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

可重复使用运载火箭（RLV）的成功发展取决于碳纤维增强塑料（CFRP）等复合材料的广泛应用，从而实现运载火箭结构的重量减轻。对减轻重量的最大贡献可能是将CFRPs用于占总发射重量90%以上的低温燃料箱。研究了低温环境对粘接接头力学特性和燃油泄漏的影响。研究结果有助于提高复合材料油箱结构效率的碳纤维复合材料搭接接头的应用。对胶粘剂层合板制备的DCB试样进行了热试验，以阐明热循环、温度水平和胶粘剂厚度对断裂韧性的影响。结果表明，低温环境对层合板断裂韧性的影响不大，但热循环可能在层合板自由边缘附近诱发基体裂纹。制作了单搭接和双搭接接头试件，在拉伸载荷作用下进行了试验，发现循环热载荷后接头的承载能力有所下降。结果被认为是由于在关节的自由边缘部分的界面退化，这是微观验证。这说明了在低温环境下对搭接接头进行边缘处理的重要性。原先预测和预测的横向裂缝的广泛积累被证明是由粘合剂中随机定向短纤维放置的存在所避免的。为了研究包括搭接节点在内的层合板的燃油泄漏情况，对具有累积损伤的层合板进行了气氦泄漏试验。具有横向裂纹的层合板透气性随裂纹密度的增加而增加，分析证实了这一点。然而，泄漏率仍然低于实际燃料箱的可接受量，只要裂缝不相互连接形成贯穿厚度的泄漏路径。从力学试验结果来看，低温环境对搭接和层合板力学性能的影响是可控的，渗漏率也较低。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