Control performance of servo-controlled testing machines in compression and creep tests

Abstract

Control performance of a closed-loop servo-controlled testing machine is discussed analytically and numerically. A simple model of the servo-controlled testing machine is proposed and the performance for a compression test is discussed. The control variable in this case is strain or a linear combination of stress and strain. Numerical simulation is also carried out assuming a non-linear rheological model which reproduces the post-failure behaviour of rock; this applies not only to Class I but also to Class II characteristics. The approximate required condition for stability of the system can be expressed by t 0>1000· T v where t 0 and T v are the loading time to the strength failure point and the time constant of the servo-valve, respectively. Control performance in a creep test is also discussed analytically and numerically. via analytical discussion, it is found that the system is stable in the primary and the secondary creep regions, and unstable in the tertiary creep region. In the tertiary creep region, the stress or the force decreases with an increase in strain rate towards the final failure. Deviation from the true creep curve which can be obtained under perfectly constant load is calculated by computer simulation. The error is negligible if the residual life, that is, the time remaining to final failure, is larger than about 10 x T v .