Papers by Keyword: Ductile-Brittle Transition

Abstract: In the present work, a cellular automata finite element model (CAFE) was developed to model the ductile-brittle transition of a Grade A ship plate steel. Therefore, ductile and brittle cellular automata (CA) arrays of cells were created in the model to integrate material data at microstructural level, along with the ductile and brittle fracture processes. Microstructural data was analysed with Weibull distributions and incorporated in CAFE model using random number generators, along with ductile and brittle fracture parameters. Ductile fracture was modelled with Rousselier damage model; hence damage model parameters were calibrated with experimental data. Brittle fracture was modelled with Beremin model, and four different cleavage particles, found in a Grade A ship plate steel, were incorporated in CAFE model in order to model a competition of particles nucleating microcracks of critical size in the damage regions of Impact Charpy tests and four-point double-notch bend tests performed at low temperature. The mechanical properties the plate steel was measured in the transition region and incorporated in CAFE model, along with ductile-brittle transition rules. The present CAFE model was able to simulate distributions of microcracks in the notch region of four-point double-notch bend models (in the transition region), which correlated with experimental data. CAFE model was also able to simulate microvoids in the notch region of Charpy specimens along with the load-displacement Charpy curve for room test temperature, with very good agreement with experimental data. Once CAFE model was validated at micro and structural level, it was applied to model the typical scatter of impact Charpy energy values in the transition region of Grade A ship plate steel with good agreement with the measured ductile-brittle transition curved of the plate steel. Keywords: cellular automata, finite element modelling, ductile-brittle transition, damage modelling.

Abstract: In this work, a study of the structure, impact toughness and mechanisms of destruction of steels 08Mn2Si and 07Cr25Ni13 was carried out using optical and electron microscopy. It was found that with a decrease in temperature, the level of impact toughness of the studied steels significantly decreases, so at a temperature of t = -100°C, the impact toughness of 08Mn2Si and 07Cr25Ni13 steel decreases by 83% and 45%, respectively, relative to the temperature t = +20°C. For steel 07Cr25Ni13, in the entire considered range, only the tough component is present in the fracture and there is no temperature of ductile-brittle transition, whereas for steel 08Mn2Si, already at temperatures below t = -41°C (ductile-brittle transition of steel 08Mn2Si), the brittle component begins to dominate and significantly the likelihood of its fragile destruction increases. On the basis of the obtained electronic photographs of the kinks, an artificial neural network (ANN) for the classification of kinks was trained and tested for intermediate test temperatures. When recognizing the viscous component in the fractures of 08Mn2Si steel obtained using the 3D-printing technology by electric arc surfacing, the recognition error does not exceed 7% with the use of ANN.

Abstract: The investigation results on the problem of the reliability of high-strength low-carbon maraging steel products have been generalized. The influence of a method for remelting on the reliability behavior is shown and the ways for the reliability behavior improvement are suggested. The study of the reasons for decreasing KCU during heat treatment shows that in addition to the precipitation of phases causing brittleness at cooling, chromium zones at heating, and formation of chemical and structure inhomogeneity in the two-phase region, the main reason is the remelting method with the parameters which predetermine the variation in grain size in the structure, a small number of interstitial elements (IE), retained austenite in the structure, and lower level of KCU of the steel prepared by VAR both after quenching and after TST. Shows influence of the quenching temperature on the amount of retained austenite and level of impact strength (KCU), of the time of aging on the work of the crack development (KCV) at the temperature of maximal development of brittleness in steel 08Cr15Ni5Cu2Ti and on the position of brittleness transition temperature prepared by ESR and VAR. After cooling down to the liquid nitrogen temperature, the VAR-steel is less liable to brittle fracture after maximal strengthening aging and more reliable after 1.5h-aging (KCV is twice as much as that in ESR-steel despite the low KCU level). The science-based regimes are developed for stamped semi-finished items from steel 08Cr15Ni5Cu2Ti allowing guaranteeing the proper quality and reliability of functioning of the items made from them.

Abstract: Analysis of the absorbed impact energy of an Al-Mg-Sc alloy after different thermo-mechanical processing routes was investigated between-196°C and 20°C. The material with a grain size of ∼ 22 μm in cast condition and with an average grain size of 0.7 μm produced by was produced by equal-channel angular pressing (ECAP) exhibits well-defined ductile-brittle transition in the temperature interval-60...-100°C, however, even at-196°C the value impact energy of fine-grained alloy is higher by a factor of 2 in comparison with coarse-grained state. The impact toughness of the hot rolled alloy linearly decreases with decreasing temperature. The influence of different microstructures on impact toughness and fracture behavior of alloy is discussed.

