Tensile shear loading test specimens

Fig. 2. Illustrations of forces to which adhesive bonds are subjected, (a) A standard lap shear specimen where the black area shows the adhesive. The adherends are usually 25 mm wide and the lap area is 312.5 mm. The arrows show the direction of the normal apphcation of load, (b) A peel test where the loading configuration, shown by the arrows, is for a 180° peel test, (c) A double cantilever beam test specimen used in the evaluation of the resistance to crack propagation of an adhesive. The normal application of load is shown by the arrows. This load is appHed by a tensile testing machine or other...

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 ... 

The optimized results depicted in Figs 2 and 3 will only be possible if the adhesive connection exhibits a certain stiffness. The load-bearing behaviour of adhesive layers is normally tested in tensile shear with small beech specimens (Fig. 4). The adhesive layer is typically b = 20 mm wide and 10 mm long according to Eurocode 5 [7]. The force (F) is measured in newtons and the shear deformation v in millimetres. 

Figure 5. The requisite load-bearing behaviour of the adhesive layers (left), as tested with small tensile-shear beech specimens (right).

It is very desirable to fabricate a standard test specimen in the dame cycle as the part being bonded. This specimen should be designed for a test method that is indicative of the prime structural loading requirement. For example, if the critical item is normally loaded in tensile shear, the specimen should be of the lap-shear type. 

The standard test for measuring the shear behavior of fabrics is the shear-frame test, also known as trellis-frame test, or the picture-frame test, as shown in Fig. 6.12. In this test, a fabric specimen is clamped with the yams typically directed perpendicular and parallel to the four clamping bars. Shear deformation is developed by fixing one corner and applying a tensile load on the opposing corner. The deformation of the fabric in the shear-frame test is shown in Fig. 6.13. 

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 lap joint test is the most commonly used adhesive test, likely because test specimens are simple to construct and resemble the geometry of many practical joints. While this test is commonly referred to as the lap shear test, this is generally a misnomer, since failure is often more closely related to the induced tensile stresses than to the shear stresses. Further, it is conventional to report the apparent adhesive strength as the load at failure divided by the area of overlap, even though the maximum stress will almost always differ markedly from this average value. Thus, while the lap joint test is commonly performed, the results from this test must be interpreted with caution. Issues associated with the single-lap joint test are discussed in ASTM D4896. 

Fracture tests on adhesive joints are always more complex than tests on bulk adhesive specimens and a G rather than a K approach is invariably followed for their analysis. The adhesive is present as a thin layer, it may be constrained by the presence of nearby substrates and the failure paths may be influenced by the poor adhesion with the substrate. Fracture tests on adhesive joints are commonly conducted in mode I (tensile opening mode), mode II (inplane shear mode), and mixed-mode I/n (combinations of mode I and n). The key to success in all LEFM fi"acture mechanic tests is to ensure that the substrates do not deform plastically during loading. Plastic deformation of the substrates would violate the assumptions of LEFM and invahdate the results. In mode I, the DCB and TDCB test specimens are almost universally employed and these tests have been standardized. Either test maybe used for the determination of Gic- The choice of which to use is likely to depend upon factors such as the toughness of the adhesive, the properties of the substrate material employed, and whether or not crack length... 

The lap-joint shear strength test method is chosen here for weld quality evaluation. The dimensions and weld configuration of the specimen are shown in Figure 1. In order to reduce bending dming the strength test, the weld line is oriented parallel with the tensile loading direction. More information on this technique can be found in reference [8]. 

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