1、Designation: D 7249/D 7249M 06Standard Test Method forFacing Properties of Sandwich Constructions by LongBeam Flexure1This standard is issued under the fixed designation D 7249/D 7249M; the number immediately following the designation indicates theyear of original adoption or, in the case of revisio
2、n, the year of last revision. A number in parentheses indicates the year of lastreapproval. A superscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers determination of facing prop-erties of flat sandwich constructions subjected t
3、o 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 balsawood and foams) as well as those with discontinuou
4、s bondingsurfaces (such as honeycomb).1.2 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard. Within the text theinch-pound units are shown in brackets. The values stated ineach system are not exact equivalents; therefore, each systemmust be used indepe
5、ndently of the other. Combining valuesfrom the two systems may result in nonconformance with thestandard.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
6、 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 configuration may be found in Test Method D 5467.2. Referenced Document
7、s2.1 ASTM Standards:2C 274 Terminology of Structural Sandwich ConstructionsC 393 Test Method for Flexural Properties of SandwichConstructionsD 3878 Terminology for Composite MaterialsD 5229/D 5229M Test Method for Moisture AbsorptionProperties and Equilibrium Conditioning of Polymer Ma-trix Composit
8、e MaterialsD 5467/D 5467M Test Method for Compressive Propertiesof Unidirectional Polymer Matrix Composite MaterialsUsing a Sandwich BeamD 7250 Practice for Determining Sandwich Beam Flexuraland Shear StiffnessE6 Terminology Relating to Methods of Mechanical Test-ingE 122 Practice for Calculating Sa
9、mple Size to Estimate,With a Specified Tolerable Error, the Average for aCharacteristic of a Lot or ProcessE 177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE 251 Test Methods for Performance Characteristics ofMetallic Bonded Resistance Strain GagesE 456 Terminology Relating
10、to Quality and StatisticsE 1309 Guide for Identification of Fiber-ReinforcedPolymer-Matrix Composite Materials in DatabasesE 1434 Guide for Recording Mechanical Test Data of Fiber-Reinforced Composite Materials in Databases3. Terminology3.1 DefinitionsTerminology D 3878 defines terms relatingto high
11、-modulus fibers and their composites. TerminologyC 274 defines terms relating to structural sandwich construc-tions. Terminology D 883 defines terms relating to plastics.Terminology E6defines terms relating to mechanical testing.Terminology E 456 and Practice E 177 define terms relating tostatistics
12、. In the event of a conflict between terms, TerminologyD 3878 shall have precedence over the other terminologies.3.2 Symbols:b = specimen widthc = core thicknessCV = coefficient of variation statistic of a samplepopulation for a given property (in percent)d = sandwich total thicknessDF,nom= effectiv
13、e sandwich flexural stiffnessEf= effective facing chord moduluse = measuring strain in facingFu= facing ultimate strength (tensile or compressive)Fs= core shear allowable strengthFc= core compression allowable strengthk = core shear strength factor to ensure facing failure1This test method is under
14、the jurisdiction of ASTM Committee D30 onComposite Materials and is the direct responsibility of Subcommittee D30.09 onSandwich Construction.Current edition approved Sept. 1, 2006. Published October 2006.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Se
15、rvice at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.l = length of loading spanL = length of su
16、pport spanlpad= length of loading padn = number of specimensP = applied forcePmax= maximum force carried by test specimen beforefailureSn1= standard deviation statistic of a sample popula-tion for a given propertys = facing stresst = facing thicknessx1= test result for an individual specimen from th
17、esample 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 subjecting a long beam ofsandwich construction to a bending moment normal to theplane of the sandwich, using a 4-point load
18、ing 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 one of thefacesheets. Failure of the sandwich core or the core-to-facesheet bond preceding failure of one of the facesheets
19、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 through-thicknesscompression or shear failure of the core that precedes failure ofthe facing.5. Significance and Use5.1 Flexu
20、re tests on flat sandwich construction may beconducted to determine the sandwich flexural stiffness, the coreshear strength, and shear modulus, 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
21、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 stiffness using Standard Practice D 7250.Due to the curvature of the flexural test specimen when loaded,facesheet compression s
22、trength from this test may not beequivalent to the facesheet compression strength of sandwichstructures subjected to pure edgewise (in-plane) compression.5.3 Core shear strength and shear modulus are best deter-mined in accordance with Test Method C 273 provided barecore material is available. Test
23、Method C 393 may also be usedto determine core shear strength. Standard Practice D 7250may be used to calculate the flexural and shear stiffness ofsandwich beams.5.4 This test method can be used to produce facing strengthdata for structural design allowables, material specifications,and research and
24、 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 be reported include the following: facing material,core material, adhesive material, methods of material fabrica-tion, facing stacking
