Microstructure and Fracture of Engineering Polymers and Composites
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MICROSTRUCTURE AND FRACTURE OF ENGINEERING POLYMERS AND COMPOSITES
K. FRIEDRICH Polymer & Composites Group, Technical 2100 Hamburg 90, West-Germany
University
Hamburg-Harburg,
ABSTRACT The fracture properties of engineering polymers and composites are strongly affected by two major areas of influence. The first one covers the microstructural parameters of the material, whereas the second one includes the external testing conditions. In this contribution, it is mainly outlined how area one can determine the fracture characteristics. An introductory section illustrates the variety of microstructural details, for example molecular structure and semicrystalline polymer morphology, and filler related factors, such as volume fraction of reinforcing fibers, their orientation etc. In the following part, effects of these parameters on fracture mechanical properties are discussed. It is distinguished between the fracture behavior of unfilled engineering polymers, of short fiber reinforced, injection molded thermoplastics, and of continuous fiber composite laminates. In the latter group, special emphasis is given to the effect of new, high temperature resistant thermoplastic matrices, for instance PEEK, on the interlaminar fracture energy of the composites.
INTRODUCTION If materials in use as structural components are subjected to high mechanical loadings, besides the demands for high stiffness and strength very often high values of their fracture toughness are required. This is not only valid for the very large group of metallic materials but also for ceramics and engineering polymers as well as their composites. In all these cases the mechanical properties and the failure behavior are strongly influenced by the microstructural parameters of the particular material. In this contribution the correlation between microstructure and fracture toughness of engineering polymers and their fiber reinforced composites will be outlined in more detail.
MICROSTRUCTURE Engineering
Polymers
With respect to the basic structure of engineering polymers, it must be distinguished between the melt-formable thermoplastics and the thermosetting resins. Because of the chemical crosslinking of their molecules, the polymers in the latter group are usually stiffer, harder and more brittle than the thermoplastics. Here, an uncrosslinked state predominates in which the molecules are either randomly arranged (amorphous) or partly ordered next. to each other. In this semicrystalline condition a morphological structure is built up which very often Mat. Res. Soc. Symp. Proc. Vol. 79. 1987 Materials Research Society
358
Fig. 1: Different morphologies in semi-crystalline isotactic polypropylene: (a) rapidly quenched from the melt, hence fine spherulitic, (b) coarse spherulitic due to isothermal crystallization at 1301C [1]
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b)
Fig. 2: (a) Transmission electron micrograph of crazes (C) in PET, modified in toughness by ionomer particles (dark); (b) mechanis
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