Molecular dynamics modeling yield stresses

The deformation or ploughing modes can also be well described for plastic and possibly even brittle fracture systems using modem numerical techniques. As with the elastomeric systems the models basically include geometric terms, such as 6, some load and various parameters such as an interface shear stress but more importantly a relatively accessible bulk deformation or dissipation property of the material. For the case of elastomers, an appropriate viscoelastic loss tangent is sufficient and for a ductile polymer some pressure dependent yield stress. There are many examples in the literature where good correlations have been obtained between a bulk mechanical test and a frictional response. Properly, it has been seen as the domain of others, perhaps polymer scientists, to seek to provide interrelationships between molecular structure and deformation dynamics and the consequent bulk material responses. 

Two principal approaches have been used to model the yield behaviour of polymers. The first approach addresses the temperature and strain-rate dependence of the yield stress in terms of the Eyring equation for thermally activated processes [39]. This approach has been applied to many amorphous and crystalline polymers (see Section 12.5.1) and links have been established with molecular relaxation processes determined by dynamic mechanical and dielectric measurements and with non-linear viscoelastic behaviour determined by creep and stress relaxation. The Eyring approach assumes that the yield process is velocity controlled, i.e. the yield process relates to existing thermally activated processes that are accelerated by the application of the yield stress to the point where the rate of plastic deformation reaches the applied macroscopic strain rate. This approach has... 

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