Mechanics of defect evolution during machining of brittle materials

An indentation fracture mechanics based model is proposed to represent the effects of grain boundary interfaces in ceramic grinding processes. First, it is observed that weak grain boundary interfaces may aid in reducing the residual damage left behind after a ceramic grinding process, and can significantly enhance the failure strength of the finished parts. It is also observed that sufficiently weak grain boundary interfaces can also alter the mode of material removal from transgranular fracture to intergranular fracture and its associated grain dislodgment.; For ceramics with transgranular fracture as the material removal mode, a simple stress based defect evolution model is developed to assess the influence of various process parameters on material removal rate (MRR) and induced damage during ceramic grinding processes. Model predictions for median and lateral damage zones under normal indentations are first compared to experimental observations. The proposed model is then extended to simulate real grinding situations. The potential of a new design avenue involving intermittent unloading is investigated next. For pyrex glass, it is observed that intermittent unloading can facilitate significant increase in force per abrasive grit without increasing the associated surface and sub-surface fragmentation in the finished part.; The potential of intermittent unloading is further investigated by the proposed fracture mechanics models. First, an analytical fracture mechanics solution is obtained to predict the size of median crack and verified against the experimental observations as well as damage model. Second, two fracture mechanics models are proposed to simulate traditional grinding and intermittent unloading grinding process respectively. The ratio of the total median crack length after intermittent unloading to that at the same maximum load in the traditional grinding is identified as the key parameter to evaluate the effect of intermittent unloading. It is observed that when the intermittent unloading load is a proper portion of the maximum load, it provides the maximum shielding effect.; A simple but rather accurate method is developed to estimate thermal stresses in electronic assemblies with different layer lengths. For layered electronics with thin adhesives, analytical expressions are obtained for interfacial shear stress and peeling stress, and they agree well with the finite element analysis, especially when the moduli of adhesive layers are significantly lower than the moduli of the other layers.