Anisotropic mechanical behaviour

Since some earlier work based on anisotropic elasticity theory had not been successful in describing the observed mechanical behaviour of NiAl (for an overview see [11]), several studies have addressed dislocation processes on the atomic length scale [6, 7, 8]. Their findings are encouraging for the use of atomistic methods, since they could explain several of the experimental observations. Nevertheless, most of the quantitative data they obtained are somewhat suspicious. For example, the Peierls stresses of the (100) and (111) dislocations are rather similar [6] and far too low to explain the measured yield stresses in hard oriented crystals. 

The simplest mechanical properties are those of homogeneous isotropic and purely elastic materials their mechanical response can be defined by only two constants, e.g. the Young modulus E and the Poisson ratio v. For anisotropic, oriented-amorphous, crystalline and oriented-crystalline materials more constants are required to describe the mechanical behaviour. 

The measured mechanical behaviour is extremely anisotropic and comparatively close to that expected on theoretical grounds. Any further progress in this respect will be marginal. 

During the course of these and related studies, notably those concerned with the temperature dependence of the mechanical anisotropy and the identification of relaxation processes in structural terms, it became apparent that the aggregate model was successful in low density polyethylene because it described effectively the influence of the very anisotropic x-relaxation process on the mechanical behaviour. Stachurski and Ward were even able to extend the aggregate model to deal with the anisotropy of dynamic loss factor. (See Chapter 9 for further discussion.) It was, however, more in the spirit of the original conception of the aggregate model that it would deal with mechanical anisotropy in glassy polymers, where morphology was of secondary importance. 

McLamore, R. Gray, K.E. 1967. The mechanical behaviour of anisotropic sedimentary rocks. Amer. Soc. Meek Engrs. Trans., Series B 62-76. 

Experimental studies of anisotropic mechanical behaviour and their interpretation... 

EXPERIMENTAL STUDIES OF ANISOTROPIC MECHANICAL BEHAVIOUR 147 which leads to a birefringence A , given by... 

It can be seen that the highly oriented polymer is very anisotropic, and that the measured values are quite similar to the predicted values. The mechanical behaviour (Figure 12) shows a very pronounced dependence on temperature and even at the highest draw ratios the a and y relaxations are clearly observed in both extension and shear. The axial modulus approaches the theoretical value for the chain axis, yet there is strong temperature dependence. These observations have been explained along the following lines. 

Anisotropic mechanical behaviour as seen by the naked eye (a) and by stress-strain measurements (b).Three samples are compared, each having the same amount of filler particles, but with different particle distribution as indicated in the figure. In figure (a) the arrows indicate the chain alignment. 

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