The study on the fracture of multiple flaw specimens
Abstract
The fracturing processes of a rock-model material with three and sixteen flaws are investigated in uniaxial compression. From the tests, wing and secondary cracks are observed. Wing cracks are tensile cracks that initiate at or near the tips of the flaw and propagate in a stable manner towards the direction of maximum compression. Secondary cracks are shear cracks that initiate from the tips of the flaw; they initially propagate in a stable manner but may become unstable near coalescence. From the tests two different types of secondary cracks are observed: quasi-coplanar and oblique secondary cracks. Oblique secondary cracks are a new type of secondary crack observed in this study. Quasi-coplanar secondary cracks initiate and propagate along a direction parallel or quasi-parallel to the plane of the flaws. Oblique secondary cracks initiate and propagate at an angle with the flaws, in the opposite direction of initiation of the wing crack. The geometry of the flaws determines the stress at which wing and secondary cracks occur. Wing and secondary cracks initiate at higher stresses with higher flaw angles, overlapping ratios and spacing. In three flaw specimens, initiation of the cracks occurs at lower stress in left-stepping than in right stepping geometries. A total of nine different types of coalescence are observed in the three flaw specimens, and four different types in the sixteen flaw specimens. In sixteen flaw specimens, columnar coalescence types are generally predominant. A strong correlation is observed between flaw geometry and type of coalescence, which applies to specimens with two, three, and sixteen flaws. Micro-observations of wing and secondary cracks can be used to distinguish tensile and shear cracks. Tensile cracks are characterized by a narrower damage area (about 10 to 20 μm) than shear cracks (about 100 μm), propagation along the boundary of crystals and across crystals, and unbroken crystals on the surface of the cracks. The characteristics of shear cracks are wider damage area, separation of grains and crystals around the pores, generation of columns by microtensile cracks at an angle with the direction of shearing, breakage of such columns and formation of angular crystals with further shearing, and discontinuous contact between the crack surface due to dilation.
Degree
Ph.D.
Advisors
Bobet, Purdue University.
Subject Area
Civil engineering
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