Influence of rate of change in stress field to the fracture phenomena

在金属的力学试验中，施加的应力通常是单轴的。但对于岩石来说，三主应力通常不为零，因为三轴应力状态可以有效地模拟覆盖层应力。通常的三轴试验是在围压下对岩石施加应力进行的。然而，在大多数实际情况下，岩石的断裂是由于地下开口等自由表面的产生而发生的。然后减小最大应力（拉应力为正）。在本研究中，我们引入了一种新的实验方法——抑制试验。新的试验从静水应力状态开始，轴向应变率保持恒定。围压以不同的压降速率同时降低。岩石试件剧烈断裂，而在常规密闭试验中，岩石试件在较高围压下表现出延性。

In the mechanical testing of metals, the applied stress is usually uniaxial. However, for rocks, three principal stresses are commonly non zero, because the triaxial stress state can effectively simulate overburden stress. The usual triaxial tests are carried out by stressing the rock under confining pressure. However, in most practical situations, the fracture of the rock occurs due to a creation of the free surfaces such as an under-ground openings. Then the maximum stress (tensile stress is taken to be positive) should be decreased. In this research, we introduce an new experimental procedure, depressing test. The new test start from the hydrostatic stress state, where the axial strain rate is kept constant. The confining pressure is decreased at various depressing rate simultaneously. Rock specimens fractured violently, while in usual confined test rock specimen behave ductile under relatively high confining pressure.