Strength of discontinuous reinforced composites: II. Isotropic crystalline polymers

Crystalline polymers are heterogeneous materials consisting of two distinct phases. They may therefore be treated as molecular versions of engineering composite materials. This paper summarizes the physical model which, when used in conjunction with composite theory, yields a complete calculational format for stiffness, expansion strain, and yield strength. The important input parameters to the calculation are the mechanical properties of the individual constituent phases (crystalline and amorphous), the crystallite aspect ratios, the volume fraction crystallinity, allowable failure strains for a continuous crystal system, and a measure of stress concentrations and strength reduction caused by the discontinuous nature of the actual crystalline reinforcing phase. An expression for the strength reduction factor is developed and details of the stiffness and yield strength calculations are presented for high density polyethylene (HDPE). Comparison with experiment for HDPE yields excellent agreement well within the necessary design accuracy. Agreement with experiment in the case of low density polyethylene (LDPE) is not as good, but within expectations considering the degree of theory refinement and the poorer morphological data base for LDPE.