Detection of fatigue crack initiation pre-cursor in meta-stable SS 304LN using Rayleigh surface wave harmonic generation

Nonlinear Rayleigh wave is allowed to propagate through cyclically damaged 304LN stainless steel for the characterization of microstructure evolved during plastic deformation. SS304LN is widely used in nuclear and petrochemical industries in piping components, heat exchangers etc. to withstand damages arising due to low cycle fatigue, thermal aging, corrosion etc. An initial monochromatic ultrasonic Rayleigh wave when interacts with degraded microstructure in material, distorts and generates higher-order harmonics. Interrupted low-cycle fatigue tests were carried out at different strain amplitudes and at each interruption, the distortion in the ultrasound wave was measured by evaluating the nonlinearity parameter which is the ratio of the amplitude of the second harmonic to the square of the amplitude of the fundamental frequency component till failure. This study shows the effectiveness of beta to characterize the microstructural evolution as well as the generation of strain-induced alpha-martensite in meta-stable 304LN stainless steel during cyclic deformation and fatigue damage accumulation.