Research On Residual Strength And Fatigue Life Of Cracked Metal Graded Composites

Metal graded composite refers to a novel integral material which contains more than two types of metal and formed by additive manufacturing technology.Based on the force condition and service function in the practical part of structure,metal graded composite can exploit the advantages of the components to the full via optimizing the gradient to ensure a continuous transition of the mechanical properties between different metals.Considering the transition,significant differences exist between metal graded composite and traditional homogeneous material when dealing the fracture and fatigue issues.From this stand,this paper researches the fatigue crack initiation and fatigue crack propagation and residual strength of the graded metal materials by experimental study and numerical simulation.The main contents include:(1)The fatigue test on three forms of high-strength steel specimens,which is based on 300 M and covered with Aer Met100 coating,has been conducted to obtain the fatigue properties of metal graded coating materials.Polycrystalline model considering randomness of microscopic metal has been established by Python and the initiation process of micro cracks has been predicted with the usage of multi-scale method.Based on the model,influence of the parameters such as the composition distribution and the meso-structure on the initiation life has been discussed.(2)The equivalent forms of component content change of elastic modulus,elastoplastic constitutive,fracture toughness and crack propagation of the material in the transition zone were deduced.A layer model was established to analyze the stress and strain in cracked graded composite and the modified J-integration was used to calculate the energy release rate on the crack tip,which lay a foundation for the prediction of residual strength and propagation life.(3)Quasi-static tests on the cracked TC11-TC4 and TA15-TA2 graded beams have been done,and the propagation behavior was simulated by finite element method.Referring to TC11-TC4 short beam specimens which can been treated as elastic condition,the fracture toughness of TC11 and TC4 has been measured and a residual strength prediction model has been founded using the K-criterion to simulate the quasi-static crack propagation in graded composite.While considering TA15-TA2 specimens,the stress was analysis with elastoplastic constitutive.In this model,the maximum hoop stress ration criterion has been taken into consideration to calculate the crack growth directions to interpret the fact that the crack stops propagating as soon as entering TA2 component.(4)Fatigue tests on the cracked TC11-TC4 and TA15-TA2 graded short beams was carried out to obtain the crack growth rate of the four types of titanium alloys and its distribution in the transition layer.Paris-formula was used to establish the propagation life prediction model.Based on this model,the fatigue propagation processes have calculated when the crack located on both sides of the specimen respectively and the influences of module and propagation property gradients on the propagation life have been discussed which can offer guidance for structural optimization.