Experimental Study On Fatigue Performance Of Q690D High-Strength And Its Joints

Fatigue failure is the result of damage accumulation of structural components.There is no obvious sign berore the failure,and sudden fracture often leads to heavy casualties and economic losses.As the main form of failure of steel structure,which has attracted great attention from the engineering and academic circles.At present,the research on fatigue performance and fatigue design of high strength steel is at the beginning stage,and there is no systematic theoretical results yet.Based on the revision of code for desigh of steel structures(GB50017-2003),the fatigue performance of Q690D high-strength steel and its welded joints and bolted joints is studied in this paper.In this paper the fatigue performance of Q690D base metal,butt joints,cruciform joints,perforated plate and bolted connection were investigated.The S-N design curves were fitted and compared with the corresponding fatigue detail design curves of Eurocode3,BS7608 and AISC360 specifications,which provided a comprehensive assessment for fatigue properties.Based on the phenomenon that fracture position of the welded joint was near the weld seam,the hardness distribution of butt-welded joint was tested and the effect of hardness on fatigue performance was discussed.Results show that an obvious softening zone,where fatigue crack is prone to occur,can be observed in the heat affected zone(HAZ),reducing the fatigue life.In addition,electron microscopy was adopted for observing the fracture morphology of base metal and welded joint,and the fracture features at different stages were identified.On the basis of the different phenomena that the fracture location of the perforated plate is located at the minimum net section and the bolt connection is in front of the first row hole of the main plate,the influences of stress concentration and bolt pretightening force on the crack initiation position and fatigue life were discussed.The researches show that stress concentration restricts the location of the crack initiation near the maximum stress point,and that the bolt pretightening weakens the stress concentration on the edge of the bolt hole,prevents the initiation of fatigue crack at the net cross section,and transfers the fatigue crack to the front of the hole,thus improving the fatigue strength.The fatigue test data of five different types of structures subjected to axial stress with a stress ratio of 0.1 was also collected.Based on the comparison between a large number of experimental data and the comparison with the design curve,the factors affecting the fatigue strength and the applicability of the specification were discussed.Finally,on the basis of the test,the full-coupling damage finite element method was used to introduce the influence of damage in the stress-strain field,and the cumulative damage process of the parent metal specimen was simulated and compared with the test results,which verified the applicability of this method to the standard small component fatigue life analysis.