闭合裂纹非共线混频超声检测试验研究

针对结构中微裂纹检测难题,发展了一种闭合裂纹非共线混频超声检测方法。在对非共线混频超声检测机理分析基础上,进行了结构中疲劳裂纹混频非线性超声检测实验。对有无裂纹试件中检测信号进行了滤波和时频分析,结果表明,可根据信号滤波后时域波形中是否存在明显的混频波包或时频分析中是否存在明显的和频分量,实现有无闭合裂纹的判识;通过移动激励探头的位置,控制两列入射声波在试件中的交汇位置,实现试件中不同深度位置的混频非线性检测。并根据测得的混频非线性系数沿试件深度方向上分布,实现了闭合裂纹沿深度方向上长度的测量。研究工作为结构中微裂纹定量评价做了有益探索。      

应用混频超声检测微小缺陷

高端飞行器的可靠性往往受限于其发动机内部盘、板等构的结构强度和使用寿命,因此对这类部件的无损检测在制造业上具有巨大需求。超声检测作为一种应用广泛、高效、环保的检测方法,常常被应用于这类构件的检测中。但是,大厚度盘、板类构件内的微小缺陷反射能力弱,常规超声脉冲反射法无法进行有效检测。为实现大厚度盘、板类构件内部微小缺陷的识别和定位,采用共线异侧纵波混频法,通过和差频信号特征识别微小缺陷;研究缺陷埋深变化对混频效果的影响,通过测量和差频信号幅值变化,实现微小缺陷的深度定位。结果表明：该方法可有效识别7075铝合金中埋深80 mm的?0.2 mm横孔微缺陷,且可实现微小缺陷的深度定位。     

接触缺陷的振动调制超声导波检测技术研究

针对常规线性超声检测方法无法实现板结构接触类缺陷(如微裂纹、脱粘等)检测问题,将超声导波技术与振动声调制技术相结合,利用稀疏分布传感器发展了一种板结构中接触缺陷非线性超声检测方法。通过低频振动改变缺陷的接触状况,使得通过接触面的高频导波信号的相位和幅值受到调制。对受低频振动调制的超声导波二维时间序列进行时频分析,由于接触类缺陷的存在,在振动调制超声导波序列的时频分布上出现明显的低频振动频率分量。利用提取出的低频振动频率下的超声导波信号,进行了结构接触缺陷成像处理。检测试验表明,基于振动声调制的超声导波缺陷成像方法可以实现结构中的接触类缺陷检测。      

基于频散补偿和分数阶微分的多模式兰姆波分离

由于兰姆波的多模和频散特性,实际检测时在同一激发频率下存在多种模式的混合信号,而各模式信号有不同的频散特性,使得在时频混叠的情况下兰姆波的检测变得十分复杂.本文在频散补偿的基础上,通过时延函数建模,依靠不同模式频散趋势的差异性,将时频混叠信号的分离问题转化为部分模式混叠信号的分离问题.基于分数阶微分的理论,用信号幅值谱分数阶微分极大值和对应频率分别与微分阶次拟合多项式实现特征参数的提取并依靠特征参数重建幅值谱.结合相位谱重构时域信号以实现部分混叠信号中频散补偿后的模式的分离.最后恢复频散获得分离后的兰姆波信号.仿真和实验结果表明,本文方法不仅可以实现时频混叠多模式兰姆波信号的分离,更能保证分离精度,有助于复杂多模式频散信号的分离与处理的进一步研究.     

调制转移光谱光频率标准系统中电光参数的优化研究

根据窄带相位调制光在非线性吸收介质中的近简并四波混频的物理机理,分析了采用电光调制器实现调制转移光谱(MTS)技术的电磁场激励及其动态演化过程,建立了MTS中频率色散谱和吸收谱的理论模型. 研究结果表明:以分子吸收谱线色散谱信号作为频率误差鉴别信号时,系统调制频率取0.72倍谱线线宽附近可以得到最佳的色散信号强度;解调相位的变化对吸收谱的影响很大,但对色散谱而言,解调相位在90°附近的小范围波动对频率误差信号解调效果影响不大;调制度在0.5–1区域可以得到较好的综合稳频效果. 关键词： 光频率标准 调制转移光谱 频率稳定 电光参数优化     

