1、Designation: D7249/D7249M 121Standard Test Method forFacing Properties of Sandwich Constructions by LongBeam Flexure1This standard is issued under the fixed designation D7249/D7249M; the number immediately following the designation indicates theyear of original adoption or, in the case of revision,
2、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.1NOTEFigure 6 was corrected editorially in March 2014.1. Scope1.1 This test method covers determination of facing prop
3、-erties of flat sandwich constructions subjected to flexure insuch a manner that the applied moments produce curvature ofthe sandwich facing planes and result in compressive andtensile forces in the facings. Permissible core material formsinclude those with continuous bonding surfaces (such as balsa
4、wood and foams) as well as those with discontinuous bondingsurfaces (such as honeycomb).1.2 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 independent
5、ly of the other. Combiningvalues from the two systems may result in 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 t
6、he user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.NOTE 1Alternate procedures for determining the compressivestrength of unidirectional polymer matrix composites materials in asandwich beam configurat
7、ion may be found in Test Method D5467/D5467M.2. Referenced Documents2.1 ASTM Standards:2C274 Terminology of Structural Sandwich ConstructionsC393 Test Method for Flexural Properties of SandwichConstructionsD3878 Terminology for Composite MaterialsD5229/D5229M Test Method for MoistureAbsorption Prop-
8、erties and Equilibrium Conditioning of Polymer MatrixComposite MaterialsD5467/D5467M Test Method for Compressive Properties ofUnidirectional Polymer Matrix Composite Materials Us-ing a Sandwich BeamD7250/D7250M Practice for Determining Sandwich BeamFlexural and Shear StiffnessE6 Terminology Relating
9、 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 MethodsE251 Test Methods for Performance Characteristics of Me-tallic
10、Bonded Resistance Strain GaugesE456 Terminology Relating to Quality and StatisticsE1309 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 Defi
11、nitionsTerminology D3878 defines terms relatingto high-modulus fibers and their composites. TerminologyC274 defines terms relating to structural sandwich construc-tions. Terminology C393 defines terms relating to plastics.Terminology E6 defines terms relating to mechanical testing.Terminology E456 a
12、nd Practice E177 define terms relating tostatistics. In the event of a conflict between terms, TerminologyD3878 shall have precedence over the other terminologies.3.2 Symbols:b = specimen widthc = core thicknessCV = coefficient of variation statistic of a sample popu-lation for a given property (in
13、percent)d = sandwich total thickness1This test method is under the jurisdiction of ASTM Committee D30 onComposite Materials and is the direct responsibility of Subcommittee D30.09 onSandwich Construction.Current edition approved Aug. 1, 2012. Published December 2012. Originallyapproved in 2006. Last
14、 previous edition approved in 2006 as D7249/D7249M 06.DOI: 10.1520/D7249_D7249M-12.2For 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 Document Summary pa
15、ge onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1DF,nom= effective sandwich flexural stiffnessEf= effective facing chord modulus = measuring strain in facingFu= facing ultimate strength (tensile or compressive)Fs=
16、 core shear allowable strengthFc= core compression allowable strengthk = core shear strength factor to ensure facing failurel = length of loading spanL = length of support spanlpad= length of loading padn = number of specimensP = applied forcePmax= maximum force carried by test specimen beforefailur
17、eSn1= standard deviation statistic of a sample populationfor a given property = facing stresst = facing thicknessx1= test result for an individual specimen from thesample population for a given propertyx = mean or average (estimate of mean) of a samplepopulation for a given property4. Summary of Tes
18、t Method4.1 This test method consists of subjecting a long beam ofsandwich construction to a bending moment normal to theplane of the sandwich, using a 4-point loading fixture. Deflec-tion and strain versus force measurements are recorded.4.2 The only acceptable failure modes for sandwichfacesheet s
19、trength are those which are internal to one of thefacesheets. Failure of the sandwich core or the core-to-facesheet bond preceding failure of one of the facesheets is notan acceptable failure mode. Careful post-test inspection of thespecimen is required as facing failure occurring in proximity tothe
