ASTM D6416/D6416M-16(2024)

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: D6416/D6416M − 16 (Reapproved 2024)
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 2. Referenced Documents
1.1 This test method determines the two-dimensional flex- 2.1 ASTM Standards:
ural properties of sandwich composite plates subjected to a C365/C365M Test Method for Flatwise Compressive Prop-
distributed load. The test fixture uses a relatively large square erties of Sandwich Cores
panel sample which is simply supported all around and has the C393 Test Method for Core Shear Properties of Sandwich
distributed load provided by a water-filled bladder. This type of Constructions by Beam Flexure
loading differs from the procedure of Test Method C393, where D792 Test Methods for Density and Specific Gravity (Rela-
concentrated loads induce one-dimensional, simple bending in tive Density) of Plastics by Displacement
beam specimens. D2584 Test Method for Ignition Loss of Cured Reinforced
Resins
1.2 This test method is applicable to composite structures of
D2734 Test Methods for Void Content of Reinforced Plastics
the sandwich type which involve a relatively thick layer of core
D3171 Test Methods for Constituent Content of Composite
material bonded on both faces with an adhesive to thin-face
Materials
sheets composed of a denser, higher-modulus material,
D3878 Terminology for Composite Materials
typically, a polymer matrix reinforced with high-modulus
E4 Practices for Force Calibration and Verification of Test-
fibers.
ing Machines
1.3 The values stated in either SI units or inch-pound units
E6 Terminology Relating to Methods of Mechanical Testing
are to be regarded separately as standard. Within the text the
E251 Test Methods for Performance Characteristics of Me-
inch-pound units are shown in brackets. The values stated in
tallic Bonded Resistance Strain Gages
each system are not exact equivalents; therefore, each system
E1237 Guide for Installing Bonded Resistance Strain Gages
must be used independently of the other. Combining values
2.2 ASTM Adjunct:
from the two systems may result in nonconformance with the
Sandwich Plate Test Fixture and Hydromat Pressure Blad-
standard.
der, ASTM D6416/D6416M
1.4 This standard does not purport to address all of the
3. Terminology
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- 3.1 Terminology D3878 defines terms relating to high-
priate safety, health, and environmental practices and deter- modulus fibers and their composites, as well as terms relating
mine the applicability of regulatory limitations prior to use. to sandwich constructions. Terminology E6 defines terms
1.5 This international standard was developed in accor- relating to mechanical testing. In the event of a conflict
dance with internationally recognized principles on standard- between terms, Terminology D3878 shall have precedence
ization established in the Decision on Principles for the over the other terminology standards.
Development of International Standards, Guides and Recom-
3.2 Definitions of Terms Specific to This Standard:
mendations issued by the World Trade Organization Technical
3.2.1 bending stiffness, n—the sandwich property which
Barriers to Trade (TBT) Committee.
resists bending deflections.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
This test method is under the jurisdiction of ASTM Committee D30 on contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Composite Materials and is the direct responsibility of Subcommittee D30.09 on Standards volume information, refer to the standard’s Document Summary page on
Sandwich Construction. the ASTM website.
Current edition approved May 1, 2024. Published May 2024. Originally Detailed drawings for the fabrication of the 500–mm test fixture and pressure
approved in 1999. Last previous edition approved in 2016 as D6416/ bladder shown in Fig. 3 and Fig. 4 are available from ASTM Headquarters,
ɛ1
D6416M – 16 . DOI: 10.1520/D6416_D6416M-16R24. www.astm.org. Order Adjunct No. ADJD6416-E-PDF.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6416/D6416M − 16 (2024)
3.2.2 core, n—a centrally located layer of a sandwich water-filled bladder. Pressure on the panel is increased by
construction, usually low density, which separates and stabi- moving the platens of the test frame. The test measures the
lizes the facings and transmits shear between the facings and two-dimensional flexural response of a sandwich composite
provides most of the shear rigidity of the construction. plate in terms of deflections and strains when subjected to a
well-defined distributed load.
3.2.3 face sheet, n—the outermost layer or composite com-
ponent of a sandwich construction, generally thin and of high
4.2 Panel deflection at load is monitored by a centrally
density, which resists most of the edgewise loads and flatwise
positioned LVDT which contacts the tension-side surface.
bending moments: synonymous with face, skin, and facing.
4.3 Load is monitored by both a crosshead-mounted load
3.2.4 footprint, n—the enclosed area of the face sheet
cell, in series with the test fixture, and a pressure transducer in
surface of a sandwich panel in contact with the pressure
the pressure bladder itself. Since the pressure bladder is also at
bladder during loading.
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-
4.4 Strain can be monitored with strategically placed strain
bolted, mild steel bar stock.
gage rosettes bonded to the tension-side face-sheet surface. A
3.2.6 isotropic material, n—a material having essentially the
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—the sandwich property which resists
shear distortions: synonymous with shear rigidity. nisms of sandwich structures loaded in a similar manner. Test
data should be useful for design and engineering, material
3.2.10 test panel, n—a square coupon of sandwich construc-
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,
width of the test panel structure between supports.