用分数阶微分实现时频重叠多模式兰姆波的模式分离

为了分离时频重叠多模式超声兰姆波,提出了一种用分数阶微分理论实现多模式兰姆波模式分离的方法。以时频重叠的S1和A1模式混合信号为例,用赛利斯分布作为幅值谱的模型,首先对信号做频散补偿,由补偿后信号幅值谱分数阶微分推导了幅值谱特征参数的计算式并根据信号模型重建幅值谱,然后结合兰姆波的频散特性推导出对应的相位谱,并通过幅值谱和相位谱重构各模式的信号,实现模式分离。对1 mm钢板中仿真时频重叠的S₁和A₁模式信号分离结果显示分离出信号的幅值和带宽相对误差小于6%,中心频率相对误差小于0.25%。实验结果也证实了方法的可行性。因此当混合信号幅值谱不完全交叠时,本方法可以实现多模式兰姆波信号的分离,有助于多模式兰姆波频散信号的分析和识别。      

阵列调制随机共振在微弱信号特征提取方面的应用

论述了阵列调制随机共振方法在强噪声背景下多频微弱信号特征提取中的工作原理和实现步骤;采用预先设定系统参数的多个并联非耦合随机共振单元形成阵列,将被测强噪声背景下的多频微弱信号分别与不同频率的载波进行调制,生成多个差频均为0.01Hz的信号作为各对应随机共振单元的激励信号,采用龙格-库塔算法求取各单元输出并进行频谱分析,根据0.01Hz处的信噪比判断在微弱信号中是否存在载波频率与差频值之和大小的频率分量,最后综合各个随机共振单元的检测结果生成微弱信号的频率特征向量;仿真结果表明,阵列调制随机共振在微弱信号特征提取方面效果明显,具有很好的应用前景。     

超声非线性效应表征的动态小波指纹分析方法

针对结构早期损伤超声非线性检测中损伤表征问题,发展了一种基于动态小波指纹的超声信号分析方法,从超声信号的动态小波指纹分布中提取出一种可用于结构早期损伤表征的超声非线性特征参数。研究了小波基函数及分析尺度对超声非线性效应提取效果的影响,优选出对结构早期损伤敏感的小波基函数以及尺度范围。将提出的动态小波指纹分析方法应用于二次谐波及混频非线性超声检测信号分析,结果表明,动态小波指纹分析方法可有效提取出检测信号中的二次谐波及混频分量,基于小波指纹分布的非线性特征参数可用于板结构中微裂纹的定量表征.本文研究工作为结构早期损伤超声非线性检测中的弱非线性效应提取作了有益探索。      

金属基复合结构粘接强度的非线性超声混频检测

采用非线性超声混频方法对金属基复合结构的粘接强度进行表征研究。以不同固化剂含量的铝合金丙烯酸酯粘接结构作为检测对象,基于非线性超声混频的共振条件及其作用机制,并结合粘接固化机理,开展了非线性超声混频实验。非线性超声混频模式选取两列横波生成和频纵波的方式,实验测量并计算不同基频周期数下的超声非线性参量。通过拉伸实验标定粘接结构的粘接强度,并采用扫描电镜分析拉伸断面微观结构,进而分析超声非线性参量与粘接强度的关系。从实验结果可以看出在不同周期的基频信号激励下,超声非线性参量随着粘接强度的增大均呈现出减小的趋势。研究表明非线性超声混频信号对金属基复合结构粘接强度的变化比较敏感,可以适用于类似结构件粘接强度状态弱化的表征。     

