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光纤光栅在超声结构健康监测中的应用与展望  PDF

  • 吴奇 1
  • 兰伍霖 1
  • 张含琦 1
  • 芦吉云 2
1. 南京航空航天大学机械结构力学及控制国家重点实验室 南京,210016; 2. 南京航空航天大学民航学院 南京,210016

中图分类号: TP212

最近更新:2022-08-25

DOI:10.16450/j.cnki.issn.1004-6801.2022.04.001

  • 全文
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  • 参考文献
  • 作者
  • 出版信息
目录contents

摘要

使用光纤光栅探测包含损伤信息的高频、小能量超声导波,是结构健康监测的关键技术和重点发展方向之一。首先,阐述了光纤光栅探测超声的基本原理,介绍了同时满足高灵敏度和大带宽的相移光纤光栅,揭示了光栅监测的角度相关性以及安装技术;其次,介绍了宽带光源解调法与可调谐激光解调法,将高速的小幅布拉格波长漂移转换为电压输出;然后,介绍了光纤光栅在冲击监测、声发射监测、声‑超声监测三个方面的应用,着重以航空航天复合材料健康监测为例,讨论了损伤判断、定位及分类等功能的实现;最后,对光纤光栅在超声结构健康监测中的应用进行了总结和展望。

∗ 国家重点研发计划资助项目(2021YFF0501800);国家自然科学基金资助项目(11972016,1201101471,U1933202);机械结构力学及控制国家重点实验室开放基金资助项目(MCMS‑I‑0521G04)

引 言

结构健康监测(structural health monitoring,简称SHM)是一种通过监测结构的特性变化评估其损伤状态、程度、位置并预测剩余寿命的技

1。该技术已被广泛应用于航空航天、道路桥梁及工业生产等领域,以提高结构安全性,降低维护成本,延长结构服役时间。在众多结构健康监测技术中,超声结构健康监测通过探测并分析结构中超声信号的衰减、模态转换等特性,实现对内部损伤的有效诊2。超声结构健康监测根据超声激励方式的不同可分为被动监测和主动监3‑4。前者通过监测环境激励或结构内部自发产生的激励实时响应结构破坏的产生,因此不需要额外激励源,且不可重复;后者则需要驱动器主动向结构中施加超声激励,因此能够实现可控的信号输入,对结构进行重复的、非实时的损伤识别。冲击和声发射监测是典型的被动监测技术,而声‑超声监测是典型的主动监测技术。

传感器性能很大程度上影响着探测到的超声信号质量,决定了结构健康状态监测的准确性。传统的超声传感器由压电(lead zirconate titanate,简称PZT)材料制成,虽然稳定性和灵敏度

5,但尺寸大,易受电磁干扰,高低温性能差且长距离传输信号衰减严重。因此,亟需研发一种新型的、性能优异的超声传感器。1978年,光纤布拉格光6(fiber Bragg grating,简称FBG)通过驻波干涉法成功研制之后,已被应用于应变、压力、温度等静态物理量的测7‑9。近年来,通过完善FBG的超声传感理论并研发解调技术,FBG已能满足超声监测的高解调分辨10和超千赫兹带11的要求,成功在固12与液13中实现了超声监测。相较PZT传感器,FBG质量轻、体积小,可嵌入材料内部监测;抗电磁干扰能力强,温度适用范围广,能够在严苛环境中正常工作;可多路复用,适用于搭建传感网络,因此在工程应用中具有巨大的潜力。

笔者将揭示FBG和相移光纤光栅(phase shifted fiber Bragg grating,简称PSFBG)的超声传感原理,分析FBG与导波的耦合特性;总结主流的FBG宽带光源解调

