The failure lives of rock were observed under constant uniaxial tensile stress, and the results were analyzed on the assumption that the failure process of rock is a stochastic process. When the logarithms of probability of survival were plotted against the failure lives, a curve opening upwards was obtained on P-t diagram. This upwards concave curve on P-t diagram means that the failure process is neither serial nor cumulative, but consists of parallel Poisson's processes of 1st order.
It is suggested that this upwards concave curve is caused from the stochastic dispersion of the rate constant of failure by various factors, rather than the coexistence of a few parallel failure processes with definitely differen trate constants of failure. The experimental errors such as the deviation of loading axis, fluctuation of humidity, etc., and the dispersion of a few macroscopic pre-existing cracks are suggested as the important factors among those affecting failure.
Based on the test results., the probability distribution of the rate constant of failure was graphically analysed, and expressed as a discrete distribution of four rate constants.
On the assumption that the failure process under constant load is not different from that under constant rate of stress, the fluctuation of uniaxial tensile strength of rock was estimated from the probability distribution of failure lives of the same rock under constant tensile stress. The estimated coefficient of variation of uniaxial tensile strength was not much different from the one obtained by the uniaxial tension test.
