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)
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
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
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
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
(1) |

图1 FBG原理示意图
Fig.1 Schematic of FBG
当FBG受轴向应变作用时,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,产生漂
(2) |
其中: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。
温度变化对波长漂移的影响为
(3) |
其中:为光纤热膨胀系数;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,引起的快速漂移。因此,类似于应变的测量,通过确定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
随着光栅制造技术的发展,啁啾光纤光

图2 FBG与PSFBG的光谱图
Fig.2 Spectra of FBG and PSFBG
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

图3 PSFBG对超声波的灵敏度分布特
Fig.3 Sensitivity distribution properties of PSFBG to ultrasonic wave
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的漂移超出解调系统的动态范围。悬臂式粘贴可克服上述问
宽带光源解调又称为功率解调,原理如
(4) |
其中:为光电探测器的影响因子;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,与待测量呈线性关系。
根据上述解调原理,Perez
可调谐激光解调又称为边缘滤波解调,原理如
(5) |
其中: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
由
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
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
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

图6 FBG在机翼盒段冲击监测中的应
Fig.6 Impact monitoring on wing using FB
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

图7 PSFBG监测到的CFRP累积声发射
Fig.7 Accumulative AE hits of CFRP monitored by PSFB
与冲击监测类似,定位也是声发射的重要研究内容。Kim
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
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
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

图8 疲劳裂纹的非线性声‑超声信号包
Fig.8 Envelop of nonlinear acousto‑ultrasonic signal of fatigue crac
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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