超声调频激励信号在粗晶奥氏体不锈钢焊缝中的传播特征

Owing to the typical structure characteristic of coarse-grained austenitic stainless steel welded joint, there will be serious scattering and attenuation during the propagation of the conventional excitation ultrasonic wave, along with the complex behaviors including beam skewing, splitting, and curved propagating, which have serious effect on the signal-to-noise ratio (SNR) and positioning accuracy, and make the defect identification difficult. A novel frequency modulation coded signal with large time width, large bandwidth and complex frequency component can obtain high detection SNR, resolution and detection accuracy by modulated coded designing and pulse compression arithmetic, which can make the difficulties mentioned above be solved. The attenuation characteristics and propagation behaviors of ultrasonic frequency modulation excitation signal in coarse-grained austenitic stainless anisotropic welded joint will be studied in this project. Based on the analysis on the interaction mechanism of the coded signal in coarse-grained structure, the attenuation model will be established, and the relation between frequency component and attenuation coefficient will be obtained. The optimization frequency modulated signal will be designed, and then the analysis of ultrasound propagation behavior, the prediction of beam propagation path and defect positioning correction will be carried out by beam propagation theory in anisotropic medium. This research has an important theoretical significance on the further understanding of the propagation characteristics of sound wave with composite frequency in coarse-grained anisotropic medium and the advancement of the ultrasonic coded excitation method, and will be of practical application value in improving the quality evaluation of the welded structures.

由于粗晶奥氏体不锈钢焊接接头的典型组织特征，常规超声激励的声波在传播过程中会产生严重的散射和衰减，并伴随声束偏转、分离和曲线传播等复杂现象，严重影响缺陷检测的信噪比和定位精度，使缺陷识别困难。一种具有较大时宽、带宽和复杂频率成分的调频编码信号，可利用调制编码设计及脉冲压缩算法，同时获得较高的检测信噪比、分辨力和检测精度，有望解决上述焊缝检测的难点问题。本项目拟研究新型的超声调频激励信号在粗晶奥氏体不锈钢各向异性焊缝中的衰减特性和传播行为。通过建立复合频率声波在粗晶组织中的衰减模型，获得调频信号的频率成分与衰减系数的对应关系；在此基础上，优化设计调频激励信号，结合各向异性介质中声线传播理论，实现其传播行为分析、传播路径预测以及缺陷定位修正。本项研究对于深入了解复合频率声波在粗晶各向异性介质中的传播特征，提高此类焊缝缺陷检测能力，完善和推进超声调频激励检测技术具有重要的理论意义。

在航空航天、尖端武器、核电等领域的重要设备中，以奥氏体不锈钢为主的粗晶材料在一些关键部位的焊接结构中得到了广泛应用。奥氏体不锈钢焊接接头通常具有局部统一排列的粗大柱状晶组织，呈现出典型的不均匀性和各向异性，导致超声波在其中传播时会产生声束畸变、分离和曲线传播等复杂的物理现象。在常规超声脉冲检测中，由于检测距离、检测信噪比、时间分辨力之间一直存在难以调和的矛盾问题，在粗晶材料检测中体现尤为突出。基于以上问题，本研究创新设计了一种适用于粗晶材料的调频编码激励信号，研究了调频编码激励信号在粗晶材料中的衰减特征，建立了焊缝材料相对衰减系数的测量方法，阐明了晶粒组织、各向异性特征与超声频率成分等对声波衰减系数的影响规律，发现奥氏体不锈钢焊缝的衰减系数与超声信号频率的平方成正比关系；通过调频编码信号参数选择方法、超声检测系统的频谱特性，提出了基于超声探头频谱特性的非线性调频编码信号的设计方法，利用信号检测仿真与实验，验证了该信号与超声系统良好匹配特性，以及具备综合提升粗晶材料中检测信噪比、时间分辨力的能力；进一步研究了超声调频信号在粗晶奥氏体不锈钢各向异性焊接接头中的传播行为，建立了调频编码信号的声场传播模型及可视化仿真程序，相比于常规超声脉冲，调频编码信号具有更强的辐射声场，更长的传输距离以及更低的散射，在后续的厚壁奥氏体不锈钢焊缝缺陷的调频激励检测与定量识别研究中有效的验证了这类信号的检测适用性。通过本项目研究为大厚度粗晶焊缝的缺陷检测提供了一种新的技术途径，也进一步完善和推进了超声调频激励检测技术的实际工程应用。