EXPERIMENTAL AND ANALYTICAL STUDIES IN CRACK INITIATION UNDER FATIGUE.1. EXPERIMENTAL STUDIES

Mechanistic investigations of damage evolution before crack initiation in an amorphous polymer show that damage consists of a core of highly dense crazing and a peripheral less dense zone of crazing. Damage cha racterization is carried out at consecutive configurations of the dama ge zone. Analysis of the kinematics of damage at different times invol ves comparisons of the inertia moments of damage distributions. The re sults indicate that damage evolution between consecutive configuration s can be approximated by a linear transformation of the space variable s. Thus, the process of damage growth can be described by translation and deformation of the damage zone. The growth rates of the damage zon e movements decrease until crack initiation. In all cases, the average densities exhibit a damping type behavior with the number of cycles. The crack initiates within a core zone immediately ahead of the stress concentrator. The experimental results suggest that damage density wi thin the core zone is independent of the loading conditions considered herein. This value is approximately equal to the damage density aroun d the crack tip during slow crack propagation. The crack length at ini tiation is found to increase exponentially with the stress level. A si mple decaying exponential relationship relates the crack initiation ti mes and the applied stress level. This result is consistent with the f racture models based on absolute reaction theories.