New Method for Detecting Steel Strip Stress Distribution Based on Laser Ultrasonic Guided Waves

Objective The flatness of a strip mainly refers to its apparent flatness, and the longitudinal internal stress is an effective index to characterize the flatness defect. In the cold rolling process of a strip steel, the automatic shape control is realized mainly by detecting the transverse distribution of tensile stress, and the strip shape meter is mainly used to measure the tensile stress distribution on line. However, the sensitivity and resolution of the contact shape meter are relatively low and the roll surface is easy to scratch, and thus it is difficult to maintain and calibrate. In addition, the detection accuracy of the non-contact shape meter is relatively low. Therefore, it is urgent to explore a new non-contact test method for the internal stress of a steel plate. In order to realize the non-contact detection of internal stress in the steel plate production process, this paper proposes a method for detecting the internal stress of a steel plate based on laser ultrasonic guided waves. Methods The theoretical basis of stress detection based on the ultrasonic method is the acoustic-elastic effect. The change of internal stress in the strip causes the velocity change of an ultrasonic guided wave, and the Lamb wave is the main guided wave in a steel plate. According to the moving characteristics of particles, the Lamb wave vibration patterns include a symmetric mode and an antisymmetric mode. The group velocity dispersion curve of the guided wave in the silicon steel plate under zero stress can be obtained according to the dispersion equation. In the 0.2 mm thick silicon steel plate, the Lamb waves are dominated by the A0 antisymmetric mode and the S0 symmetric mode. By fixing the distance between excitation light and detection light, the measurement of the guided wave group velocity is transformed into the measurement of the guided wave propagation time. The relationship between guided wave propagation time and tensile stress is obtained by applying different tension on both ends of the strip steel sample. The key of the acoustic elastic stress measurement lies in the accurate measurement of the ultrasonic propagation time difference. In order to accurately measure the time difference, the waveform correlation method is used to calculate the time difference. A laser ultrasonic guided wave stress testing device is set up in the laboratory, and a software system for signal acquisition, processing, and analysis is developed. The pre-calculated tensile load is applied to both ends of the experimental sample by the tensile loading device unit and the internal stress fields of the steel plate under various working conditions required by the experiment are obtained. The laser ultrasonic signal detection unit collects the laser ultrasonic guided wave signals under various loading conditions. The characterization ability and correlation law of the laser ultrasonic guided wave to the internal stress in the steel plate are experimentally studied. Results and Discussions Due to the limitation of the detection principle of the two-wave mixing interferometer, the detection laser axis must be perpendicular to the steel plate surface, and the detection laser is more sensitive to the out-of-plane displacement of guided waves. Therefore, the laser ultrasonic guided wave signals collected in experiments are mainly the antisymmetric modes. The laser ultrasonic guided wave signal in a 0.2 mm thick silicon steel plate is mainly at the low frequency, and its energy is mainly concentrated between 20 kHz and 1. 9 MHz (Fig. 6(b)) . Therefore, the laser ultrasonic guided wave signal is mainly the A0 antisymmetric mode. With the increase of tension, the arrival time of the head wave is advanced, that is, the phase velocity of the laser ultrasonic guided wave increases with the increase of tension (Fig. 8(a)) . With the increase of tension, the wave packet shifts to the right, that is, the group velocity of the laser ultrasonic guided wave decreases with the increase of tension (Fig. 8( b)) . The change of phase velocity or group velocity of the laser ultrasonic guided wave can be used to characterize the internal stress distribution in the strip. Tensile tests are carried out on the silicon steel plates with different thicknesses. The results show that there is an obvious linear relationship between the time difference of the head wave advance and the wave packet delay and the tensile stress of the laser ultrasonic guided wave in silicon steel plates with different thicknesses (Fig. 11) . Conclusions In this paper, a method for detecting the internal stress of a steel plate based on laser ultrasonic guided waves is proposed. The experimental results show that a pulsed laser can excite broadband laser ultrasonic guided waves in the silicon steel plate, and the A0 mode is the main mode. The phase velocity of the laser ultrasonic guided wave increases with the increase of tensile force, while the group velocity decreases with the increase of tensile force. There is an obvious linear relationship between tensile stress and the time difference of head wave advance and wave packet delay of the laser ultrasonic guided wave signal. The laser ultrasonic guided wave testing method can be used to realize the non-destructive, non-contact, and high-precision detection of internal stress of a strip steel, and this method may become a new method for the on-line stress detection of strip steels.