25、sequence and overall thickness, coregeometry (cell size), core density, adhesive thickness, speci-men geometry, specimen preparation, specimen conditioning,environment of testing, specimen alignment, loading proce-dure, speed of testing, facing void content, adhesive voidcontent, and facing volume p
26、ercent reinforcement. Further,facing strength may be different between precured/bonded andco-cured facesheets of the same material.NOTE 2Concentrated forces on beams with thin facings and lowdensity cores can produce results that are difficult to interpret, especiallyclose to the failure point. Wide
27、r 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 four-point bending test is the span length divided bythe sandwich thickness should be greater than 20 (L/d 20) with the ratioof facing
28、 thickness to core thickness less than 0.1 (t/c 0.1).6. Interferences6.1 Material and Specimen PreparationPoor materialfabrication practices and damage induced by improper speci-men machining are known causes of high data scatter incomposites and sandwich structures in general. A specificmaterial fa
29、ctor that affects sandwich cores is variability in coredensity. Important aspects of sandwich core specimen prepa-ration that contribute to data scatter include the existence ofjoints, voids or other core discontinuities, out-of-plane curva-ture, and surface roughness.6.2 GeometrySpecific geometric
30、factors that affect sand-wich facing strength include facing thickness, core cell geom-etry, and facing surface flatness (toolside or bagside surface incompression).6.3 EnvironmentResults are affected by the environmen-tal conditions under which specimens are conditioned, as wellas the conditions un
31、der which the tests are conducted. Speci-mens tested in various environments can exhibit significantdifferences in both strength behavior and failure mode. Criticalenvironments must be assessed independently for each specificcombination of core material, facing material, and core-to-facing interfaci
32、al 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 near the loading points thereby resulting infacesheet failure due to local stresses. In other cases, facingfailure can cause l
33、ocal 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 specimen as the failed specimenslook very similar for both sequences.7. Apparatus7.1 Micrometers and Cali
34、persA micrometer having a flatanvil interface, or a caliper of suitable size, shall be used. Theinstrument(s) shall have an accuracy of 625 mm 60.001 in.for thickness measurement, and an accuracy of 6250 mm60.010 in. for length and width measurement.NOTE 4The accuracies given above are based on achi
35、eving measure-ments that are within 1 % of the sample length, width and thickness.7.2 Loading Fixtures7.2.1 Standard ConfigurationThe standard loading fixtureshall consist of a 4-point loading configuration with twoD 7249/D 7249M 062support bars that span the specimen width located below thespecimen
36、, 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 supportbars fixed in place in the test machine. The standard loadingfixture shall have the centerlines of the support bars separated
37、by 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 configurations (see Fig. 2) are considered non-standardand details of the fixture geometry shall be documented in thetest repor
38、t. 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 versionsof Test Method C 393, b) because some sandwich paneldesigns require the use of non-standard loading configurationsto ac
39、hieve facesheet failure modes, and c) load-deflection datafrom non-standard configurations may be used with StandardPractice D 7250 to obtain sandwich beam flexural and shearstiffnesses.7.2.3 Support and Loading BarsThe bars shall be de-signed to allow free rotation of the specimen at the loading an
40、dsupport 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 the bars andthe test specimen during loading shall be considered significantdeflection. The recommended configuration has a 25
41、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 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 ra
42、diusof 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 the loading bars donot contact the sides of the V-groove during the test. Loadingbars consisting of 25 mm 1.0 in. diameter stee
43、l 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 specimen deflects when cylin-drical loading bars without V-grooved loading pads are used(e.g., rolling supports).7.2.4 Pressure Pad
44、sRubber 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 used between the loading bars and specimento prevent local damage to the facings.7.3 Testing MachineThe testing machine shall be i
45、naccordance with Practices E 4 and shall satisfy the followingrequirements:7.3.1 Testing Machine ConfigurationThe testing machineshall 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 hea
46、d acontrolled velocity with respect to the stationary head. Thevelocity of the movable head shall be capable of beingregulated in accordance with 11.4.7.3.3 Force IndicatorThe testing machine force-sensingdevice shall be capable of indicating the total force beingcarried by the test specimen. This d
47、evice 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
48、 span by aproperly calibrated device having an accuracy of 61% orbetter.NOTE 5The use of crosshead or actuator displacement for the beamFIG. 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-Poi
49、nt (Quarter-Span)SS/24-Point (Third-Span)3FIG. 2 Loading ConfigurationsFIG. 3 Standard 4-Point Loading ConfigurationD 7249/D 7249M 063mid-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 by means of bonded resistance straingages. One axial gage element shall be located on each face atthe center of the specimen, with the gage aligned with thespecimen length axis. S
