Study On Effect Of Prefabricated Stop-Line On Fatigue Resistance Of DT4C Steel And Its Mechanism

Aiming at the problem that fatigue crack growth life of defective metal components decreases seriously,this paper takes cold rolled DT4C steel as an object,and studies the influence of prefabricated stop-crack line parameters on fatigue crack growth performance and fracture mechanism of E-CT specimens of cracked DT4C steel.The main research contents are as follows:(1)The stop-crack line is prefabricated on the fatigue crack propagation path of specimens with cracks,and the stress distribution along the crack tip in the process of fatigue crack propagation is simulated.The ANSYS Workbench finite element model is established.The simulation results show that the stop-crack line has a certain influence on the stress distribution along the crack propagation path,and the shape and position of the stop-crack line is proposed.(2)Taking fatigue crack growth life as a criterion,the effect of stop-crack line location(distance L_a from the top of the stop-crack line to the root of U-notch)on crack growth rate and fatigue life of plate specimens was studied.By means of fatigue crack growth test,the variation trend of crack growth rate before and after reaching the crack stop line and its influence on fatigue life of E-CT specimens with different locations of crack stop line(L_a is6.4 mm,7 mm,7.6 mm,respectively)are studied.The relationship between L_a value and crack growth performance and fatigue life was obtained.Compared with the fatigue properties of the base material,the test results show that the fatigue crack growth life of the sample increases with the increase of L_a value,and the fatigue resistance of the sample is the best when L_a=7.6 mm.(3)The fatigue fracture morphology of base metal and specimens with stop-crack line were analyzed by scanning electron microscopy(SEM).The relationship between fracture micro-characteristics and crack growth performance and life is verified.The experimental results show that obvious fatigue striation characteristics can be observed in the stable crack growth region.When the crack growth length is the same,the spacing between the cracks decreases with the increase of L_a.At the same time,some specimens have secondary cracks in the process of crack propagation.The instantaneous fracture zone of the specimen fracture is characterized by dimple fracture.