20、 loading points can be caused by local through-thicknesscompression or shear failure of the core that precedes failure ofthe facing.5. Significance and Use5.1 Flexure tests on flat sandwich construction may beconducted to determine the sandwich flexural stiffness, the coreshear strength, and shear m
21、odulus, or the facingscompressiveand tensile strengths. Tests to evaluate core shear strength mayalso be used to evaluate core-to-facing bonds.5.2 This test method is limited to obtaining the strength andstiffness of the sandwich panel facings, and to obtainingload-deflection data for use in calcula
22、ting sandwich beamflexural and shear stiffness using Standard Practice D7250/D7250M. Due to the curvature of the flexural test specimenwhen loaded, facesheet compression strength from this testmay not be equivalent to the facesheet compression strength ofsandwich structures subjected to pure edgewis
23、e (in-plane)compression.5.3 Core shear strength and shear modulus are best deter-mined in accordance with Test Method C273 provided barecore material is available. Test Method C393 may also be usedto determine core shear strength. Standard Practice D7250/D7250M may be used to calculate the flexural
24、and shearstiffness of sandwich beams.5.4 This test method can be used to produce facing strengthdata for structural design allowables, material specifications,and research and development applications; it may also be usedas a quality control test for bonded sandwich panels.5.5 Factors that influence
25、 the facing strength and shalltherefore be reported include the following: facing material,core material, adhesive material, methods of materialfabrication, facing stacking sequence and overall thickness,core geometry (cell size), core density, adhesive thickness,specimen geometry, specimen preparat
26、ion, specimenconditioning, environment of testing, specimen alignment,loading procedure, speed of testing, facing void content,adhesive void content, and facing volume percent reinforce-ment. Further, facing strength may be different betweenprecured/bonded and co-cured facesheets of the same materia
27、l.NOTE 2Concentrated forces on beams with thin facings and lowdensity cores can produce results that are difficult to interpret, especiallyclose to the failure point. Wider loading blocks and rubber pressure padsmay assist in distributing the forces.NOTE 3To ensure that simple sandwich beam theory i
28、s valid, a goodrule of thumb for the four-point bending test is the span length divided bythe sandwich thickness should be greater than 20 (L/d 20) with the ratioof facing thickness to core thickness less than 0.1 (t/c 0.1).6. Interferences6.1 Material and Specimen PreparationPoor material fab-ricat
29、ion practices and damage induced by improper specimenmachining are known causes of high data scatter in compositesand sandwich structures in general. A specific material factorthat affects sandwich cores is variability in core density.Important aspects of sandwich core specimen preparation thatcontr
30、ibute to data scatter include the existence of joints, voidsor other core discontinuities, out-of-plane curvature, and sur-face roughness.6.2 GeometrySpecific geometric factors that affect sand-wich facing strength include facing thickness, core cellgeometry, and facing surface flatness (toolside or
31、 bagsidesurface in compression).6.3 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 both strength behavi
32、or and failure mode. Criticalenvironments must be assessed independently for each specificcombination of core material, facing material, and core-to-facing interfacial adhesive (if used) that is tested.6.4 Core MaterialIf the core material has insufficientshear or compressive strength, it is possibl
33、e that the core maylocally crush at or near the loading points 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 determ
34、ine the failure sequence in a post-mortem inspection of the specimen as the failed specimenslook very similar for both sequences.7. Apparatus7.1 Micrometers and CalipersA micrometer having a flatanvil interface, or a caliper of suitable size, shall be used. TheD7249/D7249M 1212instrument(s) shall ha
35、ve an accuracy of 625 m 60.001 in.for thickness measurement, and an accuracy of 6250 m60.010 in. for length and width measurement.NOTE 4The accuracies given above are based on achieving measure-ments that are within 1 % of the sample length, width and thickness.7.2 Loading Fixtures7.2.1 Standard Con
