In-plane & interlaminar shear properties

Shear testing of composite materials has proven to be one of the most difficult areas of mechanical property testing. While shear modulus measurements are considered accurate, the biggest difficulty is in measuring shear strength. The presence of edges, material coupling, non-pure shear loading, non-linear behaviour, imperfect stress distributions or the presence of normal stresses make shear strength determination questionable. It is still the situation that none of the available test methods is without its inaccuracies. Therefore, application of shear strength data to structural applications should be reviewed carefully on a case-by-case basis to understand the consequence of the potential inaccuracies of the values being used.

The following shear tests are available for in-plane shear measurements:

ASTM D 3518/D 3518M uses a +/-45° laminate loaded in tension. The in-plane shear stress in the gauge section is a function of the average applied tensile stress. While this shear stress state may not be pure, it does mimic the stress state within a structural laminate. Measurement of biaxial strain using either extensometers or strain gauges will allow shear modulus to be calculated. This test method can be used for unidirectional and woven fabric materials but cannot be used for discontinuous composites. The stress-strain curve from this test is highly non-linear and the test methods indicate the strain range for modulus calculation and the shear strain which should be used to identify a shear strength.

The Iosipescu or V-notched shear test, ASTM D 5379M, uses a rectangular beam with symmetrical centrally located V-notches. The beam is loaded by a special fixture applying a shear loading at the V notch. Either in-plane or out-of-plane shear properties may be evaluated, depending upon the orientation of the material co-ordinate system relative to the loading axis. The test is not usually used for multi-directional materials but is successfully applied to discontinuous reinforced materials. The notched specimen is loaded by introducing a relative displacement between two halves of the test fixture.

Strain gauges oriented at +/-45 to the loading axis away from the notches and along the loading axis are used to determine the shear response. The specimen notches influence the shear strain along the loading direction, making the shear distribution more uniform than would be the case without the notches.

ASTM D 4255 is termed the rail shear test and loads the laminate by using long rails to apply the shear load while reducing the normal load. While the standard is restricted to in-plane testing, it is capable of testing for either material shear or multi-directional laminate shear properties. The test method is limited to the determination of the modulus or initial shear stress-strain response, because the method cannot sustain the higher loads to failure in high strength multi-directional laminates. The shear stress state is not uniform in the rail shear test and failures can be identified outside the gauge section, hence reliable strength data cannot always be obtained.

The 10° off-axis shear test represents a simple test method. It uses a straight-sided, rectangular specimen where the fibres are unidirectional and oriented 10° off the loading axis. This specimen is not in a state of pure shear so that the test produces results of generally higher modulus and significantly lower strengths than the other shear test methods. It also is not suitable for shear evaluation of multi-directional laminates.

The short beam shear test is perhaps the most common test for interlaminar shear properties. It has several standards including ASTM D2344 and SACMA SRM 8R-94. The specimen is a short, relatively deep, flat laminate loaded in 3 point bending with a narrow span. The intent is to minimise flexural stresses while maximising in-plane shear stresses. However, the contact stresses induced at the load points greatly interfere with the stress distribution, often leading to failure under the loading nose. This test is still used for interlaminar design allowables for structural design criteria because of lack of availability of other methods. However, the V-notched beam is beginning to be the test method of preference to determine interlaminar failure strength. The short beam strength test should only be used for qualitative testing such as material process development.