Fatigue Crack Propagation Behavior under Two-Step Loading in Polycarbonate

Abstract

Fatigue tests were made on pre-cracked specimens of polycarbonate under two-step loading and the effects of stress change on fatigue crack propagation and closure were investigated. Fatigue life was also examined and discussed. The main results obtained are summarized as follows:
(1) Remarkable acceleration of fatigue crack propagation was found to occur at the first cycle after the stress increase. The acceleration factor of crack propagation was much higher than that in metals.
(2) Retardation of fatigue crack propagation was observed when the stress was decreased, and the maximum retardation was found to occur immediately after the stress decrease.
(3) The crack opening ratio U measured at the specimen surface was not affected appreciably by the stress change. This fact suggested that the acceleration and retardation in polycarbonate could not be attributed to the crack closure behavior.
(4) The value of cumulative cycle ratio was smaller than unity in the case of stress increase, but was larger than unity in the case of stress decrease. This can be explained by the crack propagation curves and the acceleration or retardation behavior of fatigue crack propagation.