36、figurationThe standard loading fixtureshall consist of a 4-point loading configuration with twosupport bars that span the specimen width located below thespecimen, and two loading bars that span the specimen widthlocated on the top of the specimen (Fig. 1), The force shall beapplied vertically throu
37、gh the loading bars, with the supportbars fixed in place in the test machine. The standard loadingfixture shall have the centerlines of the support bars separatedby a distance of 560 mm 22.0 in. and the centerlines of theloading bars separated by a distance of 100 mm 4.0 in.7.2.2 Non-Standard Config
38、urationsAll other loading fix-ture configurations (see Fig. 2) are considered non-standardand details of the fixture geometry shall be documented in thetest report. Figs. 3-5 show typical test fixtures. Non-standard 3-and 4-point loading configurations have been retained withinthis standard a) for h
39、istorical continuity with previous versionsof Test Method C393, b) because some sandwich panel designsrequire the use of non-standard loading configurations toachieve facesheet failure modes, and c) load-deflection datafrom non-standard configurations may be used with StandardPractice D7250/D7250M t
40、o obtain sandwich beam flexural andshear stiffnesses.7.2.3 Support and Loading BarsThe bars shall be de-signed to allow free rotation of the specimen at the loading andsupport points. The bars shall have sufficient stiffness to avoidsignificant deflection of the bars under load; any obviousbowing of
41、 the bars or any gaps occurring between the bars andthe test specimen during loading shall be considered significantdeflection. The recommended configuration has a 25 mm 1.0in. wide flat steel loading block to contact the specimen(through rubber pressure pads) and is loaded via either acylindrical p
42、ivot (see Fig. 3) or a V-shaped bar riding in aV-groove in the top of the flat-bottomed steel loading pad. Thetips of the V-shaped loading bars shall have a minimum radiusof 3 mm 0.12 in. The V-groove in the loading pad shall havea radius larger than the loading bar tip and the angular openingof the
43、 groove shall be such that the sides of the loading bars donot contact the sides of the V-groove during the test. Loadingbars consisting of 25 mm 1.0 in. diameter steel cylinders mayalso be used, but there is a greater risk of local specimencrushing with cylindrical bars. Also, the load and support
44、spanlengths tend to increase as the specimen deflects when cylin-drical loading bars without V-grooved loading pads are used(e.g., rolling supports).7.2.4 Pressure PadsRubber pressure pads having a ShoreA durometer of 60, a width of 25 mm 1.0 in., a nominalthickness of 3 mm 0.125 in. and spanning th
45、e full width of thespecimen shall be used between the loading bars and specimento prevent local damage to the facings.7.3 Testing MachineThe testing machine shall be in ac-cordance with Practices E4 and shall satisfy the followingrequirements:7.3.1 Testing Machine ConfigurationThe testing machinesha
46、ll have both an essentially stationary head and a movablehead.7.3.2 Drive MechanismThe testing machine drive mecha-nism shall be capable of imparting to the movable head acontrolled velocity with respect to the stationary head. Thevelocity of the movable head shall be capable of beingregulated in ac
47、cordance with 11.4.7.3.3 Force IndicatorThe testing machine force-sensingdevice shall be capable of indicating the total force beingFIG. 1 Test Specimen and FixtureConfiguration Support Span (S) Load Span (L)Standard 4-Point 560 mm 22.0 in. 100 mm 4.0 in.Non-Standard 3-Point (Mid-span)S 0.04-Point (
48、Quarter-Span)SS/24-Point (Third-Span)3FIG. 2 Loading ConfigurationsFIG. 3 Standard 4-Point Loading ConfigurationD7249/D7249M 1213carried by the test specimen. This device shall be essentiallyfree from inertia-lag at the specified rate of testing and shallindicate the force with an accuracy over the
49、force range(s) ofinterest of within 6 1 % of the indicated value.7.4 Deflectometer (LVDT)The deflection of the specimenshall be measured in the center of the support span by aproperly calibrated device having an accuracy of 61% orbetter.NOTE 5The use of crosshead or actuator displacement for the beammid-span deflection produces inaccurate results; the direct measurementof the deflection of the mid-span of the beam must be made by a suitableinstrument.7.5 Strain-Indicating DeviceStrain data, when required,shall be determined
