Damage And Fracture Of Notched Tensile Specimens Of Metallic Materials Behavior

In this paper, a series of tensile fracture and unloaded testes are carried out on four kinds of bar specimens with different root radius for different materials. By the measurement of the macro mechanical parameters, fracture surface and metallographical observations of and unloaded specimens, combing with the detailed FEM calculations of the distributions of stress and strain ahead of the notch with different root radius, the macro mechanical properties and the local damage and fracture mechanisms of the four kinds of tensile notched specimens of different materials are investigated. The main results are as follows.1. By the FEM calculations of the stress and strain distributions of four tensile specimens with different root radius for different materials, It is shown that the normal stress σ_yy, thestress triaxiality σ_m/(σ|—) and the equivalent plastic strain ε_p increase with the increase of the load P/P_gy. The distributions of stress and strain before and after the global yielding of the specimens are different. The normal stress σ_yy, the stress triaxialityσ_m/(σ|—) and the equivalent plastic strain ε_p ahead of the notch increase with decreasingroot radius of notches. The distribution of stress and strain of the specimens with a big root radius (R ≥ 2mm) and a small root radius (R ≤ 2mm) are different.2. With decreasing the notch root radius, the macro mechanical parameters which reflect the strength of the material increase. The essential reasons for the differences of material strength between the four kinds of materials is explained by the chemical composition and microstructure.3. It is shown that the mechanisms of damage and fracture of metals with different composition and microstructure are different by the investigation on the ductile damage fracture mechanism. The strength, plasticity and toughness of the four different materials are different due to the different ductile damage and fracture mechanisms. The mechanical properties may not probably be described and simulated by the Rice-Tracey and Gurson damage model due to the problems and limits of these models.4. With decreasing notch root radius, the intensity of the stress and strain fields ahead of the tip of the notch increases, which accelerates the initiation, growth and coalescence of the micro-voids. So the toughness and the plasticity of the materials decrease.5. It is shown that the ductile fracture strain depends on the stress triaxiality by measuring the local ductile fracture curves of the four kinds of materials. The fracture strain decrease with increasing the stress triaxiality.6. It is shown that there are local equivalent plastic strain ε_p , Miss stress and normalstress S₂₂ concentrations in the surroundings of the voids by the local mechanicalanalysis of the distributions of local stress and strain and damage evolution in the surroundings of the voids. The growth of primary big void, and the nucleation and growth of secondary smaller voids between the primary big voids are promoted by theconcentrations of ε_p, Miss stress, normal stress S₂₂. The coalescence of big voids ispromoted due to the local the high equivalent plastic strain ε_p between the big voids.Generally speaking, the big voids related to the debonding between the big MnS inclusions and matrix grow more faster. The voids whose location are special also grow more faster. The coalescence of those big voids is fast, which controls the ductile fracture. The effect of the orientation of voids on the damage evolution is small, The more higher the stress triaxiality is, the more higher the growth rate of void fraction and the dimensions of single void are.