RESUMO
Ultrasonic pulse technology has been widely employed for thickness measurement in the industrial field. Generally, the ultrasonic transducer is required to be perpendicular to the test sample surface when using the longitudinal wave. However, the transducer posture deviation (i.e., incident angle) of ultrasonic beam relative to the local surface normal is always inevitable. In addition, the received ultrasonic pressure will be weakened significantly because of the variation of echo propagation path and the attenuation of echo amplitude. As a result, the incident angle induced thickness error is generated. Therefore, it is necessary to investigate and quantify the negative effect of incident angle on ultrasonic thickness measurement. This paper focuses on the incident angle identification based on the first-echo energy attenuation. First, the influence mechanism of incident angle on pulse-echo pressure is analyzed theoretically by adopting an equivalent transducer model. Furthermore, a novel approach for the incident identification based on the first-echo energy attenuation is developed. A compensation model is established to correct the incident angle induced thickness error. To verify the feasibility of the proposed method, a series of incident angle calibrations and experiments were designed using the developed ultrasonic pulse measurement system. It was indicated that the proposed angle identification and error compensation approach had the capacity to improve the measurement accuracy of ultrasonic thickness.