Issue 33

A. Winkler et alii, Frattura ed Integrità Strutturale, 33 (2015) 262-288; DOI: 10.3221/IGF-ESIS.33.32 263 I NTRODUCTION lastics are in terms of engineering history relatively young materials. Compared to the amount of research available for metals, the inventions of pioneers such as Leo Baekland 2 , Wallace Carothers 3 and Stephanie Kwolek 4 have barely had the tip of the proverbial ice berg scraped off. It is therefore no wonder that literature in this field is being updated at a rapid pace. Despite this, most material models for plastics in FEA are created from a phenomenological point of view and even then they are extremely difficult to calibrate. For fatigue, matters are even worse, where attempts to classify fatigue of plastics have not strayed far from modified SN analysis with very mixed results. When working with these types of materials it is important to begin from a point of trying to understand the linguistic origins of the terminology used in technical correspondence. It may at first seem odd to begin the treatment of an engineering field from a humanist point of view, but the benefit will become clear in an organic fashion. The words polymers and plastics are often used incorrectly, leading to inevitable confusion at an early stage in one of the most complex engineering fields. The word polymer is a compound of the two greek words poli (meaning many) and meros (meaning parts). A polymer can therefore simply be thought of as a material consisting of multiple building blocks. The word plastics refers to polymers which can be shaped or moulded in a manufacturing process into a large variety of geometric shapes. To summarize somewhat simplistically: all plastics are polymers, but not all polymers are plastics. Fig. 1 depicts a schematic representation of how ethylene monomers form polyethylene macromolecules through polymerization. Figure 1 : Schematic formation of PolyEthylene (PE) molecules. T HE PHYSICAL AND CHEMICAL STRUCTURE OF POLYMERS he following section provides a basic overview of the physical and chemical structure of polymer materials. We believe that having some understanding of these structures will lead to an improved understanding why some algorithms will be better suited than others. The intent is not to cover this topic in great detail, as it does not fit the scope of this publication. The curious reader is instead referred to sources such as [1–6] for an extensive coverage of polymer chemistry and polymer physics. The properties of a polymer (mechanical, thermal, electrical, chemical to mention just a few) are directly governed by their molecular and supramolecular structure [7, 8]. By this we mean the shape of the molecule, and how the molecules act together as groups. The supramolecular structure can be further refined by distinguishing between its chemical and physical structure. Here the chemical structure pertains to the way the individual molecules are built, whereas the physical structure refers to the way the molecules arrange themselves. The manufacturing process, where the polymerisation occurs (which is a chemical reaction) determines the chemical structure of polymers. During that process, the main valence bonds, or rather covalent bonds serve as connecting constituents between the bond partners. Molecular and supramolecular physical structure is the result of partial valence forces. Both structures have a number of governing parameters assigned to them. For the chemical structure, these are: 2 Velox, Bakelite 3 Polyamide 6, Polyamide 66 4 Kevlar P T