As the state-of-the-art equipment is now available in the Laboratory, rigorous research work has been undertaken to find out how the procedure adopted in strength tests should affect the final result. One of the first tests were performed to evaluate the effects of the selected operating of the stiff tester INSTRON Rock Testing System on the actual shape of the stress-strain curve obtained during an uniaxial compression test. The curves were analysed that were obtained during the experiment when the applied force was steadily increased (SO mode), when the applied strain rates were constant: longitudinal strains (DP mode) or hoop strains (DO mode) or when the piston position was varied at the constant speed (PT mode). Experiments have shown that in the case of uniaxial compression tests, only the application of operating mode involving the constant rate of the hoop strain allows for obtaining the complete plot expressing the relationship between stress and longitudinal strain, including its post-critical section.
From the standpoint of test methodology, of particular importance is the Wawersik classification assuming the existence of two types of post-critical behaviour of rock during the uniaxial compression test.
Experiments conducted in the Laboratory have revealed that the sample shape and the conditions during the uniaxial compression test have a major bearing on the actual form of the stress-strain curve.