Mechanical Characteristics and Fuel Leakage of Laminate Adhesive Joints under Cryogenic Conditions

可重复使用运载火箭(RLV)的成功研制依赖于碳纤维增强塑料(CFRP)等复合材料的广泛应用实现运载火箭结构的轻量化。对减轻重量的最大贡献可以通过将CFRP用于占据发射总重量90%以上的低温燃料箱来实现。在本研究中，评估了低温环境对胶接搭接接头的机械特性和燃油泄漏的影响。研究结果有助于CFRP搭接接头的应用，以提高复合燃料箱的结构效率。对由胶粘层压板制成的DCB试件进行了热试验，以明确热循环、温度水平和胶粘剂厚度对断裂韧性的影响。结果表明，低温环境对测得的断裂韧性…影响不大更多，但表明热循环可能会导致层合板自由边缘附近的基质裂纹。还制作了单搭接和双搭接接头试件，并在拉伸载荷下进行了试验，结果导致循环热载荷后的承载能力下降。结果被认为是由于接头自由边缘部分的界面退化所致，这一点经过了显微镜验证。这表明搭接接头边缘处理的重要性，特别是在低温环境下。由于粘结剂中随机排列的短纤维的存在，避免了最初预测和理解的横向裂纹的广泛积累。为了调查包括搭接接头在内的层合板的燃料泄漏，对具有累积损伤的层合板进行了气体氦泄漏试验。结果表明，含横向裂纹的层合板的渗透率随裂纹密度的增加而增加，分析结果证实了这一点。然而，对于实际的燃料箱，只要裂纹不相互连接形成贯穿厚度的泄漏路径，泄漏率就保持在可接受的水平以下。根据本文获得的力学测试结果，低温环境对搭接接头和层合板力学性能的影响是可控的，同时观察到的低泄漏率。较少

The successful development of the Reusable Launch Vehicles (RLV) depends on the weight reduction of the vehicle structure achieved by the wide application of composite materials such as Carbon Fiber Reinforced Plastics (CFRP). The largest contribution to the weight reduction may be attained by the utilization of CFRPs for cryogenic fuel tanks that occupies more than 90% of the gross launch weight. In the present study, the effects of cryogenic environment on the mechanical characteristics and fuel leakage of adhesive lap joints are assessed. The results of the study contribute to the utilization of CFRP lap joints that can increase the structural efficiency of the composite fuel tanks.The DCB specimens fabricated from adhesively bonded laminates were subjected to the thermal tests to clarify the effects of thermal cycles, temperature levels, and thickness of adhesives on the fracture toughness. The results showed little effect of cryogenic environment on the measured fracture toughness … More , but indicated that the thermal cycles may induce matrix cracks near the free edges of the laminates. The single-lap and double-lap joint specimens were also fabricated and tested under tensile loadings, resulting in the decrease of load carrying capacities after the cyclic thermal loadings. The consequences were assumed to be due to the interface degradation at the free edge portions of the joints, which were microscopically verified. This suggested the importance of the edge treatment of the lap joints especially under cryogenic environment. The wide accumulation of transverse cracks that were originally predicted and apprehended were shown to be avoided by the existence of randomly oriented short fiber placements in the adhesives.In order to investigate the fuel leakage through the laminates including the lap joints, laminates with accumulated damages were subjected to gas helium leakage tests. Laminates with transverse cracks exhibited increased permeability associated with the increase of crack density, which was verified by the analysis. However the leakage rates were shown to remain below the acceptable amount for the practical fuel tanks, as long as the cracks were not inter-connected to form through-thickness leakage paths.With the results of mechanical tests obtained herein, the effects of cryogenic environment on the mechanical performances of the lap-joints and the laminates may be referred to as controllable, together with the observed low leakage rates. Less