Decohesion models

The cohesive surface description presented here has some similarities to the thermal decohesion model of Leevers [56], which is based on a modified strip model to account for thermal effects, but a constant craze stress is assumed. Leevers focuses on dynamic fracture. The thermal decohesion model assumes that heat generated during the widening of the strip diffuses into the surrounding bulk and that decohesion happens when the melt temperature is reached over a critical length. This critical length is identified as the molecular chain contour. 

Because the events which it deals with are transient and relatively inaccessible, it is very difficult to estimate how realistic the adiabatic decohesion model is. Ultimately it can only be judged by how well it succeeds in predicting high-rate decohesion and fracture behaviour. The new numerical model offers a flexible method for predicting decohesion under a wide range of cohesive surface displacement vs. time histories — of which the present situation represents the simplest possible — and for a variety of thermo-mechanical material properties. 

Thus, invoking Eqn. (1) shows that the application of die thermal decohesion model to... 

Other materials for which the thermal decohesion model appears to work well include polyamide 6 and unplasticised polyfvinyl chloride) — surprisingly, in the latter case, since this polymer does not have a very well-defined melting point. 

For a polyoxymethylene (POM), on the other hand, impact data (Fig. 3) do not conform to the thermd decohesion model at all. The minimum predicted impact fracture resistance of this material is f/D,min = 2.21 kJ m"2 at 23°C, but most impact fracture toughness results (Fig. 3) are at least 50% higher than this and there is no sign of a region of -2/3 power impact speed dependence, l ere is a further increase in at displacement rates far too low to be considered as impact or for die thermal decohesion mechanism to be viable. Viscoelastic crack blunting is a much more likely explanation. It seems most likely that the craze mechanics assumed by the thermal decohesion model simply do not apply to POM. 

The investigation concluded that it was the first rather than the second explanation which accounted for the fracture events. However, here we consider only the issue of geometry dependence in a single material. Gq is always to be calculated using Eqn. (1) the question is whether it is constant and, if not, whether its geometry dependence can be accounted for using the thermal decohesion model. 

The results are shown in Fig. 4. Clearly for MDPE they favour correlation on the basis of the thermal decohesion model rather than on the basis of a unique Gc value. 

The friermal decohesion model for impact and dynamic fracture in thermoplastics asserts that fracture resistance is not constant, and is strongly supported by experimental data. It also emphasises that the nature of resistance to impact britfle fracture is profoundly different from that to other brittle fracture modes. The model can be used, however, to construct an alternative scheme for transferring data between impact configurations. This scheme has been demonstrated for a practical impact failure problem. 

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