α稳定噪声下一类周期势系统的振动共振

将多个低频微弱信号、高频信号和加性α稳定噪声共同激励的一类周期势系统作为研究模型,以平均信噪比增益(MSNRI)为性能指标,对α稳定噪声环境下周期势系统中的振动共振现象进行了研究,分别探究了α稳定噪声的特征参数α、对称参数β、加性噪声强度放大系数D、高频信号幅值B以及频率?对振动共振输出效应的影响.研究结果表明:1)在不同分布的α稳定噪声环境下,固定频率?(或幅值B),当幅值B(或频率?)逐渐增大时,MSNRI-B(或MSNRI-?)曲线出现多个峰值,即存在多个B区间(或?区间)可诱导振动共振,并且这些区间不会随噪声分布参数α或β的变化而变化;2)当加性噪声强度放大系数D发生变化时,幅值B和频率?的共振区间没有随着D的变化而变化,表明只有高频信号能量向待测低频信号转移,噪声能量并没有向待测低频信号转移.另外当幅值B、频率?固定时,随着D的逐渐增大,依然可以实现微弱信号的检测,表明振动共振可以克服工业现场噪声强度不可调控的缺点.本文研究结果提供了一种新的微弱信号检测方法,在信号处理领域有着潜在的应用价值.     

Microcrack localization using a collinear Lamb wave frequency-mixing technique in a thin plate

A novel Lamb wave frequency-mixing technique is proposed for locating microcracks in a thin plate,which does not require the resonance condition of Lamb wave mixing and can accurately locate the microcracks through only one-time sensing.Based on the bilinear stress-strain constitutive model,a two-dimensional finite element(FE)model is built to investigate the frequency-mixing response induced by the interaction between two primary Lamb waves and a microcrack.When two primary Lamb waves of A0 and S0 modes with different frequencies excited on the same side of the plate simultaneously impinge on the examined microcrack,under the modulation of the contact acoustic nonlinearity,the microcrack itself can be deemed as the secondary sound source and it will radiate the Lamb waves of new combined frequencies.Based on the time of flight of the generated A0 mode at difference frequency,an indicator named normalized amplitude index(NAI)is defined to directly locate the multi-microcracks in the given plate.It is found that the number and location of the microcracks can be intuitively visualized by using the NAI based frequency-mixing technique.It is also demonstrated that the proposed frequency mixing technique is a promising approach for the microcrack localization.     

振动声调制技术下微裂纹的非线性响应分析

采用非线性弹簧模型对振动声调制技术下微裂纹产生的非线性调制效应进行解释。从理论上分析了振动声调制技术下微裂纹产生非线性调制现象的原理,将微裂纹的开合状态等效为非线性弹簧。利用有限元软件构建并仿真微裂纹非线性弹簧模型,分别观察不同激励下裂纹界面的开合状态及混合声波的传播特性。结果表明,微裂纹界面随低频声波呈周期性开合,使得通过裂纹界面的高频声波与低频声波相互调制而产生非线性现象。在以上研究基础上,开展振动声调制实验研究。对比仿真结果,非线性调制系数R在实验条件下和仿真随低频激励幅值的变化趋势相同,证明了该模型的正确性。该模型为振动声调制技术应用于微裂纹的超声非线性检测提供了理论依据。      

Micro-crack detection of nonlinear Lamb wave propagation in three-dimensional plates with mixed-frequency excitation

We propose a nonlinear ultrasonic technique by using the mixed-frequency signals excited Lamb waves to conduct micro-crack detection in thin plate structures. Simulation models of three-dimensional(3D) aluminum plates and composite laminates are established by ABAQUS software, where the aluminum plate contains buried crack and composite laminates comprises cohesive element whose thickness is zero to simulate delamination damage. The interactions between the S_0 mode Lamb wave and the buried micro-cracks of various dimensions are simulated by using the finite element method.Fourier frequency spectrum analysis is applied to the received time domain signal and fundamental frequency amplitudes,and sum and difference frequencies are extracted and simulated. Simulation results indicate that nonlinear Lamb waves have different sensitivities to various crack sizes. There is a positive correlation among crack length, height, and sum and difference frequency amplitudes for an aluminum plate, with both amplitudes decreasing as crack thickness increased, i.e.,nonlinear effect weakens as the micro-crack becomes thicker. The amplitudes of sum and difference frequency are positively correlated with the length and width of the zero-thickness cohesive element in the composite laminates. Furthermore,amplitude ratio change is investigated and it can be used as an effective tool to detect inner defects in thin 3D plates.     

