1、Designation: D7956/D7956M 14Standard Practice forCompressive Testing of Thin Damaged Laminates Using aSandwich Long Beam Fixture Specimen1This standard is issued under the fixed designation D7956/D7956M; the number immediately following the designation indicates theyear of original adoption or, in t
2、he 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 test-ing thin damaged multidirection
3、al polymer matrix compositelaminates reinforced by high-modulus fibers using a sandwichlong beam flexure specimen. It provides a test configuration inwhich the core does not constrain any protruding back sidedamage. It is limited to testing of monolithic solid laminateswhich are too thin to be teste
4、d using typical anti-bucklingfixtures. It does not cover compressive testing of damagedsandwich panel facings. The composite material forms arelimited to continuous-fiber or discontinuous-fiber (tape orfabric, or both) reinforced composites in which the laminate isbalanced and symmetric with respect
5、 to the test direction1.2 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard. The values stated ineach system may not be exact equivalents; therefore, eachsystem shall be used independently of the other. Combiningvalues from the two systems may result i
6、n non-conformancewith the standard.1.2.1 Within the text the inch-pound units are shown inbrackets.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and healt
7、h practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2C274 Terminology of Structural Sandwich ConstructionsD883 Terminology Relating to PlasticsD3878 Terminology for Composite MaterialsD3410 Test Method for Compressive Properti
8、es of PolymerMatrix Composite Materials with Unsupported Gage Sec-tion by Shear LoadingD6264/D6264M Test Method for Measuring the DamageResistance of a Fiber-Reinforced Polymer-Matrix Com-posite to a Concentrated Quasi-Static Indentation ForceD7136/D7136M Test Method for Measuring the DamageResistan
9、ce of a Fiber-Reinforced Polymer Matrix Com-posite to a Drop-Weight Impact EventD7137/D7137M Test Method for Compressive ResidualStrength Properties of Damaged Polymer Matrix Compos-ite PlatesD7249/D7249M Test Method for Facing Properties of Sand-wich Constructions by Long Beam FlexureE6 Terminology
10、 Relating to Methods of Mechanical TestingE122 Practice for Calculating Sample Size to Estimate, WithSpecified Precision, the Average for a Characteristic of aLot or ProcessE177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE456 Terminology Relating to Quality and StatisticsE13
11、09 Guide for Identification of Fiber-ReinforcedPolymer-Matrix Composite Materials in DatabasesE1434 Guide for Recording Mechanical Test Data of Fiber-Reinforced Composite Materials in Databases3. Terminology3.1 DefinitionsTerminology D3878 defines terms relatingto high-modulus fibers and their compo
12、sites. TerminologyC274 defines terms relating to structural sandwich construc-tions. Terminology D883 defines terms relating to plastics.Terminology E6 defines terms relating to mechanical testing.Terminology E456 and Practice E177 define terms relating tostatistics. In the event of a conflict betwe
13、en terms, TerminologyD3878 shall have precedence over the other terminologies.4. Summary of Practice4.1 This practice consists of fabricating a compositelaminate, damaging the laminate using either Test MethodD6264/D6264M or Test Method D7136/D7136M, bonding theimpacted or indented side of the lamin
14、ate onto core and a backside facing to form a sandwich panel, and testing the damagedlaminate in compression using Test Method D7249/D7249M.1This test method is under the jurisdiction of ASTM Committee D30 onComposite Materials and is the direct responsibility of Subcommittee D30.09 onSandwich Const
15、ruction.Current edition approved Aug. 1, 2014. Published September 2014. DOI:10.1520/D7956_D7956M-14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards D
16、ocument Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States15. Significance and Use5.1 This practice provides a standard method of testingdamaged composite laminates which are too thin to be testedusing typ
17、ical anti-buckling fixtures, such as those used in TestMethod D7137/D7137M. The laminate is first impacted orindented in order to produce a damage state representative ofactual monolithic solid laminate structure. Impacting or staticindentation is not performed on an assembled sandwich panel,as the
18、damage state is altered by energy absorption in the coreand by support of the core during the impact or indentationevent. After damaging, the laminate is bonded onto the corewith the impacted or indentation side of the laminate againstthe core, and with a localized un-bonded area encompassingthe dam
