Failure of engineering materials under cyclic loading typically consists of three stages, crack incubation, crack growth and fracture. In the crack incubation stage, the material has no detectable cracks so that fatigue damage cannot be quantified in this stage by existing nondestructive testing methods. Unfortunately, in many cases of practical interest, the incubation stage may take up as much as 80% of the total fatigue life of the material. It is therefore critically important to develop new techniques that are capable of quantifying the degree of fatigue damage in the crack incubation stage. To meet such a challenge, this project is to develop a methodology based on nonlinear ultrasound that is capable of nondestructively quantifying early fatigue damage in metallic materials, thus predicting fatigue life. Specifically, the project will (1) develop a physics-based model that relates quantitatively the amplitude of the second order harmonic to the fatigue-induced cumulative plastic strain in the material, and (2) develop innovative and robust measuring techniques to accurately measure the second order harmonic generated by fatigue damage in the crack incubation stage. The methodology developed in this project will enable us to acquire and use information about civil and mechanical structures to improve their safety, reliability, cost and performance.
