Study On Sealing Ability Of Metal Ring And Low Cycle Fatigue Behavior Of N18 Cladding-Tube Used For Nuclear Fuel

Metal sealing ring is the key element of a pressure vessel, and its sealing properties work nearly on safety and reliability of the pressure vessel. Metal O-ring and C-ring of Inconel718 alloy have been widely used as sealing elements of nuclear pressure vessels. O-rings of some traditional materials have been or will be used as the sealing materials of common pressure vessels. Therefore, systematical research on mechanics behaviors and springback properties of metal sealing rings has engineering significance.Based on elastic-plastic and contact analysis methods of ANSYS and MSC.MARC software, the finite analysis about mechanics behaviors of Inconel718 alloy O-ring and C-ring was carried out. It's opened out that the restriction of groove in nuclear pressure vessel could cause decrease of springback amount and expansion of high stress area of a O-ring, and a reasonable range of compressing ratio between 8 % and 12 % was recommended. For mechanics behaviors of contact force, springback amount and stress response, impact of spring stiffness on the C-ring and effect of elevated temperures on the O-ring were investigated.Compressing-springback tests on tube specimens of various materials (ss304, 2A12, 42CrMo, 60Si2MnA and Inconel718) and various geometry dimensions of O-ring were carried out. The impacts of inside, and outside diameters, compressing ratio and uniaxial mechanic properties of material on sealing ability of metal O-ring were conducted.A big springback amount database of O-ring was obtained by ANSYS software. A set of dimensionless analysis method to build prediction models of the metal O-ring springback was given. Investigation results show that when compressing ratioλis between 4% and 20%, relationship between compressing ratio and springback ratio as well as relationship between diameter-thickness ratioγand springback ratio r fits with linear law. Further more, TSM,TSM-α-I and TSM-α- II models were presented for predicting springback amount of the O-ring and they have well precision. TSM-α-I model was firstly recommended for sealing engineering. The parameters of these models can be obtained by finite analysis methods based on a material constitutive relationship or straightly by cyclic compressing-springback tests of a few O-ring specimens. Based on springback amounts database built in this thesis, STSM,STSM-αand CTSM models were presented, and they have better precision to predict the springback amount of O-rings of various materials with various geometry demensions.To meet engineering design requirement, an engineering application based on STSM model was developed by Visual Basic language.A set of optimized and self-invented grips was developed, and it is the precondition of realizing fatigue tests of small tube specimens at elevated temperatures. The cycling and fatigue performance of small cladding-tubes of N18 alloy used for fuel rods in nuclear reactor have attracted extensive attentions. This thesis carried out experimental research on cycling behavior and low cycle fatigue behavior under strain cycling of the cladding-tubes at elevated temperatures, and relative study was rarely reported in the country and abroad.A series of uniaxial monotonic tests and strain cycling tests at 20°C, 200°C, 300°Cand 400°C on small tubes of N18 alloy were performed. Uniaxial and cyclic constitutive models of the tubes were obtained. The relationship between stress ratioλ_σand strain amplitudesΔε/2 fits with power-law, and its parameters vary linearly with temperature. It is shown that the material behaves cyclic softening at 20°C, and it behaves cyclic hardening changed to cyclic softening with increasing strain amplitudes at 400°C. In additional N18 alloy does not follow Mason Law at 20°C, 200°C, 300°Cand400°C.Based on cyclic constitutive equation and half-life constitutive equation, M-C, GB-MSC and GB-half life equations to predict fatigue life of the tubes at 400°C were obtained, and they have better precision. It's opened out that relationship between temperature factor of fatigue life and plastic strain amplitude of the tubes fits with power-law, and then a new model for predicting fatigue life of the N18 alloy tubes at elevated temperatures was presented.