层状复合材料疲劳损伤引起的界面性质变化、材料的力学参数变化及微裂纹扩展等因素将影响到兰姆波的传播特性，进而将显著地影响到兰姆波的二次谐波发生效应。考虑层状复合材料的各向异性因素，建立了非线性超声兰姆波在其中传播的理论模型；在此基础上分析了疲劳导致的材料力学参数改变(包括弹性常数和各向异性等)及界面性质变化等因素对兰姆波传播特性及其二次谐波发生效应所产生的影响。理论研究表明，兰姆波的二次谐波发生效应可对层状复合材料的疲劳损伤进行更为准确的评价。建立了非线性超声兰姆波实验测量系统，可将其与疲劳试验机结合起来以构成疲劳损伤的在线评价实验系统；根据理论分析所给出的非线性超声兰姆波的实验条件，在疲劳载荷作用过程中，测量了兰姆波二次谐波的幅频特性，引入了可准确表征层状复合材料疲劳损伤的非线性兰姆波应力波因子。实验研究表明，将非线性兰姆波应力波因子作为损伤参量,可准确评价层状复合材料的早期疲劳损伤程度。本项目提出的非线性超声兰姆波评价方法综合考虑了各种疲劳损伤形式的累积效应及总体效果，可对层状复合材料的疲劳损伤进行更为准确的评价，尤其是在预报层状复合材料的早期疲劳损伤方面具有重要的实际意义。

Change in the interfacial properties and the mechanical parameters, and initiation and propagation of micro-cracks in materials, caused by fatigue-induced damage in layered composites, will influence the features of Lamb wave propagation. Further these changes will remarkably influence the effect of second-harmonic generation of Lamb wave propagation. The theoretical model of nonlinear ultrasonic Lamb wave propagation has been established when the anisotropy of materials of layered composites is taken into account. Based on this model, we have theoretically investigated the influences of changes in mechanical parameters of materials (including elastic constants and anisotropy, etc.) and interfacial properties, induced by fatigue, on the effect of second-harmonic generation by Lamb wave propagation. The theoretical analyses indicate that the effect of second-harmonic generation of Lamb wave propagation has a potential for accurately assessing fatigue-induced damage in layered composites. Based on the Ritec SNAP 5000 system, an experimental measurement system for nonlinear Lamb waves has been established. This system may be used for in-situ assessment of fatigue damage in layered composites while it is combined with an appropriate fatigue test machine. Based on the experimental conditions for nonlinear Lamb waves, obtained by the theoretical analyses, the experimental measurements are performed for the amplitude-frequency curves of second harmonics of Lamb wave propagation in the solid plates subject to cyclic loading. Further a parameter (namely the nonlinear Lamb wave stress-wave factor) is introduced, which may be used to accurately characterize the degree of fatigue damage in layered composites. The experimental results have shown that, as a damage parameter, the nonlinear Lamb wave stress-wave factor can be used to assess the early stages of fatigue damage in layered composites. The nonlinear Lamb wave approach for assessment of fatigue damage in layered composites, which is proposed in this project, synthetically considers the cumulative effect of each aspect of fatigue as well as its overall effect. Theoretical and experimental results indicate that we can more accurately assess fatigue damage in layered composites using the proposed approach. Especially, it is of practical significance that the nonlinear Lamb wave approach can be used to predict the early stages of fatigue damage in layered composites.

层状复合材料疲劳损伤引起的界面性质变化、材料的力学参数变化及微裂纹扩展等因素将影响到兰姆波的传播特性，进而将显著地影响到兰姆波的二次谐波发生效应。考虑层状复合材料的各向异性因素，建立了非线性超声兰姆波在其中传播的理论模型；在此基础上分析了疲劳导致的材料力学参数改变(包括弹性常数和各向异性等)及界面性质变化等因素对兰姆波传播特性及其二次谐波发生效应所产生的影响。理论研究表明，兰姆波的二次谐波发生效应可对层状复合材料的疲劳损伤进行更为准确的评价。建立了非线性超声兰姆波实验测量系统，可将其与疲劳试验机结合起来以构成疲劳损伤的在线评价实验系统；根据理论分析所给出的非线性超声兰姆波的实验条件，在疲劳载荷作用过程中，测量了兰姆波二次谐波的幅频特性，引入了可准确表征层状复合材料疲劳损伤的非线性兰姆波应力波因子。实验研究表明，将非线性兰姆波应力波因子作为损伤参量,可准确评价层状复合材料的早期疲劳损伤程度。本项目提出的非线性超声兰姆波评价方法综合考虑了各种疲劳损伤形式的累积效应及总体效果，可对层状复合材料的疲劳损伤进行更为准确的评价，尤其是在预报层状复合材料的早期疲劳损伤方面具有重要的实际意义。