Fatigue Characteristics of Inada Granite under Confining Pressure.

Cyclic loading tests on Inada Granite were carried out under 0, 5 and 25 MPa of confining pressure to examine the next two effects: the effect of confining pressure on the characteristics of strength and deformation; and the effect of stress amplitude on fatigue life. The main results are as follows.1) The fatigue strength rises as the confining pressure is increased. However, the rate of the increase of the fatigue life Nf to the decrease of the maximum differential stress σDmax is almost identical despite of the different level of confining pressure. A same curve can represent the relationship between σDcσDmax (difference between static strength σDc and σDmax) and log Nf under the two levels of confining pressure (5, 25 MPa). That is, this curve can be used to predict the fatigue strength from the static strength.2) Three stages, namely. transient, steady and tertiary stage, are observed in fatigue deformation. Under the 25 MPa of confining pressure, the transient stage is shorter and the steady stage is relatively longer. As the confining pressure is increased, dilatant strain is restrained and the rate of the increase of volumatric strain per one cycle decreases.3) When the maximum differential stress is fixed at a constant value, the fatigue life increases as the stress amplitude decreases. This tendency becomes more evident as the value of the maximum differential stress becomes lower.4) When the maximum differential stress in fatigue tests and creep tests are the same, time to failure in fatigue is shorter than that in creep in the range of the maximum differential stress lower than a certain point.