抄録
Fatigue crack propagation behavior under single and multiple overloading conditions in polycarbonate was studied fractographically. The results obtained are summarized as follows:
(1) The fracture surface of the crack extension accelerated by a single peak overload consists of two different regions, one contains many fine tearing lines oriented parallel to the crack propagation direction, and the other contains many voids. The former is similar to that found in the striation, and the latter is similar to that found in the discontinuous growth band.
(2) The amount of crack extension accelerated by the overload is almost constant inside the specimen, but decreases markedly near the boundary between the interior fracture surface and the shear lips formed at the specimen surface layers.
(3) The acceleration factor of crack propagation due to the overload is dominantly dependent on the ratio of peak to baseline stress, and is not affected appreciably by the crack propagation history.
(4) The degree of retardation of crack propagation after a single peak overload shows the dependence on baseline stress, which can be explained from the viewpoint of crack propagation mechanisms.
(5) The width of characteristic retardation region which is produced after the multiple overloads is about equal to the size of plastic zone at the crack tip produced by the overloads.
