Failure locus Subject

It is also important to determine the locus of failure of thin films subjected to many mechanical tests depending on peel,... 

Thus, a visual assessment of the fracture surfaces of a joint which has been subjected to environmental attack does not indicate exactly the locus of failure and techniques such as Auger and X-ray photoelectron spectroscopy are needed to identify precisely the locus of failure. However, it does indicate that we should concentrate our attention on the interfacial regions if we are to understand and prevent such environmental failure of adhesive joints. 

PSAs are usually characterized by their three common properties of tack, peel resistance, and shear resistance, which can be determined by standard laboratory test procedures. Each of these properties is affected by temperature change. Indeed, as already noted, most adhesives exhibit different properties when subjected to temperature, and in the case of PSAs the bond is almost always thermoplastic, softening at elevated temperature and becoming more brittle at lower temperatures. As the temperature increases, the adhesive mass softens and wets the substrate more rapidly and completely, and the tack and peel properties increase. At some point, however, the adhesive has softened to the point that its cohesive strength is less than the substrate adhesion, and the adhesive mass splits, failing cohesively, that is, the locus of failure has changed from adhesion to cohesive within the adhesive. As the temperature continues to increase, the adhesive mass continues to soften, and the tack and peel values continue to go down. 

This type of experiment has been a subject of scientific investigation for some time. The failure behaviour is presented as a locus of breaking strength and the breaking strain, i.e., failure envelope [5]. Examples of the failure envelopes are shown in Figure 2.3a for four different grades of NBR [4]. 

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