a b s t r a c tThe question which type of signals can be determinately related to catastrophic rupture in heterogeneous brittle media still remains open. Here we report a specific precursor of catastrophic rupture, i.e. a power-law singularity of responses, based on rock experiments. Our experimental observations show that the singularity with power exponent À b, where b¼0.5170.10 (mean 7 s.d.), appears ahead of catastrophic rupture in some rocks, and the singularity does not appear at all for gradual failure. It is indicated that the power-law singularity can emerge well only close to catastrophic rupture and thus it could serve as a specific warning for catastrophic rupture. To address the potential forewarning of imminent catastrophic rupture, a fitting process based on the data before catastrophic rupture was developed to determine the two unknowns related to the occurrence, catastrophe point U F and the power exponent À b u . It is demonstrated that the power-law singularity appears only in the vicinity of catastrophe, and that the power-law singularity does not occur for gradual failure.

In order to explore a prior warning to catastrophic rupture of heterogeneous media, like rocks, the present study investigates the relationship between surface strain localization and catastrophic rupture. Instrumented observations on the evolution of surface strain field and the catastrophic rupture of a rock under uniaxial compression were carried out. It is found that the evolution of surface strain field displays two phases: at the early stage, the strain field keeps nearly uniform with weak fluctuations increasing slowly; but at the stage prior to catastrophic rupture, a certain accelerating localization develops and a localized zone emerges. Based on the measurements, an analysis was performed with local mean-field approximation. More importantly, it is found that the scale of localized zone is closely related to the catastrophic rupture strain and the rupture strain can be calculated in accord with the local-mean-field model satisfactorily. This provides a possible clue to the forecast of catastrophic rupture.

Evolution of localized damage zone is a key to catastrophic rupture in heterogeneous materials. In the present article, the evolutions of strain fields of rock specimens are investigated experimentally. The observed evolution of fluctuations and autocorrelations of strain fields under uniaxial compression demonstrates that the localization of deformation always appears ahead of catastrophic rupture. In particular, the localization evolves pronouncedly with increasing deformation in the rock experiments. By means of the definition of the zone with high strain rate and likely damage localization, it is found that the size of the localized zone decreases from the sample size at peak load to an eventual value. Actually, the deformation field beyond peak load is bound to suffer bifurcation, namely an elastic unloading part and a continuing but localized damage part will co-exist in series in a specimen. To describe this continuous bifurcation and localization process observed in experiments, a model on continuum mechanics is developed. The model can explain why the decreasing width of localized zone can lead stable deformation to unstable, but it still has not provided the complete equations governing the evolution of the localized zone.