Failure mechanism of Yuen Long marbles using direct observation

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Failure mechanism of Yuen Long marbles using direct observation


Author: Lam, Wai-tong
Title: Failure mechanism of Yuen Long marbles using direct observation
Degree: M.Sc.
Year: 2000
Subject: Marble
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Dept. of Civil and Structural Engineering
Pages: 1 v. (various pagings) : ill. ; 30 cm
Language: English
InnoPac Record:
Abstract: Brittle materials, such as rock may contain numerous micro-flaws (cracks, pores or weak inclusions). When they are loaded in compression, cracks may nucleate from these flaws. These cracks then continue to grow with increasing axial compression. Their propagation and interaction may lead to the collapse of the material. The mechanics of compressive brittle fracture is more complex than tensile fracture and is less well understood. A lot of observations had been done on the failure process of real rock specimen using indirect methods. For example, Scholz [1972] conducted a series of tests to detect the micro-fracture activities within the rock specimens using the acoustic emission under the UC test. Tapponnier and Brace [1976] loaded the rock specimen to a different stress level than cut the rock specimen into a thin section for microscopic observation. However, these methods can not provide full observation on the failure process of rock. Therefore, the purpose of this study is to investigate the failure process of rock using direct observation method under compression load on Yuen Long marbles (YLM). Most of the YLM consists of brown or white color impurity bands (IB), which are the larger sizes of calcite. A small compression machine, which can be rotated during compression, was designed and manufactured in our Department in order to incorporate to a stereomicroscope for direct observation. The test specimens are prismatic with the size of 10mm x 10mm x 24 mm. The entire process of crack initiation, propagation and final failure mode was observed from the TV through a CCTV system of the microscope and was recorded by a video recorder. Based on the experimental studies, it was observed that once the stress release the color of the specimen image becomes light. In general, there are three types of failure mode being observed in YLM. Type 1: Axial tensile split failure. This occurs when tensile crack initiates at one end of the specimen and along the grain boundary, then propagate with the increasing loading in the direction parallel to the axial load. The growth rate of crack slows down when it is close to the other end of the specimen due to the end effect of the specimen. At this moment, the axial load can still be increased until the specimen splits into two columns. Type 2: Combination failure ( tensile crack and shear sliding ). This occurs in those specimens containing impurity band ( IB ). Tensile cracks initiate at the IB or from one end of the specimen first and propagate horizontally. When the growth tensile cracks is close to the ends of the specimen, a sliding occurs along the IB and the specimen fails finally. For crack initiates from the IB, it is observed that several microcracks initiate at an angle of 60o to the IB and the color of IB image become brighter until it slides. Type 3: Mixed mode failure ( tensile crack and shear crack ) occurs in those specimens containing the discontinuous IB, where a shear crack induces between the intact portion of the IB. With this direct observation, the failure process can be understood more.

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