纤维复合材料层合板内埋光纤光栅传感器的保护技术

内埋的光纤Bragg光栅（FBG）传感器的存活率及测试精度是其在线监测纤维增强树脂基复合材料制备和服役状态的重要前提。采用[90₁₁/0₁₁]的碳纤维预浸料铺层方式,在层合板0°和45°方向的典型位置埋入FBG温度和应变传感器,采用模压成型工艺制备复合材料层合板。在异向铺排（光纤光栅方向与碳纤维方向不同）的45°方向光纤光栅传感器内埋于碳纤维预浸料层间的过程中,对其采用4种不同的保护方式。通过对比实验结果发现:当对异向铺排的FBG传感器不采取保护措施时,在加热加压复合材料时光纤光栅容易失活;整层铺设同向预浸料以保护异向铺排的FBG传感器的方式改变了具有特定铺层参数复合材料的力学性能;采用窄长条同向预浸料上下包埋保护FBG传感器的方式增大了应变光栅测量结果的系统误差;采用窄长条同向预浸料上下包埋并在邻近铺层开凹槽的保护方式能明显提高内埋光纤光栅的存活率及测试精度。      

布拉格光栅对固化残余应力的监测

由于复合材料在固化过程中产生的残余应力会严重影响材料的使用,并且会带来安全隐患,因此固化残余应力一直都备受关注。首先在研究和测试布拉格光栅温度和应力灵敏度的基础上,将两根光栅分别以平行于碳纤维方向和垂直于碳纤维方向置入铺层的正中间,以此获得复合材料不同方向的残余应力变化情况。监测结果表明,平行方向的光栅在监测固化的过程中受到树脂和纤维的共同作用,且冷却后材料在平行方向显示的残余应力仅有-8.8MPa,而在垂直方向的光栅在监测过程中主要受到树脂的固化影响,收缩应变较大,且冷却后的光栅由于树脂固化不均匀,致使光栅信号分裂为3段。     

布拉格光栅监测不同厚度方向的固化残余应力

复合材料在固化过程中产生的残余应力会严重影响材料的使用和安全。首先在研究和测试了布拉格光栅温度和应力灵敏度的基础上,将3根光栅分别以平行于碳纤维的方向置入预浸料的上、中、下3个位置,以此获得复合材料不同厚度位置的残余应力变化情况。监测结果表明,上、下层光栅监测到的预浸料固化过程几乎完全一致,而在降温时下层的布拉格光栅残余应力则比上层的要大得多,中间层布拉格光栅在固化过程中测得的残余应力最小,但在降温过程中的热残余应力不易释放而导致最后的残余应力也较大。     

模具对复合材料构件固化变形的影响分析

通过光纤光栅的方法实验研究了在热压罐成型工艺过程中,复合材料构件由金属固化模具与复合材料构件热不匹配导致的沿厚度方向和面内的固化残余应力发展,得到了固化后残余应力沿构件厚度方向和面内的分布情况,并分析了该残余应力分布的产生机制以及对构件固化后变形的影响.结果表明:复合材料与模具之间的热不匹配导致的固化残余应变沿构件厚度方向呈梯度分布,靠近模具端大于远离模具端,并且该应变会引起构件固化后的翘曲变形,变形以沿纤维方向为主.     

模具对复合材料构件固化变形的影响分析

通过光纤光栅的方法实验研究了在热压罐成型工艺过程中, 复合材料构件由金属固化模具与复合材料构件热不匹配导致的沿厚度方向和面内的固化残余应力发展, 得到了固化后残余应力沿构件厚度方向和面内的分布情况, 并分析了该残余应力分布的产生机制以及对构件固化后变形的影响。结果表明: 复合材料与模具之间的热不匹配导致的固化残余应变沿构件厚度方向呈梯度分布, 靠近模具端大于远离模具端, 并且该应变会引起构件固化后的翘曲变形, 变形以沿纤维方向为主。     

基于内埋光纤Bragg光栅传感器的复合材料固化过程监测

为了全面了解复合材料的固化特性,在对碳纤维增强树脂基复合材料固化变形进行数值仿真分析的基础上,将自行设计的光纤Bragg光栅（FBG）传感器埋入复合材料中,实时在线监测复合材料固化过程中温度和应变的演变。预浸料铺层方式为[0₁₁/90₁₁],分别在层合板0°和45°方向的典型位置埋入FBG温度和应变传感器,采用热模压方式固化成型复合材料层合板,并对成型后的层合板进行连续2次降温处理,实时记录固化过程中FBG传感器中心波长的变化。结果表明:在相同的温度条件下,复合材料在第1次降温初始阶段的压应变绝对值明显小于在第2次降温初始阶段的压应变绝对值,表明复合材料在第1次降温过程中仍在进行FBG传感器可检的“后固化”反应;此外,层合板变形的FBG传感器监测数据与有限元模拟结果吻合良好。因此,采用内埋FBG传感器的方法能够实时监测复合材料固化过程,为更全面地分析复合材料固化特性提供了一种可靠有效的方法。      

