Anisotropic mechanical behaviour orientation

An oriented polymer is in the strictest terms an anisotropic non-linearly viscoelastic material. A comprehensive understanding of anisotropic mechanical behaviour is therefore a very considerable task. In this chapter, we will restrict the discussion to cases where the... 

The fibre reinforced concrete we studied presents a partially oriented fibre structure which gives it an 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. 

Because of such orientation the products may be anisotropic in behaviour, with mechanical properties differing if measured in different directions (tensile strength will be higher in the direction of orientation, and impact strength also will be affected—fracture can take place more easily parallel to the direction of orientation). 

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. 

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. 

In partially crystalline polymers the creep behaviour of the oriented material varies systematically with structure. Anisotropic creep studies on oriented materuds, whilst considerably more complicated, can more readily lead to understanding of deformation mechanisms than do creep studies in isotropic materials. 

Although the anisotropy of conductivity in metallic oriented-]CH) or PPV-H2SO4 samples is nearly 100, the behaviour of the MC is identical whether the current is parallel or perpendicular to the chain axis. This suggests that high quality oriented conducting polymers behave as anisotropic three-dimensional systems in which the charge transport mechanism is nearly identical in both parallel and perpendicular directions to the chain axis. 

Even though liquid crystals are fluids, the fact that orientational order exists ensures that all directions in the fluid are not equivalent. This has a profound effect on all the properties of the phase, producing a complex response to external factors such as electric fields and mechanical distortions. Yet it is this combination of factors, namely the flow properties of fluids and the anisotropic behaviour normally absent in fluids, that makes the behaviour of liquid crystals both intrinsically interesting and ripe for technical applications. 

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