19、age site. Fig. 1 illustrates the adhesive removal to avoidthe damaged area and the assembly of the sandwich specimenwith the impacted damaged laminate flipped over from theimpacting or indentation orientation. The final assembledsandwich specimen is then tested using a long beam flexuresetup with th
20、e damaged laminate being on the compressionside. 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-planeforces is one of the major design concerns of many structuresmade of advanced composite
21、 laminates. Knowledge of thedamage resistance and damage tolerance properties of alaminated composite plate is useful for product developmentand material selection.5.3 The residual strength data obtained using this testmethod is used in research and development activities as wellas for design allowa
22、bles; however the results are specific to thegeometry and physical conditions tested and are generally notscalable to other configurations.5.4 The properties obtained using this test method canprovide guidance in regard to the anticipated damage tolerancecapability of composite structures of similar
23、 material,thickness, stacking sequence, and so forth. However, it must beunderstood that the damage tolerance of a composite structureis highly dependent upon several factors including geometry,stiffness, support conditions, and so forth. Significant differ-ences in the relationships between the exi
24、stent damage stateand the residual compressive strength can result due todifferences in these parameters. For example, residual strengthand stiffness properties obtained using this test method wouldmore likely reflect the damage tolerance characteristics of anun-stiffened monolithic skin or web than
25、 that of a skin attachedto substructure which resists out-of-plane deformation.5.5 The reporting section requires items that tend to influ-ence residual compressive strength to be reported; theseinclude the following: material, methods of materialfabrication, accuracy of lay-up orientation, laminate
26、 stackingsequence and overall thickness, specimen geometry, specimenpreparation, specimen conditioning, environment of testing,void content, volume percent reinforcement, type, size andFIG. 1 Sandwich Specimen AssemblyD7956/D7956M 142location of damage (including method of non-destructiveinspection
27、(NDI), fixture geometry, time at temperature, andspeed of testing.5.6 Properties that result from the residual strength assess-ment include the following: compressive residual strengthFCAI.6. Interferences6.1 The response of a damaged specimen is dependent uponmany factors, such as laminate thicknes
28、s, ply thickness, stack-ing sequence, environment, damage type, damage geometry,damage location, and loading/support conditions.Consequently, comparisons cannot be made between materialsunless identical test configurations, test conditions, and lami-nate configurations are used. Therefore, all detai
29、ls of the testconfiguration shall be reported in the results. Specific structuralconfigurations and boundary conditions must be consideredwhen applying the data generated using this test method todesign applications.6.2 Material OrthotropyThe degree of laminate orthot-ropy strongly affects the failu
30、re mode and measured compres-sive residual strength.6.3 Thickness ScalingThick composite structures do notnecessarily fail at the same strengths as thin structures with thesame laminate orientation (that is, strength does not alwaysscale linearly with thickness). Further, the damage state for agiven
31、 level of impact or indentation energy or measuredsurface dent depth varies with laminate thickness. Thus, datagathered using this test method may not translate directly intoequivalent thick-structure properties.6.4 Damage Geometry and LocationThe size, shape, andlocation of damage (both within the
32、plane of the plate andthrough-the-thickness) can significantly affect the deformationand strength behavior of the specimen. Edge effects, boundaryconstraints, and the damaged stress/strain field can interact ifthe damage size becomes too large relative to the length andwidth dimensions of the specim
33、en.6.5 EnvironmentResults are affected by the environmentalconditions under which specimens are conditioned, as well asthe conditions under which the tests are conducted. Specimenstested in various environments can exhibit significant differ-ences in stiffness. Critical environments must be assessed
34、independently for each specific combination of core material,facing material, and core-to-facing interfacial adhesive (ifused) that is tested.6.6 Core MaterialIf the core material has insufficientshear or compressive strength, it is possible that the core maylocally crush at or near the loading poin
