ASTM D6790/D6790M-22

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Designation: D6790/D6790M − 16 D6790/D6790M − 22
Standard Test Method for
Determining Poisson’s Ratio of Honeycomb Cores
This standard is issued under the fixed designation D6790/D6790M; 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 the determination of the sandwich honeycomb core Poisson’s ratio from the anticlastic curvature
radii,radii; see Fig. 1.
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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
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:
C271/C271M Test Method for Density of Sandwich Core Materials
D883 Terminology Relating to Plastics
D3878 Terminology for Composite Materials
D5229/D5229M Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite
Materials
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
E456 Terminology Relating to Quality and Statistics
3. Terminology
3.1 Definitions—Terminology D3878 defines terms relating to high-modulus fibers and their composites, as well as terms relating
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, 2016May 1, 2022. Published April 2016May 2022. Originally approved in 2002. Last previous edition approved in 20142016 as
D6790 - 14.D6790/D6790M – 16. DOI: 10.1520/D6790_D6790M-16.10.1520/D6790_D6790M-22.
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 Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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D6790/D6790M − 22
FIG. 1 Anticlastic Curvature
to sandwich constructions. Terminology D883 defines terms relating to plastics. 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:
c = chord measurement
CV = coefficient of variation statistic of a sample population for a given property (in percent)
d = depth measurement
R = anticlastic curvature radius
a
R = cylinder radius
c
S = standard deviation statistic of a sample population for a given property
n-1
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
v = Poisson’s ratio
4. Summary of Test Method
4.1 This test method determines the Poisson’s ratio of sandwich honeycomb core by bending the core around a cylinder and taking
measurements of the anticlastic curvature that occurs.
5. Significance and Use
5.1 Certain sandwich panel analyses require the Poisson’s ratio of the honeycomb core. It is not possible to measure the
honeycomb’s Poisson’s ratio by conventional methods.
5.2 This test method provides a standard method of determining the Poisson’s ratio of honeycomb core materials for design
properties, material specifications, research and development applications, and quality assurance.
5.3 Factors that influence the Poisson’s ratio of honeycomb core materials and shall therefore be reported include the following:
core material, methods of material fabrication, core geometry, core thickness, core thickness uniformity, cell wall thickness,
specimen geometry, specimen preparation, and specimen conditioning.
6. Interferences
6.1 The test method described here is one means of obtaining the Poisson’s ratio of honeycomb core. However, this test method
has not been widely used, and it is in its conceptual stage.
6.2 Material and Specimen Preparation—Poor material fabrication practices and damage induced by improper specimen
machining are known causes of high data scatter for sandwich cores. Important aspects of sandwich core specimen preparation that
D6790/D6790M − 22
contribute to data scatter include the existence of joints, voids or other core discontinuities, out-of-plane curvature, and surface
roughness. Non-uniform edge geometry can influence the measurement of the chord distance described in 11.6.
6.3 Core Geometry—Core-specific geometric factors that affect Poisson’s ratio include core cell size, uniformity of core cell
geometry, core cell wall thickness, core cell wall thickness uniformity, core thickness, and core thickness uniformity.
6.4 Environment—Results are affected by the environmental conditions under which specimens are conditioned, as well as the
conditions under which the core is tested. Specimens tested in various environments can exhibit differences in Poisson’s ratio.
6.5 Specimen-Cylinder Contact—Results may vary if the specimen is too rigid in bending to permit intimate contact of its surface
with the cylinder along the specimen center line without fracturing the core. Use of a larger cylinder radius is recommended if the
midpoints of two parallel ends of the core specimen cannot be made to contact the standard cylinder using light hand pressure.
6.6 Measurement Devices—Measurements for anticlastic curvature depth described in 11.6 may be affected by the width of the
scale described in 7.3 where it contacts the specimen.
7. Apparatus
7.1 Micrometers and Calipers—- A micrometer having a flat anvil interface, or a caliper of suitable size, shall be used. The
accuracy of the instrument(s) shall be suitable for reading to within 0.5 % of the sample length, width and thickness. For typical
specimen geometries, an instrument with an accuracy of 60.025 mm [60.001 in.] is desirable for thickness measurement, whereas
an instrument with an accuracy of 60.25 mm [60.01 in.] is desirable for length and width measurement.
7.2 Cylinders—A cylinder shall be utilized to bend the core to a defined radius of curvature. Standard tests shall utilize a cylinder
of 305 mm [12 in.] radius. This radius is generally acceptable for testing of cores up to 25 mm [1.0 in.] in thickness. A larger
cylinder radius shall be used if the midpoints of two parallel ends of the core specimen cannot be made to contact the standard
cylinder using light hand pressure without fracturing the core. Tests conducted using non-standard cylinder radii must be
designated as such, with the cylinder radius reported with any test results.
7.3 Scale—A scale is required that is capable of measuring length with an accuracy of 61 mm [60.05 in.].
7.4 Straight Edge—A straight edge is required that is capable of measuring length with an accuracy of 61 mm [6 0.05 in.].
8. Sampling and Test Specimens
8.1 Sampling—Test at least five specimens per test condition unless valid results can be gained through the use of fewer specimens,
such 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 Test Specimens:
8.2.1 Core Geometry—Core test specimens shall be square. The length and width should be great enough to obtain an anticlastic
curvature when the specimen is bent over the cylinder. A specimen size of 300 by 300 mm [12 by 12 in.] in planar dimensions
is recommended. For honeycomb cores, the planar dimension cross-section is defined to be in the facing plane (in regard to the
orientation that the core would be placed in a structural sandwich construction) and is perpendicular to the orientation of the cell
walls. Larger specimen sizes may be required if the cylinder radius is larger than 305 mm [12 in.].
8.2.2 Core Thickness—Various core thicknesses should be tested. A core thickness of 13 mm (0.50 in.)[0.50 in.] is recommended
as a starting thickness.
8.2.3 Core Preparation and Machining—Prepare the core material samples so that the facing plane surfaces are parallel to each
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