A novel laser ultrasonic thickness measurement method for metal plate based on spectral analysis

Thickness measurement is highly useful in industrials both for controlling the thickness of the product and for material characterization considering both mechanical and chemical properties. Longitudinal ultrasonic velocity can be used to measure material thickness provided that sufficient accuracy can be achieved. A new laser ultrasonic thickness measurement method based on spectral analysis was proposed in this paper and was used for accurate thickness measurement for metal plate. A one-layer model was introduced which laser ultrasonic transmits into, deducing the expression of ultrasonic reflection coefficient amplitude spectrum (URCAS), from which we obtained the method for metal plate thickness measurement; then a laser ultrasonic system was set up to receive the laser ultrasonic transmitting into the plate. Meanwhile we developed a new laser ultrasonic data processing algorithm so that the frequency values of the peaks in frequency domain could be easily identified, then the velocity of the longitudinal ultrasonic could be calculated by the frequency difference of two adjacent peaks; finally, based on the velocity of the longitudinal ultrasonic and the experiment data the thickness of the metal plate could be calculated by inversion algorithm. The relative errors of the thickness measurement were below 0.66% for 5052 aluminum alloy; also we found that, the measurement of thickness is more accurate as the alloy thickness decreases.

I. Introduction

Thickness measurement of products is an important part in industrial processes, since thickness is a key factor affecting the pass rate of the products. For example, in the metal industrials, the thickness of the rolled steel determines the pass rate of the steel products; in the process of material composition, the thickness of the layers have a major influence on the physical and chemical properties of the products. With the development of science and technology, non-destructive testing (NDT) technologies were used by more and more factories to control the product quality and adjust production process [1]–[3]. As one of the NDT technologies, traditional ultrasonic testing has been used to determine the thickness. the inner defects and the residual stress in metal plate for a long time[4]–[9].