Abstract: In the present study the crack initiation and propagation is investigated in the ductile-brittle transition region by means of a microscopic model. In particular, the particles in the process zone in front of the crack tip are resolved discretely in finite element simulations. The competing mechanisms of particle debonding and possible subsequent void growth as well as particle breakage and cleavage of the metallic matrix are incorporated explicitly in the micromechanical model by means of a cohesive zone. This approach accounts for the complex interactions of the mechanisms and allows to simulate all stages of crack initiation and propagation at all relevant temperatures.

Abstract: Silicon carbide, a high-strength material, has a ductile-brittle transition mechanism. In order to establish a reasonable silicon carbide abrasive belt grinding parameters to obtain high precision silicon carbide free-surface efficiently, a series of finite element simulations were conducted to comprehend the single point diamond grinding of silicon carbide using professional analysis software of nonlinear finite element in this paper. According to the differences of cutting parameter, such as cutting depth, cutting deformation of the chip and the maximum cutting force were studied. For the free-form surface with higher accuracy, the data showed that ductile machining of silicon carbide is more efficient along with the larger rake angle, the higher cutting speed and the smaller cutting depth.

Abstract: The effects of temperature and strain rate on the stress–strain relationship and deformation twinning, during tensile tests, in polycrystalline Fe–5%Si alloy were studied. We performed tensile tests over a wide range of temperatures from 173 to 273 K, and strain rates ranging from 0.0001 to 0.1 /s, to clarify the relationship between total elongation and ratio of deformation twins, and the dependence of deformation twinning on grain orientation. All tensile specimens were fractured in a completely brittle manner. The total elongation decreased as the temperature decreased and the strain rate increased. The presence of deformation twins, in all fractured specimens, was confirmed by scanning electron microscope–electron backscattering diffraction analyses. The area fraction of the deformation twins increased as the total elongation decreased. However, a strong influence from the grain orientation on twinning activity was not observed for all temperatures and strain rates. A previous study on Fe–Si alloy single crystals showed that deformation twins form easily in <001>-oriented single crystals, but not in <111>-oriented single crystals. Our observations, on the dependence of deformation twinning on grain orientation in polycrystalline Fe–5%Si alloy, did not agree with those from single crystals. The present findings suggest that grain orientation does not play an important role in determining the occurrence of deformation twinning; not even in polycrystals. It is believed that the stress concentration, due to piled-up dislocations, during tensile deformation, cannot be relieved by the slip at low temperatures or high strain rates, and thus significantly affects deformation twinning.

Abstract: The fracture behavior transition due to the change of strain rate in 5%Si magnetic steel with dislocation microstructures was studied. The Si steel was multi-passed rolled at 800°C to a various reductions up to 50%. The room temperature tensile deformation was conducted at various strain rates from 10^-5/s to 10⁰/s. All rolled steels were fractured in ductile manners with local elongation (necking) at slower strain rate. When strain rate was faster, the local elongation disappeared and the fracture manner was turned to brittle. The strain rate at which fracture mechanism changed from ductile to brittle increased with the increasing of the reduction. On the other hand, the almost fully recrystallized Si steel was fractured in the brittle manner at any strain rate and the transition strain rate was not found. The fractured tensile specimen with no local elongations contains deformation twins; whereas these deformation twins were not observed in the fractured specimen with local elongations. This result indicates that dislocation structure evolved during rolling suppressed the twinning and that the dislocation structure is effective for the enhancement of toughness in Si steel.

Abstract: Structural integrity of the reactor pressure vessel of pressurized water reactors is one of the key safety issues in nuclear power operation. Integrity may be jeopardized during operational transients. The problem is compounded by radiation damage of the vessel structural materials. Structural integrity assessment as an interdisciplinary field is primarily based on materials science and fracture mechanics. The paper gives an overview on the service induced damage processes and associated changes of mechanical properties, the prediction of degradation and the assessment of the entire component against brittle fracture with a special focus on how the evolution of materials science and engineering has contributed to reactor vessel structural integrity assessment.