5.1.2 Panel face-sheet strain at load,
3.3.2 A = effective contact area of the pressure bladder
eff
5.1.3 Panel bending stiffness,
when compressed against the test panel.
5.1.4 Panel shear stiffness,
3.3.3 B = test panel bending stiffness.
5.1.5 Panel strength, and
3.3.4 c = core thickness.
5.1.6 Panel failure modes.
3.3.5 ε = normal face sheet strain, x component.
x
3.3.6 ε = normal face sheet strain, y component.
y
6. Interferences
3.3.7 f = face sheet thickness.
3.3.8 F = total normal force applied to a test panel as
m
6.1 Material and Specimen Preparation—Poor material fab-
measured by the test machine load cell.
rication practices, lack of control of fiber alignment, and
3.3.9 h = average overall thickness of the test panel.
damage induced by improper coupon machining are known
3.3.10 N = the number of included terms of the series.
causes of high material data scatter in composites in general.
3.3.11 P = experimentally measured bladder pressure.
m
Specific material factors that affect sandwich composites in-
3.3.12 φ = width of the unloaded border area of a test panel
clude variability in core density and degree of cure of resin in
between the edge supports and the effective footprint boundary.
both face sheet matrix material and core bonding adhesive.
3.3.13 S = test panel shear stiffness.
Important aspects of sandwich panel specimen preparation that
3.3.14 ω = experimentally determined deflection at center
e
contribute to data scatter are incomplete wetout of face sheet
of test panel.
fabric, incomplete or nonuniform core bonding of face sheets,
the non-squareness of adjacent panel edges, the misalignment
4. Summary of Test Method
of core and face sheet elements, the existence of joints or other
4.1 A square test panel is simply supported on all four edges core and face sheet discontinuities, out-of-plane curvature, and
and uniformly loaded over a portion of its surface by a surface roughness.
D6416/D6416M − 16 (2024)
6.2 Test Fixture Characteristics—Configuration of the panel To minimize the error, the edges of soft-core panels should be
edge-constraint structure can have a significant effect on test reinforced in accordance with 8.3.2.
results. Correct interpretation of test data depends on the
7. Apparatus
fixture supporting the test panel in such a manner that the
boundary conditions consistent with simple support can be 7.1 Procedures A, B, and C—A schematic diagram illustrat-
assumed to apply. Panel edge support journals must be copla- ing the key components of the test method apparatus appears in
nar and perpendicular to the loading axis. Given the fixture Fig. 1.
itself has sufficient rigidity, erroneous conclusions about panel 7.1.1 Testing Machine—The testing machine shall be in
conformance with Practices E4 and shall satisfy the following
strength and stiffness might be drawn if insufficient torque has
been applied to the fasteners securing the lower panel edge requirements:
7.1.1.1 Testing Machine Heads—The testing machine shall
support frame. In general, panels with more flexural rigidity
have both an essentially stationary head and a movable head.
and shear rigidity require more bolt torque to approach simple
7.1.1.2 Drive Mechanism—The testing machine drive
support.
mechanism shall be capable of imparting to the movable head
6.3 Pressure Bladder Characteristics—When a pressure
a controlled velocity with respect to the stationary head. The
bladder is used to introduce normal load to a plate, the response
velocity of the movable head shall be capable of being
of the plate is dependent on the resulting pressure distribution.
regulated in accordance with 11.3.
The true function of the pressure bladder is to convert the
7.1.1.3 Load Indicator—The testing machine load-sensing
absolute load applied by the test machine into a pressure field
device shall be capable of indicating the total load being
that can be specified by a relatively simple mathematical
carried by the test specimen. This device shall be essentially
model. With the hydromat-style bladder, two simplifying
free from inertia-lag at the specified rate of testing and shall
assumptions are permitted: (1) the shape of the contact area is
indicate the load with an accuracy over the load range(s) of
a readily definable geometric shape (or combination of shapes)
interest of within 61 % of the indicated value. The load
and (2) the pressure is constant within the boundaries of the
range(s) of interest may be fairly low for bending and shear
contact area. The pressure distribution is then characterized
modulus evaluation or much higher for strength evaluation, or
merely by the magnitude of the pressure and the size of the
both, as required.
footprint. Obviously, the size and shape of the pressure bladder
7.1.2 Loading Fixture—As illustrated in the schematic dia-
have a significant effect on test results in terms of the observed
gram of Fig. 1, the loading fixture has two parts, a rigid,
strains and deflections. Some errors in data interpretation are
overhead upper panel support structure, which is attached to
possible insofar as the actual pressure distribution differs from
the load cell on the load frame crosshead, and a rigid lower
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 support
structure is not centered with respect to the actuator of the test
machine, or if the plane defined by the pa
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