超声兰姆波二次谐波发生效应的理论、实验及应用研究进展

Advances in theoretical,experimental and applied studies on second-harmonic generation of ultrasonic Lamb waves

材料退化或微损伤的早期无损检测和评价可以保障关键结构的安全服役.因此,开展缺陷演化的早期评价,甚至缺陷产生前的早期检测是非常重要的.通常,线性超声检测方法对材料微观缺陷不敏感.非线性超声技术被广泛认为可以表征材料微观结构变化.二次谐波发生效应作为超声传播过程中的一种典型非线性声学效应,可以用来评价材料的性能退化程度和检测早期的微缺陷.此外,超声兰姆波被广泛应用于大型工业结构的健康监测及材料的无损检测和评价.基于超声兰姆波的无损检测方法能够检测出结构不可达或隐蔽区域,并具有低成本、高效率的优势.兰姆波二次谐波结合了兰姆波的优点和二次谐波对微损伤的高敏感特性,近年来受到越来越多的关注和研究.然而,由于兰姆波的频散和多模式特性,一般认为兰姆波传播过程中不存在强烈的非线性效应,实验上也很难对兰姆波二次谐波进行观察与测量.近年来,虽然兰姆波二次谐波的理论和实验研究取得了重要的进展,但该领域仍存在一系列科学和技术难题.本文系统梳理了超声兰姆波二次谐波发生效应的理论研究历程,综述了兰姆波二次谐波的实验研究进展以及在不同类型损伤评价中的应用,讨论了超声兰姆波二次谐波近年来研究中出现的争议问题,最后展望了兰姆波二次谐波的未来研究重点及发展趋势.

Nondestructive evaluation and testing of material degradation or micro-damages in an early stage are essential for the maintenance of the safety-critical structures.Therefore,it is important to detect defects in a very early stage or even better to detect changes in the microstructure before the defect initiation even occurs.Linear ultrasonic characteristics are not sufficiently sensitive to the kind of microscopic degradation of the material.However,on the other hand,even very small imperfections can produce very significant excess nonlinearity,which can be orders of magnitude higher than the intrinsic nonlinearity of the intact material.The excess nonlinearity is produced mainly by the strong local nonlinearity of microcracks.In order to assess the damage in components at an early stage,the use of nonlinear acoustics has been shown to be the leading and one of the most promising techniques.Second harmonic generation is a typical acoustic nonlinear effect of ultrasonic waves,which can be used to assess the material degradation and detect the micro-defects in their early stages.Furthermore,ultrasonic Lamb wave has been widely used in large-scale industrial structures for health monitoring and material nondestructive testing.The possibility to inspect inaccessible or hidden areas and great cost-effectiveness are the key advantages of ultrasonic Lamb wave-based damage detection approaches.Second harmonic Lamb wave,which combines the advantages of Lamb waves and the high sensitivity of second harmonic waves to microdamage,is drawing increasing attention in recent years.The measurement of second harmonic Lamb waves is investigated as a promising feature extraction technique to characterize material nonlinearity in plate-like structures.However,the second harmonic fields of Lamb waves are much more complex due to the dispersion and multi-mode nature of propagating Lamb waves.In general,the effect of second harmonic generation is often very small and can easily be overlooked experimentally.Consequently,proper mode tuning with physically based features is essential to enhance the efficiency of second harmonic Lamb wave generation and reception.In experimental work,it is also difficult to observe and measure the second harmonic Lamb waves.Although significant development has been made in theoretical analysis and experimental investigation of second Lamb waves,the use of second harmonic Lamb wave for structural damage assessments is still limited by a series of scientific and technical difficulties.In this paper,the theoretical and experimental advances concerning second harmonic Lamb wave are systematically reviewed.First,theoretical analysis of second harmonic Lamb waves based on nonlinear reflection of acoustic waves is reviewed.Then,a general approach of perturbation method and normal modal expansion for theoretical study of the second harmonic Lamb waves propagation in a plate is introduced.The physical insight of nonlinear features among different Lamb modes is clarified.As revealed in these pieces of theoretical work,the second harmonic modes of Lamb waves display an accumulative effect when conditions of phase matching and non-zero power transfer from the fundamental mode to the second harmonic mode are satisfied.The conditions for strong nonlinear effect of Lamb wave are discussed.Experimental observation and measurement of second harmonic Lamb waves are reviewed.Cumulative effect and efficiency of second harmonic generation of Lamb waves are discussed.The guideline for choosing the Lamb modes with higher sensitivity to material nonlinearity is provided.Based on the theoretical and experimental studies,the applications of second harmonic Lamb waves for material damage evaluation are summarized.In addition,some obscure understanding and disputed issues during the study in this research area are clarified.Finally,prospective priorities and research trends of second harmonic Lamb waves are discussed.