双层粘接板界面的超声非线性谐振特性分析

采用声-力-电类比建立粘接界面非线性力学行为的等效非线性振荡电路,以求解双层粘接板的超声非线性谐振频率。理论上导出非线性谐振频率方程,确定双层粘接板非线性谐振频率与激励幅度、三阶弹性劲度系数的解析关系。双层粘接铝板的超声实验发现:在不良粘接情形下,超声谐振频率发生偏移,其值大于粘接完好区,且激发了较强的三次谐波,但二次谐波幅度变化不大。实验结果表明三次谐波幅度上升,超声谐振频率也显著增大,与理论导出非线性谐振频率变化规律相吻合,且三次谐波与基波、二次谐波的比值反映了非线性谐振频率变化趋势,证实粘接层三阶劲度系数是产生非线性共振频率偏移的主要因素。      

Chirp excitation of ultrasonic guided waves

Most ultrasonic guided wave methods require tone burst excitations to achieve some degree of mode purity while maintaining temporal resolution. In addition, it is often desirable to acquire data using multiple frequencies, particularly during method development when the best frequency for a specific application is not known. However, this process is inconvenient and time-consuming, particularly if extensive signal averaging at each excitation frequency is required to achieve a satisfactory signal-to-noise ratio. Both acquisition time and data storage requirements may be prohibitive if responses from many narrowband tone burst excitations are measured. Here chirp excitations are utilized to address the need to both test at multiple frequencies and achieve a high signal-to-noise ratio to minimize acquisition time. A broadband chirp is used to acquire data at a wide range of frequencies, and deconvolution is applied to extract multiple narrowband responses. After optimizing the frequency and duration of the desired tone burst excitation, a long-time narrowband chirp is used as the actual excitation, and the desired tone burst response is similarly extracted during post-processing. Results are shown that demonstrate the efficacy of both broadband and narrowband chirp excitations.     

Multiple solutions of asymmetric potential bistable energy harvesters: numerical simulation and experimental validation

In this paper, we investigate the multiple solutions of nonlinear asymmetric potential bistable energy harvesters (BEHs) under harmonic excitations. Basins of attraction under certain excitations explain the existence of multiple solutions in the asymmetric potential BEHs and indicate that the asymmetric system has a higher probability to oscillate in the deeper potential well under low and moderate excitation levels. Thus, the appearance of asymmetric potentials in BEHs has a negative influence on the output performance. Average output powers under different excitation frequencies and initial conditions illustrate that the asymmetric potential BEHs are more likely to achieve high-energy branch (HEB) with initial conditions in the shallower potential well, and the probability is influenced by the degree of asymmetry of the BEHs. Finally, experiments are carried out, and results under constant and sweep frequency excitations demonstrate that the output performance will be actually improved for the asymmetric potential BEHs if the initial oscillations are from the shallower potential well.     

基于等效弹性模量的微裂纹-超声波非线性作用多阶段模型

提出了一种基于等效弹性模量的微裂纹-超声波非线性作用多阶段模型.该模型将微裂纹微观层面的界面几何特征和宏观层面的界面相对运动统一为介观单元弹性模量的变化,利用等效弹性模量表征损伤区域的"应力-应变",然后利用分段函数来描述微裂纹-超声波非线性相互作用,最后通过有限元仿真对波动方程进行求解,验证了模型的有效性,获得了超声波在经过微裂纹后传播的非线性波动规律.仿真结果表明本文提出的模型相比于双线性刚度模型、接触面模型,能更好地体现一个谐波周期内超声波经过微裂纹损伤区域时波形会发生畸变.同时,仿真实验还分析了裂纹倾角、裂纹长度和超声波激励幅值对超声波经过微裂纹后产生的二次和三次谐波的幅值的影响.最后,对比分析了该模型的仿真计算结果与实验测试结果,表明本文提出的多阶段模型与实验测试均能较好地体现微裂纹-超声波非线性作用产生的二次谐波信号,且结果基本一致,验证了模型的有效性.该模型为开展超声波非线性效应定量检测微裂纹提供了一种新的仿真手段.