2000 年 49 巻 4 号 p. 440-447
Fatigue crack propagation tests in vulcanized natural rubber sheets were conducted by using narrow, wide plate specimens under the constant displacement condition. The J integral was kept constant during fatigue crack propagation. The fatigue crack propagation rate was expressed as a power function of the J-integral range, and the power exponent was 1.81 under the ratio R=0 of the minimum to maximum displacement. When R is greater than 0.23, the crack propagation rate is lower than that predicted by the power function. This reduction may be caused by the fiber structure of molecules formed at the crack tip which is not relaxed even at the minimum load. Microcracks are formed ahead of the tip of a crack propagating at high rates. Fatigue fracture surfaces are rough and are accompanied by secondary cracks. No fatigue striation corresponding the crack propagation rate was observed on the fatigue fracture surface.