Mechanical properties and cracking behaviour of marble and moulded gypsum under high strain rates
Date of Issue2015
School of Civil and Environmental Engineering
The research on the mechanical properties and the cracking behaviour of rocks under dynamic loadings and the underlying fracturing mechanisms is significant to solve the dynamic rock engineering problems. One of the most important issues to address is how and to what extent the strain rate (loading rate) influences the mechanical properties and the cracking behaviour of rock or rocklike materials. Both experimental and numerical approaches are adopted in the present research. Intact and non-intact specimens containing open flaws are used to investigate the influences of the discontinuity in rock masses. In experimental studies, two different brittle materials, high quality marble and moulded gypsum, representing different types of rocks are used. The mechanical properties of both intact and non-intact specimens including the compressive strength, the tensile strength, the elastic modulus, and the failure strain are tested under different strain rates by using a servo compression machine and a split Hopkinson pressure bar (SHPB) system. Experimental results reveal that these mechanical properties all show strain rate-dependence to different degrees. The size and geometry effects are also found in dynamic tests. Furthermore, the cracking behaviours of these specimens under dynamic loadings are investigated and compared with those under quasi-static loadings. The results indicate that the cracking behaviour is also significantly affected by the strain rate. The first cracks (or white patches) of single-flawed specimens are similar, while the secondary cracks are much different which lead to distinct failure modes: the diagonal failure under quasi-static loadings and the “X” shaped failure under dynamic loadings. Different levels of fragments are induced by various strain rates. In addition, a numerical analysis using AUTODYN taking into account the loading rate and the strain-rate dependent model is also performed. The material model is derived from experimental results on the mechanical properties versus strain rates. The numerical results confirm the experimental observations, hence providing plausible explanations of the different fracturing phenomena in the single-flawed marble specimens under different loading conditions. Towards the end of the thesis, a future research plan is also outlined to improve and extend the present works.