Static and Dynamic Mechanical Behavior of Quasi-brittle Solid with Heat-driven Fracture

• Main Authors: Hung-wei Liu, 劉峵瑋
• Other Authors: Yao-chung Chen
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/35529084885699768467

博士 === 國立臺灣科技大學 === 營建工程系 === 103 === The early development of Taiwan transportation infrastructure was mostly located on the side of mountain region forming a circumferential turnpike and railway network. The rapid densification of this network continues. At present, National highway design considers environmental and economical impacts. To reduce damage to sensitive ecosystems brought on by construction, bridges and tunnels are considered to be the environmentally sustainable options. Previously, a lot of emphasis has been given to safety and economy of excavation of rock mass without particular attention to excavation in heated rock and to effects of tunnel fires. However, thermo-induced damage affects the structural and material integrity of civil engineering structures; the damage can induce direct or indirect extensive structural collapse. This study aims to give engineers and tunnel designers a reference for protecting the tunnel structure and surrounding rock, increasing construction efficiency as well as decreasing casualties and cost in the project life cycle. In this study, the complicated engineering problem was simplified into two issues; geo-thermal rock and tunnel fire studied under three topics: 1. Pseudo-static mechanics test after one-dimensional heat-driven Fracture(HdF), 2. Dynamic mechanics test after individual heat-driven damage, and 3. Heat-driven damage test with linear transient thermal loading (LTTL) on inner hollow surface (HIS) for investigating the failure mechanism. The intended purpose of the method was to predict the effects of heat treatment on the static and dynamic mechanical properties and characteristic width of process zones, as well as the establishment of theoretical model is to predict fracture occurring time and position on the topic of HdD with LTTL on HIS. The numerical and experimental methods were used to verify the theoretical model. In addition, using the theoretical model discusses the fracture influence of increasing temperature rate and the radius ratio of outer and inner hole. The results of the 1st topic showed that in the macroscopic, all of the results can be regressed by a continuous equation; therefore, the regression equations obtained from the results of continued heat-damaged specimen pieces represented more accurate prediction equations. Moreover, a critical damage temperature is approximately 500-600°C. For a temperature range between room temperature and approximately 500-600°C; the variation of all of the mechanical properties decreased by approximately 7.6-14.5% per 100°C, but they decreased by approximately 29-37% per 100°C between 500-600°C and the highest temperature used in the tests. On investigating the microscopic results, for specimen at room temperature, micro-cracks cluster around the top of SCB sample (near the loading position) before approximate loading level of 80%, then micro-cracks will cluster around the tip of pre-existing crack during loading level of 80% and 100%. The micro-cracks around the tip of pre-existing crack dominate the SCB fracture behavior. However, for the higher temperature specimen (approximately 450°C), before loading level of 80%, micro-cracks do not cluster at the top of Semi-circular Bend (SCB) sample, because the thermal-induced defects existed within the SCB sample, the stress field near top of SCB sample is a compressive situation, the micro-cracks will be closing, hence the acoustic emission sensors will receive new signals from the top of SCB sample during this stage. Moreover, the study attempted to calculate fracture toughness of quasi-brittle material using synchronized nondestructive techniques. Currently, only some sample results were obtained. The results on 2nd topic showed that the dynamic mechanical parameters increase with the loading rate and decrease with the heat treatment in general. The parameters are hardly affected between room temperature and 450°C. the parameters decrease sharply between 450 and 600°C, it is probably related with the phase change of Quartz from the ? to ? phase, in which volume increases by 0.4% at approximately 573°C. In addition, the average velocity of crack propagation increases with the temperature. On investigation of the results of the 3rd topic, comparison of the temperature and stresses between simulation analysis and theoretical results obtained the similar patterns; for experimental results, the temperature of heat conduction, the micro-crack localization position by AE, as well as the macro-crack initiation location from ESPI are the similar as the theoretical results. The localization position clusters between approximately nondimensional radius (?? of 4.5 and 6.5 using acoustical technique and the initiation location emerges at approximately ? of 5. Then, using the theoretical model to understand the fracture behavior with varying increasing temperature rate (M) and the radius ratio of outer- and inner-hole (?搓); the ?搭 decreases and failure temperature increases with the M and ?搓 increases. To investigate the coefficient ?燁, the ?搭 decreases with ?燁 increases, when the ?燁<5, the failure time and the ?搭 increase.