摘要:超声检测技术凭借它快速、高效率、适用范围大的优点在无损检测领域得到了非常广泛的应用。基于超声波在传播过程的波速、衰减等信息的传统检测方法能够评价和检测结构的宏观损伤和缺陷，然而它不能有效地检测和评估材料的轻微损伤。

材料的性能退化总是伴随着某种形式的非线性力学行为,这可能会导致超声波传播的非线性,即高频谐波的产生。对于金属材料的早期力学性能退化等问题，本篇文章从理论和试验两个方面研究了金属材料在周期性疲劳负载等作用下的超声非线性特性,并提供了金属材料的非线性系数的测量和信号处理方法，从而利用二次谐波的非线性系数来检测金属材料（铝合金）的早期疲劳损伤。实验结果表明本文利用傅立叶变换（FFT）分析超声信号,分析计算非线性系数的变化规律。研究结果表明：在高周疲劳加载模式下,非线性超声测试的非线性系数随疲劳载荷的增加而变大,非线性系数与载荷应力和疲劳周次有密切的关系，非线性系数与载荷方式和疲劳周次有密切的关系。综上，超声非线性系数可以很好地反映出金属的早期力学性能退化。

关键词:无损检测;力学性能退化;铝合金;非线性系数

Abstract:Ultrasonic non-destructive testing technique has been very widely used in industrial production by virtue of its fast, high efficiency, and extensive application.. Based on wave velocity, attenuation and other propagating characteristics,  the  traditional ultrasonic testing technique can be applied to detect macro damage and defects in structue. However, it can not effectively detect and evaluate the micro damage and mechanical degradation of materials and structures.

The degradation of the material is always accompanied by some form of nonlinear mechanical behavior, which may lead to non-linear propagation of ultrasound, that is, the generation of high frequency harmonics. In present paper, for the mechanical propertie degradation of metal materials the ultrasonic nonlinear characteristics of aluminum alloy under fatigue load were studied., It also provides a nonlinear measurement method for metalsand signal processing techniques. The nonlinear coefficients related to magnitude of the second harmonic were used to detect the early fatigue damage in metals . In this paper, the fast Fourier transforms were performed on ultrasonic signal and the nonlinear coefficients were calculated. The results show that the nonlinear coefficient of ultrasonic ncrease with the magnitude of fatigue loadingand has a linear relationship with the propagation distance. Furthermore, the nonlinear coefficient is closely related to fatigue loading mode and the fatigue cycle.Overall, the nonlinear ultrasonic coefficient can be used to detect micro damage and property degradation in metal.

Key words: nondestructive testing； mechanical properties degradation； aluminum alloy；nonlinear coefficient；second harmonic

目录

第一章绪论 1

1.1材料或结构力学性能退化检测的意义和检测方法 1

1.1.1材料或结构力学性能退化检测的意义 1

1.1.2金属力学性能退化检测的发展历程 2

1.2非线性超声检测方法的特色及优点 4

1.3介绍主要研究内容 6

第二章非线性超声检测的相关理论基础 7

2.1非线性超声检测的相关理论 7

2.1.1超声波概论 7

2.1.2超声波的传播 8

2.1.3非线性超声波检测的理论基础