Direct Observation of the Mechanisms of Fatigue Crack Growth

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A.Investigation of the effect of frequency, temperature and material on size effect of fatigue crack propagation.The same material (Ni-base superalloy in this year) was tasted in the three research groups.(1) At University of Sheffield, the automated observation of the initiation and propagation of small fatigue cracks in large specimens is conducted at both room temperature and 800*C.The effect of micro-structure on the initiation and propagation of small fatigue cracks is being made clear.(2) At Kyushu University, the automated observation of the fatigue crack propagation of small specimens in the Instron-type scanning electron microscopes connected to servohydraulic fatigue testing machine is conducted at 50 thousand magnifications. The relationship between the deformation of the crack tip (crack opening-closing behavior). micro-structure and fatigue crack growth ratio is being made clear.(3) At Osaka University, the dynamic observation of fatigue crack in the Osaka-type scanning electron microscopes connected to servohydraulic fatigue testing machine is conducted under variable amplitude loads. The effect of variable amplitude loads on short fatigue crack growth is being made clear.B.Investigation of the relationship between the crack shape and specimen size by 3-D surface crack propagation analyzes.(4) At Kyushu University, the analytical method based on Boundary ElementMethod, which is extremely precise, for stress intensity factors of 3-D surface crack is developed.(5) Comparing the result of (4) and that by Finite ElementMethod at University of Sheffield, the precision of FEM is confirmed to be suit for the prediction of the 3-D crack growth.