Stress Evaluation of Welded Joints with Metal Magnetic Memory Testing Based on Tension–Compression Fatigue Test

Metal magnetic memory testing (MMMT) is an effective nondestructive technique for fatigue damage monitoring of weldments because of its capacity for stress evaluation. An experimental investigation of the effect of the applied fatigue stress on MMMT signals, including the tangential component B(x) and the normal component B(z), during tension–compression fatigue tests in welded joints was carried out systematically. The B(x) and B(z) signals at different fatigue cycles and fatigue stresses were collected and analyzed, and the results showed that there was a peak of B(x) and abnormal peaks of B(z) that existed at the welded joint before loading. After loading, the peak of B(x) and the abnormal peaks of B(z) reversed, and the B(x) signals moved upward and the B(z) signals rotated anticlockwise dramatically in the first few fatigue cycles. After the fatigue cycle number was larger than 1000, B(x) and B(z) were stable, with very little fluctuation. In addition, the characteristics of B(x) signals, the mean value, and the peak value of the average of B(x) had an extremely significant linear relationship with the applied fatigue stress during the stable stage of the fatigue test, which indicates that MMMT is a feasible method for fatigue stress evaluation and even residual fatigue life estimation for weldments in service.