基于横波衍射法应用激光-电磁超声检测内部缺陷

为了有效评估出工件内部缺陷的深度,提出了脉冲激光激励和电磁超声换能器接收的非接触式检测方法。分析了超声波的烧蚀激励原理,体波作用于缺陷后的衍射现象,以及电磁超声的接收过程。依据横波衍射理论,当电磁超声换能器处于缺陷正上方时,衍射信号的渡越时间将取得最小值,基于此推导了缺陷深度计算公式并讨论了检测盲区。搭建了工件内部缺陷的激光-电磁超声检测系统,先后测量了内部含有圆孔和不同倾角裂纹的工件试样。在固定激励点的前提下,移动电磁超声换能器,观察信号渡越时间的变化规律,以及分析衍射横波的相位特征。提取衍射横波的最小渡越时间并求得缺陷深度值,其相对误差均在±3之内。实验结果表明,激光-电磁超声检测方法能够有效测量出缺陷深度,可作为接触式压电超声检测技术的一种补充方案,应用于无法满足耦合条件的场合。

Detection internal defects by laser electromagnetic ultrasound based on shear wave diffraction

In order to effectively evaluate the depth of the internal defects of the workpiece,a combined non contact detection method of pulsed laser excitation and electromagnetic ultrasonic receiving waves is proposed.The principle of ablation excitation of ultrasound waves is analyzed,and the diffraction phenomenon of body waves acting on defects and the reception process of electromagnetic ultrasound are analyzed.According to the theory of transverse wave diffraction,when the electromagnetic ultrasonic transducer (EMAT) is directly above the defect,the transit time of the diffracted transverse wave obtains the minimum value.The calculation formula of the defect depth is derived and the detection blind zone is discussed.A laser electromagnetic ultrasonic testing system for internal defects of workpieces is built to detect internal defects in workpieces,and the workpieces with circular hole and cracks with different inclination angles are measured successively.In the case of a fixed excitation point,the signal transit time varies with the position of the EMAT,and the phase characteristics of the diffracted transverse wave are analyzed.The defect depth value is obtained according to the minimum transit time of the diffracted transverse wave,and the measurement error of the defect depth is within ±3%.The experimental results show that the laser electromagnetic ultrasonic testing method can effectively measure the defect depth of defects and can be used as a supplementary scheme of contact piezoelectric ultrasonic testing technology for applications where coupling conditions cannot be met.