Research On Fracture Behavior Of Ultra-High Strength Steel And Welded Joint

The ultra-high strength steel is widely used in aerospace and military industry,at present it is also used in construction industry,automotive industry and bridge construction,etc.However,there are two phenomena in the application of the ultra-strength steel:one is that the low fracture toughness of the ultra-high strength steel and the machining defects will lead to fracture accident between whiles;the other is that the laboratory specimen of high stress triaxiality was used to assess the practical structures of low stress triaxiality,and this may lead to a conservative assessment result which will lead to a waste of the ultra-high strength steel.Aiming at these problems,taking D406A steel as an example,this paper systematically studied the fracture behavior of the ultra-high strength steel and it's welded joint.Firstly,the chemical compositions of D406A steel,the non-metallic inclusion and microstructure were tested,and their effects on the fracture toughness were analyzed.The basic mechanical properties of D406A steel were tested,too.The welding condition and welding material of D406A steel were recommended after that;the mechanical properties and microhardness of welded joint were also tested. In accordance with the fracture toughness test standard of BS7448, the D406A steel which is 8-10mm thick and its fracture toughness and J-Resistance curve of welded joint were tested by using three-point bending specimens,and the effect of mismatching on the fracture toughness of welded joint was analyzed.The surface crack fracture toughness KIe of D406A steel were tested. But the test results can not satisfy the validity criterions, so the elasto-plastio fracture mechanics test and finite element analysis (FEA) had been conducted to calculate the ductile fracture toughness JIe of material. Then surface-crack fracture toughness KIe was computed on the fracture mechanics method from JIe, and the conservation of loop J-integration and the J-dominant characteristic of surface crack were studied.Thus,the problem that testing the surface crack fracture toughness KIe by using small specimens was successfully solved. The surface crack fracture toughness of welded joint was tested (the surface cracks was in the seam center and in the point which was 1mm away from the fusion line),the effect of mismatching on the fracture toughness of welded joint was thoroughly studied.The local approach was used to study the fracture behavior of the ultra-high strength steel.Comparison was made between the stress field near crack tip of different specimens and the HRR field,and the different restraint intensity of all specimens was analysed.The shallow surface crack fracture toughness was tested, and the fracture toughness probability distribution between the deep and shallow surface cracks was compared.Weibull parameter estimation procedure was used to calculate the shape parameter and scales parameter of Weibull function of D406A steel,and then a prediction of the probability distribution of shallow surface crack fracture toughness was successfully made by using deep surface crack sample.The finite element method (FEM) was used to analyze the changes of J-integral that took place in the base metal and HAZ of welded joint with surface crack and while the Weibull stress changed,also the fracture toughness probability distribution of the two was compared ,and the base metal was used to make a successful prediction of the HAZ fracture toughness probability distribution.