A Study Of Fracture Mechanisms In Brittle Materials Under Borehole Blasting

Blasting has been widely employed in the areas of excavation, construction, civil engineering, oil and gas industry due to its high efficiency and low cost. It is, therefore, important to investigate the mechanisms of fractures induced by explosion and especially by borehole blasting. The fracture processes in brittle materials by explosion are very complex which involves many areas of sciences. A comprehensive study is carried out in this thesis on fracture mechanisms in brittle materials under borehole blasting, which mainly consists of the following parts:Three response regions, i.e. comminuted region, cracked region and elastic region were formed in brittle materials such as rocks and concretes subjected to borehole blasting. Based on these observations, spherical and cylindrical cavity expansion models with an initial radius were constrcted, which are different from the previous models without an initial radius. To cater for compressibility or dilatancy of the material in comminuted region, dilatant-kinematic relation is introduced in the models. On the other hand, numerical simulations are performed using LS-DYNA into which a material constitutive relation developed by our group has been incorporated. It is shown that the numerical results are in good agreement with those predictions from the theoretical models. Moreover, from both cavity expansion theoretical models and numerical simulations a critical radial stress can be derived above which a cavity is possible to expand.An analytical model based upon the above developed cavity expansion theory was proposed to analyze the penetration of a following through bomb (FTB) into concrete targets with pre-drilled hole. It transpires that the model predictions are in good agreement with available test data. It also transpires that the optimized desgn of a dual warhead system can be achieved in terms of penetration depth.vs. impact velocity and ratio of following through bomb (FTB) diameter to forward shaped charge (FSC) diameter.The effects of pulse parameters such as pulse peak, loading rate and unloading rate on crack initiation and growth in brittlt materials under borehole blasting have been evaluated numerically. It is found that higher peak amplitude induces more extensive comminuted zone which absorbes more energy and is not conducive to crack formulation and propagation, whilst lower peak value is not powerful enough to produce fractures. With the same impulse and peak amplitude, trapezoid pulse is found to more effective than triangle pulse in terms of crack propagation. It is also found that the loading and unloading rates have significant influences on the number and the length of cracks, and that pulse with higher loading rate and lower unloading rate will cause longer but fewer cracks, and that the effect of borehole blasting is satisfactory if the critical radial stress predicted from the cavity expansion model is taken as the lower bound of peak amplitude.Numerical simulations with LS-DYNA software into which a material constitutive relation developed by our group has been incorporated are performed to examine the cracking patterns. The user subroutine code with the constitutive relation is used to study the dynamic fracture behavior of brittle materials. It is shown that the numerical simulations are in good agreement with the experimental observations for granite subjected to borehole blasting.Numerical simulations with ABAQUS/STANDARD are performed to invesitigate the behavior of gas driven fracture propogation, the pressure inside crack and the crack width. It is found that the denser the radial cracks are, the higher critical pressure is needed. It is also found that crack caused by the combined effect of gas pressure in borehole and gas pressure penetrated into the crack is longer than that solely caused by gas pressure penetrated into crack.