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Since the inception of structural bonding, peel tests have been an integral part of adhesive performance specifications, and have played an important role in the development of adhesives [6, 8]. The emphasis has been on the development of higher peel strengths, often at the expense of environmental resistance. Peel resistance is defined as the average force per unit test specimen width, measured along the bond line that is required to separate progressively two adherend members of a bonded joint. The incessant demand for higher peel stresses has provided an impetus for the development of suitable peel tests. Engineers and designers now have a wide selection of peel tests and associated standards at their disposal.
Peel stresses arise if one or both adherends are flexible, and hence the test methods are suitable for use in determining peel resistance of thin metallic adherends (e.g. thin aluminium alloy sheet). The popularity of these methods can be attributed to ease of use and a physical resemblance to actual in-service debonding problems. Specimen fabrication, testing and data reduction are relatively straightforward. Adhesion is measured by peeling a thin adhering layer from a rigid substrate or from an identical layer. The peel force is a measure of fracture energy, although the data generated has little use in the stress analysis of a bonded joint [7]. Peel tests are normally used to compare adhesives.
Peel strength has been observed to increase with both adherend and adhesive thickness, but decreases with increasing adhesive stiffness [7]. Steel adherends are known to give higher peel strengths than aluminium adherends for an identical thickness. This is because the higher modulus material is less likely to deform out-of-plane, thus reducing peel stresses. A major disadvantage of peel tests is that excessive extension of flexible adherends contributes to failure at the adhesive/adherend interface.
In this section, three popular peel tests will be described (see also Table 1). These are listed below:
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