Continual observation of crack growth behavior by ultrasonic inspection in two-roller rolling fatigue test

An ultrasonic inspection system was proposed to observe a subsurface crack growth behavior continually in two-roller rolling fatigue test. A 50 MHz delay line contact probe was used in the system to detect clearly small cracks near the contact surface and to conduct the ultrasonic inspection while the test rollers were kept attached to the rolling fatigue testing machine. The ultrasonic wave pattern was analyzed by digital image processing to predict the position and shape of cracks. The proposed system was adequate to the inspection of a crack which had about 1.5 mm circumferential extent parallel to the contact surface in the subsurface region and inadequate to the continual observation of cracks which originated at the contact surface. This result suggests that cracks originating at the contact surface hardly have a large enough extent to reflect the ultrasonic wave in the crack growth process but grow to failure very rapidly only in the final stage of the process.