基于相控阵激光超声的裂纹衍射增强试验研究

针对超声衍射时差法(TOFD)存在检测精度较差、区域检测可靠性不够和信号信噪比(signal-to-noise ratio, SNR)低等问题,提出了一种基于光纤皮秒激光器和高速旋转镜的相控阵激光超声裂纹检测方法。利用有限元方法模拟热弹机制,建立二维瞬态激光超声力-固耦合模型产生横波与纵波在缺陷处发生的衍射现象,分析了裂纹尖端不同奇异点、声波不同中心频率和相控阵激励源不同位置对声波衍射的影响,通过衍射信号的信噪比和位移幅值两个计算指标来分析变化规律,并进行了试验验证。结果表明:数值模拟与试验结果有较好的一致性,相控阵激光源较传统单束激光源对衍射信号幅值和信噪比有明显的增强作用,纵波衍射信号信噪比较理想;衍射信号幅值随裂纹尖端奇异点增加和声波中心频率减小而增大;信噪比随尖端奇异点增加而增大,随声波中心频率一定范围增加无明显变化,随激光源距离的增加呈现先增加后减小的趋势;缺陷定量分析时计算出的裂纹长度与实际裂纹的误差均不超过6.8%。

Tests for crack diffraction enhancement based on phased array laser ultrasound

Here,aiming at problems of poor detection accuracy,low reliability of area detection and low signal-to-noise ratio(SNR)of time of flight diffraction(TOFD),a phased array laser ultrasonic crack detection method based on fiber picosecond laser and high-speed rotating mirror was proposed.The finite element method was used to simulate thermoelastic mechanism,establish a 2D transient laser ultrasonic force-structure coupling model and produce diffraction phenomenon of transverse wave and longitudinal wave at defect.Effects of different singular points at crack tip,different center frequencies of acoustic wave and different positions of phased array excitation source on acoustic wave diffraction were analyzed.2 calculation indexes of SNR and displacement amplitude of diffraction signal were used to analyze their variation laws and test verifications were done.The results showed that numerical simulation results agree better with test ones;compared with the traditional single beam laser source,the phased array laser source can obviously enhance amplitude and SNR of diffraction signal,and SNR of longitudinal wave diffraction signal is ideal;diffraction signal amplitude increases with increase in singular points at crack tip and decrease in central frequency of acoustic wave;SNR increases with increase in singular points at crack tip,SNR has no obvious change with increase in central frequency of acoustic wave within a certain range,it increases firstly and then decreases with increase in distance of laser source;the error between the calculated crack length and the actual crack length during defect quantitative analysis is not more than 6.8%.