Numerical Investigations On Strengthening-toughening And Failure Mechanisms Of Metallic Materials With Gradient Structure

Gradient structural metallic materials inspired from the intelligence of nature break the long-standing Achilles heel of metallic material-i.e.the strength-ductility trade-off of material science.How to achieve the"strong and tough"mechanical performance through the gradient structure and how to improve such strengthening-toughening performance have become the principal problems to be solved.Furthermore,during the process of engineering applications,there is a spatial gradient microstructural distribution between the surface strengthing layer and the core substrate area of surface strengthening axle steel for the railway.In addition,surface treatments tend to introduce residual stress distributions in the surface area of the material.The spatial gradient distribution of microstructure leads to the difference in mechanical properties of materials at different depth and the existence of the residual stress field could also significantly affects the failure behavior of materials during application.How to describe the influence of combined gradient structure with residual stress on crack-growth behavior in surface strengthening materials is also the focus of this dissertation.Based on the above problems of gradient structural materials,the main contents of this dissertation are as follow:Firstly,in allusion to the problem of the strengthening-toughening mechanism of gradient structural metallic materials,this dissertation employed the finite element model based on three-dimensional Voronoi polyhedron to explore the internal mechanism of the excellent uniaxial tensile properties of the gradient structural materials and discuss the further enhancement of the strengthening-toughening effect.By cperformance onstructing a multi-layer grain size gradient model,the"strong and tough" of gradient structural material was realized at the simulation level.Through the distributions of von Mises stress and equivalent strain of the complete deformation process,we illustrated the different roles of the fine-grained area and the coarse-grained area in the overall excellent mechanical performance of grain-size gradient materials.Based on the different coarse grain volume fraction,fine grain grain-size and the degree of grain-size gradient,we discussed in detail the effective strategies to further improve the strengthening-toughening properties of gradient structural materials.The statistical result of a large number of numerical investigations showed that the appropriate adjustment of the grain-size gradient geometry information could enhance the strengthening-toughening effect of the gradient structural material.Secondly,based on complete testing methods,the axle steel S38C of the EMU train subjected to surface induction treatment was used to obtain the spatial distributions of microstructure,microhardness and the uniaxial tensile mechanical properties in the 8 mm thick region from the outer surface of the axle.The test results showed that the surface treatment produced a hybrid gradient microstructural feature of the axle steel.Such microstructural feature made the axle steel show the spatial gradient distribution of the strength and hardness gradually decreasing and the plasticity gradually increasing from the surface to the core in the radial direction.In addition,the tests used a small sample to measure the residual stress distributions in the axial direction within the depth of 30 mm from the surface.The test results showed that there is a large residual compressive stress distribution in the outermost layer of the axle gradient structure.With the increasing depth,the residual stress was successively transforming into residual tensile stress and residual compressive stress(both of which are small).Finally,considering the high-frequency alternating load that the axle is subjected to in daily operations,in-situ experimental studies and numerical investigations on fatigue failure behavior of gradient structural materials were carried out in combination with three-point bending specimens.The results of the in-situ fatigue tests showed that the fatigue initiation life of the sample with a surface gradient structure is significantly improved compared with the sample from the core area.With the growth of the crack,the relatively poor plasticity of the gradient structure accelerated its crack growth rate,at which time the core substrate sample exhibited a lower rate of fatigue crack growth.Furthermore,by using the experimentally determined key mechanical properties parameters and the multi-layered model of gradient structure,the numerical investigations of the fatigue crack growth behavior of the axle steel were carried out.The simulation results revealed that the gradient structure and residual compressive stress could significantly alter the stress distribution near a crack tip,which could also influence the size of the local plastic zone during fatigue crack-growth.In addition,the influence of combined gradient structure with residual stress on crack-growth behavior in surface strengthening materials was fully explained by the cyclic J integral theory under small-scale yield condition.In summary,this dissertation systematically studied the strengthening-toughening mechanism and failure mechanism of gradient structural metallic materials by numerical investigations combined with experiments and theory.The relevant research results could be used to guide performance design and engineering applications of gradient structural metallic materials.