Soft, stiff and servo-controlled testing machines: a review with reference to rock failure

Testing machines have become increasingly sophisticated and versatile since they were first introduced in the early eighteenth century. Two recent advances in testing machine technology are of particular importance for laboratory studies of rock failure: the development of machines with stiff frames and the use of feedback control systems. Force has been traditionally regarded as the independent variable in materials testing but the inevitable outcome of a rock mechanics test with a constant loading rate is violent uncontrolled failure at the peak of the force-displacement curve. Rock failure can be controlled if displacement is regarded as the independent variable and a constant displacement rate can be achieved in a testing machine with a stiff frame or a feedback control system. This review contains a brief history of testing machines and a detailed discussion of the principles involved in controlling rock failure with stiff and servo-controlled testing machines. The development of stiff testing machines is followed from 1935 to the present day and the rock specimen stiffness and testing machine stiffness (factors that are neither fixed nor independent) are discussed in detail. In a closed-loop servo-controlled testing machine, the ability to control rock failure is governed by the fast response time and correct choice of feedback signal. These factors are explained and examples are given of the precise control that can now be achieved in experimental studies of rock failure that were impractical or impossible several years ago.