Development of a Non-collinear Nonlinear Ultrasonic-Based Technique for the Assessment of Crack Tip Deformation

Non-collinear nonlinear ultrasonic (NCNLU) wave mixing technique has been established to study the localized plastic deformation at the crack tip during fatigue. A pair of ultrasonic shear wave was mixed non-collinearly to obtain a longitudinal wave of frequency equal to the sum of the two shear wave frequencies under a resonant condition. Experiments were carried out on notched 9Cr-1Mo 3-point bend specimen during high-cycle fatigue. The variation of the NCNLU parameter with the stress accumulation at the crack tip during the fatigue crack initiation and propagation and mapping of the deformation zone around the crack tip are described in this paper.