多级网格加筋壳的低缺陷敏感性能研究与优化设计

As a dominating failure mechanism of large-diameter grid stiffened cylindrical shells, compression-induced buckling is known to be very sensitive to initial geometrical imperfections. It is a great challenge to develop a type of stiffened shells with high load-carrying capacity and low imperfection sensitivity for Chinese new-generation Launch Vehicles and future Heavy Lift Launch Vehicles. Recent research has shown that biological materials and many civil engineering structures such as the Eiffel Tower are composed of hierarchical sub/micro-structures and they all exhibit excellent tolerance of flaws. Inspired by these research findings, this application proposes a new structural concept for investigating multi-level grid stiffened shells that focuses on the characterization of the failure modes and failure strengths at different stiffener levels of structural hierarchy. We seek to use state-of-the-art numerical and experimental approaches to unveil the scientific facts underpinning the low imperfection-sensitivity of these multi-level grid stiffened shells. Based on this exploration, a novel optimization method is expected to be developed for directly identifying the most unfavorable imperfection among several base shapes of geometrical imperfection such as dimple-shape imperfection. With the proposed multi-level model and the multi-objective optimization approach, combined with plate homogenization method by Kalamkarov and proposed multiple smeared stiffener approach, we aim to deepen the understanding of the optimum design of this new type of stiffened shells and to obtain a couple of innovative and realizable launch vehicle designs by taking into account both load-carrying-capacity and imperfection-sensitivity objectives.

初始几何缺陷对大直径轴压网格加筋圆柱壳的承载力影响极大，如何给出承载力高、缺陷敏感度低的优秀网格加筋设计是我国未来重型运载火箭结构设计中的突出难题。生物体结构和埃菲尔铁塔等大型土木工程结构具有丰富的结构层次，它们都具有较好容忍缺陷的性质。本申请从如何适当丰富网格加筋的结构层次获得启发，提出并研究多级网格加筋新型结构方案，通过数值和实验分析研究不同级网格加筋参数对其失效模式和破坏强度的影响规律，考虑不同类型初始缺陷，理解并揭示其低缺陷敏感度的力学机理；考虑侧向凹陷等高发缺陷，通过搜索最不利"基形状"组合，发展更客观的结构轴压承载性能的评价方法；结合板壳均匀化方法与多次刚度等效分析，发展多层次和多目标优化算法，建立多级网格加筋优化设计方法，获得更优设计。我们希望通过本研究获得该类新型网格加筋结构承载性能与优化设计的规律性认识，并能得到一到两种可用于未来运载火箭工程实施的创新网格加筋结构方案。

对于薄壁筒壳设计，同时考虑结构减重和抗缺陷干扰很困难。追求抗缺陷的结构设计，往往增重。因此，对于大直径筒壳初始几何缺陷显著增加的现状，亟需发现创新的筒壳结构构型，用以同步达到结构减重和抗缺陷干扰。.项目组提出并研究了多级网格加筋结构参数对极限承载力的影响规律，验证了“失稳局部化”是多级网格加筋结构优异承载力的力学机理，提出了变截面筋条等多级网格加筋新颖方案，并验证了其强抗缺陷的能力。.项目组开展了多级网格加筋结构低缺陷敏感度研究，结果表明：多级加筋柱壳将缺陷引发的初始局部出平面变形限制在主筋格栅中，降低了对初始几何缺陷的敏感性；提出“最不利多点侧向扰动载荷法”预测筒壳结构承载力折减因子，结果表明：本项目方法能够包络筒壳轴压承载力，验证了其准确性和可信性。.项目组提出了基于渐进均匀化方法的快速高精度刚度等效方法，算例结果验证了NIAH方法预测结果的准确性，可提高求解效率。提出了基于等效模型的多级网格加筋壳抗缺陷优化方法，结果表明：未考虑缺陷影响的最优设计极易对缺陷敏感；通过选型优化可有效增加多级加筋壳抗缺陷的能力。建立了基于自适应等效策略的多级网格加筋壳分层次优化方法，算例结果验证了分层次优化分析框架的有效性并表现出极高的优化效率。.项目组在结构力学、航空航天等领域权威学术期刊：AIAA J., Compos. Struct., Int. J. Solids Struct.等发表论文45篇，其中32篇为SCI检索论文，相关成果得到了国内外知名学者的高度评价；获授权发明专利5项（包括两项国际专利）。多层级网格加筋结构被写入我国航天行业标准《运载火箭网格加筋结构强度设计要求》。此外，多级网格加筋构型已成功应用在我国海军某重点型号的关键承力舱段；“最不利多点扰动载荷法”被用于我国新一代大型运载火箭长征五号的承力结构研制。依托本项目的部分研究成果，获得了2017年教育部高等学校科学研究优秀成果奖技术发明奖一等奖（第一完成人王博、第三完成人郝鹏、第六完成人程耿东，均是本项目研究成员）。