Mechanical fatigue of polymers: A new approach to characterize the S-N behavior on the basis of macroscopic crack growth mechanism

This work demonstrates that a continuum-based characterization of mechanical fatigue fracture of polymers, in the absence of thermal effects, is possible on the basis of macroscopic crack growth mechanism. The macroscopic mechanism is that the fractional remaining fatigue life of a given polymer specimen is uniquely related to the fractional size of the remaining uncracked section, which includes the intact craze length, anytime during fatigue. A crack growth functional is proposed and is the used to represent the cyclic fatigue crack advance due to the increasing net-section stress amplitude in fatigue. A crack growth equation, which is consistent with Frost, Dugdale and Weibull's observations of macroscopic crack growth mechanics, is proposed as a base equation. A compact constitutive equation for the stress-life (S-N) fatigue behavior of polymers is then derived. The constitutive equation accurately describes the mechanical S-N fatigue behavior of a large number of polymers under test conditions in which the thermal effects were absent. The effect of temperature is rationalized on the basis of beta relaxation, glass transition and the variation of strength with temperature, depending on the polymer. The constitutive equation is then expanded to create a master equation to include the mean-stress effects, which allowed the prediction of stress-life behavior and endurance limit for any mean stress solely from the S-N behavior of fully reversed fatigue data. It is demonstrated that the present the S-N constitutive equations can quantitatively and accurately describe the polymer fatigue phenomenon. (C) 2016 Elsevier Ltd. All rights reserved.