Issue 33

A. Winkler et alii, Frattura ed Integrità Strutturale, 33 (2015) 262-288; DOI: 10.3221/IGF-ESIS.33.32 277 Figure 21. Three-dimensional scan of the void in Fig. 14. Fig. 21 shows a three-dimensional scan of the void geometry with units in micrometres. The severity of the void as a stress raiser and its manifestation in the form of reduced structural integrity is evident. The previous section serves to visualize the morphological complexity of plastics, and how this directly governs properties and structural performance. Particularly with respect to fatigue, it is clear that any uniaxial approach to fatigue life prediction applied will meet with limited success. The role of the manufacturing process skin also becomes more tangible and with it, the explanation for bulk initiating fatigue cracks increasingly obvious, especially from a polymer physics point of view. T EMPERATURE part from morphological differences with the way in which fatigue presents itself in plastics as compared to metals, a further significant differentiator with respect to classical materials is temperature. The difference stems from the fact that plastics are poor conductors, while at the same time, they dissipate a relatively large amount of deformation energy into heat. In other words, self-heating becomes an issue. The key behaviour is yet again related to the morphology. Plastics consist of long chained molecules that are strongly intertwined. Applying a deformation to the material will typically incur two distinct responses. Firstly, there exists some quantity of reversible energy storage in the material, which is comparable to the straightening of a coiled spring. This is morphologically equivalent to the stretching of the chains. The reversible energy storage mechanism is on a macroscopic scale interpreted as elasticity, but since the internal structure changes as a result of the deformation, the elastic behaviour is hardly ever linear. Secondly, irreversible energy is being dissipated as a result of the long chains sliding along one another. This is typically internal frictional behaviour, which is comparable to plasticity. However, in plastics this frictional behaviour is strongly rate dependent, and thus on a macroscopic scale we can interpret this behaviour as viscosity. The internal mechanisms acting here are complex, and the behaviour can be construed to be anywhere in between viscoelastic and viscoplastic. Let us clarify the terms in our interpretation, because there are often subject to inconsistent use. We term the behaviour viscoelastic when the material returns to its original state over an arbitrarily long period of time when we remove the load. If the material it does not return to its original state, we call the behaviour viscoplastic. The standard way of modelling of plastics in numerical analysis is often performed with the assumption of viscoelasticity, but the actual behaviour is a non-trivial combination of both viscoelasticity and viscoplasticity. The chains will be somewhat locked in place while being pulled alongside other chains, but at the same time no real anchor exists. So after A