Study On The Influence Law Of Stop-line On Fatigue Crack Propagation Performance Of Q345 Steel And Welded Joint

Metal components often bear fatigue load during service,which eventually leads to fatigue failure.Double stop-hole is a common method to prolong the fatigue life of cracked components by changing the stress distribution and the crack tip passivation.However,no consensus has been reached on the mechanism of life extension and the application standard of this method.In this paper,the Q345 low carbon steel is taken as the research object,and the standard compact tensile specimen is used to achieve the purpose of life extension by prefabricating double stop hole and new stop-line model,and the parameter optimization and its life extension mechanism are studied.The main research contents are as follows:(1)Fatigue crack propagation tests were carried out at different locations of the compact tensile specimen.The comparative analysis showed that the preset double stop-hole in the compact tensile specimen could achieve the purpose of life extension,and the location of the stop-hole had a great influence on the life extension effect.The closer the double stop-holes was to the crack tip,the more significant the life extension effect was.(2)Based on the double stop-holes model,the combined model of hole and line is put forward,and its life extension effect is obviously better than that of the double stop-holes model.It is found that the crack stop line will form the effect of stress shielding,so that the expansion rate will decrease before the crack stop line.After the crack arrest line,the second crack initiation occurs,which increases the fatigue life of the specimen.At the same time,it was found that the location of the crack arrest line had a significant effect on the life prolongation effect.When the crack arrest line was located 6mm ahead of the crack tip,the effect was the most significant,and the life prolongation was about 64.8%.Through ANSYS simulation of the crack growth process,it is proved that there is a stress shielding region in front of the crack stop line,which slows down the crack growth rate,and the stress intensity factors corresponding to different crack lengths of the base metal and the sample containing the crack stop line are obtained.The exponential model is used to fit the ANSYS simulation results,and the a-N curve is obtained,and the fitting results are in good agreement with the test results.(3)The Q345 welded joint was used to make the standard compact tensile specimen,and the stop-line was pre-prefabricated 6mm before the crack tip.Compared with the specimen without stop-line,the fatigue life of the specimen was effectively extended by 86.4%,indicating the reliability of the stop-line model.The life extension effect of the welded joint is different from that of the base metal,which is related to the effect of the stop-line itself,the residual stress and the passage of the expansion path through the heat-affected zone.