Multiple surface cracks coalescence mechanism in granite

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Multiple surface cracks coalescence mechanism in granite

 

Author: Yin, Peng
Title: Multiple surface cracks coalescence mechanism in granite
Degree: Ph.D.
Year: 2014
Subject: Granite -- Analysis.
Rock mechanics.
Fracture mechanics.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: xxxiv, 255 p. ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2696090
URI: http://theses.lib.polyu.edu.hk/handle/200/7423
Abstract: This thesis studies experimentally and numerically the crack growth and coalescence mechanism between two parallel pre-existing surface cracks in granite specimen under uniaxial compression. Geometrical parameters were varied, including crack length 2c, bridge angle β (inclination between the inner tips of the two pre-existing surface cracks), crack length 2c, bridge length ratio b/c (bridge length, 2b, divided by crack length), crack angle α (inclination of pre-existing surface crack), and crack depth ratio d/t (crack depth, d, divided by specimen thickness). Different experimental techniques including CCD camera photography, digital speckle correlation method (DSCM), and acoustic emission (AE) technique were used in the present study. This is the first study to classify the nature of cracks by DSCM. In this dissertation, three main contributions have been made: (1) the development of two criteria which are critical strain criterion (γmax/ε1, γmax represents maximum shear strain and ε1 represents maximum principal strain) for surface crack classification and the ratio of AE events of different types (tensile, shear and collapse) for internal cracking classification; (2) providing a fundamental understanding of the nature of crack initiation and propagation in solids containing surface fracture; (3) providing a fundamental understanding of the effect of surface crack geometry on crack coalescence in solids. In total, five crack types are observed on specimen surface including two types of primary cracks: wing crack and anti-wing crack and three types of secondary cracks which initiate from wing crack or anti-wing crack. In addition, petal crack is observed inside the specimen. Petal crack plays an important role in the initiation, propagation and coalescence of pre-existing surface cracks. On the specimen surface, prior to the appearance of all macrocracks, white patches (internal microcracks) appear first before they turn to macrocracks. Anti-wing cracks occur earlier than other types of cracks on specimen surface. Secondary cracks occur at the stage close to the failure of specimen. Both wing cracks and anti-wing cracks are classified as Ts mode "tensile dominant mixed mode", different from the identifications of previous researchers by visual inspection. Anti-wing cracks may change to "TsS" (tensile dominant mixed mode plus shear slippage) mode during their propagation. Secondary cracks are mainly classified as TsS mode. In contrast to the results in 2-D crack coalescence, the coalescence mechanism of two 3-D pre-existing surface cracks involves both the linkage by cracking on the specimen surface and the linkage between petal cracks inside the specimen.
The AE event measurement reveals that owing to the heterogeneity of the granite specimen, the growth paths of the anti-wing cracks may not be the same even from the same pre-existing surface crack. A new criterion based on the ratio of AE events of different types (tensile, shear and collapse) is proposed to identify the nature of cracks inside the rock. Wing crack growth inside the specimen is classified as Mode I crack. Anti-wing crack is classified as Mode III crack, differing from the classification by DSCM (tensile crack). Petal crack is also classified as Mode III crack. The proposed criterion provides a better approach to interpret the nature of cracking inside the specimen. The effects of crack geometric parameters on surface crack coalescence pattern and stresses are studied in the present study. Nine types of crack coalescences patterns are identified. A regime classification for the occurrence of surface crack coalescence in b/c-β space is proposed and can be used as a benchmark-solution for further studies. In numerical simulations, four regions with relative large tensile stress are observed on the surface of the specimen where two regions locate at the regions of wing cracks and other two at the regions of anti-wing cracks. While in 2-D case there are only two relative large tensile stress regions. The growth process of anti-wing crack observed by numerical simulations is from the surface of the specimen to the interior. Both smaller value of homogeneity index m and larger value of σc/σt (compressive strength over tensile strengths) facilitate the occurrence of anti-wing crack.

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