1、Designation: D7956/D7956M 14D7956/D7956M 16Standard Practice forCompressive Testing of Thin Damaged Laminates Using aSandwich Long Beam FixtureFlexure Specimen1This standard is issued under the fixed designation D7956/D7956M; the number immediately following the designation indicates theyear of orig
2、inal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of lastreapproval. A superscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers an approach for compressive testing thin d
3、amaged multidirectional polymer matrix compositelaminates reinforced by high-modulus fibers using a sandwich long beam flexure specimen. It provides a test configuration inwhich the core does not constrain any protruding back side damage. It is limited to testing of monolithic solid laminates whicha
4、re too thin to be tested using typical anti-buckling fixtures. It does not cover compressive testing of damaged sandwich panelfacings. The composite material forms are limited to continuous-fiber or discontinuous-fiber (tape or fabric, or both) reinforcedcomposites in which the laminate is balanced
5、and symmetric with respect to the test direction1.2 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in eachsystem may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from
6、 thetwo systems may result in non-conformance with the standard.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 responsibilityof the user of this standard to establis
7、h appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:2C274 Terminology of Structural Sandwich Constructions (Withdrawn 2016)3D883 Terminology Relating to PlasticsD3878 Terminology for Composite Mater
8、ialsD3410 Test Method for Compressive Properties of Polymer Matrix Composite Materials with Unsupported Gage Section byShear LoadingD6264/D6264M Test Method for Measuring the Damage Resistance of a Fiber-Reinforced Polymer-Matrix Composite to aConcentrated Quasi-Static Indentation ForceD7136/D7136M
9、Test Method for Measuring the Damage Resistance of a Fiber-Reinforced Polymer Matrix Composite to aDrop-Weight Impact EventD7137/D7137M Test Method for Compressive Residual Strength Properties of Damaged Polymer Matrix Composite PlatesD7249/D7249M Test Method for Facing Properties of Sandwich Constr
10、uctions by Long Beam FlexureE6 Terminology Relating to Methods of Mechanical TestingE122 Practice for Calculating Sample Size to Estimate, With Specified Precision, the Average for a Characteristic of a Lot orProcessE177 Practice for Use of the Terms Precision and Bias in ASTM Test MethodsE456 Termi
11、nology Relating to Quality and StatisticsE1309 Guide for Identification of Fiber-Reinforced Polymer-Matrix Composite Materials in Databases (Withdrawn 2015)3E1434 Guide for Recording Mechanical Test Data of Fiber-Reinforced Composite Materials in Databases (Withdrawn 2015)31 This test method is unde
12、r the jurisdiction of ASTM Committee D30 on Composite Materials and is the direct responsibility of Subcommittee D30.09 on SandwichConstruction.Current edition approved Aug. 1, 2014Sept. 1, 2016. Published September 2014September 2016. Originally approved in 2014. Last previous edition approved in 2
13、014as -14. DOI: 10.1520/D7956_D7956M-14.10.1520/D7956_D7956M-16.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM websi
14、te.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior
15、editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13. Terminology3.1 DefinitionsTerminology D387
16、8 defines terms relating to high-modulus fibers and their composites.Terminologycomposites, as C274 defineswell as terms relating to structural sandwich constructions. Terminology D883 definesterms relating to plastics.Terminology E6 defines terms relating to mechanical testing.Terminology E456 and
17、Practice E177 defineterms relating to statistics. In the event of a conflict between terms, Terminology D3878 shall have precedence over the otherterminologies.4. Summary of Practice4.1 This practice consists of fabricating a composite laminate, damaging the laminate using either Test Method D6264/D
18、6264Mor Test Method D7136/D7136M, bonding the impacted or indented side of the laminate onto core and a back side facing to forma sandwich panel, and testing the damaged laminate in compression using Test Method D7249/D7249M.5. Significance and Use5.1 This practice provides a standard method of test
19、ing damaged composite laminates which are too thin to be tested usingtypical anti-buckling fixtures, such as those used in Test Method D7137/D7137M. The laminate is first impacted or indented inorder to produce a damage state representative of actual monolithic solid laminate structure. Impacting or
20、 static indentation is notperformed on an assembled sandwich panel, as the damage state is altered by energy absorption in the core and by support of thecore during the impact or indentation event.After damaging, the laminate is bonded onto the core with the impacted or indentationside of the lamina
21、te against the core, and with a localized un-bonded area encompassing the damage site. Fig. 1 illustrates theadhesive removal to avoid the damaged area and the assembly of the sandwich specimen with the impacted damaged laminateflipped over from the impacting or indentation orientation.The final ass
