Grundmann Neele, Brüning Hauke, Tserpes Konstantinos, Strohbach Tim, Mayer Bernd
Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, Wiener Straße 12, 28359 Bremen, Germany.
Laboratory of Technology & Strength of Materials, Department of Mechanical Engineering & Aeronautics, University of Patras, Patras 26500, Greece.
Sensors (Basel). 2020 Mar 17;20(6):1665. doi: 10.3390/s20061665.
The increasing utilization of carbon fiber reinforced plastic (CFRP) in the aeronautical industry calls for a structural health monitoring (SHM) system for adhesively bonded CFRP joints. Optical glass fiber with inscribed fiber Bragg gratings (FBGs) is a promising technology for a SHM system. This paper investigates the intrusive effect of embedding optical glass fibers carrying FBGs on adhesive bond strength and adhesive layer thickness and quality. Embedding the optical glass fibers directly in the adhesive bond has the advantage of directly monitoring the targeted structure but poses the risk of significantly reducing the bond strength. Optical glass fibers with different cladding diameters (50, 80, 125 µm) and coating types (polyimide, with a thickness of 3-8 µm, and acrylate, with a thickness of ~35 µm) are embedded in structural and repair film adhesives here. Without embedded optical glass fibers, the film adhesives have an adhesive layer thickness of ~90 µm (structural) and ~100 µm (repair) after curing. The intrusive effect of the fiber embedding on the adhesive bond strength is investigated here with quasi static and fatigue single lap joint (SLJ) tensile shear tests. Also, the influence of hydrothermal aging procedures on the quasi static tensile shear strength is investigated. It is found that optical glass fibers with a total diameter (glass fiber cladding + coating) of ~145 µm significantly reduce the quasi static tensile shear strength and increase the adhesive layer thickness and number of air inclusions (or pores) in the structural film adhesive joints. In the repair adhesive joints, no significant reduction of quasi static tensile shear strength is caused by the embedding of any of the tested fiber types and diameters. However, an increase in the adhesive layer thickness is detected. In both adhesive films, no effect on the quasi-static tensile shear strength is detected when embedding optical glass fibers with total diameters <100 µm. The applied aging regime only affects the repair film adhesive joints, and the structural film adhesive joints show no significant reduction. A polyimide-coated 80 µm optical glass fiber is selected for fatigue SLJ tensile shear tests in combination with the more sensitive structural film adhesive. No significant differences between the S-N curves and tensile shear fatigue strength of the reference samples without embedded optical fibers and the samples carrying the polyimide-coated 80 µm optical glass fibers are detected. Thus, it is concluded that the influences of embedding optical glass fibers with total diameters <100 µm on the fatigue limit of the tested film adhesive joints is negligible.
碳纤维增强塑料（CFRP）在航空工业中的使用日益增加，这就需要一种用于CFRP粘接接头的结构健康监测（SHM）系统。带有写入光纤布拉格光栅（FBG）的光学玻璃纤维是一种很有前景的SHM系统技术。本文研究了嵌入带有FBG的光学玻璃纤维对粘接强度、粘接层厚度和质量的侵入效应。将光学玻璃纤维直接嵌入粘接剂中具有直接监测目标结构的优点，但存在显著降低粘接强度的风险。这里将不同包层直径（50、80、125 µm）和涂层类型（厚度为3 - 8 µm的聚酰亚胺和厚度约为35 µm的丙烯酸酯）的光学玻璃纤维嵌入结构和修复薄膜胶粘剂中。在未嵌入光学玻璃纤维的情况下，固化后薄膜胶粘剂的粘接层厚度约为90 µm（结构胶粘剂）和100 µm（修复胶粘剂）。本文通过准静态和疲劳单搭接接头（SLJ）拉伸剪切试验研究了纤维嵌入对粘接强度的侵入效应。此外，还研究了湿热老化过程对准静态拉伸剪切强度的影响。结果发现，总直径（玻璃纤维包层 + 涂层）约为145 µm的光学玻璃纤维会显著降低结构薄膜胶粘剂接头的准静态拉伸剪切强度，并增加粘接层厚度和空气夹杂物（或孔隙）数量。在修复胶粘剂接头中，任何测试纤维类型和直径的嵌入均未导致准静态拉伸剪切强度显著降低。然而，检测到粘接层厚度有所增加。在两种胶粘剂薄膜中，嵌入总直径<100 µm的光学玻璃纤维时，未检测到对准静态拉伸剪切强度有影响。所施加的老化条件仅影响修复薄膜胶粘剂接头，而结构薄膜胶粘剂接头未出现显著降低。选择了一根涂覆聚酰亚胺的80 µm光学玻璃纤维与更敏感的结构薄膜胶粘剂一起进行疲劳SLJ拉伸剪切试验。未检测到未嵌入光纤的参考样品与带有涂覆聚酰亚胺的80 µm光学玻璃纤维的样品在S - N曲线和拉伸剪切疲劳强度方面存在显著差异。因此，可以得出结论，总直径<100 µm的光学玻璃纤维嵌入对测试薄膜胶粘剂接头疲劳极限的影响可忽略不计。
