The Effects of Stress on the Detection of Fatigue Cracks by Ultrasonic Technique

Abstract

Fatigue cracks with different surface length were induced under a tension-to-tension load in a three-point bending configuration. These cracks were induced in flat plates (24″ x 6″ x 5/8″) of Ti-6A1-4V in the annealed condition at different loads, number of cycles, but constant frequency. Using the ultrasonic Delta Scan method, operating at 5 Megacycles, no correlation was found between the received ultrasonic amplitude to crack length. However, the test results showed that those cracks induced at higher stress levels and cycles generally produced larger amplitudes. In almost all the cases wider cracks of the same length produce larger amplitudes, but the crack widths are generally wider when generated at higher stress levels. After cycling two cracks with a compressive load of approximately 44 Kpsi and 34 kilocycles, without extending the crack length, the ultrasonic amplitude becomes two to three times larger. The compressional cycles have either increased the crack cross-sectional area (by increasing the crack depth and/or the amount of retardation) and/or the crack opening (by decreasing the compressive residual stress field around the crack or direct physical deformation of the asperities at the crack interface). In any event, the minimum detectable crack size on these Ti specimens appears to be strongly dependent on the load spectrum used to generate the cracks. Finally, the amplitude of ultrasonic shear waves, operating at 5 Megacycles, was monitored from zero to 27.3 Kpsi of applied tensile load. This test showed that an applied load of few Kpsi can increase the amplitude severalfold.

Work performed on independent research and development (IRAD) funds of Convair Aerospace/Fort Worth Operation.