22、embled sandwich specimen is then tested using a long beamflexure setup with the damaged laminate being on the compression side. The sandwich panel configuration is used as a form ofanti-buckling support for the thin damaged laminate.5.2 Susceptibility to damage from concentrated out-of-plane forces
23、is one of the major design concerns of many structures madeof advanced composite laminates. Knowledge of the damage resistance and damage tolerance properties of a laminated compositeplate is useful for product development and material selection.5.3 The residual strength data obtained using this tes
24、t method is used in research and development activities as well as for designallowables; however the results are specific to the geometry and physical conditions tested and are generally not scalable to otherconfigurations.FIG. 1 Sandwich Specimen AssemblyD7956/D7956M 1625.4 The properties obtained
25、using this test method can provide guidance in regard to the anticipated damage tolerance capabilityof composite structures of similar material, thickness, stacking sequence, and so forth. However, it must be understood that thedamage tolerance of a composite structure is highly dependent upon sever
26、al factors including geometry, stiffness, supportconditions, and so forth. Significant differences in the relationships between the existent damage state and the residual compressivestrength can result due to differences in these parameters. For example, residual strength and stiffness properties ob
27、tained usingthis test method would more likely reflect the damage tolerance characteristics of an un-stiffened monolithic skin or web than thatof a skin attached to substructure which resists out-of-plane deformation.5.5 The reporting section requires items that tend to influence residual compressiv
28、e strength to be reported; these include thefollowing: material, methods of material fabrication, accuracy of lay-up orientation, laminate stacking sequence and overallthickness, specimen geometry, specimen preparation, specimen conditioning, environment of testing, void content, volume percentreinf
29、orcement, type, size and location of damage (including method of non-destructive inspection (NDI), fixture geometry, timeat temperature, and speed of testing.5.6 Properties that result from the residual strength assessment include the following: compressive residual strength FCAI.6. Interferences6.1
30、 The response of a damaged specimen is dependent upon many factors, such as laminate thickness, ply thickness, stackingsequence, environment, damage type, damage geometry, damage location, and loading/support conditions. Consequently,comparisons cannot be made between materials unless identical test
31、 configurations, test conditions, and laminate configurationsare used. Therefore, all details of the test configuration shall be reported in the results. Specific structural configurations andboundary conditions must be considered when applying the data generated using this test method to design app
32、lications.6.2 Material OrthotropyThe degree of laminate orthotropy strongly affects the failure mode and measured compressiveresidual strength.6.3 Thickness ScalingThick composite structures do not necessarily fail at the same strengths as thin structures with the samelaminate orientation (that is,
33、strength does not always scale linearly with thickness). Further, the damage state for a given level ofimpact or indentation energy or measured surface dent depth varies with laminate thickness. Thus, data gathered using this testmethod may not translate directly into equivalent thick-structure prop
34、erties.6.4 Damage Geometry and LocationThe size, shape, and location of damage (both within the plane of the plate andthrough-the-thickness) can significantly affect the deformation and strength behavior of the specimen. Edge effects, boundaryconstraints, and the damaged stress/strain field can inte
35、ract if the damage size becomes too large relative to the length and widthdimensions of the specimen.6.5 EnvironmentResults are affected by the environmental conditions under which specimens are conditioned, as well as theconditions under which the tests are conducted. Specimens tested in various en
36、vironments can exhibit significant differences instiffness. Critical environments must be assessed independently for each specific combination of core material, facing material, andcore-to-facing interfacial adhesive (if used) that is tested.6.6 Core MaterialIf the core material has insufficient she
37、ar or compressive strength, it is possible that the core may locallycrush at or near the loading points thereby resulting in facesheet failure due to local stresses. In other cases, facing failure can causelocal core crushing. When there is both facing and core failure in the vicinity of one of the
38、loading points, it can be difficult todetermine the failure sequence in a post-mortem inspection of the specimen as the failed specimens look very similar for bothsequences.6.7 DamagingRefer to Test Methods D6264/D6264M or D7136/D7136M for indentation or impact related interferences.7. Sampling and
39、Test Specimens7.1 SamplingTest at least five specimens per test condition unless valid results can be gained through the use of fewerspecimens, as in the case of a designed experiment. For statistically significant data, consult the procedures outlined in PracticeE122. Report the method of sampling.
