-
图 1所示为光纤光栅传感基本原理。当光从FBG通过时,与布喇格相位条件相匹配的光会反射回去,剩余波分波长的光反射很微弱,因此大部分光继续沿光纤向前传输,光纤布喇格光栅方程为:
$ \lambda_{\mathrm{B}}=2 n_{\mathrm{eff}} \varLambda $
(1) 式中,λB为布喇格波长,neff为纤芯有效折射率,Λ为光栅周期。
温度和应力变化会影响光栅纤芯折射率及光栅周期,从而导致光纤光栅中心波长的变化。当光栅所处的环境温度保持不变,仅受外力作用而产生轴向应变时,中心波长变化量ΔλB与应变量ε的关系可表示为:
$ \Delta \lambda_{\mathrm{B}}=\lambda_{\mathrm{B}}\left(1-P_{\mathrm{e}}\right) \varepsilon=K_{\varepsilon} \varepsilon $
(2) 式中,ΔλB为波长变化量,Pe为光纤材料的弹光系数,Kε为应变灵敏度系数,对于普通石英光纤一般约为1.2pm/με[12-14]。
-
近年来,随着先进复合材料使用量和使用范围的增加,其成型后的残余应变释放过程得到了越来越多的关注。复合材料在热压罐固化成型过程中及成型后,由于温度变化会引起一系列复杂的热化学物理过程,伴随着残余应力的产生发生了构件的形变。复合材料的纤维和树脂之间,模具与构件之间存在着热膨胀系数的差异,再加上构件由外部至内部的尺寸以及热传导系数等因素造成了在成型过程中构件存在着温度的梯度分布。构件成型后热残余应力释放导致的材料的残余应变对复合材料构件的后期质量存在着不可忽略的影响[15]。
FBG不仅可用于实现碳纤维复合材料构件固化全程的在线监测,在构件服役后,还能够监测复合材料的健康状态[16-17]。与其它类型的传感器相比,FBG传感器优势在于光纤的直径较细,裸纤直径仅有125μm,对被测构件的干扰较小,其传输损耗很小,低于0.2dB/km,且不受电磁干扰,同时还具有优异的抗化学腐蚀的能力[18]。树脂基复合材料构件在固化成型后残余应力释放导致构件发生形变期间,材料表面的温度、应变等都在发生变化,传统的电阻应变传感器无法剔除温度对测量结果的影响,而利用FBG传感器可同时实现应变与温度的监测,实现对应变监测结果的实时补偿。所以,FBG传感器是复合材料固化残余应变监测的最佳选择[19]。
-
为进一步验证FBG测量数据的可靠性,本文中利用ABAQUS软件构建了环形构件在工件出罐后降温过程中表面残余应力的变化的有限元模型。该有限元模型包括传热分析和力学分析两大模块,二者顺序耦合,传热分析是基于傅里叶热传导定律和能量平衡关系,并借助子程序求解得到温度场;将传热模块的分析结果作为初始条件导入力学分析中,得到相应的热应变。
由于环形构件属于轴对称模型,故在仿真过程中为简化计算,仅取构件截面的部分矩形2维模型进行顺序耦合热应力分析。
仿真过程中材料参量参考T700碳纤维,热传导分析中使用预定义场设置各节点初始温度为180℃,并在模型上边缘设置外温为15℃的热交换条件,热对流系数为20W/(m2·K),可以得到如图 10所示的温度分布结果。
可以看到, 由材料外表面到材料内部,因不同位置的热边界条件不同,不同深度的材料传热不均匀,成型反应存在明显温度梯度分布,导致材料除了释放的热应力还残留着一部分残余应力在构件内部,最终材料为平衡这部分残余应力会发生相应的残余应变。
将上述传热分析中模型的温度场时间变化历程以预定义场的形式导入力学分析,作为应力应变分析的初始条件,将边界条件设置为对称边界条件,网格属性设置为轴对称应力网格,分析步设置为通用分析步,通过作业功能模块进行分析求解。
由图 11~图 13中的仿真分析结果可知,仿真建立的模型表面应力变化趋势与FBG传感阵列中间部位轴向监测结果有较高的一致性。
根据仿真结果分析可知,构件表面残余应力随时间延长而逐渐减小,温度越低减小得越慢。整个历程中模型外表面残余应变变化量为11.33MPa,代入胡克定律:
$ \sigma=E \cdot \varepsilon $
(3) 式中,E为材料模量,σ为材料表面应变。计算得到该历程中模型表面残余应变大小为47.10με。
