Research On The Creep Behavior Of PMMA Immersed In Liquid Scintillator And Stability Of Spherical Pressure Vessels

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.