1、Designation: D7249/D7249M 161Standard 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.1NOTEEquation 4 was corrected editorially in April 2017.1. Scope1.1 This test method covers determination of facing pr
3、op-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 bal
4、sawood 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 independe
5、ntly 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
6、 the 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 configur
7、ation may be found in Test Method D5467/D5467M.1.4 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by th
8、e World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2C393 Test Method for Flexural Properties of SandwichConstructionsD3878 Terminology for Composite MaterialsD5229/D5229M Test Method for MoistureAbsorption Prop-erties and Equilibrium Condi
9、tioning 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 to Methods of Mechanical Te
10、stingE122 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 Bonded Resistance Strain Gag
11、esE456 Terminology Relating to Quality and Statistics3. Terminology3.1 DefinitionsTerminology D3878 defines terms relatingto high-modulus fibers and their composites. Terminology E6defines terms relating to mechanical testing. Terminology E456and Practice E177 define terms relating to statistics. In
12、 theevent of a conflict between terms, Terminology D3878 shallhave 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 percent)1This test method is under the jurisdiction of A
13、STM Committee D30 onComposite Materials and is the direct responsibility of Subcommittee D30.09 onSandwich Construction.Current edition approved July 1, 2016. Published July 2016. Originally approvedin 2006. Last previous edition approved in 2012 as D7249/D7249M 12. DOI:10.1520/D7249_D7249M-16E01.2F
14、or 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 page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C70
15、0, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the Wo
16、rld Trade Organization Technical Barriers to Trade (TBT) Committee.1d = sandwich total thicknessDF,nom= effective sandwich flexural stiffnessEf= effective facing chord modulus = measuring strain in facingFu= facing ultimate strength (tensile or compressive)Fs= core shear allowable strengthFc= core c
17、ompression 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 beforefailureSn1= standard deviation statistic of a
18、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 Test Method4.1 This test method consists of
19、 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 strength are those which are internal to
20、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 loading points can be caused by local t
21、hrough-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 modulus, or the facingscompressiveand ten
22、sile 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 calculating sandwich beamflexural and shear sti
23、ffness 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 edgewise (in-plane)compression.5.3 Core shear s
24、trength 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 and shearstiffness of sandwich beams.5.4
25、 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 the facing strength and shalltherefore
26、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 preparation, specimenconditioning, environment o
27、f 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 material.NOTE 2Concentrated forces on beams wit
28、h 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 is valid, a goodrule of thumb for the fou
29、r-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-rication practices and damage induced by impr
30、oper 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 thatcontribute to data scatter include the existe
31、nce 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 bagsidesurface in compression).6.3 Envi
32、ronmentResults 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 behavior and failure mode. Criticalenvironment
33、s 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 possible that the core maylocally crush at or n
34、ear 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 determine the failure sequence in a post-morte
35、m inspection of the specimen as the failed specimenslook very similar for both sequences.D7249/D7249M 16127. Apparatus7.1 Micrometers and CalipersA micrometer having a flatanvil interface, or a caliper of suitable size, shall be used. Theinstrument(s) shall have an accuracy of 625 m 60.001 in.for th
36、ickness 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 ConfigurationThe standard loading fixturesh
37、all 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 through the loading bars, with the supportbar
38、s 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 ConfigurationsAll other loading fix-ture confi
39、gurations (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 historical continuity with previous versi
40、onsof 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 to obtain sandwich beam flexural andshear
41、 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 the bars or any gaps occurring between
42、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 pivot (see Fig. 3) or a V-shaped bar ridi
43、ng 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 groove shall be such that the sides of
44、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 spanlengths tend to increase as the spec
45、imen 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 the full width of thespecimen shall be use
46、d 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 machineshall have both an essentially stationary h
47、ead 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. TheFIG. 1 Test Specimen and FixtureConfiguration Support Span (S) Load Span (L)Standard 4-Point 560 mm 22.0 in.
48、100 mm 4.0 in.Non-Standard 3-Point (Mid-span)S 0.04-Point (Quarter-Span)SS/24-Point (Third-Span)3FIG. 2 Loading ConfigurationsFIG. 3 Standard 4-Point Loading ConfigurationD7249/D7249M 1613velocity of the movable head shall be capable of beingregulated in accordance with 11.4.7.3.3 Force IndicatorThe
49、 testing machine force-sensingdevice shall be capable of indicating the total force beingcarried 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 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 actu
