Numerical Study on Ductile-to-Brittle Transition of Steel and its Behavior under Residual Stresses

Abstract

The transition of fracture mode from ductile to brittle (DBT) is a crucial phenomenon of structural materials, e.g. body centered cubic (BCC) metals, which are normally brittle at low temperatures or high loading rates, become ductile at high temperatures or low loading rates. The DBT has aroused great concerns in science and engineering in the past years. The prediction of DBT will benefit for the application of materials in arctic engineering, particularly where avoiding the occurrence of catastrophic brittle fracture is one of the major concerns for the material or structures working at the low temperature. To this end, some important aspects pertaining to the DBT will be addressed in the present PhD thesis: (i) revealing the intrinsic mechanisms of DBT and searching a physically-motivated variable to capture the temperature dependent fracture toughness in the transition regime, (ii) exploring a framework for the modelling of DBT by implementing the micromechanical approach with this physically-based variable, (iii) studying the DBT of material or components under an important circumstance, e.g., residual stresses, through applying this framework. The mechanism of DBT can be fundamentally discovered by studying its reverse process, e.g., brittle-to-ductile transition (BDT), in which an intrinsically brittle material fractures in a ductile manner. The BDT is not an intrinsic phenomenon of material, and depends not only on the strain rate but also on the constraint at crack tip. However, few work has been performed on studying the effect of constraint on BDT. A dislocation mobility based continuum model is employed to model the BDT behavior of single-crystal iron under different loading rates. Two scenarios of T-stress implementation in the model have been adopted to study the constraint effect on the BDT. It is found that the change of the stress distribution ahead of crack tip due to the T-stress dictates the fracture toughness in the BDT transition region. Lower constraint leads to a higher fracture toughness in the transition region, a smoother transition curve and a lower critical BDT temperature, and also a higher fracture toughness at the critical BDT temperature. A quantitative relation between fracture toughness and T-stress has been established such that the BDT curve with constraint can be estimated from a reference BDT curve. A solution to build a temperature-dependent effective surface energy law has been proposed, which could facilitate the understanding of the change of the fracture toughness in the transition region. It is still a challenge to numerically achieve the interactive competition between ductile damage and brittle fracture in transition region. In addition, since two types of fracture occur at two independent material length scales, it is difficult to process them with the same mesh size by using finite element method. A framework of modelling DBT of a thermal mechanical controlled-rolling (TMCR) steel is explored by using the cellular automata finite element (CAFE) method. The statistical feature of material's microstructure is incorporated in the modelling. It is found that DBT curve cannot be reproduced with only one temperature dependent flow property, for which another temperature dependent variable must be considered. A temperature dependent effective surface energy based on typical cleavage fracture stage is proposed and obtained through a continuum approach. The DBT of TMCR steel is simulated by using CAFE method implemented with a temperature dependent effective surface energy. It is found that numerical simulation is able to produce a full transition curve, especially with scattered absorbed energies in the transition region represented. It is also observed that simulation results can reproduce a comparable DBT curve contrasting to the experimental results. The effect of residual stresses on fracture of materials or structures has been widely studied. However, its influence on DBT has rarely been investigated so far. Employing the eigenstrain method residual stresses are introduced into a bi-material specimen, where two configurations of crack and interface, e.g., one with interface perpendicular and one parallel to the crack extension, are designed to study the influence of residual stress. The DBT of the biomaterial specimen in the presence of residual stresses is numerically studied by using the CAFE method where temperature dependent surface energy is implemented. It is found that residual stresses generated in the two configurations affect the DBT with a similar manner. The DBT curves generally shift to higher temperature due to the decrease of absorbed energy with the increase of residual stress. Residual stress induces a significant change of the DBT curve at higher temperature, e.g., the upper-shelf, however its influence decays with the decrease of temperature. It is found that the decrease of absorbed energy in both configurations is caused by the additional constraint on the notch root induced by the residual stress, which can facilitates the fracture.