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Shear test methods

Shear testing is one of the most complex areas of testing with a choice of very different test methods. This is unlike the alternatives tensile or compression tests, which were all similar in philosophy and could be harmonised. For example, the simplest tests - interlaminar shear beam shear (ILSS) measures the interlayer strength, while plate twist only provides in-plane shear modulus. In a recent NPL paper, it was suggested that plate twist and V notch (Isopescu) should be used together to minimise cost as modulus can be obtained from the former test, avoiding the cost of strain gauges, and shear strength from the later test without using gauges. Care must also be taken that the test is chosen according to the required shear plane. A summary of the shear test methods is given below.

Test Advantages Disadvantages interlaminar shear strength, ILSS (EN ISO 14130) simple jigs, specimen and analysis used, Robot potential for QA testing Interlaminar strength only, frequent invalid failure modes, tensile ±45° shear test (EN ISO 14129) double V notched shear test (ASTM D5379) double notch shear test (ASTM 10° off-axis test rail shear tests. torsion tests (rod, tube, strip).

Interlaminar shear strength, ILSS (EN ISO 14130)

This standard was based on ISO 4585, which it replaced with input from ASTM D2344, CRAG 100, EN 2377 and EN 2563. The only significant change from ISO 4585 is for a 2 mm thick standard specimen to accommodate existing preferences when testing prepregs. Other thicknesses are accommodated by the existing scaling rules. The test only provides an "apparent" shear strength using an assumed isotropic material stress distribution, that also excludes the effect of other stresses due to the flexural loading and contact points. There is an increased emphasis in the standard on accepting only interlaminar shear failures as valid results. It frequently gives invalid failure modes and is most reliable for aligned unidirectional material. It has the advantages of being small specimens, simple preparation, easily conditioned and the testing is easily automated with robots.

In-plane shear modulus by plate twist (BS EN ISO 15310)

The test method was developed by NPL to give a cheap, simple and adaptable to different composite materials (c.f. limited application of tensile ±45° shear test. The test is similar to a flexure test and has the same benefits of low loads, large (linear) specimen deflections, cheap specimen and simple test equipment (see Figure 18.6). The test has been standardised for plywood for many years and has featured in many reviews over the last 25 years. Results are determined from load and displacement data, so that the high cost of using strain gauges is avoided.

Recently, a revised analysis developed at NPL allowed the loading and support points to be positioned in-board of the actual specimen plate corner with a major effect on the ease and speed of the test. No special lay-up is required for the specimen but as the shear loading is developed in flexure, the test material should appear "homogeneous" through the thickness and have orthotropic symmetry. Therefore, for a 0°/90° lay-up there should be several layers rather than three.

Precision data obtained in a round-robin exercise organised by the NPL supported the original "New Work Item" (NWI) submission by the UK. Eight sites took part in validating the method using six generic material representing the full range of available materials. It has also been used with metal and ceramic matrix composites, and monolithic materials.

Tensile ±45° shear test (EN ISO 14129)

The new standard is based on ASTM D 3518 with input from prEN 6031 and CRAG 101. It applies to all fibres but requires a lay-up of aligned prepreg or fabric at ± 45° to the specimen axis. In common with the ASTM revision, the test will be terminated at 5% shear strain with a subsequent shortening of the normally excessive test duration for tougher matrix systems, which gives a very flat load-time response for materials with high shear failure strains. The 5% limit also minimises fibre rotation and heating effects. The peak load at or before 5% strain is taken as the shear strength.

It has been reported that the result is dependent on the number of layers, or shearing interfaces, in the specimen. This may have technical and harmonisation implications for the Airbus Industries/prEN 6031 version, which is the only version of this test using 1mm thick specimens (cf 2mm in all others). It is recommended that the number of shearing faces (e.g. interfaces) should be kept constant when ply thickness vary from the normal 0.125 mm plies.

Double V notched shear test (ASTM D5379)

This test is frequently called the Isopescu test after one of the early developers, or the ARCAN test when the test plane is loaded in a mixed mode, not in direct shear. Modulus determination is relatively straight forward once the small strain gauges are applied in the centre of the specimen. The measurement of shear strength needs additional care as for some composite materials and for some directions the failure is initiated by localised tensile stresses rather than shear stress. Re-design of the loading jig in the recommended ASTM drawing for this standard has been proposed by National Physical Laboratory (NPL) to reduce the tendency for unsymmetrical loading of the specimen.

Double notch shear test (ASTM D3846, also new work UK programme)

This ASTM test method developed by the plastics committee (D20) is not supported for preparation as an ISO method by the composites committee within ASTM (D30). Research at NPL found that it could be used for more directions in more materials than ILSS, while obtaining a correct Interlaminar shear failures. It is frequently required as 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 a preferred stress distribution, neither rod or tube tests have been standardised. The specimen does exist within the dynamic mechanical properties test methods - torsion pendulum.