Research On Structural Form Election And Mechanical Behaviors Of Materials And Joints For Neutrino Detectors

The research on neutrinos is a very hot topic in the world.In 2015,a Japanese scientist and a Canadian scientist won the Noble Prize in Physics together,because of their contribution to the research on neutrinos.In 2016,a Chinese scientist won the Fundamental Physic Prize for the first time.In order to conduct neutrino experiments,neutrino detectors are needed.In this paper,the structural forms of neutrino detectors are designed and studied for the Jiangmen and the Jinping neutrino experiments.Polymethyl methacrylate(PMMA,acrylic)is a kind of crucial materials used in neutrino detectors.For the purpose of extending the use of PMMA to the structural engineering applications,the basic mechanical behavior and the fatigue crack propagation behavior of PMMA are investigated.The works and results are concluded as follows:(1)An acrylic ball supported by a double-layer stainless steel latticed shell is designed and optimized for the Jiangmen neutrino experiment.In the optimized scheme,the total weight decreases by 35%,the maximum stress on the acrylic ball decreases by 12.6%,the maximum axial load on the braces decreases by 6.3%,and the maximun structrural displacement decreases by 5.8%.While the difference of the liquid levels inside and outside the acrylic ball is 3 m,the maximum stress on the ball decreases by 43.6%.While the temperature increases by 10℃,the maximum stress on the ball decreases by 20%.Under the seismic action,the maximum stress on the columns reaches 178.6 MPa.When the joint at the the acrylic ball with the highest stress fails,the maximum axial load on the stainless steel brace increases by 9.6%.(2)Two structural schemes for the kilo-ton Jinping detector are proposed.A 1-ton small detector is designed and analyzed using the finite element method(FEM).The results show that the maximum stress on the acrylic ball of the 1-ton detector is 2.10 MPa,less than 5 MPa.The maximum stress on the stainless steel truss is 128.73 MPa,less than the design strength.In the stability analysis,the coefficient of the first buckling mode is 23.3 and the safety factor considering the double nonlinearities is 6.9.The results satisfy the requirement.(3)Three acrylic and stainless steel joints are designed for the Jiangmen detector.The ultimate bearing capacities are experimentally studied.The ABAQUS software is used to model the test.The results show that the ultimate bearing capacities of the three joints are 288 kN,325 kN,and 513 kN,respectively.The joints remain elastic before failures.The creep phenomena of acrylics are pronounced under the long-time loading.The polymerization of acrylic and stainless steel should be avoided in the process of fabricating the joints.The maximum stress on the acrylic is less than 10 MPa,which satisfies the using requirement of the Jiangmen detector.(4)Three stainless steel fasteners are designed and the bearing capacities are studied by experimental method.The ABAQUS software is employed to study the stress distribution on each part of the fasteners.The results show that all the ultimate bearing capacities of the three fasteners exceed 2000 kN and the safety factor is higher than 14.The maximum Mises stresses on the ball screw,the ball hat,the bottom hat,and the ball cover are 49 MPa,87 MPa,44 MPa,and 35 MPa,respectively.(5)The quasi-static uniaxial tensile test and the plain-stain fracture toughness(FT,KIC)test are conducted on acrylics.The effect of temeperture on the mechanical behavior of acrylics is studied.The results show that the stress-strain curves for the specimens exhibit the nonlinear characteristic.They have no yield points and the failures are brittle.With the increase of temperature,the ultimate strengths and the initial moduli of elasticiy decrease.The nonlinear elsatic expression of the ZWT model can fit the test data well.The KIC value of the base-material specimen is lowest at +20 ℃,equal to 1.45 MPa m1/2,and highest at-20 ℃,equal to 2.90 MPa·m1/2.The KIC value of the specimen with joint area is lowest at +40 ℃,equal to 1.24 MPa·m1/2,and highest at-40 ℃,equal to 2.21 MPa·m1/2.The smoother the fracture surface is,the higher the KIC will be.When the arc-shaped stripes along the crack propagation direction appear and accumulate,the KIC value of the specimen will decrease.When the radial-pattern stripes perpendicular to the edge of fatigue crack appear,the KIC value will decrease further.(6)The fracture mechanics analysis on the acrylic and stainless steel joints are performed using the ANSYS software.The largest crack depth on the acrylics under the load 6 times the design value is investigated.The results show that the maximum crack depth on the patched and the main acrylic plates should be limited to 7 mm and 0.4 mm,respectively.(7)The 2-D and 3-D FE analyses of fatigue crack propagation on acrylic plates are performed using the ANSYS software.The results show that the values calculated by the extended FEM and the remeshing method are close to each other for the 2-D fatigue crack propagation problems.The remeshing method is well capable of solving the 3-D problems.