Shear Strength Tests

The shear strength of a material is another very important material property to consider. If a material is to be exploited to its fijll potential then its mechanical behaviour in all planes should be considered. Indeed most materials are subjected to a mixture of forces either during their manufacture or application including shear stresses. Conventional shear strength testing involves the use of a hollow or solid cylindrical bar which is subjected to a torque loading (Figure 1.4). Although this form of testing is used for some brittle specimens it can present difficulties with specimen design thus its use is limited and alternatives have been devised.

Aoki and Sato (1976) studied the mechanical fracture behaviour of hollow and solid blackboard chalk cylinders under hydrostatic pressure when subjected to a torque loading, in order to gain a better understanding of the behaviour of the material when formed by hydrostatic extrusion. By raising the pressure above a critical value they showed distinct difierences in the deformation and fracture characteristics of specimens and were able to determine the point of initiation of crack propagation and its subsequent helical propagation with increase in twist angle.

Taylor et al (1967) on reporting the mechanical properties of reactor graphite examined the influence of fast neutron irradiation on the shear strength and found it to cause an increase. They used a double shear apparatus comprising of an interlocking T and U piece (Figure 1.5), where the test piece was sheared by applying a direct tensile-puU perpendicular to the axis of the specimen. Unfortunately the test configuration was not discussed and it is doubtful whether an accurate measure of the shear strength is in fact obtained particularly as it is possible for misalignment and subsequent bending effects to be introduced adding a tensile component to the test.

Within the forestry industry the measuring of the internal bond strength of particleboard (the tensile strength perpendicular to the plane of the board) is an important quality control tool. In addition, it reveals the quality of the glue bond and hence allows estimates of related properties. The standard test involves the lamination of specimens between steel or hardwood blocks to facilitate the application of a direct tensile-pull. Suchsland (1977) devised an alternative test which still retains the advantages of the original test, namely that failure occurs in the weakest plane and that both thick and thin specimens can be tested. The test which showed good correlation with the standard test.

Figure 1.4 Schematic representation of a shear strength test through the loading of a cylindrical bar.

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was less time-consuming and was simpler to perform and is known as the compression shear test. The centre plane of the particleboard was oriented at 45° to the direction of an applied compressive force by means of a simple cutting and laminating operation (Figure 1.6). Thus planes parallel to the centre plane of the specimen became planes of maximum shear stress. The test showed less variability because there were no stresses generated at the glue-lines between the platens and specimen as is the case with the standard test.

To obtain the ultimate shear strength of a material the loading configuration should produce a uniform pure shear stress state in the section of the specimen under test throughout the elastic and plastic ranges until failure at the section occurs (losipescu, 1967). Such conditions are reported to be met when a symmetrically notched beam specimen is loaded in a special way (Figure 1.7). The introduction of two symmetrical 90° notches creates isostatic lines which cross the section at 45° and hence indicate a state of pure shear. The notches also ensure that failure occurs at this section and by varying the depth of the notches the stress distribution across the section can be made uniform (losipescu, 1967). Rabie (1981) discussed the principles of the test and verified the stresses generated using photo-elastic analysis with Araldite test specimens. He then designed a test rig (Figure 1.8) for measuring the shear strength of plaster beam specimens through the application of a compressive load. Balance weights were used to ensure that the dead weight of the fixture passed through the roots of the notches and hence the central section of the specimen experienced a zero bending moment. A notch depth equal to one quarter that of the beam depth was found to produce the optimum shear stress distribution. When the shear strength values obtained were compared with modulus of rupture strengths the former were found to be lower. However, because of the different volumes of material subjected to the applied stresses and the differing nature of these stresses this was easily explained (Rudnick a/, 1963; Stanley, 1985).

Since the adoption of the losipescu in-plane shear test its use has been widely accepted, particularly for very brittle materials because of the advantages over the torsion test. Walrath and Adams (1984) verified the applicability of the test for composite materials. Through a modification of the test rig they gained several improvements and were able to measure the shear strain of the test specimens. Seerat-un-Nabi et al (1990) used this test to study the shear properties of SiC-fibre-reinforced Pyrex matrix composites. By mounting two torque strain gauges at ±45° to the specimen axis between the notches

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Figure 1.7 Schematic representation of an losipescu (1967) shear test specimen with 90° symmetrically introduced notches. Pi and ? 2 represent the applied

compressive loads and Si and S: the isostatic lines which cross the notched section at 45° indicating a state of pure shear.

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Figure 1.8 Schematic representation of a fixture used to test the shear strength of plaster beam specimens through the application of a compressive load Rabie (1981).

they were able to measure the shear moduli of the materials in addition to the shear strength. Two types of composite weave pattern were investigated to measure the in-plane and interlaminar shear properties. For both matrices the shear moduli were found to be identical whereas in the case of the shear strength both composites exhibited two values. The lower stress was linked to the onset of matrix and interface cracking in and around the notch areas, whereas the higher value represented the ultimate shear strength where a critical damage level had been reached above which all flexural rigidity was lost. This latter value was found to be equivalent to short-beam shear strength measurements made using a three-point bend. Thus the losipescu method provides useful details about the fi'acture mechanisms of composite materials in addition to accurate values of shear strength.