A novel process for discontinuous deformation in perfectly elastic materials is established in the first part of this two-part thesis. A discontinuous deformation for an elastic material will involve a "jump" in stretch from one state to another. In such a process, it is established that the work per unit volume of the elastomer required to perform the stretch will be greater than the corresponding area under the stress-strain curve for loading and will depend only upon the end states of deformation. The important implication of this analysis is that a cyclic process of discontinuous deformation will involve heat build-up and dissipation due to a difference between the work required to stretch and the work recovered on contraction. Experimental demonstrations indicate a large amount of heat build-up in a rubber belt that is subjected to cyclic deformation involving discontinuities. Elastomers have tremendous strain energy potential. However, in order to utilize this potential, losses associated with discontinuities should be minimized. This research sheds light on how that can be achieved by performing the stretch and the contraction process in as many small steps as possible.
The second part of this thesis describes efforts to understand the phenomenon of environmental stress cracking (ESC) in polycarbonate. Environmental stress crazing or cracking is the failure of inherently ductile polymers under the combined effects of stresses and solvent environments. Polycarbonate is studied under a variety of environmental conditions in this research. Thermodynamics of the system, effects of residual latent energy and orientation in the polymer, tendency of the environmental agent to swell the polymer and the nature of morphological damage are some of the effects characterized. Interestingly, the formation of micro-cracks rather than crazes is observed in the systems studied. Under commensurate exposure conditions over different stress states, it appears that the component of stress normal to the direction of cracks determines the crack patterns. Constant hydrostatic stress, as predicted by the modified Flory-Huggins equation, is not observed to have an influence. Experimental findings indicate that ESC is a stochastic process, influenced by a surface flaw induced mechanism.