14与可调谐激光解调15;介绍FBG在冲击监测、声发射监测及声‑超声监测3个方面的应用,并主要针对光纤光栅在航空航天复合材料超声监测中的实例展开讨论。

1 光纤光栅技术

1.1 光纤布拉格光栅

FBG是光纤纤芯的折射率发生轴向周期性调制而形成的衍射光

16,如图1所示。当光纤的一端输入宽带光源时,FBG会选择性地反射特定波长范围内的入射光,并透射其余波长的光。反射光谱中存在一个反射峰,其中心波长(亦称为布拉格波长λB)由光栅周期Λ和有效折射率neff决定,其表达17

λB=2neffΛ (1)

图1  FBG原理示意图

Fig.1  Schematic of FBG

当FBG受轴向应变Δεz作用时,几何效应与光弹效应会使光栅周期和有效折射率发生变化,λB产生漂

18。应变引起的波长漂移量ΔλBεΔεz成正比

ΔλBε=1-pεΔεzλB (2)

其中:pε为有效光弹系数。

此外,当FBG受到温度影响时,热膨胀和热光效应也会导致光栅周期和折射率发生改变。因此,FBG可实现应变和温度的监测。

温度变化ΔT对波长漂移ΔλBT的影响为

ΔλBT=α+ξΔTλB (3)

其中:α为光纤热膨胀系数;ξ为热光系数。

超声导波的实质是一种在有界介质中传播的机械应力波,会引起局部的应变高速变化,从而调制FBG,引起λB的快速漂移。因此,类似于应变的测量,通过确定λB的漂移量可实现超声的监测。研究表明,光栅栅长与超声波波长的比例影响了FBG对超声的响

19‑21:①当超声波长远大于光栅栅长时,FBG波长漂移函数退化为式(2),即ΔλB与超声应变成正比;②当超声波波长与光栅栅长接近时,ΔλB小于同等静态应变作用时产生的中心波长漂移量,且随着光栅栅长减小而变小;③当超声波波长远小于光栅栅长时,在整段光栅上应变平均值为0,λB并不会产漂移。因此,短栅长的FBG具有更大的超声探测带22。另一方面,因为FBG传感器的监测灵敏度一般与其光谱主峰线性区域的斜率呈正相关,所以短栅长FBG虽然满足了超声监测带宽的要求,但其光谱中线性段斜率较缓,降低了监测灵敏度。综上,FBG的带宽与灵敏度之间是相互制约的关系,即在相同的光栅调制深度下,仅通过调节光栅长度难以满足监测要求。

1.2 相移光纤光栅

随着光栅制造技术的发展,啁啾光纤光

23、长周期光纤光24、超结构光纤光25等特殊FBG相继被研制出来以应用于超声监测。其中,PSFBG被认为是最适用于超声监测的光栅结构。PSFBG是通过在栅区中心插入一个π相移而制成的,相移区与两端的均匀FBG形成了一个类似法布里‑珀罗的结22

图2展示了在相同的光栅长度与调制深度下仿真的FBG和PSFBG光谱。PSFBG反射光谱中心出现了一个狭窄的反射谷(透射峰),其线性区域呈现更陡的斜率。因此,PSFBG相比FBG有更高的灵敏

27。此外,光功率集中在PSFBG的中心相移区,因此有效光栅栅长更短。该特性使PSFBG拥有更宽的超声带宽,能监测兆赫兹的信28,同时满足大带宽和高灵敏度的高性能超声监测要求。

图2  FBG与PSFBG的光谱图

Fig.2  Spectra of FBG and PSFBG

1.3 光纤光栅与导波的耦合特性

因为在实际的超声监测中,需要在试件上不同位置、不同的测试条件下布置FBG,所以需要研究FBG超声监测的角度相关性与安装方法。

研究表明,FBG和PSFBG具有相似的超声角度相关性,可通过平板和光纤中的应变传递和转换机理进行解

29。如图3所示,当超声激励源与FBG距离较远时,FBG对沿着光纤轴向传播的超声敏感,但对垂直于光纤方向上的超声不敏感,与角度呈近似于三角函数关30。然而,当超声激励源与FBG距离较近且垂直于光纤时,泊松效应使得光栅仍能被调制,探测到的超声波具有相反的相31

图3  PSFBG对超声波的灵敏度分布特[

30]

