Fracture Toughness Control in Functionally Gradient Material

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The international collaborative work between Tohoku University and The University of Washington on the evaluation of fracture toughness of FGMs were performed for three years with sharing research subjects between both contributors. Tohoku University took charge of the fabrication of FGM test pieces, testing of thermal shock property by burner heating test, as well as the valuation of the fracture toughness of FGMs by newly devised repeated Vickers indentation technique. The University of Washington, on the other hand, took part in the precise assessment of the fracture resistance that will be induced by the composition-graded layr. An unconventioanl hybrid approach using simultaneously optical measurement and numerical aclculation.The burner heating thermal shock test on a FGM of zirconia/stainless steel showed that the initiation of cracks depends on the thermal stress generation and on defect distribution within the material. The formation of vertical cracks on the ceramic surface during cooling period of the burner heating test were observed, and the propagation of the cracks was found to be effectively suppressed by the graded layr. The repeated Vickers indentation test provides us easy and fast assessment of the fracture toughness of FGMs. Micromechanical and fracture mechanical considerations showed that the repeated Vickers indentation method is suitable for the assessment of the fracture toughness of FGMs particularly for the thin FGM layrs. The hybrid method provides us the more detailed information on the propagation of the cracks in the graded layr, and will be a viable approach for the clarification of the mechanisms of the crack propagation and its cessation.The present collaborative work has provied fundamanetal information on the fracture mechanics of FGMs, and will contribute much to the progress of the FGM researches.