| In this paper, a series of four point normal and reverse bending experiments with different applied loads are carried out for notched specimens of three heat-treat microstructure of two steels, which introduce different levels of initial damage. The residual stress and work hardening are removed by tempering the specimens at high temperature. Then the specimens are loaded by four point bending and unloaded at different loads at room temperature. By measurement of mechanical parameters and microscopic observation, finite element method(FEM) calculation and micro-simulation, the effects of initial damage on the ductile initiation toughness of notched specimens with different microstructures of steels and the mechanisms are investigated. The main results are as follows:(1)For material A (fine grain of 16Mn steel), when the pro-load ratio Po/Pgy≤0.861, the ductile fracture initiation toughness Pi/Pgy decreases slowly with the increment of the pro-load ratio Po/Pgy, and in the case of Po/Pgy≥0.861, the Pi/Pgy decreases rapidly with the increase of the Po/Pgy. For material B (coarse grain of CF steel) and material C (coarse grain of 16Mn steel), the Pi/Pgy does not essentially change with the Po/Pgy during the Po/Pgy≤0.861, but it decreases rapidly with the Po/Pgy after the Po/Pgy≥0.861. The ductile fracture initiation toughness of the material B is the highest and that of material A is the lowest in the range of the Po/Pgy.(2) During Po/Pgy<0.861, the initial void area fraction f0 (damage amount) slowly increases with increment of the Po/Pgy. But after Po/Pgy>O.861, the fo increases rapidly with the Po/Pgy. At the same Po/Pgy , the level of initial damage of material A is obviously higher than that of the material C, that is, the material A with fine grains is easy to damage and sensitive for the damage. The level of initial damage of material B is the lowest.(3) The larger initial damage in materials makes the ductile fracture initiation toughness low. The larger the initial damage amount is, the larger the decrease of the ductile fracture initiation toughness. At the same Po/Pgy, the material with larger initial damage amount has lower ductile fracture initiation toughness. The reason for the lower ductile fracture initiation toughness of the material with larger initial damage amount is that the micro-cavity initial damage has been induced into the materials in pre-load process, and during the subsequent loaded process, the strong localized high stress/strain between and around cavities are produced and promote the growth of initial voids at high rate. It also causes small, secondary voids tonucleate and grow, and easy to coalesce with large voids. So the ductile fracture initiation toughness of material becomes lower.(4) The growth and coalescence of a few large voids ahead of notches usually leads to the ductile fracture initiation of material, that is, the ductile initiation is dominated by the large inclusions or voids. However, the Gurson model, a widely-used model to describe the micro-mechanical effects of damage in ductile metals, only uses the parameter of micro-void volume fraction fv to characterize the effect of damage voids on material properties, and does not consider the influences of the size, shape and distribution of cavities, so it is necessary to modify or develop a more appropriate and accurate model.(5) For material B and C, they have nearly the same grain size, but material C has larger, elongated inclusions. During the pre-loading, some elongated MnS inclusions will debond from the matrix and the elongated larger initial defects are formed. This makes the ductile fracture initiation toughness low in the loading process later. Therefore, during the design and making of materials, it is necessary to decrease the size and number of inclusions, especially larger, elongated ones as possible as you could and avoid machining part with large deformation.(6) For material A and material C, they have the same chemical composition andinclusion, but have different grain sizes. Because the material A with fine grain has larger initial damage amount, the ductile fracture initiation toughness Pi/Pgy of fine grain material A is lower than that of coarse grain material B. That indicates that fine grain material is sensitive to initial damage and easy to damage in the deformation process. These lead to the decrease of its ductile fracture initiation toughness Therefore, the effect of initial damage on subsequent mechanical properties should be considered during the processing for the fine grain material.(7) The reason that fine grain material is easy to form initial damage is that the stress at the interface between inclusion and matrix is higher due to its high yield strength. Therefore, it is easier for inclusion to debond from matrix and to nucleate voids.(8) The mechanism of effect of initial void damage on ductile fracture initiation toughness of steel is studied by using finite element method to simulate the evolution of pre-damage voids during the subsequent loading process. The results show that, the initial voids with large size grow fast, and there is deformation localization among these large voids. The deformation localization can promote the nucleation and growth of secondary voids, which leads to the coalescences of the large voids and the ductile fracture initiation of material. The large initial voids dominate the ductile fracture initiation of material. With increasing the amount of initial damage, the number and size of large voids increase This causes the decrease of strain for void coalescence (ductile fracture initiation), and this is the mesomechanical reason that the ductile fracture initiation toughness Pi/Pgy decreases with increasing the pre-load ratio...
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