1、Designation: D7249/D7249M 121D7249/D7249M 16Standard 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 cas
2、e 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 NOTEFigure 6 was corrected editorially in March 2014.1. Scope1.1 This test method covers determinatio
3、n of facing properties of flat sandwich constructions subjected to flexure in such amanner that the applied moments produce curvature of the sandwich facing planes and result in compressive and tensile forces inthe facings. Permissible core material forms include those with continuous bonding surfac
4、es (such as balsa wood and foams) aswell as those with discontinuous bonding surfaces (such as honeycomb).1.2 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
5、 be used independently of the other. Combining values from 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
6、 the responsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.NOTE 1Alternate procedures for determining the compressive strength of unidirectional polymer matrix composites materials in a sa
7、ndwich beamconfiguration may be found in Test Method D5467/D5467M.2. Referenced Documents2.1 ASTM Standards:2C274 Terminology of Structural Sandwich Constructions (Withdrawn 2016)3C393 Test Method for Flexural Properties of Sandwich ConstructionsD3878 Terminology for Composite MaterialsD5229/D5229M
8、Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix CompositeMaterialsD5467/D5467M Test Method for Compressive Properties of Unidirectional Polymer Matrix Composite Materials Using aSandwich BeamD7250/D7250M Practice for Determining Sandwich Beam Flexural an
9、d Shear StiffnessE6 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 MethodsE251 Test Methods for
10、 Performance Characteristics of Metallic Bonded Resistance Strain GagesE456 Terminology 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-Reinfo
11、rced Composite Materials in Databases (Withdrawn 2015)31 This test method is under 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, 2012July 1, 2016. Published December 2012J
12、uly 2016. Originally approved in 2006. Last previous edition approved in 20062012 asD7249/D7249M 06.D7249/D7249M 12. DOI: 10.1520/D7249_D7249M-12.10.1520/D7249_D7249M-16.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annua
13、l Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.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 t
14、echnically possible to adequately depict all changes accurately, ASTM recommends that users consult prior 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,
15、PO Box C700, West Conshohocken, PA 19428-2959. United States13. Terminology3.1 DefinitionsTerminology D3878 defines terms relating to high-modulus fibers and their composites. Terminology C274defines terms relating to structural sandwich constructions. Terminology C393 defines terms relating to plas
16、tics. Terminology E6defines terms relating to mechanical testing. Terminology E456 and Practice E177 define terms relating to statistics. In the eventof a conflict between terms, Terminology D3878 shall have precedence over the other terminologies.3.2 Symbols:b = specimen widthc = core thicknessCV =
17、 coefficient of variation statistic of a sample population for a given property (in percent)d = 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
18、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 before failureSn1 = s
19、tandard deviation statistic of a sample population for a given property = facing stresst = facing thicknessx1 = test result for an individual specimen from the sample population for a given propertyx = mean or average (estimate of mean) of a sample population for a given property4. Summary of Test M
20、ethod4.1 This test method consists of subjecting a long beam of sandwich construction to a bending moment normal to the plane ofthe sandwich, using a 4-point loading fixture. Deflection and strain versus force measurements are recorded.4.2 The only acceptable failure modes for sandwich facesheet str
21、ength are those which are internal to one of the facesheets.Failure of the sandwich core or the core-to-facesheet bond preceding failure of one of the facesheets is not an acceptable failuremode. Careful post-test inspection of the specimen is required as facing failure occurring in proximity to the
22、 loading points canbe caused by local through-thickness compression or shear failure of the core that precedes failure of the facing.5. Significance and Use5.1 Flexure tests on flat sandwich construction may be conducted to determine the sandwich flexural stiffness, the core shearstrength, and shear
23、 modulus, or the facings compressive and tensile strengths. Tests to evaluate core shear strength may also beused to evaluate core-to-facing bonds.5.2 This test method is limited to obtaining the strength and stiffness of the sandwich panel facings, and to obtainingload-deflection data for use in ca
24、lculating sandwich beam flexural and shear stiffness using Standard Practice D7250/D7250M. Dueto the curvature of the flexural test specimen when loaded, facesheet compression strength from this test may not be equivalentto the facesheet compression strength of sandwich structures subjected to pure
25、edgewise (in-plane) compression.5.3 Core shear strength and shear modulus are best determined in accordance with Test Method C273 provided bare corematerial is available. Test Method C393 may also be used to determine core shear strength. Standard Practice D7250/D7250M maybe used to calculate the fl
26、exural and shear stiffness of sandwich beams.5.4 This test method can be used to produce facing strength data for structural design allowables, material specifications, andresearch and development applications; it may also be used as a quality control test for bonded sandwich panels.5.5 Factors that
27、 influence the facing strength and shall therefore be reported include the following: facing material, corematerial, adhesive material, methods of material fabrication, facing stacking sequence and overall thickness, core geometry (cellsize), core density, adhesive thickness, specimen geometry, spec
28、imen preparation, specimen conditioning, environment of testing,specimen alignment, loading procedure, speed of testing, facing void content, adhesive void content, and facing volume percentreinforcement. Further, facing strength may be different between precured/bonded and co-cured facesheets of th