40、7.2 Specimen and Fixture GeometryThe test requestor shall specify all specimen dimensions and materials along with theloading fixture dimensions. Refer to Test Method D7249/D7249M for sandwich beam specimen sizing requirements andguidelines.7.2.1 SpecimenThe test specimens shall be rectangular in cr
41、oss section. The width of the specimen shall be at least three (3)times the width of the major damage area (as determined by NDI). The major damage area is defined as the region of impactdamage that generally extends through the entire laminate thickness; generally it does not include splitting or d
42、elamination of thesurface ply on the opposite side from the impact (typically, the major damage area is approximately circular on the NDI scanimage). Any such backside damage that is not included in the major damage area should not be greater than one-half (12) thespecimen width nor extend to near t
43、he edges of the specimen. If there is uncertainty over the damage area to use for specimensizing, the test lab should consult the test requestor.D7956/D7956M 163NOTE 1The recommended specimen width is five (5) times the damage width, however this may not be practical in all cases. cases, particularl
44、y whenthere is backside ply splitting or delamination, or both. Also, it may not be possible to accurately predict the damage sizes prior to fabrication of the testspecimens. specimens; therefore a pre-test impact survey program is recommended prior to specimen fabrication. As impact or indentation
45、damagediameters are often on the order of 25 mm 1.0 inch,1.0 in., a typical specimen width is 127 mm 5.0 inch.in. Specimen width should be minimizedto avoid excessive anti-clastic bending.bending; a maximum width to support span ratio of 0.3 is recommended.7.2.2 Loading FixtureThe loading span shoul
46、d be increased from the Test Method D7249/D7249M standard specimendimension of 100 mm 4.0 in to the minimum of a dimension equal to the specimen width or three (3) times the dimension ofthe damage measured in the axial direction of the specimen. The support span may have to be modified from the Test
47、 MethodD7249/D7249M standard specimen dimension in order for the specimen to produce the desired facesheet failure mode.7.3 Back Side FacingsThe back side facings may use the same laminate as the laminate to be tested in compression, or mayuse a similar laminate. The back side facing should have equ
48、al or greater stiffness (Et) than the top (compressive) facing and shallhave greater tensile strength than the anticipated compressive strength of the damaged laminate to be tested. The back side facingshould have a similar coefficient of thermal expansion as the top facing to avoid panel warpage du
49、ring fabrication.NOTE 2Metallic sheets are not recommended for the back facings as it can be difficult to machine a sandwich panel with dissimilar materials for thetwo facings.7.4 CoreThe core material for the sandwich panels shall have sufficient shear and compression strength to avoid failures inthe core prior to facing failure. Refer to Test Method D7249/D7249M for core material selection guidelines7.5 AdhesiveThe adhesive material for bonding the facings to the core of the sandwich panel shall have sufficient shearstrength at the test temperature to
