Effect of Mean Stress on Fatigue Crack Propagation in Fiber Reinforced Polycarbonates

Abstract

A fatigue experiment has been carried out on the glass fiber reinforced polycarbonates under the load condition of combined cyclic and mean stresses to study the effect of mean stress on the crack propagation behavior.
The results obtained are summarized as follows:
(1) It was not possible to clarify which one, the maximum stress (σ_max) or the stress amplitude (σ_a), affected the fatigue crack propagation more. But under a given condition of constant σ_a, the crack propagated faster with increasing mean stress due to an increase in σ_max. In spite of the difference in fiber content, all the materials showed a similar tendency. On the other hand, under a given condition of constant σ_max, the crack propagated slower with increasing mean stress due to a decrease in σ_a.
(2) In the case of change in mean stress under the condition of variable σ_max and constant σ_a, the relation between the crack propagation rate and the stress intensity factor range followed the Forman's formula at low mean stresses, but at high mean stresses the relation follows the formula only when the corrected fracture toughness values were used. In the latter case, the constant C' depended on the mean stress. Moreover, the fracture surfaces were examined macroscopically as well as microscopically. The different tear line patterns were observed, by changing the mean stress, on the macro-fractographs of the nonreinforced material. The change in pattern due to the debonding between fiber and matrix and the plastic flow in the matrix were observed, by changing the mean stress, on the micro-fractographs of the reinforced ones.