composite panel, and panel test specimen.
3.3 Symbols: 5.1.2 Panel face-sheet strain at load,
5.1.3 Panel bending stiffness,
3.3.1 a = support span of the test fixture or the length and
width of the test panel structure between supports. 5.1.4 Panel shear stiffness,
5.1.5 Panel strength, and
3.3.2 A = effective contact area of the pressure bladder
eff
when compressed against the test panel. 5.1.6 Panel failure modes.
3.3.3 B = test panel bending stiffness.
6. Interferences
3.3.4 c = core thickness.
6.1 Material and Specimen Preparation—Poor material
3.3.5 e = normal face sheet strain, x component.
x
fabrication practices, lack of control of fiber alignment, and
3.3.6 e = normal face sheet strain, y component.
y
damage induced by improper coupon machining are known
3.3.7 f = face sheet thickness.
causes of high material data scatter in composites in general.
3.3.8 F = total normal force applied to a test panel as
m
Specific material factors that affect sandwich composites in-
measured by the test machine load cell.
clude variability in core density and degree of cure of resin in
3.3.9 h = average overall thickness of the test panel.
both face sheet matrix material and core bonding adhesive.
3.3.10 N = the number of included terms of the series.
Important aspects of sandwich panel specimen preparation that
3.3.11 P = experimentally measured bladder pressure.
m
contribute to data scatter are incomplete wetout of face sheet
3.3.12 f = width of the unloaded border area of a test panel
fabric, incomplete or nonuniform core bonding of face sheets,
between the edge supports and the effective footprint boundary.
the non-squareness of adjacent panel edges, the misalignment
3.3.13 S = test panel shear stiffness.
of core and face sheet elements, the existence of joints or other
3.3.14 v = experimentally determined deflection at center
e
core and face sheet discontinuities, out-of-plane curvature, and
of test panel.
surface roughness.
4. Summary of Test Method
6.2 Test Fixture Characteristics—Configuration of the
4.1 A square test panel is simply supported on all four edges panel edge-constraint structure can have a significant effect on
and uniformly loaded over a portion of its surface by a test results. Correct interpretation of test data depends on the
water-filled bladder. Pressure on the panel is increased by fixture supporting the test panel in such a manner that the
moving the platens of the test frame. The test measures the boundary conditions consistent with simple support can be
two-dimensional flexural response of a sandwich composite assumed to apply. Panel edge support journals must be copla-
plate in terms of deflections and strains when subjected to a nar and perpendicular to the loading axis. Given the fixture
well-defined distributed load. itself has sufficient rigidity, erroneous conclusions about panel
4.2 Panel deflection at load is monitored by a centrally strength and stiffness might be drawn if insufficient torque has
positioned LVDT which contacts the tension-side surface. been applied to the fasteners securing the lower panel edge
4.3 Load is monitored by both a crosshead-mounted load support frame. In general, panels with more flexural rigidity
cell, in series with the test fixture, and a pressure transducer in and shear rigidity require more bolt torque to approach simple
the pressure bladder itself. Since the pressure bladder is also at support.
all times in series with the load cell and test fixture, the 6.3 Pressure Bladder Characteristics—When a pressure
effective contact area of the pressure field is continuously bladder is used to introduce normal load to a plate, the response
monitored as the load/pressure quotient. of the plate is dependent on the resulting pressure distribution.
4.4 Strain can be monitored with strategically placed strain The true function of the pressure bladder is to convert the
gage rosettes bonded to the tension-side face-sheet surface. A absolute load applied by the test machine into a pressure field
typical arrangement has four rosettes equally spaced along one that can be specified by a relatively simple mathematical
D 6416/D 6416M
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.
NOTE 1—The error in the hydromat model has mainly to do with details
of the footprint shape, since the effective contact area can be calculated at
any time by dividing the absolute applied load by the bladder pressure. A
secondary error arises from the non-zero bending stiffness of the fiber-
reinforced industrial belting fabric that results in a narrow band of varying
pressure at the very edge of the footprint. Calibration tests using a steel
plate equipped with strain gages are recommended for each bladder unit
to verify that the errors in the pressure distribution model are negligible
(see Section 9).
6.4 Tolerances—Test panels need to meet the dimensional
and squareness tolerances specified in 8.2 to ensure proper
edge support and constraint.
6.5 System Alignment—Errors can result if the panel sup-
port structure is not centered with respect to the actuator of the
test machine, or if the plane defined by the panel edge-bearing
FIG. 1 Elements of the Two-Dimensional Sandwich Plate Flexural
surfaces is not perpendicular to the loading axis of the test
Test
machine. Errors can also result if the pressure bladder is not
centered properly with respect to fixture and actuator or if the
indicate the load with an accuracy over the load range(s) of
edges of the bladder clamping bars are not parallel to the panel
interest of within 61 % of the indicated value. The load
edge-support journals.
range(s) of interest may be fairly low for bending and shear
6.6 Other System Characteristics—When attempting to
modulus evaluation or much higher for strength evaluation, or
measure panel surface deflection, an error results which is an
both, as required.
artifact of the test. It arises as normal load is applied, to the
7.1.2 Loading Fixture—As illustrated in the schematic dia-
extent that the edges of the sandwich specimen are compressed
gram of Fig. 1, the loading fixture has two parts, a rigid,
from the reactive line loads generated by the upper and lower
overhead upper panel support structure, which is attached to
panel support structure. This direct rigid-body addition affects
the load cell on the load frame crosshead, and a rigid lower
any LVDT positioned to contact the tension-side panel surface.
panel edge support frame which bolts to the upper panel
To minimize the error, the edges of soft-core panels should be
support structure at the corners. A square sandwich composite
reinforced in accordance with 8.3.2.
panel specimen is constrained at the edges when captured from
above and below by these two fixture elements. All bearing
7. Apparatus
surfaces are hardened steel rods with a circular cross-section,
7.1 Procedures A, B, and C—A schematic diagram illustrat- 12.7 mm [0.5 in.] in diameter. The support span for each
ing the key components of the test method apparatus appears in dimension of the fixture is defined in Fig. 2. That the loading
Fig. 1. fixture constrains the test panel at all four edges is shown in the
7.1.1 Testing Machine—The testing machine shall be in photographs of Figs. 3 and 4. Panel flexural response is thus
conformance with Practices E 4 and shall satisfy the following two-dimensional under normal loading. The length of the
requirements: support spans should be equal in both dimensions. Simply
7.1.1.1 Testing Machine Heads—The testing machine shall
supported boundar
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