29、e same material.NOTE 2Concentrated forces on beams with thin facings and low density cores can produce results that are difficult to interpret, especially close toD7249/D7249M 162the failure point. Wider loading blocks and rubber pressure pads may assist in distributing the forces.NOTE 3To ensure th
30、at simple sandwich beam theory is valid, a good rule of thumb for the four-point bending test is the span length divided by thesandwich thickness should be greater than 20 (L/d 20) with the ratio of facing thickness to core thickness less than 0.1 (t/c 0.1).6. Interferences6.1 Material and Specimen
31、PreparationPoor material fabrication practices and damage induced by improper specimenmachining are known causes of high data scatter in composites and sandwich structures in general. A specific material factor thataffects sandwich cores is variability in core density. Important aspects of sandwich
32、core specimen preparation that contribute todata scatter include the existence of joints, voids or other core discontinuities, out-of-plane curvature, and surface roughness.6.2 GeometrySpecific geometric factors that affect sandwich facing strength include facing thickness, core cell geometry, andfa
33、cing surface flatness (toolside or bagside surface in compression).6.3 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 environments can exhibit significant
34、 differences inboth strength behavior and failure mode. Critical environments must be assessed independently for each specific combination ofcore material, facing material, and core-to-facing interfacial adhesive (if used) that is tested.6.4 Core MaterialIf the core material has insufficient shear o
35、r 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 load
36、ing 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.7. Apparatus7.1 Micrometers and CalipersA micrometer having a flat anvil interface, or a caliper of suitable size, shall be used. Th
37、einstrument(s) shall have an accuracy of 625 m 60.001 in. for thickness measurement, and an accuracy of 6250 m 60.010in. for length and width measurement.NOTE 4The accuracies given above are based on achieving measurements that are within 1 % of the sample length, width and thickness.7.2 Loading Fix
38、tures7.2.1 Standard ConfigurationThe standard loading fixture shall consist of a 4-point loading configuration with two supportbars that span the specimen width located below the specimen, and two loading bars that span the specimen width located on thetop of the specimen (Fig. 1), The force shall b
39、e applied vertically through the loading bars, with the support bars fixed in placein the test machine. The standard loading fixture shall have the centerlines of the support bars separated by a distance of 560 mm22.0 in. and the centerlines of the loading bars separated by a distance of 100 mm 4.0
40、in.7.2.2 Non-Standard ConfigurationsAll other loading fixture configurations (see Fig. 2) are considered non-standard anddetails of the fixture geometry shall be documented in the test report. Figs. 3-5 show typical test fixtures. Non-standard 3- and4-point loading configurations have been retained
41、within this standard a) for historical continuity with previous versions of TestMethod C393, b) because some sandwich panel designs require the use of non-standard loading configurations to achieve facesheetfailure modes, and c) load-deflection data from non-standard configurations may be used with
42、Standard Practice D7250/D7250Mto obtain sandwich beam flexural and shear stiffnesses.7.2.3 Support and Loading BarsThe bars shall be designed to allow free rotation of the specimen at the loading and supportpoints. The bars shall have sufficient stiffness to avoid significant deflection of the bars
43、under load; any obvious bowing of the barsor any gaps occurring between the bars and the test specimen during loading shall be considered significant deflection. Therecommended configuration has a 25 mm 1.0 in. wide flat steel loading block to contact the specimen (through rubber pressurepads) and i
44、s loaded via either a cylindrical pivot (see Fig. 3) or a V-shaped bar riding in a V-groove in the top of the flat-bottomedsteel loading pad. The tips of the V-shaped loading bars shall have a minimum radius of 3 mm 0.12 in. The V-groove in theloading pad shall have a radius larger than the loading
45、bar tip and the angular opening of the groove shall be such that the sidesof the loading bars do not contact the sides of the V-groove during the test. Loading bars consisting of 25 mm 1.0 in. diameterFIG. 1 Test Specimen and FixtureD7249/D7249M 163steel cylinders may also be used, but there is a gr
46、eater risk of local specimen crushing with cylindrical bars. Also, the load andsupport span lengths tend to increase as the specimen deflects when cylindrical loading bars without V-grooved loading pads areused (e.g., rolling supports).Configuration Support Span (S) Load Span (L)Standard 4-Point 560
47、 mm 22.0 in. 100 mm 4.0 in.Non-Standard 3-Point (Mid-span)S 0.04-Point (Quarter-Span)S S/24-Point (Third-Span)S S/3FIG. 2 Loading ConfigurationsFIG. 3 Standard 4-Point Loading ConfigurationFIG. 4 3-Point Mid-Span Loading Configuration (Non-Standard)D7249/D7249M 1647.2.4 Pressure PadsRubber pressure
48、pads having a Shore A durometer of 60, a width of 25 mm 1.0 in., a nominal thicknessof 3 mm 0.125 in. and spanning the full width of the specimen shall be used between the loading bars and specimen to preventlocal damage to the facings.7.3 Testing MachineThe testing machine shall be in accordance wi
49、th Practices E4 and shall satisfy the followingrequirements:7.3.1 Testing Machine ConfigurationThe testing machine shall have both an essentially stationary head and a movable head.7.3.2 Drive MechanismThe testing machine drive mechanism shall be capable of imparting to the movable head a controlledvelocity with respect to the stationary head. The velocity of the movable head shall be capable of being regulated in accordancewith 11.4.7.3.3 Force IndicatorThe testing machine force-sensing device shall be capable of indicatin
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