Typical values of Gc and Kc

From the various methods discussed above values of the fracture energy, Gc, and, for LEFM, the stress intensity factor, Kc may be obtained. 

For example. Fig. 7.9 shows the results from studying crack growth in a simple-extension joint (Fig. 7.7) consisting of a crosslinked styrene-butadiene rubbery adhesive (a non-linear-elastic material) adhering to a rigidly supported poly(ethylene terephthalate) substrate [103]. From Equation 7.44, with ks = 7T, the data is plotted as t/dc versus and the linear relationship which is 

A final example is illustrated in Fig. 7.11 and shows the experimentally determined values of compliance, C, versus crack length, a, for a tapered double cantilever beam joint (Fig. 7.8) consisting of an epoxy adhesive bonding aluminium alloy substrates. As discussed in Table 7.1, this type of joint is designed to produce a constant value of dC/da and this may be readily seen. Further, the joint obeys LEFM and for the adhesive thickness, /Za, used of 0.5 mm the adhesive layer is sufficiently thin so that the experimental value of dC/da agrees well with the theoretical value deduced from Equation 7.49 given in Table 7.1. Obviously, either from the experimental or theoretical values of dClda, the value of Gu may be calculated from Equation 7.4. For those joints which obey LEFM, in the case of cracks in the centre of the adhesive layer, values of Kc may be determined via Equation 7.35 whilst for interfacial cracks Equations 7.36 to 7.39 are more appropriate. 

From the previous discussions it is obviously difficult to state exact values of Gc and Kc without defining the mode of loading, test rate and temperature, etc. Nevertheless, representative values for a range of adhesive joints tested at room temperature and a moderate rate of test are given in Table 7.2, with the aim of imparting to the reader the typical values obtained and their positions relative to other materials. 

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Table 7.2 Typical values of Gc and Kc for various joints and materials ...

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