Fatigue damage evaluation in A36 steel using nonlinear Rayleigh surface waves

This research uses nonlinear Rayleigh waves to characterize the damage due to plastic deformation in A36 steel specimens subjected to quasi-static, monotonic tension, and low cycle fatigue. A36 steel is widely used in the civil infrastructure, such as steel bridges, where fatigue damage can lead to a catastrophic failure. Plastic deformation causes the generation of higher order harmonics in an initially monochromatic Rayleigh wave signal, and this measurable change occurs before macroscopic damage such as cracks appear in a specimen. This increase in the acoustic nonlinearity is produced by plasticity-induced microstructure changes, and thus can be taken as a direct measure of damage. Experiments are conducted using a pair of wedge transducers to generate and detect tone burst ultrasonic Rayleigh surface wave signals. The amplitudes of the first and second order harmonics are measured at different propagation distances to obtain the nonlinearity parameter for a given damage state throughout the fatigue life and monotonic loading process in three specimens. The results of the nonlinear ultrasonic measurements show an increase in the measured acoustic nonlinearity, especially in the early stages of fatigue life. In addition, there is a notably close relationship between the measured acoustic nonlinearity and the cumulative plastic deformation. These results demonstrate the feasibility of using nonlinear Rayleigh waves to characterize damage associated with plastic deformation, and this quantitative information can be a useful input for life prediction models.