Fig.3  Sensitivity distribution properties of PSFBG to ultrasonic waves[

30]

常规的FBG安装方式是将光栅区域直接粘贴在待测结构上。该方法虽然操作简单,但服役中结构的大静态应变会使FBG的λB漂移超出解调系统的动态范围。悬臂式粘贴可克服上述问

32。该方法是将FBG栅区旁的光纤粘贴在结构上,使光栅区域悬空以隔绝静态应变;但超声导波仍可通过粘合点传入光纤,并被光纤引导离开高温等严苛环境,继而被FBG监33。在悬臂式粘贴的基础上,将FBG的另一边也粘贴在结构上,可形成桥式粘34。两处粘合点可在不同时刻接收原始激励与损伤信号形成自参考,有助于超声损伤信号的识别与分析。此外,研究表明,粘接剂的选择与粘接面积都会影响FBG与超声导波的耦合特35‑36,这是一个相对复杂且有待解决的问题。

2 解调技术

2.1 宽带光源解调法

宽带光源解调又称为功率解调,原理如图4所示。采用发光二极管和放大自发辐射光源等宽带光源向FBG传感器输入光信号,光信号经FBG反射和光学滤波器滤波后被光电探测器接收并转化为电压信号。由于滤波器光谱Ffilter和传感FBG光谱Fsensor存在一个重叠区域,当传感FBG在超声作用下产生波长漂移时,重叠区域的面积随之变化,输出电压变化V的表达

37

VΔλB=RDλSλFfilterλFsensorλ-ΔλBdλ (4)

其中:RD为光电探测器的影响因子;S为宽带光源的光谱密度。

图4  宽带光源解调技术

Fig.4  Demodulation using broadband light source

在一定的布拉格波长漂移范围内,V与待测量呈线性关系。

根据上述解调原理,Perez

38通过串联一个FBG作为光学滤波器设计了一种匹配光栅解调技术,成功监测到了PZT与断铅试验产生的超声信号,但灵敏度不高。笔者采用两个PSFBG形成匹配光栅结构,利用PSFBG较陡的斜边大幅提高了监测灵敏39。Cui40设计了一种具有反馈控制器的FBG解调系统,解决了反射匹配光栅解调中的非线性问题。此外,M‑Z干涉41、阵列波导光42等结构也能够代替光栅进行滤波。宽带光源解调法易实现且成本低,多路复用能力强,但宽带光源中存在较大的强度噪声与相位噪声,限制了灵敏度,较难监测低幅值或远距离的超声信号。

2.2 可调谐激光解调法

可调谐激光解调又称为边缘滤波解调,原理如图5所示。该方法采用波长可调谐激光作为光源,调节激光器波长至FBG光谱线性区域3 dB位置,光电探测器接收反射光(或透射光),其输出电压V与FBG波长变化成正

43

V=ΔλBGRDP (5)

其中:G为光谱线性斜率;P为激光的光功率。

图5  可调谐激光解调技术

Fig.5  Demodulation using tunable laser source

式(5)可知,输出电压的幅值由光功率与光栅斜率决定,高光功率和大光栅斜率有利于监测低幅值超声信号。然而,可调谐激光解调法中的强度噪声与频率噪声限制了系统的灵敏

44,因此仅通过提高P对监测灵敏度的改善有限。文献[45]提出了平衡光电探测系统,通过同时接收反射光和透射光放大了超声信号并降低了激光强度噪声,结合拥有大线性斜率的PSFBG,该系统的灵敏度相较传统的可调谐激光解调FBG提高了近30 dB。Hu46通过设置PSFBG参考臂探测噪声信号,将频率噪声从原始信号中减去,进一步提高了20 dB的信噪比。与宽带光源相比,波长可调谐激光器所需成本更高,且多路复用能力差。其优势在于对超声监测具有很高的灵敏度,可探测能量小、距离远的超声信号,满足实时监测和大范围监测的需求。

