Abstract
A testing method (membranous pressure method) for investigating fatigue strength under equi-biaxial stress/strain condition has been developed. In this method, the failure of the specimen was defined as the number of cycles when a crack penetrated a specimen. The strain range was largest at the center of the specimen and decreased with the distance from the center and crack grew to the edge of the specimen even if it was initiated at the center of the specimen. It might affect the number of cycles to failure because the strain range measured at the center of the specimen was used for evaluation. In this study, the influence of theses characteristics of the membranous pressure method was evaluated by investigating the crack initiation and growth behavior during the fatigue test. Periodic penetrate inspections were made by interrupting the test and the change in a crack size was identified. It was shown that, although the crack length at the failure was about 3 mm, it has little influence on the number of cycles to failure. It was concluded that the number of cycles to failure obtained by the membranous pressure method can be compared with those obtained by a general uni-axial fatigue test without correction.
