水电站垫层蜗壳结构中钢衬-混凝土间脱空的力学机制

A power-generating structure is the core of a hydropower house. It is usually of a steel spiral case surrounded with mass concrete, composing a steel-concrete composite system. When embedded in an unwatered condition with the top portion of it covered with a compressible membrane, a steel spiral case is believed to resist most of the internal water pressure (IWP) with only a small load being transmitted into the surrounding concrete. Meanwhile, the membrane is probably responsible for weakening the hold of concrete on the steel spiral case. There is a strong probability that the normal contact pressure between steel spiral case and concrete is inadequate, potentially causing a gap between them. This research programme will take a new look at the gapping mechanism between steel spiral case and concrete. Both physical model tests and FEM-based numerical simulations will be performed. First, we will examine the expanding behavior of steel spiral case under an IWP. Then, we will focus on how a steel spiral case separates from its surrounding concrete from the view of mechanics. Finally, we will try to clarify the key structural factors influencing the contact state between steel spiral case and concrete. In order to identify the tiny relative displacement between steel spiral case and concrete, a close range photogrammetry will be employed in the physical model tests. In addition, the research programme will investigate how to simplify the nonlinear numerical description of a membrane regarding its complex geometrical and mechanical property. It is considered that a softened contact relationship defined between steel spiral case and concrete, rather than a set of thin-layer continuum elements routinely created, would help to achieve this goal in the numerical simulations. We believe that, the results can offer vital support for preventing a gap between steel spiral case and concrete to a large extent in engineering practices. The findings have important implications for overcoming engineers' doubts about a membrane-caused unstable operational state of a hydrogenerator.

水电站厂房发电核心结构是钢蜗壳（钢衬）-混凝土组合体系，钢蜗壳上表面铺设软垫层在降低了内水压力外传至混凝土结构的同时，也削弱了混凝土对钢蜗壳的包裹和嵌固作用，体现为钢衬-混凝土间较小的法向接触压力甚至脱空状态。本项目拟围绕"钢衬-混凝土间脱空的力学机制"这一科学问题，开展物理模型试验和有限元数值模拟研究，阐明受内水压力作用下钢衬的膨胀力学行为，解释其与外围混凝土分离的力学机制，揭示影响钢衬-混凝土间接触状态的关键结构因素。物理模型试验中拟借助近景摄影测量技术，以克服钢衬-混凝土间微小相对位移难于捕捉的困难；数值模拟中拟采用"软"接触关系代替"实体单元+力学本构模型"的常规方式描述垫层材料，以从几何和力学两方面简化其原本复杂的非线性数值描述过程。研究成果可以为工程实践中最大程度上避免钢衬-混凝土间出现脱空状态提供科学依据，以期消除工程界有关钢蜗壳上表面铺设垫层可能影响机组稳定运行的顾虑。

水电站厂房发电核心结构是钢蜗壳（钢衬）-混凝土组合体系，钢蜗壳上表面铺设软垫层在降低了内水压力外传至混凝土结构的同时，也削弱了混凝土对钢蜗壳的包裹和嵌固作用，体现为钢衬-混凝土间较小的法向接触压力甚至脱空状态。本项目围绕“钢衬-混凝土间脱空的力学机制”这一科学问题，采用非线性有限元数值模拟技术，基于ABAQUS成功模拟了单次加-卸压循环中垫层材料的应力-应变响应关系，并发展了一种基于软接触关系的垫层模拟新技术，从几何和力学两方面简化了其原本复杂的非线性数值描述过程。数值模拟成果揭示了削弱混凝土对钢衬包裹和嵌固作用的关键结构因素：①钢衬-混凝土间较小的摩擦系数；②子午断面内垫层较大的铺设包角；③垫层较小的压缩刚度。同时，也研究了非平滑接触面如何影响钢衬-混凝土间的滑移及传力行为，数值模拟结果给出了削坡构造附近交替出现的局部脱空区和集中传力区，并阐明了其成因。研究成果可以为工程实践中最大程度上避免钢衬-混凝土间出现脱空状态提供科学依据，以期消除工程界有关钢蜗壳上表面铺设垫层可能影响机组稳定运行的顾虑。