Locking Cap Lap-weld Partition Modeling And Fatigue Life Prediction Research

In a nuclear reactor,mining machinery and other large projects,locking cap structure is widely used to strengthen the stability of the bolt connection.There will be fatigue failure caused by dynamic load,like high temperature,radiation environment and water flow.304/304 L austenitic stainless steel,for the carbon content is low,corrosion resistance and weldability is good,used in engineering areas.There exists two load conditions in locking cap weld,due to weld structure is small and irregular,the joint organization distribution,high precision modeling,and fatigue life prediction method is difficult and have important theoretical meaning and engineering value for the safety application of the locking cap.This paper carried out the locking cap lap weld modeling method and fatigue life prediction.Through metallographic,hardness,tensile test and fatigue test,the structure composition,hardness distribution and mechanical properties of two different laps are obtained.Based on the hardness test data to modify empirical formula,fitting plastic stress-strain curve of different areas.Partition modeling and fatigue life prediction method has been put forward and applied to locking structure.The main contributions and conclusions of this dissertation are summarized as:(1)Research on the weld stress form and sample preparationThrough simulation analysis of fasteners vibration test,two stress form for locking cap weld has been acquired,that is tensile and shear,then two lap joint samples have been designed.The element composition and mechanical properties of welding material 304/304 L was introduced,as well as the filler metal selection principle and tungsten inert gas(TIG)welding process parameters.(2)Research on organization and mechanical properties of lap jointThrough metallographic test,hardness test and tensile test,the joint regional material distribution of organization and mechanical properties were obtained.The weld joint area is divided into seven areas depending on the type of grain type and size,mainly for the content of austenite,ferrite and martensitic structure.Access to the seven areas of Vickers hardness value through hardness test,verified the strength of weld area is greater than the parent metal and heat affected zone.The tensile test result showed that the tensile sample has larger fracture elongation and higher tensile strength.(3)Research on the region partition modeling method of lap jointCombined with 304 stress-strain curve and hardness test result,the three stages Ramberg-Osgood stress-strain model was modified.Then stress-strain parameters(curve)of the seven areas are identified.Finite element method based on partition modeling method was proposed,and using technique creep grid rules to mesh different dimensions.The simulation results are verified compared with axial tensile test,the maximum error is less than 2%.(4)Research on locking structure fatigue life predictionThe fatigue test under stress control was taken on two joint forms of samples,fitting the relationship curve under some stress ratio R between the stress S and the fatigue damage of life N in the logarithmic coordinates,namely S-N curve.Using the finite element analysis software nCode Design Life to predict fatigue life,the simulation result was consistent with the test result.Finally,the modeling and fatigue life prediction method of lap joint weld was applied to the reactor locking cap structure,and the requirements of using 60 years was verified.