Research On Fatigue Crack Propagation Life Of 10CrNi5MoV High Strength Steel Pressure Structure

Based on 10CrNi5MoV high-strength steel pressure structure,in the specific position,such as the cone column joint shell,due to the sudden change of structural stiffness and continuity,additional bending moment is caused,forming the combined stress of compression and bending at the local joint position.While the material and process characteristics of high-strength steel itself,as well as the stress concentration effect at the weld toe position of structural joints,make the above typical joint position of pressure structure become the hot spot area of structural fracture and fatigue failure.Based on the high strength steel structure,the crack propagation problem can not be ignored.In the"Structural Fatigue Calculation Criteria"based on 10CrNi5MoV steel,Newman-Raju formula and Paris formula are used to check the fatigue crack propagation life of pressure structure.Newman-Raju formula was originally used to solve the stress intensity factor of semi-elliptical surface crack under the combined stress state of tension and bending.While the specific node position of pressure structure is under the combined stress state of compression and bending,so it is difficult to use Newman-Raju formula directly to analyze the fatigue crack propagation under the specific compression bending ratio.In this paper,the theoretical formula is analyzed and modified through numerical simulation and fatigue test under the framework of the"Structural Fatigue Calculation Criteria",and the method which is suitable for both physical mechanism and engineering practice is sought to evaluate the fatigue life of 10CrNi5MoV steel pressure structure.It mainly includes the following four aspects:1)According to the datum calculation parameters of the stress intensity factor of the semi-elliptical surface crack,the NASGRO software is used to calculate the stress intensity factor under the combined stress state of compression and bending.It is proved that the calculation of the stress intensity factor by NASGRO can not only ensure the enough accuracy but also have higher calculation efficiency.Considering the influence of crack depth to plate thickness ratio a/t,crack shape ratio a/c and compression bending ratio?on stress intensity factors,Newman-Raju formula and NASGRO are used to calculate the stress intensity factors of series size semi-elliptical surface cracks under the combined stress of compression and bending.By comparing the calculation results,the application range of Newman-Raju formula to calculate the stress intensity factor under the combined stress state of compression and bending is given.2)The bending load fatigue test and the combined load of compression and bending fatigue test were carried out on the butt welded specimens of solid thick plate respectively.The fracture positions of the specimens are all at the weld toe,and the"semi-elliptical"crack propagation morphology can be observed on the fracture surface of the specimens.Compared with the experimental data,the existence of compressive stress can slow down the propagation rate of fatigue crack.The Newman-Raju formula and Paris formula are used to calculate the fatigue crack propagation life of the specimen.Compared with the test results,the results considering the stress concentration will be more safe.If the stress concentration is not considered,the results of bending load fatigue test specimen are more consistent,while the results of combined load of compression and bending fatigue test specimen are more dangerous.3)The effect of stress concentration on crack propagation life is considered.The submodel method is used to calculate the stress concentration factors K_tm and K_tc at the weld toe of the girth weld of the pressure structure when the bending load and the compressive load are applied separately,so as to correct the nominal stress state of the structure joint position.Considering the influence of the change of the average stress caused by the compressive stress on the crack propagation life,the equivalent bending stress?_meqis introduced.By comparing with the test data,the equivalent stress of the combined stress state is calculated by Morrow transformation.The bending stress correction factor f???is introduced to show the effect of the maximum total stress on the crack stress intensity factor under the same compression bending ratio.4)Considering the influence of stress concentration effect,average stress and compression bending ratio,and the maximum total stress on the outer surface on the crack propagation rate,the Newman-Raju formula is modified,and the calculation formula of stress intensity factor for the semi-elliptical surface crack under the combined stress state of compression and bending is given.The fatigue crack propagation life calculation model of 10CrNi5MoV steel pressure structure is established by combining the Paris crack propagation rate formula.Compared with the test data,the calculation results of the model are basically within the allowable error range of the project,and the results are more safety.The fatigue crack growth life calculation model of 10CrNi5MoV steel pressure structure established in this paper is of great engineering significance to provide technical support for the fatigue life evaluation of actual products of pressure structure based on 10CrNi5MoV steel.