Fracture Mechanics Interaction of MicroーFlaws with Ceramic Grain Boundaries.

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The main objective of the present study is to elucidate the role of ceramic grain boundaries in toughening polycrystalline ceramic materials by examining the micromechanics interaction between the strengthーcontrolling small flaws and the grain boundaries of ceramics.1. Experimental detailsConducted were the measurements of the mechanical strength and the fracture toughness of oxide ceramics including MgO and Al2MgO4 with different grain sizes, as well as polycrystalline graphite materials, where we focused mainly on the influence of the ratio of (sintered grain size) / (strength-controlling flaw size) on these fracture parameters.2. Brief outline of the results and discussion(1) There exists a finite difference in the experimentally determined fracture toughness values obtained from test specimens with a macro-notch or a macro-crack and with a micro-flaw. The decrease in the grain boundary bonding forces of polycrystalline ceramics enhances this difference.(2) The macroscopic fracture … More toughness decreases, while the microscopic fracture toughness in the IM-method increases, with decreasing grain boundary bonding forces. This toughness increment in the IM-method for weak grain boundaries may result from the microscopic local relaxation of the crackーtip stress field through stress-induced microcracking along the grain boundaries.In addition, it has been disclosed in the present study that this fracture toughness difference between macro- and micro-cracked specimens is closely related to the R-curve behavior of the polycrystalline ceramic.(3) Critically reviewed was the conventional concept of the "microstructural R-curve effect" which has been introduced to explain the apparent decrease of fracture toughnss with the progressive decrease of the strength-controlling flaw size. It has been emphasized and concluded that the transition (switching) of the strength-controlling fracture origin from the artificially induced flaws to the intrinsic flaws and vice versa is most essential to dictate the fracture strength and fractue toughness of polycrystalline ceramics with defects/cracks/flaws whose dimensions are in the same order of the magnitude of sintered grains. Less