early warning and prediction of sandstone failure by determining crack initiation under compression tests

The international society of rock mechanics (isrm) suggested methods are used traditionally in laboratory testing of rocks to determine the peak strength. however, it has been widely accepted that in low porosity rocks, during compressive loading process there are five distinct stages can be identified: 1) crack closure, 2) elastic region, 3) crack initiation or stable crack growth, 4) crack damage or unstable crack growth and 5) peak strength. crack initiation is considered as the first stage of stress-induced damage in rocks. this important parameter can be used for the estimation of spalling strength in underground natural rock supports such as pillars. despite the importance of crack initiation, there is no mention of it in the isrm suggested methods for determining the strength and deformability of rocks in compression tests. accurate determination of crack initiation in low porosity rocks is still one of the challenging issues in rock mechanics. there are various methods proposed in the literature for identifying crack initiation in laboratory tests. in this review paper, these methods are reviewed, evaluated, and a simple mathematical technique is proposed using six sandstone rock samples to help for better identifying crack initiation stress. the slope of a curve is a sensitive parameter. so small changes in the slope of stress-strain curve can be used as a good indicator of five distinct stages of progressive failure in brittle low porosity rocks. when using other existing methods is difficult and sometimes impossible, this simple technique would be useful to obtain the crack initiation stress threshold.