Research On Fatigue Characteristics Of Welded Specimens Of High-strength Steel Under Complex Stresses Compressing And Bending

The cone-cylinder combined shell is typical structure and fatigue hot parts of submarines and deep-diving machines, whose fatigue problem has become particularly prominent with the increase of the structure size and the application of the new style of high-strength steel. Under working condition, due to the impact of the cone angle, the cone-cylinder combination supports complex stress of a bending stress and two pressure stresses, and at the same time, the impact of residual stress and stress concentration of welding structure are considered, so fatigue research in this part is very complicated. Currently, one pressure stress or tow pressure stresses are mostly neglected when the specimen of this typical node is used to research, but the effects of this neglect upon fatigue characteristics and fatigue life haven't been analyzed, so uncertain factors may be are produced when fatigue questions of structure are assessed.With welding specimen simulating the cone-cylinder typical part as the research object, the paper researches fatigue characteristics of the welded specimen under different loads, and discusses the influence which pressure stresses impact on the fatigue performance of the cone-cylinder combination, via test and theory analysis. It is raised at the first time that influences of pressure stresses can not be neglected when the estimating of fatigue life of the cone-cylinder combination is taken.First of all, how local stress and strain change with nominal stress is analyzed, under different forms of stress, by the way of welded specimen tests under bi-direction loads and elastic-plastic finite element calculation. It is found that when the nominal stress of the weld toe is not altered, local stress and strain values are different because of the difference of stress combination, and it obviously decreases as a result of pressure stresses, what's more the decrease is related to the proportion between pressure stress and bending stress. The results are used for estimating fatigue crack initiation life.And then, the fatigue crack growth experiments are carried under different loads for welded specimens with a prefabricated semi-elliptical surface crack, to research how pressure stresses impact on the crack growth law. The tests found that the crack shape of semi-elliptical is not obviously affected by pressure stresses, but the ratio of crack depth to half-length, a/c, has a great impact; With the analysis of Newman-Raju formula about crack growth rate, under the same stress intensity factor amplitude AK, longitudinal pressure stress decreases the value of crack growth rate, but the trend of rate withâ–³K is not changed, while circumferential pressure stress obviously increase the crack growth rate, and the trend of rate withâ–³K is more steep. The results provide a basis for estimating crack propagation life.Finally, according to the results from tests and analysis, crack initiation life and crack propagation life of weld toe are estimated, respectively basing on local stress method and fracture mechanics method, and then the whole fatigue life is calculated. With the same nominal stress of weld toe, longitudinal pressure stress and circumferential pressure stress all decrease crack initiation life and crack propagation life;when the proportion between pressure stress and bending stress is about 0.5,the decrease of fatigue life is up to 50%.Thus, during the structural fatigue life assessment, results ignoring pressure stress are somewhat dangerous.