Adherends, fracture mechanics

Fracture mechanics (qv) tests are typically used for stmctural adhesives. Thus, tests such as the double cantilever beam test (Fig. 2c), in which two thick adherends joined by an adhesive are broken by cleavage, provide information relating to stmctural flaws. Results can be reported in a number of ways. The most typical uses a quantity known as the strain energy release rate, given in energy per unit area. 

The three principal forces to which adhesive bonds are subjected are a shear force in which one adherend is forced past the other, peeling in which at least one of the adherends is flexible enough to be bent away from the adhesive bond, and cleavage force. The cleavage force is very similar to the peeling force, but the former applies when the adherends are nondeformable and the latter when the adherends are deformable. Appropriate mechanical testing of these forces are used. Fracture mechanics tests are also typically used for structural adhesives. 

The results above suggest that it may be possible to apply fracture mechanics data to determine failure loads of more complex structures, provided that (i) the adhesives used are not too ductile, (ii) bondline thickness is known and controlled, (iii) non-linear behaviour due to adherend and interface damage is limited, and (iv) the specimens employed to determine... 

Adherend fracture Failure of a bonded joint under mechanical stress in the adherend material, thus, outside the adhesive layer. Indicates that the bond strength is higher than the adherend strength. 

In the following section, fracture mechanics methods will be discussed for adhesive bonds consisting of rigid adherends, since these are most common in structural adhesive applications. 

Clearly, the peel strength is not a fundamental property for an adhesive. The value of force per unit width required to initiate or sustain peel is not only a function of the adhesive type, but also depends on the particular test method, rate of loading, thickness and stiffness of the adherend(s) and adhesive as well as other factors. Thus, peel tests generally do not yield results that may be used in quantitative design. This does not imply, however, that the peel test is not a useful test. Peel tests provide quantitative comparisons between different adhesive systems, insight into rate and temperature effects, etc. Additionally, peel tests can be used to provide fracture mechanics information as will be discussed in the next section. In the author s opinion, the latter aspect of peel tests has been perhaps most adroitly exploited by Gent and Hamed [18-20] who used peel tests in conjunction with fracture mechanics to obtain insights into time-temperature effects, the role of plasticity, and many other aspects of adhesive fracture. 

The analytical methods of fracture mechanics (both cohesive and adhesive) are described in a number of references [21-24] and will not be repeated here. However, a brief outline of one simple approach provides some insight into the concepts, principles, and methodologies involved for the reader who is not familiar with fracture mechanics. In the previous discussion of peel tests, it was noted that Gent and Hamed [ 18-20] had performed some extremely informative fracture mechanics tests using peel specimens. We consider a simplified fracture mechanic analysis of the 90° peel test shown in Fig. 17. Here we assume that the substrate is rigid and the peel adherend is very flexible and perfectly elastic. The stress distribution in the vicinity of the 90° bend is complex and difficult to determine. If the material is perfectly elastic, however, this stress distribution is... 

The discussion of the previous sections has been based on the assumptions of linear-elastic fracture mechanics. This requires that the adherends do not deform plastically, and that the deformation of the adhesive layer does not contribute significantly to the overall deformation of the system. Plastic deformation of the... 

Linear-elastic fracture mechanics can only be used to characterize fracture of an adhesive joint when the two parameters EEa/d h and EEo/o. h (in mode-I) are sufficiently small that linear-elastic deformations dominate the behavior of the Joint. The effect of the parameter EEo/o h was discussed in the previous section. In this section, the effects of large-scale plastic deformation in the adherends will be discussed. 

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