Study On Creteria Of Vibration Stress Relief Of DH36 Welded Structures And Thermal Vibration Stress Relief

Vibration stress relief(VSR)is a method for mediating residual stress in welded structures,which is a green,convenient and effective method.It has wide potential application in industries.Vibration characteristics of the structures usually change slightly after the VSR treatment,along with the decrease of residual stress.This is currently used in engineering and national standard to estimate whether or not the VSR treatment is effective.However,since the research on the relationship between the residual stress and the vibration characters is still not adequate,the current estimation is empirical to some extent,while quantitative analysis is still in need.Therefore,there are significant limitations in application of this method.In this study,along with the study of VSR treatment of marine engineering DH36 welded structures,analytical investigation,experimental study and finite element method,FEM,were employed to study the relationship between the decrease of residual stress and the variation of vibration characters.It is expected to draw a new effective and accurate route to evaluate the efficiency of VSR,and application of VSR on various welded structures were carried out.Residual stress in thin rectangle DH36 alloy welded structures was effectively reduced by VSR.The mechanical properties,hardness and microstructure of the alloy were not changed by VSR.This excluded the possibility that the variation of material properties changed the vibration characters.FEM was used to study factors that affect vibration mode and amplitude-frequency curve of beam,including exciting position,supporting position,damping and stress.When the exciting and supporting positions are given,the vibration amplitude is affected by both damping and stress,while natural frequency is affected only by stress.Therefore,natural frequency were selected as the criteria of efficiency of VSR.Natural frequency function,NFF,relating to the two direction stress was established,including combined effects of them.The results derived from this function were very similar to that obtained from dynamic analytical solution and FEM studies.Efficiency Index(EI),h,was proposed,through which variations of stress and natural frequency was connected.The value of h indicates the decrease of residual stress in welded structure after VSR treatment.FEM was used to determine the natural frequency,amplitude-frequency curve and vibration mode of symmetric and non-symmetric 2D structures with 2D stresses.Corresponding Natural Frequency Surface,NFS,and NFF were proposed,which included the combined effects of stresses in two directions.The prediction made with them was relatively accurate on the points far from the fitting sampling points,especially in the areas with high stress,indicating a good universality.Natural Frequency Curve,NFC,was proposed,which indicates the set of residual stress conditions correlating to a given natural frequency of the structure.The natural frequency-stress state of the system lied on different NFC before and after the VSR treatment.When the natural frequency decreased,it indicates that at least residual stress in one direction decreased.When the variation of natural frequency is subtle,the residual stress may decrease in two directions,or decrease on one direction while increase on the other.When the variation is significant,the stress decreases in both directions.Critical Natural Frequency,CNF,was defined as the minimum natural frequency that corresponding to the state that stresses in none of the two directions would increase.The variation of natural frequency can be used to qualitatively indicate the variation of residual stress in two directions.EI can quantitatively indicate the decrease of residual stress.The theoretical prediction met well with the experiments on rectangle DH 36 welded structures.VSR and post weld heat treatment was used and compared for large marine engineering welded tubes.In this example,the EI was 56.3%,and the measured decrease of residual stress was ~50%.The two met well with each other.This indicates that EI has very significant application value,and it can be used not only in welded planes,but also other complicated structures.In making similar decrease of stress,VSR cost less than 1/10 energy and time than thermal stress relief.By analyzing the effects of heating on natural frequency,vibration amplitude and yielding strength,thermal-vibration stress relief(TVSR)was proposed.And it was tested in high rigidity DH36 thick plates,which could hardly be treated by typical VSR.The weld seam was heated up to 150°C for the TVSR.After the TVSR treatment,the decrease of residual stress met the requirement of the welded DH36 steel plates.The TVSR method expanded the scale of application of VSR.