Application of ultrasonics to detection of fatigue cracks

An ultrasonic system was developed and used to observe the formation of fatigue cracks in centernotched sheet specimens of unalloyed aluminum, two aluminum alloys, a mild steel, and a nickel-base alloy tested in axial tensile fatigue. S-N curves of life-to-initial detectable cracks as well as life-to-fracture were obtained. With the reflection technique, fatigue cracks that ranged in length from 0.0005 to 0.005 in. were detected while the test was in progress. Cracks were detected within approximately 1 to 3 percent of total specimen life for all of the materials considered over the range of stresses considered. The through-transmission technique was utilized to measure relatively long cracks.