The Effect of Stress Ratio on the Fatigue Crack Growth in High Polymers

Abstract

The fatigue crack growth rate dc/dN can be represented in the power law of the stress intensity range ΔK as follows:dc/dN=a1(ΔK)mwhere m is the material constant and a1 is considered as a function of ΔK according to Forman et al.'s law. It has been pointed out that a1 and m, which have important physical meanings, are functions of several factors, stress ratio R, frequency, temperature, environment and so on.In investigating the mechanism of fatigue crack growth, it is necessary that the effect of those factors will be systematically studied. Not so many studies have hitherto been made on the fatigue crack growth in high polymers as have been made on those in metals. The mechanism of their fatigue crack growth has been left in the dark in this way.In the present paper, therefore, it is pointed out that the effect of stress ratio on the fatigue crack growth rate will act as a link in the chain of the study on the fatigue of high polymers.The results obtained are summarized as follows:(1) The fatigue crack growth period is strongly influenced by stress ratio.(2) The fatigue crack growth rate follows the relation as follows:dc/dN=a5eαR(ΔK)m/(1-R)KIc-ΔKwhere KIc is the fracture toughness, and a5, α and m are material constants.(3) From the microscopic observation, the fatigue crack tip deformation process at loading is found to be different from that at unloading. This difference will lead to the formation of striation.(4) From the measurement that was made on the surface roughness meter, the striation height is found to be proportional to the striation spacing.