Abstract
The absolute acoustic nonlinearity parameter β is defined by the displacement amplitudes of the fundamental and second-order harmonic frequency components of the ultrasonic wave propagating through the material. As β is a sensitive index for the micro-damage interior of industrial components at early stages, its measurement methods have been actively investigated. This study proposes a laser-ultrasonic detection method to measure β. This method provides (1) the β measurement in a noncontact and nondestructive manner, (2) inspection ability of different materials without complex calibration owing to direct ultrasonic displacement detection, and (3) applicability for the general milling machined surfaces of components owing to the use of a laser interferometer based on two-wave mixing in the photorefractive crystal. The performance of the proposed method is validated using copper and 6061 aluminum alloy specimens with sub-micrometer surface roughness. The experimental results demonstrated that the β values measured by the proposed method for the two specimens were consistent with those obtained by the conventional piezoelectric detection method and the range of previously published values.
Highlights
The nonlinear ultrasonic technique (NUT) is an innovative nondestructive evaluation (NDE) method [1,2] used to diagnose micro-damage such as thermal aging [3,4], fatigue damage [5], creep [6], and others [7,8] at an early stage in industrial components
The results indicate that this laser-ultrasonic detection method could be applicable even on rough surfaces corresponding to general milling machined surfaces
The measurement of the absolute acoustic nonlinearity parameter (β) was conducted using the laser-ultrasonic detection method, where a laser interferometer based on two-wave mixing (TWM) in the photorefractive crystal (PRC) was used as the receiver
Summary
The nonlinear ultrasonic technique (NUT) is an innovative nondestructive evaluation (NDE) method [1,2] used to diagnose micro-damage such as thermal aging [3,4], fatigue damage [5], creep [6], and others [7,8] at an early stage in industrial components. In the NUT, a relative acoustic nonlinearity parameter β0 , defined by the detected electric signal amplitudes from any ultrasonic receiver regardless of displacement, is used as an indicator [14,15] This relative β0 is limited to a relative comparison between the before and after damage of a material [16]. This absolute β measurement provides a quantitative evaluation of the material microstructural properties and micro-damage, providing the potential for quantitative NDE [17]
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