3 光纤光栅在超声结构健康监测中的应用

3.1 冲击监测

冲击是一种作用时间较短的载荷,其特点是信号幅值大、频率低。早期,FBG传感器主要用于监测较大能量的低速冲击,且很难将冲击能量和材料损伤形式相对

47‑48。Chambers49在碳纤维增强复合材料(carbon fiber reinforced plastic,简称CFRP)中嵌入FBG并开展低速冲击试验,量化了冲击能量,探索了CFRP的冲击损伤机制。Takeda50使用小直径的FBG传感器,降低了嵌入式FBG对CFRP机械性能的影响,监测到了CFRP中的冲击分层损伤,讨论了载荷与冲击损伤之间的关系。Jang51针对冲击信号的频率分量进行小波变换,建立了CFRP中分层损伤面积与FBG光谱漂移之间的相关性。

冲击定位可将损伤区域判定在一定范围内,有效降低损伤处理和结构维护的复杂度,是冲击监测中另一个关键问题。FBG传感系统冲击定位的精度主要受FBG传感器的安装方式与信号处理算法影响。在实际应用中,可通过布置FBG三角应变

52、正交应变53等结构避免FBG的超声角度相关性,并根据测量到的超声导波达到时间差来确定冲击位置。Frieden54‑55结合FBG矩形阵列安装与定位插值算法,提出了一套判别CFRP损伤大小和损伤位置的混合数值‑试验优化方法,定位精度高达到5 mm。随着算法的革新,神经网络、机器学习等算法模型被陆续应用于冲击信号处理。Wen56在CFRP表面粘合了包含4个FBG的传感网络,将冲击信号作为特征向量输入神经网络模型进行训练,输出冲击坐标的平均定位误差为2.1 mm。Sai57提出了一种由8个FBG组成的传感系统,通过粒子群优化算法求解信号时间差,将冲击定位平均误差降至1.7 mm,进一步提高了定位精度。

基于FBG的冲击监测在大型复杂结构中也得到一定的应用。Zheng

58建立了一套基于FBG的冲击监测系统,结合投影双字典算法,在碳纤维蜂窝板上实现了高精度的冲击定位。如图6所示,Jang59利用FBG的多路复用性,将包含6段不同中心波长的FBG串嵌入CFRP机翼盒段内,采用神经网络算法在强噪声干扰下实现了冲击信号的监测。Shrestha60使用一维阵列FBG布局和参考数据库算法,开发了一套可应用于全尺寸机翼的监测系统,大幅减少了所需传感器的数量,在4.2 m的机翼上冲击定位误差仅为35 mm。文献[61‑62]在CFRP机翼面板成型过程中嵌入FBG传感网络,建立了一套完整的冲击裂纹损伤监测系统,同时实现了材料内部应变和机翼面板损伤状态的监测。

图6  FBG在机翼盒段冲击监测中的应[

59]

Fig.6  Impact monitoring on wing using FBG[

59]

3.2 声发射监测

声发射是指材料内部突然释放能量而产生瞬时弹性波(应力波)的物理现

10。虽然声发射信号幅值低、频率高,但断铅试验证明了FBG传感器监测声发射信号的可行63。Tsuda32利用FBG的悬臂梁式结构成功监测到了CFRP罐在加压试验中产生的声发射信号,但受传感器共振特性的影响,很难对信号进行超声模式分析。笔者结合平衡解调法和PSFBG,监测了CFRP拉伸测试中产生的声发射信号,通过统计分析、模态分析等,识别了复合材料中基体裂纹、分层和纤维断裂这3种典型的微小损伤形64‑65,如图7所示。此外,还对三点弯试验中的复合材料基体裂纹进行PSFBG监测,通过小波分析揭示了基体裂纹声发射信号的特66。Yu67‑68结合FBG悬臂梁粘贴与平衡解调法对CFRP的损伤开展了定量评估,提出了一种基于峰值频率与幅值比的损伤因子,识别了CFRP层合板损伤类型,阐明了兰姆波模态、声发射信号幅值、峰值频率与损伤类型之间的关系。

