Fatigue crack prognosis based on combined nonlinear ultrasonic modulation measurement and numerical simulation

Abstract

Accurate prediction of fatigue crack growth and estimation of a remaining useful fatigue life (RUL) are extremely important to prevent a sudden failure of the structure. This thesis presents an experimental and numerical simulation of nonlinear ultrasonic modulation to detect fatigue crack. The basic premise of this technique is that, if a fatigue crack exists, the nonlinear interaction of a low frequency pumping wave and a high frequency probing wave produces additional responses at the frequencies equal to the sum and difference of the input frequencies. The nonlinear ultrasonic modulation is further explored for fatigue crack prognosis. First, a fatigue index is defined as a function of nonlinear ultrasonic modulation components. Second, the fatigue index is also expressed as a power function of loading cycles based on Paris-Erdogan and Nazarov-Sutin theories. Then, by curve fitting of the power function to the fatigue index values obtained at various loading cycles, the relationship between the fatigue index and the loading cycle is established. Finally, RUL corresponding to a fatigue index of one (structural failure) is estimated from the fitted power function. Note that the power function is initially estimated using the fatigue index values obtained from numerical simulation, and then continuously updated as fatigue index values from actual experimental measurements become available. Based on this framework, RUL is initially estimated from numerical simulation and continuously updated as experiment data becomes available. The effectiveness of the proposed prognosis methodology was examined based on the fatigue tests of aluminum plates with various thicknesses.