UNIAXIAL COMPRESSION TESTING USING A LINEAR COMBINATION OF STRESS AND STRAIN AS THE CONTROL VARIABLE

Abstract A new control method is proposed in which a linear combination of stress and strain, (ϵ − σ/E′), is used as the feedback signal in a closed-loop, servo-controlled testing machine. In this study, uniaxial compression tests of one Class I and seven Class II rocks were conducted by this new method under loading conditions specified as ϵ − σ/ E ′ = C · t , or ϵ = σ/ E ′ + C · t , where C is a constant. The results indicate that failure of Class II rock can be controlled by this method and stress-strain characteristics up to the strength failure point coincide with those obtained by the constant strain rate method. Naturally, Class I rock can also be controlled by this method. With this method, if stress decreases by an amount δσ, strain is decreased by an amount δσ/E′, whereas strain is monotonically increased in the constant strain rate method. This is the reason why a Class II rock which has a positively inclined portion of the stress-strain curve in the post-failure region can be controlled by the new method. The method, by which complete stress-strain curves of Class I and Class II rocks can be obtained reproducibly, is an improvement on the constant strain rate method which is currently standard for rock testing.