带有铝板复合材料层板固化过程中残余应力演化

本文对两种材料组成的层板的残余应力进行了研究.提出应用有效温差并结合线膨胀系数对带有铝板复合材料层板固化成型过程中残余应力形成进行理论分析,其中残余应力主要考虑温度产生的.采用先进的光纤技术对带有铝板复合材料层板固化成型过程中残余应力进行试验研究,即将光纤布拉格光栅(FBG)分别埋入到复合材料内部及复合材料与铝板之间,监测固化全过程残余应变的演变,并且构件最终变形与理论分析释放残余应力方向一致.     

热残余应力对内埋光纤光栅传感器性能的影响

将布拉格光纤光栅(FBG)埋植于复合材料T型加筋板结构非干涉区—三角填充区作为应变传感器对复合材料加筋板在固化过程及冲击后压缩过程中的应变变化进行监测。对比了光纤刻栅区采用UV光固化树脂涂层保护和未保护的两种FBG传感器的波谱信号变化; 分析了复合材料在固化成型过程中产生的非轴对称热残余应力对FBG传感性能的影响。结果表明, 刻栅区采用聚合物涂层保护的FBG传感器的半峰宽(FWHM)在固化过程中未发生变化, 并且聚合物涂层可以有效地消除非轴对称热残余应力对光纤光栅反射波谱的影响。在冲击后压缩过程中, 采用聚合物涂层保护的FBG传感器测得的应变与贴于试样表面的应变片测得的应变数据一致性较好。本文对埋植于复合材料加筋板三角填充区的FBG传感器在复合材料固化过程及冲击后压缩过程中应变监测的有效性及可靠性进行了有益的探索。     

纤维缠绕复合材料固化过程残余应力/应变的三维数值模拟

采用有限元分析软件ABAQUS,对具有金属内衬的纤维缠绕复合材料圆筒在固化过程中残余应力及应变的变化规律进行了模拟计算。采用FORTRAN语言编制了用以分析固化过程中残余应力的子程序,该子程序考虑了固化过程中复合材料力学性质的变化和由于树脂固化收缩产生的化学收缩应变。算例结果表明：复合材料和金属内衬的残余应力在初始阶段均接近于零,当固化到一定阶段,残余应力迅速增加并且很快达到最大值,在降温阶段释放了部分的残余应力;在整个固化过程中,金属内衬受到压应力,而纤维缠绕层受到拉应力。本文中的三维有限元模型可以得到任意时刻复合材料的温度及固化度分布,通过数值模拟可以有效地优化复合材料固化工艺参数,提高制件的质量。     

粘弹性基体复合材料层合板的固化残余应力和曲率变化

粘弹性基体复合材料层合板因铺层取向差异而产生的固化残余应力会随层合板在室温下放置时间的推移而变化, 这一过程可通过非对称层合板的曲率变化得到反映。本研究通过非对称板的曲率变化实验来观察板内残余应力的变化过程, 并根据细观复合材料力学理论和适宜的变形分析方法建立一种与残余应力最终稳定分布相对应的铺层修正折减模量矩阵, 用于进行层合板固化残余应力和曲率变化的渐近值预测, 并依此确定固化后残余应力的变化范围。     

Monitoring LCM Process by FBG Sensor Under Birefringence

Abstract: Optical fibre Bragg gratings (FBG) provide accurate and non‐intrusive strain and temperature local measurements. FBG sensors can be embedded into fibrous preforms to monitor the flow and the cure of the resin and deliver real‐time information on the ongoing process. The paper concentrates on the utilisation of the strain‐induced birefringence of the FBG to derive information on the effective stress–strain state of the composite at the end of the process cycle (Vacher 2004, Optical fiber sensors to monitor the processing and the mechanical characterization of composites. PhD thesis). During the cooling phase, the reflection spectrum from the FBG splits into two peaks because of the birefringence of the glass fibre owing to the residual stress. The paper shows that this effect can be utilised to estimate the residual stress and strain in composites manufactured by Liquid Composite Moulding technologies. The birefringence effect arising from the cooling of a [0₆,90₃]_S CFRP laminate is first characterised, and then the determination of the strains along the principal axes inside the laminate is completed by modelling the local stress–strain state because of the interaction of the optical fibre and its environment within the framework of orthotropic thermo‐elasticity and the Classical Laminated Plate Theory.     

