Composite materials modelling with defects

Heat conductivity of composite materials are severely and adversely affected by structural defects in the material. These defects are due to voids, uneven distribution of filler, agglomerates of some materials, unwetted particles, etc. Figure 15.18 shows the effect of filler concentration on thermal conductivity of polyethylene. Graphite, which is a heat conductive material, increases conductivity at a substantially lower concentration than does quartz. These data agree with the theoretical predictions of model. Figure 15.19 shows the effect of volume content and aspect ratio of carbon fiber on thermal conductivity. This figure should be compared with Figure 15.17 to see that, unlike electric conductivity which does depend on the aspect ratio of the carbon fiber, the thermal conductivity is only dependent on fiber concentration and increases as it increases. 

This paper is concerned with the modelling of defects in electromagnetlcally coupled elastic materials using the concept of material multipoles, in particular of the Induced type. The problem of eddy current flaw testing for elastic solids and the problem of the mechanical behaviour of magnetoelastic composites are dealt with in detail. 

Inorganic phases with broad composition ranges are called grossly nonstoichiometric phases, and considerable effort has been pnt into the clarification ofthe structure ofthese materials. The initial model for such materials is to suppose that they contain high populations of vacancies or interstitials, to make up the compositional imbalance. A few examples of this approach, from among the many in the literature, are chosen as illustrative. It is possible that the examples cited will be shown to have more complex defect stmctures in the future. 

In the simplest case, the structure of a material is represented or replaced by the model-like system of a matrix with embedded particles or grains. For such composites the microstnictural fields are assumed to be homogeneous, whereas for FGMs they are heterogeneous. Due to the gradients in FGMs, the "normal" approximations and models, used for traditional composites, are not directly applicable to FGM. The situation becomes even more complicated, when an FGM has gradients on several levels, i.e. macro-, micro- and nano-scale, where defects such as vacancies and dislocations start to play an important role in the transfer processes and mechanical behaviour of the specimen. 

Open-pore microcellular aluminium foams can be produced by a process known as replication . This consists in infiltration of NaCl powder preforms by a melt, which is then solidified to form a composite. The NaCl is subsequently leached out with water, to leave a network of open pores, of volume fraction roughly varying between 65 and 90% [15], The foams can be produced to feature good microstructural homogeneity over a comparatively wide range of metal alloy compositions, pore size and component shape. They furthermore serve as attractive model materials for the investigation of microstructure/property relations in metal foams because of their macroscopically uniform and fine-scale microstructure, and because the metal making the foam can be varied with relatively wide latitude and produced free of internal defects. 

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