蜗壳垫层材料压缩-回弹响应行为的试验研究及数值模拟

An Experimental and Numerical Investigation on Compression-resilience Responses of Membrane Materials in Spiral Case Structures

垫层在水电站钢蜗壳-混凝土组合结构中起传递内水压力的作用,但在当前的设计实践中,垫层通常被简化为线弹性材料。本文选取聚氨酯软木与聚乙烯闭孔泡沫等2种最常用的蜗壳垫层材料,首先采用土工轴承式单杠杆固结仪对2种材料分别进行循环加-卸压试验,材料试样由环刀切割成型,受压过程在环刀内完成,最大压力荷载定为1.0 MPa,加压与卸压过程均分10级完成,级差0.1 MPa。而后根据试验中的垫层材料试样尺寸,建立试样的平面轴对称有限元模型,在ABAQUS平台上采用HYPERFOAM和MULLINS EFFECT两者共同模拟单次加-卸压循环中2种材料的非线性响应过程,2种模型中必要的材料参数通过UNIAXIAL TEST DATA选项结合材料的压缩应力-应变试验数据确定。研究结果表明:2种材料均具有显著的非线性、不可逆的压缩特性,在循环加-卸压作用下均存在显著的残余应变;数值模拟对2种材料在单次加-卸压循环中的应力-应变响应过程的预测效果良好。认为在蜗壳结构的设计中不应忽略垫层材料的非线性力学特性,前述基于ABAQUS平台的数值模拟技术能够简便地实现垫层材料的非线性力学描述。建议垫层材料在出厂前,宜提前经历足够次数和时长的"预压"过程,最大程度上降低垫层材料残余变形对蜗壳结构受力的潜在不利影响,垫层材料的力学设计参数须基于已经历过"预压"过程的试样压缩-回弹数据确定。

Membranes play load-transferring roles between steel spiral cases and surrounding concrete in hydroelectric power plants.However,membranes are routinely recognized as linear-elastic materials in current engineering practices.The two most commonly-used membrane materials,i.e.the PU cork and PE foam,are chosen as the current study subjects.Cyclic compression-resilience tests were performed for the two materials with a lever-type consolidometer.The specimens were cut out with cutting rings and the cutting rings provide lateral confinement for the specimens in compression.The maximum pressure is 1.0 MPa with a loading/unloading interval of 0.1 MPa.Each loading/unloading cycle thus consists of 20 gradations.Then the HYPERFOAM model in conjunction with the MULLINS EFFECT option was employed to model the non-linear responses of the two materials in a single loading-unloading cycle based on the ABAQUS code.The specimen was modeled in line with its dimensions with CAX4 axisymmetric solid elements.The UNIAXIAL TEST DATA option was used to obtain the parameters for the HYPERFOAM model directly from the experimental stress-strain data.The strong evidence of the nonlinearity and irreversibility of the compressive behavior of both materials is found.The significant permanent set upon removal of the applied load was observed for both materials.The numerical simulation results fit well with the non-linear responses of the two materials in a single loading-unloading cycle.This study highlights the need to consider the non-linear compressive behavior of membrane materials in the structural design of spiral case structures.The presented simulation procedure based upon the ABAQUS code is practicable for the numerical description of the non-linear compressive behavior of membrane materials.To avoid as far as possible the potential adverse effects of permanent set on structural performance of spiral case structures,it is suggested that membrane materials should undergo sufficient duration and cycles of precompression before field service.The compressive stress-strain curves of membrane materials used for design purposes should be obtained on specimens that have undergone precompression.