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Long term fatigue behavior of Ti-6Al-4V which is widely used in biomedical applications has been investigated.Constant stress amplitude fatigue experiments (S-N curves) were carried out at ultrasonic frequency (20 kHz) with two different surface conditions (ground surface and polished surface) in ambient air and in a corrosive fluid similar to body fluid.It is found that the improvement of fatigue strength is pronounced for polished specimens below 106 cycles in ambient air due to fatigue cracks initiate from surfaces of specimens at high stress amplitudes, while surface conditions have no effect on fatigue behaviors of the material because the defects located within the specimen become favorable sites for crack initiation.The endurance limit at 108 cycles of polished Ti-6Al-4V specimens decreased by 7% if it was cycled in simulated body fluid instead of ambient air.Fracture surfaces show all fatigue failure initiated from surfaces in simulated body fluid.