In situ monitoring of the strain evolution and curing reaction of composite laminates to reduce the thermal residual stress using FBG sensor and dielectrometry

Generally, a large, thermal residual stress is generated during the curing process for composite laminates due to differences in the coefficients of thermal expansion of the respective layers. The thermal residual stress during fabrication greatly decreases the fatigue life and dimensional accuracy of the composite structures. In the present study, through a fiber bragg grating (FBG) sensor and dielectrometry in an autoclave, the strain evolution and curing reaction in composite laminates with a stacking sequence of [0₅/90₅]_S were monitored simultaneously during a conventional cure cycle and a modified cure cycle to reduce the thermal residual stress. From the study, it was verified that about 50% of the thermal residual stress during fabrication could be reduced in a composite laminate by adjusting the cure cycle; this improved the static strength and fatigue life by 16% and up to 614%, respectively, for a peak ratio of 0.9.     

Composite cure simulation scheme fully integrating internal strain measurement

This study proposes a new approach to determine key material parameters for stress/strain calculation of curing composite laminates and validate the simulation. Specifically, fiber Bragg grating (FBG) strain sensors are embedded in a composite laminate and the two key parameters for simulation, composite shrinkage strain and stiffness change during curing, are simultaneously determined from in-situ measurements by the embedded sensors. Furthermore, the simulation is validated using internal strain change during curing. This paper begins by presenting an overview of the proposed simulation scheme and by comparing it with previous approaches to highlight its advantages. Material parameter determination using a shear-lag effect at the edge of the embedded sensors is then described and the practical procedure to obtain the key parameters is demonstrated using a carbon/epoxy laminate. Finally, cure simulation is conducted for validation. Further extension to more general cases including thermoplastic composites is also discussed.     

基于FBG传感技术的复合材料T型加筋板低速冲击损伤监测

针对碳纤维增强树脂复合材料低速冲击损伤的实时监测,设计将布拉格光纤光栅（FBG）传感器埋植在复合材料T型加筋板结构的三角填充区,在线监测复合材料T型加筋板冲击损伤过程。分别将FBG传感器埋植于复合材料层合板内部和复合材料T型加筋板的三角填充区,对比FBG传感器的埋入对复合材料层合板和复合材料T型加筋板力学性能的影响。结果表明,内埋FBG传感器的复合材料层合板试样的拉伸强度比未埋植传感器的层合板试样降低了约5%,但在FBG传感器的破坏应变范围内,FBG传感器可以准确、实时地监测复合材料的应变信号。将FBG传感器埋入复合材料T型加筋板的三角填充区,内埋FBG传感器的T型加筋板样件压缩破坏载荷与未埋植的样件基本一致。通过对比T型加筋板蒙皮上冲击位置、冲击能量对FBG传感器测得的冲击过程持续时间和最大应变值的影响,表明冲击过程持续时间随着冲击能量增大而延长,最大应变值随着冲击距离的增加呈下降趋势,而最大应变值随着冲击能量的增大呈上升趋势。利用FBG传感器测得的应变信号可初步实现对复合材料T型加筋板蒙皮冲击损伤位置及冲击能量的实时监测。      

Experimental determination of residual stresses in composite laminates [02/θ2]s

The present work aims to determine the residual stresses in carbon fiber/epoxy composite laminates, by means of the incremental hole-drilling method. Based on mechanical theories of composite laminates and an elastic plate with a circular hole, the relationship between the relaxed strains on the surface of laminates and the residual stresses in laminates was established. This newly deduced theoretical formula was adapted into the incremental hole-drilling method and allowed us to further study the residual stresses in the through-thickness direction for various composite laminates. Related numerical modeling of composite laminate with a hole was built to calibrate the coefficients within the formula. Experiments were conducted and the residual stresses in composite laminates [0₂/θ₂]_s are presented. The proposed approach was validated with the consistence between our results for cross-ply laminates and those in literature.     

Characterization of microscopic damage in composite laminates and real-time monitoring by embedded optical fiber sensors

First, a methodology for observation and modeling of microscopic damage evolution in quasi-isotropic composite laminates is presented. Based on the damage observation using both an optical microscope and a soft X-ray radiography, a damage mechanics analysis is conducted to formulate the stiffness change due to transverse cracking. Then, both energy and stress criteria are combined to provide a valid procedure to predict the transverse crack evolution. The theoretical prediction is found to agree well with the experimental results for the transverse crack density as a function of strain as well as stress–strain curves. Then, another methodology is introduced using two kinds of embedded optical fiber sensors to detect and monitor the transverse crack evolution in composite laminates. One is plastic optical fibers (POF), where the loss in optical power is generated by local deformation of POF due to transverse cracking. The other is fiber Bragg grating (FBG) sensors, where the local strain distribution within the FBG gage length due to transverse cracking alters the power spectrum of the light reflected from the FBG sensors. Embedded optical fiber sensors are found to be a powerful method to detect and monitor the transverse crack evolution in composite laminates.