Experimental Research on Crack Propagation in Specimens with Open and Closed Crack Under Uniaxial Loading

Authors
A. Ghazvinian
P. Norouzi
Tarbiat Modares University
Abstract
An attempt in this paper has been made to experimentally investigate the behavior of rock like pre-cracked samples under uniaxial loading. To reach that aim, several physical models made up mixture of cement, gypsum and water with 5×10×20 cm of dimensions which contained open and closed joints with rough and smooth surface. Open and closed joints with rough and smooth surfaces were located in the center of samples. Open and closed joints with 4, 5 and 6 cm of lengths and 30, 45 and 60 degree of angles relative to loading direction were located in the center of samples. Saw cut profiles were used to make a roughness in the specimens. Experimentally results illustrate that depend on length and angle of closed or open joints, two types of cracks (wing and secondary cracks) propagated from the tips of pre-existing joints. Secondary cracks propagated after than wing cracks. With increasing internal stress due to loading, tensile stress concentration were happened earlier than shear stress concentration at the tips of pre-existing open cracks. Propagation of cracks from the tips of joint proved stress concentration was larger than another part of joints. In fact stress concentration in the central part of joint is more than joint tips. Location of crack propagation changed into the central part of joint with decreasing of closed joints angle relative to horizontal direction so stress concentration in the center more than another part of closed joints. Crack initiation stress depends on length, dip and closed or open state of joints. Wing crack initiation stress proportional to joint angle relative to loading axis and inversely proportional to joint length. In the Similar circumstances Initiation stress for closed joints more than open joints. In this research, effective parameters on secondary crack propagation were investigated. The experimental results reveal that secondary crack propagation depends on joint dip and distance between crack tips and sample sides. If minimum distance between crack tips and sample sides called (s) and width sample called (b) then for the specimens with 30 degree of dip angle and ratio secondary cracks propagated from crack tips to sample sides.

Keywords

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A. Ghazvinian, M. J. Azinfar and P. Norozi, 2012 “Mechanical response of discontinuities of different joint wall contact strengths”, Journal of Arabian Journal of Geosciences, under Press
Modares Civil Engineering journal
Volume 13, Issue 1
March and April 2013
Pages 75-87