A Nonlinear Ultrasonic Wave Mixing Method for Micro-damage Detection of 316L Austenitic Stainless Steel

The 316L austenitic stainless steel is widely used in extreme conditions of high temperatures, high pressures, and alternating loads. Under the effects of the temperature and load, the 316L stainless steel is subjected to different types of damage, such as thermal aging, creep, and fatigue. The damage accumulation may cause the sudden failure of 316L steel structures. The detection of damage, especially the early micro-damage, in 316L steel is of great practical significance. In this paper, a new nonlinear ultrasonic wave mixing (NUWM) method is proposed for the detection of early micro-damage in metal materials. This NUWM method uses the sinusoidal continuous signal excitation to enhance the intensities of nonlinear acoustic modulation responses. This NUWM method applies a frequency sweeping method to quickly select appropriate excitation frequencies of two ultrasonic waves. The basic theory of the NUWM method for damage detection of metal materials is introduced. A detection system is developed to implement the NUWM method. The 316L steel specimens with different types and degrees of damage are prepared through high-temperature fatigue and thermal aging tests, respectively. The NUWM method is applied to detect damage of these specimens. Nonlinear characteristics are extracted from the received detection signal by the frequency spectrum analysis. The extracted signal characteristics are used to define a nonlinear modulation index for quantifying micro-damage. Experimental results show that the NUWM method can effectively detect and assess the fatigue and thermal aging damage of the 316L stainless steel.