对比分析可知,仿真得到的残余应变值与监测结果的30με~90με相近。由于该仿真结果只模拟了构件由固化温度下降到室温时材料内部温度梯度的不均匀分布所产生的宏观残余应力,没有模拟构件内部纤维与树脂热膨胀系数不匹配以及树脂由于化学收缩等造成的微观残余应力,故造成了仿真结果与监测结果的差异。总体来说,仿真结果一定程度上证实了监测结果的可靠性。
基于FBG的复合材料环形构件残余应变研究
Research on surface residual strain of composite ring component based on FBG
-
摘要: 为了解决因碳纤维/环氧树脂复合材料构件中树脂基体与碳纤维间的热膨胀系数存在差异, 热压成型后, 内部未释放的残余应力会使材料构件发生残余应变, 造成构件变形、影响制品质量的问题, 采用光纤布喇格光栅(FBG)传感器在线监测实验研究和有限元仿真分析相结合的研究方法, 进行了理论分析和实验验证, 采集了碳纤维/环氧树脂复合材料环形构件热压罐成型后表面残余应变变化实时数据。结果表明, 该构件下法兰根部应变释放情况复杂, 靠近支承位置处的残余应变释放受阻, 而其余监测点位的残余应变普遍在30με~90με的范围内, 与仿真结果相符; FBG传感器能对构件成型后残余应变释放历程进行多点位实时在线监测, 并实现对构件整体应变分布的分析和预警。该研究具有一定科学研究和工程应用意义。Abstract: In order to solve the problems of component deformation and affecting manufacturing quality caused by the residual strain of material components, which is due to the residual stress after hot pressing by the difference of thermal expansion coefficient between resin matrix and carbon fiber in carbon fiber/epoxy resin composite components. The theoretical analysis and experimental verification were carried out by combining the on-line monitoring experimental research of fiber Bragg grating (FBG) sensor and finite element simulation analysis. The real-time data of surface residual strain of carbon fiber/epoxy composite ring components after hot pressing tank forming were collected. The results show that the strain release at the flange root is complex, the residual strain release near the support is blocked, and the residual strain at other monitoring points is generally 30με~90με. The results are consistent with the simulation results. FBG sensor can carry out multi-point real-time on-line monitoring of the residual strain release process after component forming, and realize the analysis and early warning of the overall strain distribution of the component. The research has certain scientific research and engineering application significance.
