利用激光超声纵波的衍射信号测量开口裂纹

激光超声的纵波声场在工件内部各个方向均有分布,相比于横波和表面波,纵波最先到达开口裂纹处,随后发生衍射以及产生波型转换现象。若提取相应含有开口裂纹信息的超声波信号,便能够达到缺陷识别的目的。基于烧蚀机制下激光超声纵波声场的分布特点,分析了衍射纵波信号的时间及相位特征,并给出了裂纹参数的表达式。建立了含开口裂纹工件的二维有限元模型,根据衍射纵波的渡越时间计算了开口裂纹的深度并进行了对比。同时还探讨了衍射纵波的相位反转现象,为检测过程中衍射纵波的识别提供了依据。最后对开口裂纹进行了激光超声测量实验,验证了利用激光超声纵波的衍射信号测量开口裂纹的可行性。

Measurement of opening cracks using diffraction signal of laser ultrasonic longitudinal wave

Laser ultrasonic longitudinal wave sound field is distributed in all directions inside the workpiece.Compared to shear wave and surface wave,longitudinal wave reaches the opening crack first,followed by diffraction and wave mode conversion.If the corresponding ultrasonic signal which contains the crack information of the opening crack is extracted,the purpose of defect identification can be achieved.Based on the distribution characteristics of the ultrasonic longitudinal wave sound field generated by laser ablation,the time and phase characteristics of the diffracted longitudinal wave are analyzed,and the expression of the crack depth is given.A two-dimensional finite element model for the sample with an open crack is established,and the depth of the open crack is calculated based on the time of flight of the diffractive longitudinal wave.At the same time,the phenomenon of phase inversion of diffractive longitudinal wave is discussed,which can provide a basis for the identification of diffractive longitudinal wave during the detection process.Finally,a laser ultrasonic measurement experiment is performed;the test results show that it is feasible to use the diffraction signal to measure the open crack.