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Shear test methods

Shear testing is one of the most complex areas of testing with achoice of very different test methods. This is unlike the alternativestensile or compression tests, which were all similar in philosophy andcould be harmonised. For example, the simplest tests - interlaminarshear beam shear(ILSS) measures the interlayer strength, whileplate twist only provides in-plane shear modulus. In arecentNPLpaper, it was suggested that plate twist and V notch(Isopescu) should be used together to minimise cost as modulus canbe obtained from the former test, avoiding the cost of strain gauges,and shear strength from the later test without using gauges. Caremust also be taken that the test is chosen according to the requiredshear plane. A summary of the shear test methods is given below.

Test Advantages Disadvantages interlaminar shearstrength,ILSS(ENISO14130) simple jigs, specimen and analysisused, Robot potential for QA testing Interlaminar strength only,frequent invalid failure modes, tensile 45 shear test (ENISO14129)double V notched shear test (ASTMD5379) double notch shear test(ASTM10 off-axis test rail shear tests. torsion tests (rod, tube, strip).

Interlaminar shear strength,ILSS(ENISO14130)

This standard was based onISO4585, which it replaced with inputfromASTMD2344,CRAG100, EN 2377 and EN 2563. The onlysignificant change fromISO4585 is for a 2 mm thick standard

specimen to accommodate existing preferences when testingprepregs. Other thicknesses are accommodated by the existingscaling rules. The test only provides an "apparent" shear strengthusing an assumed isotropic material stress distribution, that alsoexcludes the effect of other stresses due to the flexural loading andcontact points. There is an increased emphasis in the standard onaccepting only interlaminar shear failures as valid results. It frequentlygives invalid failure modes and is most reliable for alignedunidirectional material. It has the advantages of being smallspecimens, simple preparation, easily conditioned and the testing iseasily automated with robots.

In-plane shear modulus by plate twist (BS ENISO15310)

The test method was developed byNPLto give a cheap, simple andadaptable to different composite materials (c.f. limited application oftensile 45 shear test. The test is similar to a flexure test and has thesame benefits of low loads, large (linear) specimen deflections, cheap

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specimen and simple test equipment (see Figure 18.6). The test hasbeen standardised for plywood for many years and has featured inmany reviews over the last 25 years. Results are determined fromload and displacement data, so that the high cost of using straingauges is avoided.

Recently, a revised analysis developed atNPLallowed the loadingand support points to be positioned in-board of the actual specimenplate corner with a major effect on the ease and speed of the test. Nospecial lay-up is required for the specimen but as the shear loading isdeveloped in flexure, the test material should appear "homogeneous"through the thickness and have orthotropic symmetry. Therefore, fora 0/90 lay-up there should be several layers rather than three.

Precision data obtained in a round-robin exercise organised bytheNPLsupported the original "New Work Item" (NWI) submission by

the UK. Eight sites took part in validating the method using sixgeneric material representing the full range of available materials. Ithas also been used with metal and ceramic matrix composites, andmonolithic materials.

Tensile 45 shear test (ENISO14129)

The new standard is based onASTMD 3518 with input from prEN6031 andCRAG101. It applies to all fibres but requires a lay-up ofaligned prepreg or fabric at 45 to the specimen axis. In commonwith theASTMrevision, the test will be terminated at 5% shear strain

with a subsequent shortening of the normally excessive test durationfor tougher matrix systems, which gives a very flat load-time responsefor materials with high shear failure strains. The 5% limit alsominimises fibre rotation and heating effects. The peak load at orbefore 5% strain is taken as the shear strength.

It has been reported that the result is dependent on the number oflayers, or shearing interfaces, in the specimen. This may havetechnical and harmonisation implications for the AirbusIndustries/prEN 6031 version, which is the only version of this testusing 1mm thick specimens (cf 2mm in all others). It is recommended

that the number of shearing faces (e.g. interfaces) should be keptconstant when ply thickness vary from the normal 0.125 mm plies.

Double V notched shear test (ASTMD5379)

This test is frequently called the Isopescu test after one of the earlydevelopers, or theARCANtest when the test plane is loaded in a

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mixed mode, not in direct shear. Modulus determination is relativelystraight forward once the small strain gauges are applied in the centreof the specimen. The measurement of shear strength needsadditional care as for some composite materials and for somedirections the failure is initiated by localised tensile stresses rather

than shear stress. Re-design of the loading jig in therecommendedASTMdrawing for this standard has been proposed byNational Physical Laboratory (NPL) to reduce the tendency forunsymmetrical loading of the specimen.

Double notch shear test (ASTMD3846, also new work UK programme)

ThisASTMtest method developed by the plastics committee (D20) isnot supported for preparation as anISOmethod by the compositescommittee withinASTM(D30). Research atNPLfound that it couldbe used for more directions in more materials thanILSS, whileobtaining a correct Interlaminar shear failures. It is frequently requiredas a quality control test. The UK is conducting an experimental round-robin [MMS1.1] programme to obtain precision data for this method.torsion tests - although often quoted as a classic method with apreferred stress distribution, neither rod or tube tests have beenstandardised. The specimen does exist within the dynamicmechanical properties test methods - torsion pendulum.

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