Effect of Stress Rate on Fatigue Failure Characteristics of Shirahama Sandstone

In order to clarify the influence of stress rate on fatigue failure characteristics of Shirahama sandstone, loading and unloading on the rock specimens were repeated between lower (about 0.5MPa) and upper stresses under a constant stress rate. In addition, the behavior of P-wave velocity and the AE occurrence in the process of fatigue failure were investigated.As stress rate is slower, failure at the same upper stress ratio (upper stress/compressive strength at each constant stress rate) occurs at the smaller number of the cycles. Then, the number of the cycles to failure increases exponentially as the upper stress ratio decreases.In the fatigue process, tangent deformation modulus and lateral/axial strain ratio are significantly affected near the upper stress level by stress rate. Also, AE behavior is different with stress rate, but the effect of the stress rate doesn't appear on the behavior of P-wave velocity.The relation between the number of cycles to failure and upper stress ratio differs in stress rate. Then, to predict rock failure in fatigue test, we should utilize the shortest elapsed time to failure estimated from the relation between upper stress and time to failure at various stress rates.