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

在金属的机械测试中，施加的应力通常是单轴的。然而，对于岩石来说，三个主应力通常不为零，因为三轴应力状态可以有效地模拟上覆应力。通常的三轴试验是通过在围压下对岩石施加应力来进行的。然而，在大多数实际情况下，岩石的破裂是由于自由表面（例如地下开口）的产生而发生的。那么最大应力（拉应力取为正）应该减小。在这项研究中，我们引入了一种新的实验程序——抑郁测试。新的测试从静水应力状态开始，其中轴向应变率保持恒定。同时以不同的降压速率降低围压。岩石试件剧烈破裂，而在通常的约束试验中，岩石试件在相对较高的围压下表现出延性。

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.