Issue 33

A. Winkler et alii, Frattura ed Integrità Strutturale, 33 (2015) 262-288; DOI: 10.3221/IGF-ESIS.33.32 267 Figure 6: Composition of a partially crystalline structure for the example of PE [23]. C ORRELATION BETWEEN STRUCTURE AND PROPERTIES aving briefly reviewed the morphology of plastics, it becomes evident that concepts of integral mechanical properties, for a type of plastic do not apply. One can therefore not give an answer to the question “what is the isotropic Young’s modulus of PE” directly. Instead, it becomes necessary to think about plastics as multi- component materials on a constitutive level. This is the reason for which nonlinear, multilayer, multi-scale material models have proven to be much more successful at numerically reproducing force-deformation behaviour on a time scale from actual physical tests than conventional material models. The dependency of mechanical properties of plastics on its chemical and physical structure has been investigated, recorded and confirmed by various authors, see e.g. [7, 10, 24-26]. In order to visually quantify the correlation between structure and property, three distinctive diagrams are presented. The first (see Fig. 7) depicts the dependency of the UTS for PS (Polystyrene) on the orientation. The data was recorded for different densities and energy elastic residual stresses. At high degrees of orientation, it is possible to achieve a near 100% increase of the UTS. The second diagram (see Fig. 8) depicts the secant modulus for injection moulded PP as a function of the surface layer thickness fraction. The surface layer thickness is influenced by the injection moulding process. A large increase in the measured quantity can be observed with increasing surface layer thickness, which in turn can be traced back to high molecular orientation in the surface layer. The third and final diagram (see Fig. 1) shows the UTS as a function of the density for PS, this time for fixed degrees of orientation and no residual stress in the test specimens. In contrast to Fig. 6, an optimum combination of orientation and density can be observed, after which there is no apparent further increase to be gained. At this stage we can conclude that regardless of amorphous or partially crystalline states, the morphology directly influences the properties and thus needs to be accounted for in any form of investigation, phenomenological, analytical or numerical. H