Pure-shear test specimen

Figure 7.7 Adhesive joint geometries used for evaluating Gc for flexible joints, (a) Peel test specimen before and after peeling by an eunount, a. (b) Simple-extension test specimen, (c) Pure shear test specimen, (d) Blister test specimen.

Weissberg V, Arcan M (1988) A uniform pure shear testing specimen for adhesive characterisation. In ... 

Shear strength is measured via a simple single overlap shear specimen of standard dimensions (Fig. 9). In contrast to its simple appearance, the forces in a thin-adherend shear specimen can be quite complex due to the inherent offset loading of the specimen and subsequent bending in the substrates. The single overlap shear test is anything but a pure shear test, but the configuration is easy to manufacture, simple to test and is firmly entrenched in the industry as a primary examination technique for materials qualifications, inspection and process control. 

Pure shear tests are often performed with torsional bars or flexural tests with very close spans, or using grooved tensile specimens (Fig. 12.2a). They provide the shear yielding stress, ry, which can be related to ay using yielding criteria (see below). 

Another alternative is the plane strain compression test, shown in Figure 14.5c. The advantage displayed by this experiment is that the area of the specimen remains constant over the test and therefore = a . This test can be classified as a pure shear test as only two of the three sample dimensions are changed. 

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.)...

FIGURE 25.6 Examples of variable ampUtude fatigue crack growth test signals applied to pure shear specimens to investigate the effects of (a) load severity, (b) load sequence, (c) R-ratio, and (d) dwell periods on crack growth rates. A, B, and C denote peak strain levels. 

Apart from the short beam shear test, which measures the interlaminar shear properties, many different specimen geometry and loading configurations are available in the literature for the translaminar or in-plane strength measurements. These include the losipescu shear test, the 45°]5 tensile test, the [10°] off-axis tensile test, the rail-shear tests, the cross-beam sandwich test and the thin-walled tube torsion test. Since the state of shear stress in the test areas of the specimens is seldom pure or uniform in most of these techniques, the results obtained are likely to be inconsistent. In addition to the above shear tests, the transverse tension test is another simple popular method to assess the bond quality of bulk composites. Some of these methods are more widely used than others due to their simplicity in specimen preparation and data reduction methodology. 

This test has an inherent problem associated with the stress concentration and the non-linear plastic deformation induced by the loading nose of small diameter. This is schematically illustrated in Fig 3.17, where the effects of stress concentration in a thin specimen are compared with those in a thick specimen. Both specimens have the same span-to-depth ratio (SDR). The stress state is much more complex than the pure shear stress state predicted by the simple beam theory (Berg et al., 1972 ... 

In a majority of cases, a body under stress experiences neither pure shear nor pure dilatation. Generally, a mixture of both occurs. Such a situation is exemplified by uniaxial loading which, of course, may be tensile or compressive. Here a test specimen is loaded axially resulting in a change in length, AL. The axial strain, e, is related to the applied stress in an elastic deformation by Hooke s law ... 

One way of monitoring the behaviour of a material in shear is with a torsional test. The method has the advantage that, provided a circular specimen is used, a condition of pure shear can be achieved. However, torsion tests can be relatively difficult to perform unless specimens can be machined accurately to avoid warping effects. The alternative is to test a prismatic specimen in a shear box of the form outlined in Fig. 2.15. 

Pure shear stresses are those which are imposed parallel to the bond and in its plane (Figure 11.1). Single-lap shear specimens do not represent pure shear, but are practical and relatively simple to prepare. They also provide reproducible, usable results. The preparation of this specimen and method of testing are described fully in ASTM D1002-01. Two types of panels for preparing multiple specimens are described. ... 

Pre-strain was included in modulated stress testing of rubber and the dimensions of the pre-strained specimens used in calculation of the loss modulus. The loss modulus was independent of pre-strain for filled and unfilled rubbers. A test specimen geometry was chosen where pure shear could be superimposed with a small strain imparted with a shear oscillation. Again, loss modulus was mostly independent of pre-strain for filled and unfilled rubbers, including those filled with carbon black. The results enable understanding of energy dissipation mechanisms in rubber composites. ... 

The two-rail shear test is only suitable for the determination of shear moduli. The three-rail shear does give a better approximation to pure shear as the compressive load is applied parallel to the clamped edges of the specimen. 

The losipescu test (Figures 17.48 and 17.49) has two opposing V-notches to impose pure shear in the middle of the specimen (ASTM D 5379) and gives results that compare well with the torsion tube method [64,66]. 

In the method proposed in 1967 by losipescu [3], a state of uniform pure shear can be achieved within the test section of the specimen with the geometry defined in Figure 2. The value of the shear strength is simply equal to the ratio of the maximum force by the cross-sectional area between the two notch tips. Fibres can be aligned either in the direction or perpendicular to the direction of the applied force. Results obtained from this test are generally well reproducible whatever the nature of the fibres and the matrices. 

Other less well-known types of nonlinearities include interaction and intermode . In the former, stress-strain response for a fundamental load component (e.g. shear) in a multi-axial stress state is not equivalent to the stress-strain response in simple one component load test (e.g. simple shear). For example. Fig. 10.3 shows that the stress-strain curve under pure shear loading of a composite specimen varies considerably from the shear stress-strain curve obtained from an off-axis specimen. In this type of test, a unidirectional laminate is tested in uniaxial tension where the fiber axis runs 15° to the tensile loading axis. A 90° strain gage rosette is applied to the specimen oriented to the fiber direction and normal to the fiber direction and thus obtain the strain components in the fiber coordinate system. Using simple coordinate transformations, the shear response of the unidirectional composite can be found (Daniel, 1993, Hyer, 1998). At small strains in the linear range, the shear response from the two tests coincide. 

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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