Activation volume tensor

The ratio QtJf represents the volume of the polymer segment under deformation. The activation volume tensor plays an important role in nonlinear viscoelasticity. The relaxation time takes the form... 

For isotropic glasses, the activation volume tensor has two independent components. These are the bulk activation volume, which is equal to the lattice volume v, and the shear activation volume Q12. The activation volumes in uniaxial tension and compression are found to be [30]... 

In the solid state deformation, the nonlinear viscoelastic effect is most clearly shown in the yield behavior. The activation volume tensor is a key parameter. In addition to the well known dependence of yield stress on temperature and strain rate, the functional relationships between yield, stress field, and physical aging are presented. 

The observed apparent decrease in activation volume (increase in slope) with strain level in the Eyring plots can be explained by a variant of the previous model, proposed recently by Buckley [12], which recognises intrinsic anisotropy of segment diffusion because of local correlation of molecular segments. Arm (B) of the model must then be replaced by an aggregate of anisotropic flow units. The quasi-linear relation between the viscous part of the rate of deformation D and bond stretch deviatoric stress s of the previous model for PET [6], D" = Os , where 0 is a scalar function of the stress invariants then takes a new form for each unit, D = 0s , where 0 becomes a fourth rank fluidity tensor. 

That the number of slip systems to be activated is five can be explained as follows An arbitrary deformation has six independent components of the strain tensor (see section 2.4.2). Because plastic deformation does not change the volume, each one of these components is dependent on the others, and five independent components remain, corresponding to the required five slip systems. 

---