-
Key words:
- sensor technique /
- FBG /
- compound material /
- ring structure /
- residual strain /
- online health monitoring
-
[1] CHOPRA I. Status of application of smart structures technology to rotorcraft systems[J]. Journal of the American Helicopter Society, 2000, 45(4): 228-252. doi: 10.4050/JAHS.45.228 [2] KALAYCIOGLU S, SILVA D. Minimization of vibration of spacecraft appendages during shape control using smart structures[J]. Journal of Guidance, Control and Dynamics, 2000, 23(3): 558-561. doi: 10.2514/2.4566 [3] TIAN H, WANG J H, JI Y D, et al. Monitoring of curing residual stress by Bragg grating[J]. Materials Reports, 2012, 26(20): 111-114(in Chinese). [4] ZHANG G M, WANG J H, DING A X, et al. Prediction of curing deformation and residual stress of carbon fiber composite laminates[C]// China International Congress on Composite Materials. Hangzhou: Chinese Society For Composite Materials, 2017: 2(in Chinese). [5] SEROVAEV G, KOSHELEVA N. The study of internal structure of woven glass and carbon fiber reinforced composite materials with embedded fiber-optic sensors[J]. Fracture and Structural Integrity, 2019, 14(51): 225-235. [6] CAPELL T F. Applications of embedded chirped fibre Bragg grating sensors for damage and defect detection in composites and composite bonded joints[D]. Surrey, UK: University of Surrey, 2012: 16-37. [7] HOU Zh Y, LI Y, SHAO H F, et al. Study on composite and strain sensing characteristics of high intensity fiber Bragg grating and carbon fiber[J]. Semiconductor Optoelectronics, 2020, 41(1): 88-91 (in Chinese). [8] ZHANG Y A, ZHAN L H. Application of fiber Bragg grating in curing of advanced composites[J]. Fiber Reinforced Plastics/Compo-sites, 2016, 43(1): 86-91(in Chinese). [9] BO D. Embedded fiber-optic sensors in carbon fiber composites for temperature-insensitive and intensity-modulated microdisplacement measurement[C]//IEEE International Conference on Advanced Infocomm Technology. New York, USA: IEEE, 2013: 10-11. [10] AN T Y, ZHU Q R, WU H. Research on damage detection of composite materials based on fiber Bragg grating sensor[J]. Experimental Technology and Management, 2019, 36(6): 72-75(in Chinese). [11] LI B. Study on curing deformation and residual stress of composite curved surface structure[D]. Shenyang: Shenyang Aerospace University, 2019: 57-100(in Chinese). [12] WU R J, ZHENG B L. Analysis of strain transfer of surface bonded FBG sensor[J]. Instrument Technique and Sensor, 2016, 53(8): 14-17(in Chinese). [13] SUN Y J, ZHANG Q, CHENG G, et al. Analysis and experiment of strain transfer characteristics of surface mounted distributed optical fiber sensor based on optical frequency domain reflection technology[J]. Science Technology and Engineering, 2018, 18(33): 46-52(in Chinese). [14] KESAVAN K, RAVISANKAR K, SENTHIL R, et al. FBG sensor technology to interfacial strain measurement in CFRP-strengthened concrete beam[J]. Experimental Techniques, 2015, 39(5) : 21-29. doi: 10.1111/j.1747-1567.2012.00858.x [15] SUN J X, CHEN Zh Q, YANG X R, et al. Study on temperature sensitivity of CFRP under laser irradiation[J]. Laser Technology, 2021, 45(3): 280-285(in Chinese). [16] TAKEDA S, AOKI Y, NAGAO Y, et al. Damage monitoring of CFRP stiffened panels under compressive load using FBG sensors[J]. Composite Structures, 2012, 94(3): 813-819. doi: 10.1016/j.compstruct.2011.02.020 [17] MENG Q P, SUN W, WANG Y L, et al. Study on life cycle structural state of carbon fiber composites implanted with FBG[J]. Aerospace Material Technology, 2018, 48(4): 46-50(in Chinese). [18] VALERII P, NATALIA A, GRIGORII S, et al. Numerical analysis of the strain values obtained by FBG embedded in a composite material using assumptions about uniaxial stress state of the optical fiber and capillary on the Bragg grating[J]. Fracture and Structural Integrity, 2019, 13(49) : 177-189. [19] GAO L L, WANG Q L, WANG X X, et al. Protection technology of fiber Bragg grating sensor embedded in fiber composite laminate[J]. Acta Materiae Compositae Sinica, 2016, 33(11): 2485-2491(in Chinese). [20] LIU J. Research on theory, method and engineering application of optical fiber sensing strain detection[D]. Wuhan: Wuhan University of Technology, 2016: 14-33(in Chinese).