Characterization And Measurement Of Fracture Toughness Of Surface Modified Layer Of 18CrNiMo7-6 Alloy Steel

Anti-fatigue manufacturing technology is a new generation of manufacturing technology to improve the performance and fatigue strength of workpiece by controlling the surface integrity and the surface-modified layer（SML）,which is very different from the current"forming"manufacturing.The core of anti-fatigue manufacturing is to give the workpiece SML with excellent performance by surface modification technology.Characterization and measurement of fracture toughness of SMLs is the basis of anti-fatigue manufacturing technology,and has important significance for the research and development of anti-fatigue manufacturing technology.In this thesis,the characterization method for fracture toughness of SMLs was proposed by tensile test and continuous indentation test,and the fracture toughness of 18CrNiMo7-6 alloy steel base material and SMLs formed by carburizing heat treatment was characterized.The main contents are as follows:（1）Based on tensile tests,a method for characterizing fracture toughness J-integral,of surface metamorphic layers is proposed.The J-Δa resistance curve of the base material was obtained by compact tensile test,and then the fracture toughness K_JCof the base material was analyzed.The fracture toughness K_JCof18CrNiMo7-6 alloy steel base material was characterized.Based on the unilateral notch tensile test,using finite element software to simulate the tensile test process,the empirical formula of the plastic factor?_pof the SML changing with the strain hardening index n was analyzed.Using the strain hardening index of the SML obtained by stripping tension,the overall plasticity factor of the SML is given,and a characterization method of J-integral for characterizing the fracture toughness of the SML was proposed.The J-integral of fracture toughness of 18CrNiMo7-6 alloy steel with SMLs was analyzed by using the characterization method were proposed in this thesis.（2）The method for determining the fracture toughness of SMLs based on continuous indentation was proposed by layer removal.The fracture toughness of the substrate material and SMLs of 18CrNiMo7-6 alloy steel was studied by indentation test.The fracture toughness K_JCof base material was characterized by continuous indentation test and compared with the compact tensile test.The influence of penetration depth on fracture toughness was studied.Through finite element analysis,the equivalence of P-h curves of gradient contact model and homogeneous model was studied,and a continuous indentation method was proposed to characterize the fracture toughness of SMLs.The method was used to characterize the fracture toughness of the SML of carburizing heat treatment 18CrNiMo7-6 alloy steel,and the variation of the fracture toughness of the SML with depth was studied.The anisotropy of fracture toughness of SMLs of 18CrNiMo7-6 alloy steel after carburizing heat treatment was studied by continuous indentation test.