Research on Fatigue Damage in High-Strength Steel (FV520B) Using Nonlinear Ultrasonic Testing

In view of the early fatigue damage of high-strength steel FV520B, a nonlinear ultrasonic detection was performed on two types of fatigue samples using nonlinear Lamb waves. The experimental results indicated that the ultrasonic nonlinear parameter is highly sensitive to early fatigue damage in high-strength FV520B. For plate specimens, the ultrasonic nonlinear parameter increased with the number of fatigue cycles. Scanning electron microscopy (SEM) observations of the fatigue specimens revealed that as the number of fatigue cycles increased, the microstructure of the material gradually deteriorated, and the ultrasonic nonlinear parameter increased. For notched specimens, the ultrasonic nonlinear parameter increased as the size of the main crack increased. SEM observations of the fracture indicated that the ultrasonic nonlinear parameters were more consistent with the equivalent microcrack length (defined as the sum of microcrack lengths in the statistical area), as compared with the length of the main crack. It was determined that the nonlinear effect of the Lamb wave is related to the equivalent microcrack length inside the material and that the ultrasonic nonlinear parameter can effectively characterize the fatigue damage state of high-strength FV520B.