MEASURMENT OF R-CURVES FOR CERAMIC COMPOSITS,AND ITS MACROMECHANIC ANALYSIS

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In order to make clear the micromechanisms governing the fracture behavior of materials, it is necessary to pursue precise characterizations giving the depecdence of the resistance showed by the material to crack propagation on the crack length (R-curve behavior). In this work, some basic problems related to the stable propagation of cracks in brittle materials have been solved. First, a technique to introduce a sharp (mum scale diameter) notch in the specimen has been developed. Then, the problem of stabilizing the crack propagation through the use of a stabilizer apparatus using the bending geometry (according to JIS) has been overcome. In such a way, the routine procedure to obtain R-curve in even very brittle materials has been well established and applied to two type of materials : 1) composite materials reinforced on the micron scale by whiskers or fiber-like (elongated) grains and, 2) composite materials reinforced on a larger scale by long fibers.In addition to fracture mechanics characterizations, in situ spectroscopy measurements have been performed to characterize the mechanisms of fracture on the microstructural scale. As a result of this characterizations, it has been shown that mechanisms operating in the wake of the crack tip, namely, the elastic or frictional bridging, play the major role in hampering fracture propagation in brittle solids.Finally, precise characterizations of the shear resistance of fiber-reinforced materials have also been performed with particular emphasis on relating the microscopic mechanism of interface debonding to the macroscopic failure process by shearing. The results and the final interpretation on the above fracture processes have then summarized and presented in scientific papers.