图7  PSFBG监测到的CFRP累积声发射[

65]

Fig.7  Accumulative AE hits of CFRP monitored by PSFBG[

65]

与冲击监测类似,定位也是声发射的重要研究内容。Kim

69采用FBG传感系统监测了复合材料机翼断裂时发出的声发射信号,损伤点定位误差仅为14%。Mendoza70开发了一种FBG声发射监测系统(F声发射SenseTM),实现了不同飞机部件中损伤的实时监测、识别及定位等功能。Costa71在T38复合材料机翼四分之一模型中嵌入FBG阵列传感器,通过监测静态应变和动态声发射信号对机翼结构进行健康评估。除了航空航天结构,基于FBG的声发射技术也被应用于大型风力机叶72、长距离传输管73‑74等结构的监测中。

3.3 声‑超声监测

声‑超声监测是主动向结构中输入超声导波,通过探测并分析超声经过损伤区域之后发生的变化而实现结构健康监测。传统使用PZT作为超声发射器和传感器的声‑超声监测技术易受电磁干扰的影响,且信号易发生相互串扰,限制了损伤信号识别的精度。此外,PZT不具备多路复用的能力,在大型结构监测上的应用受限。Betz

75提出将FBG作为超声传感器,形成PZT/FBG混合声‑超声监测系统,可有效地区分发射器与传感器中的信号,并且可监测不同模式的兰姆76。Wu77在铝板与CFRP平板上布置多个PZT与一段多路复用的FBG,验证了该PZT/FBG混合声‑超声监测系统的损伤监测可行性,并实现了CFRP上脱粘的定位。Lam78将PZT/FBG应用在玻璃纤维复合材料的损伤监测中,通过归一化的超声响应表征了玻璃纤维复合材料分层损伤。

进一步使用具有质量轻、体积小、方向性好的宏观纤维复合材料(macro fiber composite,简称MFC)代替PZT,可建立MFC/FBG声‑超声监测系

79。Okabe80使用该系统监测了CFRP中的分层损伤,通过小波变换识别了兰姆波中的对称与反对称模态,评估了分层长度。Zhang81将MFC/FBG声‑超声监测系统应用于变截面CFRP螺旋桨叶和复合材料T型接头,采用具有大监测带宽的双频信号处理技术,监测到携带更多损伤信息的超声信号,适用于复杂的航空航天复合材料结构监测。

FBG除了用于监测声‑超声中的线性成分,还可监测其中的非线性成分。超声经过微损伤后,信号会发生畸变从而产生高阶谐波,通过提取非线性超声波信号特征可以实现微小损伤的监

82。如图8所示,笔者课题组利用PSFBG的平衡解调系统成功探测到了金属板中传播的二次谐波,阐明了疲劳裂纹对兰姆波非线性成分的调制机理,评估了疲劳裂纹的长83,还用相似的技术监测了复合材料中的非线性超声,评估了基体裂纹个数的增84

图8  疲劳裂纹的非线性声‑超声信号包[

81]

Fig.8  Envelop of nonlinear acousto‑ultrasonic signal of fatigue crack[

81]

4 结束语

近年来,光纤光栅超声结构健康监测技术取得了飞速发展,并在航空航天复合材料结构上得到了一定程度的应用。为了进一步满足超声监测所需的带宽、灵敏度、复用性和稳定性,不仅需要研发创新的FBG传感器与解调系统,还需要全面地考虑安装方法、方向灵敏度及环境干扰等问题,并针对冲击监测、声发射监测以及声‑超声监测等特定技术的需求改进和完善监测技术。后续研究工作包括:研发创新的传感器,进一步拓展动态范围,提高抗干扰能力;研发创新的解调仪,以同时保证带宽、灵敏度和复用性;针对光纤光栅超声监测的特点,研发新的数据处理算法,提高损伤判断、定位及分类的精度,满足超声结构健康监测在复杂结构、严苛环境中的实用性要求。

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