| Critically Refracted Longitudinal Wave(LCR),as a kind of longitudinal wave propagating along the surface/subsurface of the material,has been widely concerned in recent years for its characteristics of fast propagation speed,insensitivity to the surface state of the material,and sensitivity to stress and damage.The traditional LCR technology based on conventional ultrasound utilizes hard wedges with the first critical angle to achieve LCR wave excitation,which has poor adaptability to the change of material sound velocity.It brings many limitations to the practical application of LCR waves.Phased Array Ultrasonic Testing(PAUT)can realize the ultrasonic beam deflection and focusing by controlling the excitation delay time of different array elements,thereby controlling the wave field.In this paper,based on PAUT,the Phased Array-Critical Refracted Longitudinal Wave(PA-LCR)technology is proposed.Through electronic delay time control,flexible adjustment of first critical angle under different conditions can be accurately realized,so as to adapt to the change of sound velocity of different materials.Realize the effective excitation of LCR wave and improve the quality and efficiency of detection.The main research contents and conclusions are as follows:(1)With the help of finite element method,the propagation behavior of conventional LCRwave is analyzed,finding that the refracted wave field in the material mainly includes refracted longitudinal wave,head wave,and refracted transverse wave.The optimal probe frequency is determined by comparing the received signals of different frequencies.According to Snell's law and the sound velocity of common materials,the corresponding first critical angle range is calculated,and two wedges of plexiglass and polytetrafluoroethylene(PTFE)are designed.By analyzing the wave field under the two delay laws of deflection and deflection focusing,it is found that the LCR wave energy is the highest with the plane wave front incident at the first critical angle.When incident direction deviates from the first critical angle by±7°,the normalized amplitude of LCR wave reduce by about 50%,can still be effectively excited.(2)A PA-LCR system based on phased array ultrasonic pitch-and-catch is built,and a method to calculate the LCR wave velocity is proposed based on the time difference of different receiving elements.The high signal-to-noise ratio excitation and velocity measurement of LCRwaves have been successfully realized on CSK-IA carbon steel,aluminum,and 304 stainless steel test blocks,and the experimental influence factors were analyzed.Combining test condition control and signal post-processing to reduce measurement error,the deviation from the result measured by ultrasonic bulk longitudinal wave is less than 2%.This method can be applied to a variety of engineering materials.(3)The acoustic propagation model of transversely isotropic media i.e.unidirectional CFRP laminate is established,and the LCR wave fields when measuring in different fiber orientations are analyzed.The results indicate that the PA-LCR system has good adaptability to the sound velocity changes under different fiber orientations.The signal energy is significantly affected by the angle between detection direction and the fiber orientation.The main reason is that the anisotropy of CFRP may lead to the deflection of energy propagation.On this basis,part of the in-plane elastic constants were calculated by using sound velocities in different fiber directions. |
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