Fatigue Crack Detection in AISI 304 Austenitic Stainless Steel Using Nonlinear and Linear Ultrasonic Testing Methods

Abstract

Fatigue cracks are often underestimated by traditional linear ultrasonic techniques due to the poor direction as well as narrow gaps within the cracks. The nonlinear ultrasonic testing technique is a promising method that may be used to solve this problem. In this study, the effects of traditional C scan imaging in measuring fatigue cracks are analyzed by comparing their results with that of metallography. Then, the finite amplitude method of nonlinear ultrasonic testing is used to detect micro-damages, and the influences of excitation voltage on the nonlinear coefficient are investigated. The results indicated that the accuracy in C scan imaging becomes worse when the crack’s surface is not perpendicular to the incident wave or if the crack’s gap is very small. In contrast, relative nonlinear coefficients under an optimum voltage increase according to the degree of micro-damage, which was found to not be affected by the status of macro-cracks but was particularly sensitive to the accumulation of micro-damage. This study suggests that nonlinear ultrasonic techniques be implemented to achieve beneficial effects in testing fatigue damage.