考虑非线性及温度影响的超声表面波应力检测基础研究

In this research project, we will investigate the surface stress measurement of the metallic material by the ultrasonic surface wave acoustoelastic method. The main detected objects are focus on the steel structure which are widely used in most of the huge and complex mechanism. Theoretical analysis and experimental study means will be used to obtain the key points of the ultrasonic surface wave acoustoelastic stress detection method. The main contents of this research plan are shown as follow: (1) The difference identification method of the ultrasonic surface wave propagating characteristics between the stressed and unstressed object surface. In order to get the stress information much more easily, we try to optimize the ultrasonic surface wave acoustoelastic formula deduced by Husson to estimate the surface stress of the detected material. And we will use the digital correlation method to obtain the difference in time of flight between two ultrasonic signals obtained in stressed and unstressed object surface. The calculation reliability, speed and accuracy of the digital correlation method will be discussed by a digital simulation analysis method. (2) The principal factors which may result in the stress detection errors in the experimental research will be deeply discussed, such as the main frequency choosing of the surface wave transducers and the coupling condition between the transducers and the specimen surface. By abundant experimental research we will propose the error reducing method which is corresponding to each kind of experimental error. (3) The nonlinear characteristic of the ultrasonic surface wave acoustoelastic must be researched by theory and experimental means. It is possible that there is a stress concentration near the surface of the specimen because of the surface micro fluctuation which may be considered as micro crack or defect. The stress concentration can result in a plastic deformation near the surface. If the plastic deformation near the surface takes place, the acoustoelastic relationship will be nonlinear. However, the matter of the nonlinear characteristic should be presented much more distinctly in this project research. We will try to propose one kind of modified method to reduce the influence of the nonlinear errors in the surface wave acoustoelastic stress detection method. (4) Finally, in order to impulse the proposed surface stress detection method using in the industrial detection, the change rule of the ultrasonic surface wave acoustoelastic in violently temperature environment will be researched by experimental simulation. In one word, from all of the research results which will be presented after this project, we can obtain the fundamental key points of the mechanism structure stress measurement by the ultrasonic surface wave method.

以工程结构中常见材料（钢材）为主要研究对象，开展基于超声表面波声弹性效应的关键机械结构应力无损检测基础研究。研究内容包括：(1)结构在有、无应力作用时的超声表面波传播特性变化的精确表征方法，将表面波传播速度变化演化为一定声程内传播时间的变化特征，探讨采用数字相关法计算传播时间差；(2)研究影响表面波声弹性实验精度的因素，并针对主要影响因素提出有效降低测量误差的方法或措施；(3)从理论和实验角度研究钢材中表面波声弹性效应的非线性特征，分析非线性特征的形成机理，深入研究表面局部塑性现象对表面波声弹性效应的影响，探讨基于非线性声弹性效应的表面波应力检测方法；(4)实验研究极端温度条件下的表面波声弹性效应的特点，解决特殊环境中的表面波声弹性应力检测的基础问题。其研究成果可为关键机械结构的超声表面波法应力无损检测奠定基础，为大型、关键机械设备（如矿山装备）的结构安全性评估提供新途径和科学依据。

机械结构的可靠性和使用寿命与其关键构件的工程应力水平密切相关，因此结构应力检测对机械装备的安全性评估、维修加固以及后续新产品的优化设计均具有重要意义。本项目从超声表面波的声弹性物理效应出发，将超声波无损检测技术引入到结构表面应力检测中。项目遵循“表面波声弹性效应的精确表征、提取→声弹性效应的非线性特征→特殊环境下的应力检测”的研究主线。.研究了结构在应力作用下的超声表面波传播特性的变化规律的获取方法。搭建并优化了由超声信号发生仪、表面波组合探头和数字信号采集系统组成的声弹性实验研究平台。将表面波传播速度的变化特征演化为一定声程内传播时间的变化特征，提出了采用数字相关方法进行传播时间差的计算，有效地提高了声弹性效应实验的计算效率和精度。.测试了不同表面波探头频率对声弹性效应敏感度的影响，实验表明频率为2.5MHz的表面波对钢材表面的声弹性效应最为敏感，可作为常用工程材料的表面波声弹性实验的探头频率的选择依据。另外，实验研究表明介质表面粗糙度和探头压紧力等因素对耦合性能均有影响，因此在进行声弹性实验时必须保证同等的表面耦合条件，以降低耦合误差对实验准确性的影响。.针对实验过程中发现的表面波声弹性效应的非线性现象，从塑性力学角度解释了非线性特征的形成机理。由于弹性介质表面的微缺陷处易产生应力集中，从而导致弹性体表面局部应力水平偏大，提前进入塑性区，导致传播时间差与应力的关系呈非线性。该研究可为后续的结构表面应力腐蚀以及摩擦、磨损监测提供新的思路。.研究了变温环境下的超声波的传播特性变化规律。初步的实验研究表明温度在-20～70度变化时，材料的弹性模量呈增大趋势，泊松比呈下降趋势，为声弹性曲线的修正提供了理论依据。项目研究成果初步应用于工程实际，对焊接残余应力进行了无损检测，其应力检测的相对误差可控制在20％，并提出了二维应力测量中的主应力分离技术的解决方案，为装备的结构安全性评估提供了新途径。