Abstract
In order to investigate microscopically the effects of crystallinity and temperature on the fatigue crack growth behavior of polypropylene, the microfractographs were discussed based on the fatigue crack growth curves (a-N curves).
The results obtained were as follows:
(1) Two kinds of damage zone occurred at the crack tip during the fatigue crack growth process: The one in which the spherulite structure is broken down and the other not so. It was found that the former zone was accompanied with an intrusion on the specimen surface and crazes appeared in the bottom of intrusion at 20°C. However, it was difficult to find the crazes at temperatures below 0°C.
(2) The effects of crystallinity and temperature on the fatigue crack growth behavior were studied by microfractographs for the cases showing three different shapes of macroscopic a-N curves. In the case where the a-N curve showed the step-wise shape, the voids occurred in the transition from the decelerated zone to the accelerated zone of crack growth and the crack grew discontinuously. Furthermore, a different and peculiar fracture surface appeared at the non-propagating zone as well as at the accelerated zone. The specimen in which the stable crack growth hardly took place and the unstable fracture occurred suddenly, showed the flat and brittle fracture surface.
