S Palit Sagar

Nonlinear ultrasonic to assess localized plastic deformation during high cycle fatigue

Nonlinear ultrasonic is the new approach for the effective evaluation of material degradation in the area of non‐destructive evaluation (NDE). Fatigue damage of a material produces a substantial distortion of ultrasonic waves propagating through the degraded material. The wave distortion is quantified by means of a material nonlinearity parameter β that is defined as the ratio of the amplitude of the 2nd; harmonic to the square of the amplitude of the fundamental. This nonlinear parameter changes when, for instance, a distribution of microcracks appears inside the material. This feature has recently been found as a new potential application in the characterization of fatigued and degraded materials. In this present work, an attempt has been made to assess the localized plastic deformation during high cycle fatigue to locate the position of crack initiation much before the failure along the gage length of hourglass type specimen in polycrystalline copper by nonlinear ultrasonic technique.

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.