Research on Fatigue Fracture Mechanisms of Micro Materials

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Fatigue test systems with electrolic-polishing apparatus were developed to study the size effect on the fatigue strength of the micro-materials. This system has an electro-dynamic actuator to load small amplitude cyclic force. Fatigue tests were conducted after manufacturing small cross-sectional specimens by the electrolic-polishing apparatus without removing specimens from the system. Commercially pure aluminum and iron wires of 1.0 mm diameter were employed for the fatigue tests. The minimum diameter of each specimen was from 0.20 to 0.60 mm. To remove the effect of stress gradient, cyclic axial force was applied. The frequency of the cyclic force was 50 Hz with the stress ratio of 0.1. In both materials, the fatigue strength was almost independent of the specimen diameter, and the variations in the fatigue life from specimen to specimen at the same stress amplitude were large, and the crack initiation life is dominant to total fatigue life.The slip-band formation and fatigue crack-initiation processes in balk material of 70-30 brass were observed by means of atomic-force microscopy (AFM). The slip-band angle relative to the stress-axis at the specimen surface varied from 15 to 90°, and it appeared most frequently around 60°. The depth of the intrusion drastically increased with its outgrowth to a crack, and with coalescence of cracks, the width of cracks increased rapidly. The critical value of the intrusion depth for crack initiation was given as a function of the slip-band angle through a geometrical model for the slip-direction and the slip-step, and the critical value was independent of stress amplitude. From precise observations of slip-band tips, slip-bands had steep slope when they were blocked by grain boundaries, and the slip-bands descended by gradual slopes to plain surfaces when they terminated within grains.