基于非线性超声的焊接接头早期疲劳损伤的预测与微裂纹表征

The early diagnosis and prediction of fatigue damage in welded structures and joints is the key to ensure the reliability of the large structure and equipments. However, because of the complexity of the structure and the inhomogeneity of the weld joint microstructure, it is difficult to detect those structures. In the early stage of fatigue, the property degradation and microcracks appear in the internal structure is difficult to detect with traditional methods. In this project, the nonlinear acoustic method is planned to be used to investigate the acoustic wave distortion caused by the fatigue damage in welded joints, including the amplitude variation of the higher harmonics and the modulation effects between the acoustic waves. The influence of the fatigue damage and microcracks on the mechanical property of the joints will be analyzed and the corresponding mathematical and physical model will be established. And then the nonlinear propagation behavior and the nonlinear interaction mechanism of the acoustic signals in the fatigue damage area will be discussed; Controllable excitation method such as swept-frequency excitation will be used to obtain the optimal parameters, the nonlinear information of the output signal will be extracted with modern signal procession such as Hilbert-Huang transform (HHT ) and wavelet analysis. A prediction model for the fatigue damage of the welded joint will be established and a corresponding nonlinear parameter will be defined, in order to realize the early prediction and characterization of the fatigue damage and the microcracks. This research will have an important theoretical significance on the further understanding of the propagation behavior and interaction of the acoustic waves in the inhomogeneous materials, and will be of practical application value in improving the use safety of the welded structures.

焊接结构及接头疲劳损伤的早期诊断与预测是保证大型结构与设备可靠运行的关键，其难点在于结构的复杂性以及焊接接头组织的不均匀性，在疲劳的早期阶段，常规技术难以检测与发现其内部出现的性能退化及微裂纹。本项目拟采用非线性声学方法研究焊接接头疲劳损伤引起的声波的畸变，即高次谐波的变化以及声波间的调制效应等。具体方法是通过分析疲劳损伤及微裂纹的出现对接头力学性能的影响，建立相应的数学物理模型，进而探讨声信号在疲劳损伤区域的非线性传播行为及非线性相互作用机制；采用扫频激励等可控激励方法获取最佳检测参数，利用希尔伯特黄变换、小波分析等信号处理方法，提取回波信号中的非线性信息。在此基础上建立焊接接头疲劳损伤预测模型并定义相应的非线性参数，最终实现疲劳损伤及微裂纹的早期预测与表征。本项研究对于深入了解非均质材料中声波的传播行为及相互作用具有重要的理论意义，同时对于提高焊接结构的使用安全性具有实际的应用价值。

焊接结构及接头疲劳损伤的早期诊断与预测是保证大型结构与设备可靠运行的关键，其难点在于结构的复杂性以及焊接接头组织的不均匀性，在疲劳的早期阶段，常规技术难以检测与发现其内部出现的性能退化及微裂纹。本项目搭建了用于有限振幅超声非线性检测和振动声调制技术的硬件系统，设计加工了用于疲劳裂纹在线监测实验的试样，探讨了声信号在疲劳损伤区域的非线性传播行为及振动/超声在裂纹界面上调制效应及相互作用机制；研究了最佳检测参数的获取方法，以及接收信号中的非线性信息的提取方法，从而对结构损伤程度进行了表征。.研究表明，在疲劳初期、中期和后期，有限振幅波分别主要与疲劳区域中的位错、闭合裂纹和宏观裂纹相互作用。其中声波与闭合裂纹作用产生的非线性最强烈，且裂纹在声波作用下闭合区域越大，非线性效应越强。非线性系数β随着疲劳加载次数N的增长呈先上升后下降的趋势，利用实验获得的β~N曲线预测了U71Mn钢焊接接头试件的剩余疲劳寿命，平均预测误差不超过20%，最低预测误差约为5%。.对于振动声调制检测技术，调制强度峰值出现在结构共振峰两侧，其间隔为低频频率；调制强度随低频激振力增大而增加，但激振力到达临界值后，调制强度基本不再变化；低频频率为工件振动共振频率时，调制明显增强。高频幅度对结果影响较小，而不同高频频率下调制强度差别较大，采用扫频技术寻求最佳高频频率，扫频范围应至少为铝杆纵向共振基频的两倍，最大扫频速率由检测所需的频率分辨率所决定。振动/超声在界面上同时存在幅度调制和频率调制。通过对调制模型修正，提出了采用受调制的高频声波与工件中声波总能量之比对缺陷尺寸进行表征的方法。进一步试验表明，振动声调制技术可对螺栓结构松动进行有效检测，并可对焊接结构疲劳裂纹进行在线监测。.本项研究对于深入了解非均质材料中声波的传播行为及相互作用具有重要的理论意义，同时对于提高焊接结构的使用安全性具有实际的应用价值。