Creep behaviour polymeric materials

The theory outlined above is rigorous only for infinitesimal elastic deformation. Creep of polymeric materials is explicitly concerned with time dependence and implicitly with finite strains and therefore nonlinear behaviour. The nature of the non-linear behaviour is complex and varies not only from material to material but also with direction within a given sample of material, i.e. the non-linearity of behaviour is anisotropic. It is found, therefore, that on a particular definition of strain the behaviour of a sample may appear to be linear in one direction and significantly non-linear in another. Such a phenomenon is demonstrated in results presented below. 

The response of this model to creep, relaxation and recovery situations is the sum of the effects described for the previous two models and is illustrated in Fig. 2.39. It can be seen that although the exponential responses predicted in these models are not a true representation of the complex viscoelastic response of polymeric materials, the overall picture is, for many purposes, an acceptable approximation to the actual behaviour. As more and more elements are added to the model then the simulation becomes better but the mathematics become complex. 

Induced stress is always a factor in structural applications and can result from processing conditions, thermal history, phase transitions, surface degradation, and variations in the expansion coefficient of components in a composite. Since the modulus of a material is not only temperature dependent but time dependent as well, the stress relaxation behaviour of polymers and composites is of great importance to the structural engineer. Stress relaxation is also important to the polymer chemist developing new engineering plastics because relaxation times and moduli are affected by polymer structures and transition temperatures. Therefore, it is essential that polymeric materials that will be subjected to loading stress be characterised for stress relaxation and creep behaviour. 

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