动态复合加载下典型复合材料机械连接结构的失效机理研究

With the technology development of the aviation engineering and material science, composites have an increasing application prospect in aircraft structures. However, bolted joints for composite structures take a big risk of failure subject to dynamic loads or impact, for example, in the emergence landing scenario. Failure mechanism and energy dissipation characteristics of the composite joints highly depend on strain rate, fastener, joining members and load case. Especially the combined load conditions introduce more complex failure mechanism which has not been well understood. The fidelity of crashworthiness assessment is therefore limited.The attention is first paid to Hi-Lock fastener of the bolted joint. Effect of tension, shear and their combination on the failure mechanism of Hi-Lock fastener is recognized to build a general equation to descript dynamic failure behavior. The failure of joining member made up from CFRP is then focused on. The CFRP specimens are tested at low-mid range strain rate to investigate into the thermal dissipation mechanism due to dynamic loading and structure failure. Accordingly, an appropriate dynamic constitutive equation and a damaged model will be proposed afterwards. A combined load will be at last dynamically imposed on typical bolted joints for composite structures. The proposed model representing failure of Hi-lock fastener as well as joining composite structures and allowing for thermal dissipation effects was validated. The results achieved by the present work are significant and with great potential to be applied on crashworthiness assessment of aircrafts in practice.

随着航空科学技术的发展，复合材料越来越多的应用在飞机结构中，在应急坠撞等冲击载荷作用下，复合材料机械连接结构是机体结构的主要失效部位，然而，其在冲击载荷作用下的失效模式及能量吸收特性受载荷的作用状态、载荷的加载速率、母材和连接紧固件的类型等多个因素的影响。现阶段对于动态复合加载下复合材料机械连接结构的失效机理仍不明确，这成为制约复合材料机体结构抗坠撞评定的关键技术问题。本课题首先以高锁螺栓连接紧固件为对象，探寻拉-剪复合及纯拉/纯剪等不同动态加载状态对其失效模式的影响规律和机理，并构建普适性的连接紧固件动态失效方程；其次，以碳纤维树脂基复合材料为研究对象，探寻中低应变率加载范围内其失效的热耗散物理机制，提出适用的动态力学本构方程和失效本构方程；最后，通过典型的复合材料机械连接结构件动态加载实验对上述提出的本构方程进行综合验证。本课题对飞机抗坠撞评定具有重要的科学意义和工程应用前景。

我国新一代民机将大量使用复合材料，复合材料机身耐撞性是型号研制中必须关注的因素，在应急坠撞过程中机械连接紧固件和连接母材均是机体的主要失效部位，其冲击失效特性对整个机身结构的破坏过程和能量耗散有重要影响。本项目分别从理论、实验和数值模拟三个方面系统研究了动态复合加载下典型复合材料机械连接结构的失效机理和力学行为。首先从复合材料连接紧固件的冲击失效机理和基础实验方法入手，开展了系统的研究工作，揭示了载荷作用状态、加载速率对连接紧固件失效模式的影响规律，建立了综合考虑加载速率和失效模式的紧固件动态破坏准则及失效方程，发现CFBL1001-6-6 /CFNT1003CY6 高锁螺栓的失效模式和失效载荷受加载方式影响较大，受加载速度影响较小;建立的失效方程可较好地表征高锁螺栓的失效特性。然后解决了碳纤维增强树脂基复合材料的中应变率拉伸试验测试难题，给出了典型碳纤维树脂基复合材料动态力学性能本构方程和失效方程，发现两种铺层碳纤维复合材料均具有明显的应变率强化效应，材料的流动应力随应变率的提高而显著增强，均发生局部的纤维断裂和分层失效，且分层损伤面积随应变率的提高逐渐增大，而失效应变与应变率呈非线性变化关系，与应变率区间相关。最后建立了复合材料连接结构的动态失效实验方法，研究了连接结构的冲击失效机理和能量耗散规律，建立了复合材料机械连接结构的冲击仿真分析模型，基于冲击实验数据和仿真分析模型探讨了冲击速度对复合材料连接结构冲击响应的影响关系，发现连接件的失效模式受连接母材、高锁螺栓/螺母材料强度影响，当连接母材强度较弱时，拉伸过程中连接板弯角和螺栓转角都会比较大，最终是连接母材产生失效破坏；当连接母材强度较强时，拉伸过程中连接板弯角和螺栓转角均较小，最终是螺栓/螺母的连接螺纹受剪切而产生拔出失效。连接件的能量吸收随冲击速度增加而提高，失效载荷随冲击速度增加而微幅增加。研究成果将应用于国产CR929等飞机的研制，产生显著的社会和经济效益。