Performance evaluation of a magnetic NDE technique to assess residual fatigue life of the retired hydraulic cylinders

Metal magnetic memory (MMM) method is a novel and potential nondestructive evaluation (NDE) technique to assess residual fatigue life of the ferromagnetic structure, such as hydraulic cylinder. In this paper, an evaluation framework was proposed. Firstly, the MMM signals under different fatigue cycles were recorded in the tensile-tensile experiment. A representative signal characteristic Hpp was defined as an evaluation index. The monotonic change trend and abrupt change point was investigated. Statistical results show that Hpp can be used to predict residual fatigue life. Except for a few specimens, almost all specimens produced an abrupt change at the normalized fatigue life coefficient k = 0.91. This can be used to distinguish different stages of fatigue life. The sensitivity, specificity, and diagnostic performance were analyzed using Receiver Operating Characteristic (ROC) curves. The statistical results show that when the coefficient k = 0.91, the diagnostic performance is relatively best with an area under the curve (AUC) is about 0.85, and Jorden's index is about 0.75.