Experimental studies on subcritical cracking in facade rock panels

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Experimental studies on subcritical cracking in facade rock panels

 

Author: Kwok, Kui-wah
Title: Experimental studies on subcritical cracking in facade rock panels
Degree: M.Phil.
Year: 2008
Subject: Hong Kong Polytechnic University -- Dissertations.
Materials -- Fatigue.
Fracture mechanics.
Department: Dept. of Civil and Structural Engineering
Pages: xiii, 138 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2239186
URI: http://theses.lib.polyu.edu.hk/handle/200/1721
Abstract: Thin rock panels are commonly used on exterior wall cladding in high-rise buildings. It has been found that these rock panels may develop long-term time-dependent cracking, due to stress concentrations induced by periodic wind loading or thermal loading from sunshine, or stress corrosion due to environmental effects (such as acid rain). This type of cracking inducing at a stress level, much lower than the fracture toughness, can be explained by the concept of subcritical crack growth (SCG). However, existing design requirements of rock panels do not account for SCG and such crack growth behaviour in rock panels under different environmental conditions has not been known. The two main objectives of this study are to investigate the behaviour of SCG in rock panels under different environmental conditions including air, water, acid and cyclic heating, and to develop a guideline for rock panel design based on SCG. As there is no suitable testing method to study SCG on rock panels in the previous studies, a four-point bending test was firstly introduced to study SCG in panels made of granite, dolomite and Carrara marble. Experimental results show that the crack growth rate v decreased first, v attained a minimum constant value and finally increased when the crack propagated towards to the lower edge. To our knowledge, such a crack growth behavior has not been reported in the literature on SCG studies. Thus, this is the first study to fully investigate the crack growth behaviour of rock panels by using the acoustic emission (AE) technique and microscope observation. It is found that the decrease in v might be caused by the compressive stress inducing around the notch tip under bending stress. The constant v represented more microcracks grew from some of existing defects. The v increased because those microcracks joined together to form macrocracks. Apart from the crack growth behaviour, an experimental apparatus was originally developed for cyclic heating study of rock specimens. This apparatus provided heating and cooling on upper and bottom specimen surface respectively in order to simulate the real situation of facade rock panels. Due to some uncertainties, the effect of cyclic heating had not been fully understood. However, this study provides the preliminary idea for further study on the effect of cyclic heating. By comparing among all testing conditions including air, water, acid and cyclic heating, it is found that the crack growth rate was higher in acidic condition and therefore the effect of acidic condition is considered to be the critical issue on SCG. On the other hand, the failure time of rock panels was predicted based on the model developed by Chau and Shao (2006). This is the first study to adopt such model by applying the SCG parameters obtained from the four-point bending tests under corrosive cracking and cyclic healing. By comparing the results predicted from sunny, rainy and acid rain conditions, it is found that the acidic condition is the most critical case for rock panel design because of the shortest predicted failure time. As several assumptions were made in the prediction, further studies are recommended to provide more precise failure time prediction. With this concept of time prediction and experimental results of four-point bending tests, a guideline for rock panel design is finally developed.

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