Pure shear test-piece

Figure 5.1 Tearing energy vs rate for different test-piece geometries. Trouser test-piece, + pure shear test-piece, o split test-piece.

The tearing energy of a pure shear test-piece is given by Equation (5.6) ... 

Fig. 5.58 Pure shear test piece used in the study of the tearing of elastomers, (a) Specimen before crack propagation (b) after crack propagation. (After Andrews, Chapter 9, in Polymer Science, ed. Jenkins, North-Holland, 1972.)...

A good approximation to pure shear is obtained with a strip test piece strained perpendicular to its length as shown in Figure 8.18. 

The viscoelastic behaviour of rubbers is not linear stress is not proportional to strain, particularly at high strains. The non-linearity is more pronounced in tension or compression than in shear. The result in practice is that dynamic stiffness and moduli are strain dependent and the hysteresis loop will not be a perfect ellipse. If the strain in the test piece is not uniform, it is necessary to apply a shape factor in the same manner as for static tests. This is usually the case in compression and even in shear there may be bending in addition to pure shear. Relationships for shear, compression and tension taking these factors into account have been given by Payne3 and Davey and Payne4 but, because the relationships between dynamic stiffness and the basic moduli may be complex and only approximate, it may be preferable for many engineering applications to work in stiffness, particularly if products are tested. 

To enable the determination of almost pure strength values for the adhesive layer, the parameters eccentric application of load and adherend extension/ deformation must be eliminated. This is the case in the test piece geometry depicted in Figure 10.4 according to the standard ISO 11003-2 Shear testing method for thick adherends . 

The stress distribution is not even, with this test piece, and the action is not pure tension but involves peel and shear forces. The test was investigated by Painter [11] who showed that the stress is concentrated at the tips of the cones. He found that failure occurred at the interface rather than in the rubber, and the measured strengths were lower than with a plain disc test piece of similar diameter, more in line with the results of peel tests. It is probable that the result is sensitive to geometry, particularly the cone... 

Figure 5.2 DUferent types of tear test-pieee geometry (a) trouser, (b) pure shear, (c) angle and (d) split tear test-pieces.

It reduces the problems of forking as experienced with other common test-pieces such as parallel tensile strip, pure shear and trouser tear. 

The drop in the impact strength between 196 K and 24°K of the weld metal was accompanied by a change in the mode of fracture. A small area of cleavage developed at the 196 to 78 K range and increased to an area approximately 1/3 of the fracture surface at 24 K, However, there was still some plastic deformation at 24 K, All the welded specimens separated into two pieces. Figures 18 and 19 of the welded impact specimens tested at 300 and 20°K, respectively, show that fracture occurred within the grains at both temperatures, The fracture of the parent specimen appeared to be pure shear down to 24 K. The specimens did not break... 

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