Study on magnetic memory detection of weld fatigue damage by using the relative entropy theory

Abstract

In this paper, fatigue tests of different kinds of welds were carried out. The three magnetic components of MMM signal on the weld under different cycles were collected. The change law of MMM signal with the fatigue cycle was analyzed, and the relative entropy theory was introduced to establish the mapping relationship between the relative entropy and the fatigue cycle. The damage state of the weld was evaluated based on the change of the relative entropy. The results showed that the curves of initial MMM signal presented specific change law with the increase of fatigue cycle and a counter-trend change occurred at the local position after the weld failure. In addition, there was a good correspondence between the relative entropy with the fatigue stage of the weld. The value of relative entropy in the fatigue stable stage remained approximately constant and tended to be close to zero. In the final fatigue stage, the value of relative entropy increased exponentially. The relative entropy gradient and the magnetic index k were calculated to capture the time node of the deterioration of the weld more accurately and the k = 0.1 could be defined as the critical value to evaluated the stability of the weld. Hence, the relative entropy that was introduced could effectively characterize the fatigue damage degree of the weld, and realize the early warning of the weld damage.