【作者】 周凡；

【导师】 陈志平；

【作者基本信息】 浙江大学 ， 化工过程机械， 2017， 博士

【摘要】 有机玻璃因其良好的综合力学性能以及光学特性被广泛地用作承压设备的主体材料,运用到建筑结构、航空航天、医疗设备和物理研究等领域。多样化的服役环境会对有机玻璃的力学性能产生影响,特别是其蠕变性能,将直接影响到结构的长期使用寿命。因此,了解和掌握有机玻璃的在特定条件下的长期力学性能显得尤为重要,对充分合理地利用有机玻璃材料具有重要的理论意义和指导价值。而对于承受外压的有机玻璃薄壁壳体,屈曲失稳是一种常见的失效形式,将有可能引发严重的工程事故,需要进行防屈曲设计。本文采用理论分析、试验测试和数值模拟等手段,对有机玻璃在液体闪烁体(简称液闪)中的蠕变性能以及有机玻璃球壳的外压稳定性进行研究。开展的主要工作如下:(1)应用一种耐液体蠕变试验机,对浸泡在液闪中的有机玻璃试样分别进行了恒温变载以及恒载变温蠕变试验,获得其短期蠕变行为。基于短期试验数据,采用时间-应力等效原理(TSSP)以及时间-温度等效原理(TTSP)加速表征有机玻璃的长期蠕变行为;并采用Burgers蠕变模型和Findley经验方程描述有机玻璃的短期蠕变行为。(2)对浸泡在液闪中的有机玻璃试样开展蠕变断裂试验,验证了加速表征方法的可靠性与准确性;探索并得到了应力与有机玻璃断裂时间的关系式;引入非线性粘性元件构造了可以描述有机玻璃蠕变全过程的非定常Burgers蠕变模型,并编写了该模型的 User-defined Material Mechanical behavior(UMAT)子程序。(3)综合开展了有机玻璃薄壁半球壳外压屈曲试验和有限元数值模拟研究,采用高速摄像机捕捉了有机玻璃半球壳的屈曲模态,应用动态电阻应变仪测得了球壳外表面的应力分布;验证了非线性有限元屈曲分析方法对有机玻璃球壳的适用性与准确性,探讨了初始几何缺陷幅值对有机玻璃球壳屈曲临界载荷的影响;确定了基于非线性有限元屈曲分析的有机玻璃球壳外压稳定性安全系数,并给出了可以满足稳定性要求的江门中微子探测器结构方案。(4)初步开展了有机玻璃球壳的蠕变屈曲研究。推导了有机玻璃球壳的蠕变屈曲临界载荷计算公式,从理论上分析有机玻璃的蠕变行为对结构长期稳定性的影响;利用编写的非定常Burgers蠕变模型UMAT子程序,对有机玻璃球壳进行蠕变屈曲分析,得到了蠕变屈曲临界时间。更多还原

【Abstract】 Polymethyl Methacrylate(PMMA)has been widely used as the main material of pressure equipment and applied in the fields of Architectural Structure,Aerospace,Medical Equipment and Physic research.Due to different requirements,they may be exposed to various environments.It is well known that the long-term behavior of viscoelastic material influences the lifetime of the structure.Also,property deterioration of materials as a result of exposure to various special environments leads to the reduction of lifetime.Therefore,for safety applications,the creep properties of materials need to be studied.Buckling failure is a common failure form for acrylic thin-wall shells under external pressure,which may lead to serious engineering accidents.So it is of great significance to carry on design and check to prevent buckling.In this paper,methods such as experiments,numerical simulation and theoretical analysis are used to investigate the creep properties of acrylic and the stability of acrylic pressure shells.The main works are as follows.(1)To realize the function that specimens can be immersed in a liquid environment during tests,a uniaxial tensile creep test machine with a soaking tank has been built.With this new-type creep test machine,constant load and constant temperature creep tests of PMMA specimens immersed in liquid scintillator were carried out.And the short-term creep behaviors were obtained.Based on the short-term test data,Time-Stress Superposition Principles(TSSP)and Time-Temperature Superposition Principles(TTSP)are applied to evaluate the long-term creep properties of acrylic.(2)Tensile creep rupture tests of acrylic specimens immersed in liquid scintillator were performed.The reliability and accuracy of the methods of accelerated characterization are verified.The relationship between stress levels and rupture time is obtained.The linear Burgers Creep Model is modified to be an unsteady Burgers Creep Model with a nonlinear adhesive pot.The unsteady Burgers Creep Model can describe the whole creep process.Besides,the modified unsteady Burgers Creep Model was secondary development with the User-defined Material Mechanical behavior(UMAT)subroutine.(3)Experimental study and finite element numerical simulation of acrylic hemispheres were carried out.The buckling mode of acrylic hemisphere was obtained with High-speed Camera.The stress distribution was obtained by the Dynamic resistance strain gauge.And the applicability and accuracy of finite element numerical simulation method on acrylic hemispheres is verified.The effect of initial geometrical imperfection on the critical buckling loads is studied.Besides,a stability safety factor based on nonlinear finite element method is determined.And several feasible schemes for the design of acrylic vessel in Jiangmen Underground Neutrino Observatory(JUNO)are proposed.(4)A preliminary research on the creep buckling of acrylic spherical shells is carried out.Creep critical buckling load formula is derived.The effect of creep on the stability is studied by theoretical analysis.The creep buckling of acrylic spherical shells is simulated with the modified unsteady Burgers creep UMAT subroutine.And the critical creep buckling time is obtained.更多还原