Fracture mechanics theoretical considerations

Linear elastic fracture mechanics (LEFM) describes the behaviour of sharp cracks in linear, perfectly elastic materials. Since polymers are neither linear nor elastic, the utility of the theory may, at first sight, seem doubtful. In fact, the deviations from the theoretical assumptions are such that quite minor modifications to the analysis produce a precise description of crack growth in polymers within the framework of the conventional theory. The considerable resources of the subject may thus be utilised in that testing experience on other materials may be employed, together with the available analytical work. 

A study was made of gas decompression failures in elastomeric seals using a fracture mechanics approach with considerations of gas permeation. An equation is proposed for the tearing energy associated with crack growth from internal gas bubbles in a finite thickness elastic media. The effects of gas pressure, temperature, rate of decompression and mechanical strain were studied for a range of elastomers used in oil and gas sealing applications. A theoretical treatment is presented based on a fracture mechanics criterion for fracture from an internal disc shaped flaw in a thick elastic medium. Permeation theory provides a quantification of the amount of gas available internally to initiate failures. 21 refs. 

However, it is also clear from the discussions in the present chapter that there are many areas in the application of fracture mechanics to the failure of adhesive joints which are far from fully understood. For example, the theoretical problems of dealing with cracks at, or near, bimaterial interfaces and the dependence of the measured values of Gc and Kc upon the joint geometry have yet to be completely understood and modelled. Thus, if the aims of predicting the short- and long-term behaviour of adhesive joints from the behaviour of the bulk adhesive material are to be realized, considerable work remains to be done. 

However, from theoretical considerations of the mechanisms of attack a clearer picture emerges. Recalling Equation 7.8, the adhesive fracture energy, Gic, may be expressed by ... 

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