The effects of dolomite-gypsum bonded interfaces on acoustic properties and damage of rock under cyclic bending loads

Abstract

The change in the elastic wave velocities C p, C s1, C s2 and the acoustic quality factor Q of rock beams under cyclic bending loading are considered in the article. The specimen contained bonded dolomite-gypsum boundaries of complex structure. The number of loading cycles and damage parameter w were registered.The experiments were carried out on rock specimens from the Novomoskovsk gypsum field (Tula region, Russia) with bending by a three-point scheme. The upper part of the sample was mainly a layer of dolomite, which had higher strength, lower acoustic loss and higher acoustic quality factor compared to gypsum (which was located mainly in the lower part of the sample). The interface between dolomite and gypsum had a complex spotty texture with the alternation of both minerals. The experiment was carried out in a series of 100 load/unload cycles. The velocities of the longitudinal and transverse elastic waves (along and across the direction of loading), as well as the acoustic Q factor, were measured before and between cycles. The maximum load of the cycle in each subsequent series was increased in comparison with the previous series to find the mode of low cycle fatigue. The elastic wave velocities decreased and the acoustic Q factor increased with an increase in the number of fatigue cycles. Just before destruction, the Q factor showed a sharp decrease associated with the destruction of the matrix. Modeling by the finite element method confirmed the hypothesis that an increase in the quality factor is associated with a weakening of contacts at the boundaries between highquality dolomite and low-quality gypsum. The damage parameter w was estimated as the ratio of the total number of AE event count from the beginning of the experiment to the total number of AE event count at destruction. The accuracy of the regression dependencies of the damage parameter w by the acoustic properties is determined for various amounts.