Online prognosis of fatigue crack at welded joints　using nonlinear ultrasonic modulation

摘要

In this study, an online residual fatigue life estimation (prognosis) technique based on nonlinear ultrasonic modulation isdeveloped for a welded joint. When two ultrasonic input signals are applied to a structure at two distinct frequencies,resulting crack opening-and-closing generates modulation components at the summation and difference of the two inputfrequencies. For prognosis, a fatigue index is defined and extracted from nonlinear ultrasonic modulation componentsobtained from a target structure. Based on Nazarov-Sutin and Paris-Erdogan theories, the fatigue index is formulated as asimple power function of the loading cycles applied to the target structure. Next, a residual fatigue life is estimated byfitting a power function to the ultrasonic modulation data obtained up to the current loading cycles and extrapolating thefitted power function to the failure point. Finally, the proposed prognosis technique is validated using test data obtainedfrom aluminum (6061-T6) plate specimens with a welded joint. The uniqueness of this paper lies in (1) definition of afatigue index based on nonlinear ultrasonic modulation components, (2) theoretical formulation of the fatigue index as apower function of loading cycles, (3) online fatigue crack prognosis based on nonlinear ultrasonic measurements, and (4)experimental validation using welded aluminum plates under cyclic loading.