레이저 수신을 이용한 초음파 비선형 파라미터 측정의 표면 거칠기 효과 보정

Abstract

The ultrasonic nonlinear parameter can evaluate the microstructural change of the materials such as thermal aging, creep and fatigue. For these reasons, the ultrasonic nonlinear parameter has been promised as a considerable nondestructive indicator of material degradation. However, the ultrasonic nonlinear parameter measured by contact transducers has the issue of converting the measured signal to a displacement. A few studies have attempted to overcome this limitation by using the photorefractive interferometer to measure the ultrasonic nonlinear parameter. This method has an advantage compared to the contact measurement method, because the signal of the ultrasonic wave is directly proportional to the displacement of the ultrasonic wave. However, laser-detected measurement is sensitive to the surface roughness of the examined specimen, requiring the measured amplitudes of the ultrasound to compensate for the influence of surface roughness. In this study, a method that can compensate for the effect of surface roughness on the laser-detected ultrasonic wave amplitude is proposed, in which the ultrasonic amplitude is compensated by use of a correction factor that varies with the surface roughness. For the experimental verification, the measurement of the relative nonlinear parameters in Al6061 specimens with surface roughness values of 0.249 ㎛, 0.518 ㎛, 1.037 ㎛ and 3.287 ㎛ was conducted. The results showed that the ultrasonic nonlinear parameter could be measured by using laser-detection only if the influence of surface roughness in the detection of ultrasonic amplitude is addressed.