J. Botsis et C. Huang, EXPERIMENTAL AND ANALYTICAL STUDIES IN CRACK INITIATION UNDER FATIGUE.1. EXPERIMENTAL STUDIES, International journal of fracture, 65(3), 1994, pp. 219-234
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.