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

碳纤维增强塑料（CFRP）等复合材料的广泛应用，使运载器结构重量减轻，是可重复使用运载器研制成功的关键。对重量减少的最大贡献可能是通过将CFRP用于占总发射重量90%以上的低温燃料箱来实现的。在本研究中，低温环境的机械特性和燃料泄漏的粘接搭接接头的影响进行了评估。研究结果有助于利用CFRP搭接接头，可以提高结构效率的复合材料燃料tanks.DCB试样进行了热试验，以澄清热循环，温度水平，和厚度的粘合剂的断裂韧性的影响。结果表明，低温环境对断裂韧性的影响很小 ...更多信息 但指出热循环可在层合板自由边附近诱发基体裂纹。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测试，导致循环热载荷后的承载能力下降。这些后果被认为是由于接头自由边缘部分的界面退化造成的，并通过显微镜进行了验证。这表明了搭接接头边缘处理的重要性，特别是在低温环境下。横向裂纹的广泛积累，原来预测和逮捕被证明是避免的存在随机取向的短纤维的位置在adhesies.To调查通过层压板，包括搭接接头的燃料泄漏，层压板累积损伤进行了气体氦泄漏试验。分析结果表明，含横向裂纹的层合板渗透率随裂纹密度的增加而增加。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，泄漏率被示出保持在可接受的量以下。根据本文获得的力学测试结果，低温环境对搭接接头和层压板的力学性能的影响可以被称为可控的，连同观察到的低泄漏率。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