Development of Metallic Structure Fatigue Cracks by Composite Patch Repair Using Pre-embedded Fiber Bragg Grating Sensors

許多機械構件如飛機、船舶、火車等，在長期使用下，會因結構疲勞而產生裂縫，當裂縫成長至某臨界值時，其結構會有斷裂破壞之危險，應盡快加以修補。目前構件的修補技術，採用複合材料補片膠合修補方式，然而修補後貼片底下，不容易進行檢測內部構件的破壞狀況，且其疲勞破壞的發展是否有效被抑制，難以得知。由於光纖感測系統本身具有徑細質輕、敏感度高、長期穩定性佳、不受電磁雜訊干擾及與高分子材料相容性高等特性，可埋入高分子基複合材料中，形成一個智慧型的修補貼片，並可用來監測修補下之金屬結構疲勞裂縫的發展。 本研究提出利用置放在結構中的布拉格光纖光柵(Fiber Bragg Grating, FBG)感測器來監控裂縫生長的情形，以便對金屬結構做有效的斷裂破壞危險之評估。從疲勞裂縫生長實驗的結果發現，FBG垂直裂縫方式擺放，可依頻譜反射波形之峰值波長飄移量，來得知裂縫成長的狀況。在修補後之疲勞實驗發現，若裂縫持續生長時，FBG以平行裂縫方式擺放於試片與補片介面間，FBG頻譜寬度會變寬，一旦寬度由寬變窄，表示裂縫尖端已超過FBG可感測的範圍；而FBG以垂直裂縫方式擺放於補片內及外側，則可依頻譜反射波形之峰值波長飄移量，來得知裂縫的成長情形；若裂縫停止生長時，FBG之反射光頻譜波形維持不變。由此可總結，若能將FBG擺放位置在適當的配置，則可確實監測判斷裂縫之生長情況，且將此監測方法應用於大型金屬結構上，將可提高此大型結構之安全可靠度，降低因疲勞破壞產生斷裂破壞的風險。Fatigue is a common failure mechanism in aircraft structures. Composite patching is a recent technique for repairing fatigue cracked structures. However, it is not easy to know whether the crack underlying the composite patch is still growing or not. In this paper, we investigate the possibility of using optical fiber sensor for monitoring the fatigue crack under a composite repair patch. Optical fiber sensing system has the advantage of small size, high sensitivity, long term stability and immunity to electrical noise. Moreover it is also highly compatible with polymeric composites. Optical fiber can be embedded inside the polymers matrix composite material to be an intelligent repair patch that can repair and monitor the development of metallic structure fatigue crack meanwhile. This study proposes to use the Fiber Bragg Grating (FBG) to monitor the growth of cracks. For the fatigue crack growth test, the growth of cracks could be known by FBG located vertical cracks, according to the peak wavelength shift of the reflected light spectrum. For the composite patches repair test, when the crack tip grew past the grating, the degree of FBG placed parallel cracks split would decrease. While the FBG placed vertical to cracks, it was easy to know the growth of cracks, according to the peak wavelength shift of the reflected light spectrum. The FBG spectrum would remain unchanged, which is the same as the case for a arrested crack. Hence FBG can be used detect the growth of a crack if special FBG arrangement in the patch layout is employed. With its use, the reliability of large-sized structure can be greatly improved