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STRESSES FOR CRACK CLOSURE AND ITS APPLICATION TO ASSESSING STRESS-INDUCED MICROCRACK DAMAGE

PENG Jun;CAI Ming;RONG Guan;ZHOU Chuangbing;ZHAO Xingguang;State Key Laboratory of Water Resources and Hydropower Engineering Science,Wuhan University;School of Engineering,Laurentian University;School of Civil Engineering and Architecture,Nanchang University;Beijing Research Institute of Uranium Geology;  
Several important stress thresholds,termed as the crack closure stress,the crack initiation stress,the crack damage stress and the peak stress,can be identified from the progressive failure process of brittle rocks. The crack closure stress and its relationship with the stress-induced microcrack damage were studied. Firstly,various methods that have been proposed for identifying the crack closure stress were reviewed,and a new method,called as axial strain response(ASR) method,was proposed. An attractive feature of the proposed method is that it does not require subjective interpretation from the user and can be easily programmed. Secondly, the method was verified using the test data from uniaxial and triaxial compression tests. The crack closure stress determined from the ASR method was comparable with that from other methods in unconfined and confined conditions, which validated the proposed ASR method. Finally, the crack closure stress was used to evaluate the stress-induced microcrack damage of three types of rock from Forsmark and Oskarshamn sites,Sweden. By correlating the crack closure stress with the sampling depth,it was found that the crack closure stresses varied with the sampling depth.At the shallow depth,the crack closure stress increases roughly linearly with depth increasing. However,at greater depth,the crack closure stress is more or less constant. A linear relationship between the crack closure stress and thedifference of in-situ stress was also observed,indicating that the magnitude of the difference of in-situ stress may be a key factor that dictated the amount of stress-induced microcracks in the rock.
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