35、ts thereby resulting infacesheet failure due to local stresses. In other cases, facingfailure can cause local core crushing. When there is both facingand core failure in the vicinity of one of the loading points, itcan be difficult to determine the failure sequence in a post-mortem inspection of the
36、 specimen as the failed specimenslook very similar for both sequences.6.7 DamagingRefer to Test Methods D6264/D6264M orD7136/D7136M for indentation or impact related interfer-ences.7. Sampling and Test Specimens7.1 SamplingTest at least five specimens per test condi-tion unless valid results can be
37、gained through the use of fewerspecimens, as in the case of a designed experiment. Forstatistically significant data, consult the procedures outlined inPractice E122. Report the method of sampling.7.2 Specimen and Fixture GeometryThe test requestorshall specify all specimen dimensions and materials
38、along withthe loading fixture dimensions. Refer to Test Method D7249/D7249M for sandwich beam specimen sizing requirements andguidelines.7.2.1 SpecimenThe test specimens shall be rectangular incross section. The width of the specimen shall be at least three(3) times the width of the damage (as deter
39、mined by NDI).NOTE 1The recommended specimen width is five (5) times thedamage width, however this may not be practical in all cases. Also, it maynot be possible to accurately predict the damage sizes prior to fabricationof the test specimens. As impact or indentation damage diameters areoften on th
40、e order of 25 mm 1.0 inch, a typical specimen width is127 mm 5.0 inch. Specimen width should be minimized to avoidexcessive anti-clastic bending.7.2.2 Loading FixtureThe loading span should be in-creased from the Test Method D7249/D7249M standard speci-men dimension of 100 mm 4.0 in to the minimum o
41、f adimension equal to the specimen width or three (3) times thedimension of the damage measured in the axial direction of thespecimen. The support span may have to be modified from theTest Method D7249/D7249M standard specimen dimension inorder for the specimen to produce the desired facesheet failu
42、remode.7.3 Back Side FacingsThe back side facings may use thesame laminate as the laminate to be tested in compression, ormay use a similar laminate. The back side facing should haveequal or greater stiffness (Et) than the top (compressive) facingand shall have greater tensile strength than the anti
43、cipatedcompressive strength of the damaged laminate to be tested. Theback side facing should have a similar coefficient of thermalexpansion as the top facing to avoid panel warpage duringfabrication.NOTE 2Metallic sheets are not recommended for the back facings asit can be difficult to machine a san
44、dwich panel with dissimilar materialsfor the two facings.7.4 CoreThe core material for the sandwich panels shallhave sufficient shear and compression strength to avoid failuresin the core prior to facing failure. Refer to Test MethodD7249/D7249M for core material selection guidelines7.5 AdhesiveThe
45、adhesive material for bonding the fac-ings to the core of the sandwich panel shall have sufficientshear strength at the test temperature to avoid failure in thebond prior to facing failure. The adhesive used to bond thedamaged facing shall not significantly flow into the cutout areaaround the damage
46、 location.8. Procedure8.1 Fabricate Laminate and Back Side FacingsFabricatethe laminates to be tested. Also fabricate back (tension) sidefacings for the sandwich specimens.D7956/D7956M 143NOTE 3It is acceptable to fabricate larger panels with sufficientmaterial for several specimens. Mark the locati
47、on of each specimen,damage the large panel once in the center of each specimen, bond thelaminate onto the core and back facing, then machine each individualsandwich specimen.8.2 Perform DamagingImpact damage using either TestMethod D6264/D6264M or Test Method D7136/D7136M,using an impact energy or i
48、ndentation depth to be supplied bythe test requestor. Apply a single impact or indentation in thecenter of each specimen to be tested. Record the damagemeasurements per Test Method D6264/D6264M or TestMethod D7136/D7136M.8.3 Fabricate Sandwich SpecimensFabricate the sand-wich panels using the damage
49、d laminates, the back sidefacings, and the selected core and adhesive. The damagedlaminates shall be bonded to the core with the side of thelaminate which was impacted or indented being bonded to thecore (flipped upside down from the orientation during impact-ing or indention); see Fig. 1. The adhesive used to bond thedamaged laminate shall have a circular area without adhesive,centered over the damage site, with a diameter larger than thegeneral disbond area detected by NDI. A typical size for thecutout in the adhesive is 51 mm 2.0 inch (it is acceptabl
