光热调制真实微裂纹的非线性激光超声检测机理及应用研究

The present fundamental research and experiment development project is dedicated to significantly improve the limited detectability of the conventional laser ultrasonic method to real micro-cracks, especially partially closed cracks. In this project, the Non-Destructive Evaluation (NDE) of real cracks using the nonlinearity caused by the interaction between laser-generated ultrasound waves and a real crack modulated by laser irradiation will be studied theoretically and experimentally. In the theoretical part, The dynamic response of the crack under different laser irradiation loading, as well as the mechanism of nonlinear interaction between the modulated crack and the laser-induced ultrasound waves will be analyzed; A laser-ultrasonic evaluating setup for real crack under opto-thermal modulation will be build experimentally, by exploiting changes in propagating times, amplitudes, as well as spectrums of multiple acoustic modes, before/after the closure of the crack,the mechanism of the nonlinear interaction between the near-field-generated acoustic waves and the crack can be studied, so that the position and width of the crack, roughness of the crack face, as well as the information of inclusion within the crack can be obtained accordingly. Results in this project are expected to provide theoretical guidance and experimental evidence for the high-sensitivity detection and multiple-parameters extraction of a real crack by using the nonlinear laser-ultrasonic technique.

传统激光超声方法对真实的微裂纹，尤其是部分闭合裂纹的检测能力有限，且无法获得裂纹开口宽度等信息。针对这些问题，本项目拟利用由激光激发的超声波与光热调制下裂纹的相互作用引起的非线性效应，从理论和实验两方面进行真实裂纹的激光超声无损检测方法研究。理论方面，研究加载不同加热光致使裂纹闭合的动态特性，以及裂纹闭合与光致超声的非线性作用机理；实验方面，建立基于光热调制裂纹的激光超声检测装置，通过检测多种材料中光热调制致裂纹闭合前后，超声波在传播时间、幅值以及频谱上的变化，研究光致近场超声波与光热调制下裂纹相互作用引起的非线性效应，最终得到裂纹的位置、开口宽度、裂纹壁表面粗糙度以及裂纹内是否掺杂等信息，从而为利用激光超声技术进行真实裂纹的高灵敏度探测和多信息提取提供理论指导和实验依据。

本项目针对传统激光超声方法对真实微小或部分闭合裂纹检测能力有限的问题，利用不同时空调制的激光激发超声波，观测研究了真实微裂纹在光热调制下的闭合、由裂纹闭合引起的非线性混频，以及巨幅光声信号经过裂纹时的非线性改变等现象。具体地，搭建了四套全光学超声激发/检测实验装置，使用数值模拟和实验等手段，先后研究使用激光阵列源在材料中以热弹机制无损激发高信噪比声波以及窄带声波技术，非线性光声混频激发技术，基于光吸收涂覆层的巨幅声脉冲激光激发技术，以及特定方向超声体波的连续激光扫描激发技术等；通过检测脉冲激光激发的不同模式声波的幅值变化，观测了真实裂纹在幅度调制激光辐照下的闭合，并首次观测到粗糙的裂纹壁在闭合过程中发生的不可逆改变以及裂纹在施加加热光后的动态闭合响应现象；在已有非线性混频技术基础上，改进扫描光源，固定激发源和检测源，通过仅扫查加热光的手段，消除激发光和检测光热效应对裂纹闭合的影响，并获得了与同时扫查加热光和激发光相似的混频信号，得到了裂纹的位置信息；通过在材料表面施加碳悬浮颗粒涂覆层的方式在材料内激发出巨幅声波（应变比普通激光激发超声高至少一个量级），并利用巨幅声波激发裂纹迟滞非线性的激光超声技术实现了裂纹的二维扫描检测。本研究的开展，可进一步拓宽激光超声技术的应用场合，改进和完善现有非线性激光超声技术，帮助新型高精度非线性激光超声缺陷/裂纹检测技术缩短实用化周期。