In-situ tensile tests for mechanical characterization of steels with complex microstructure

Abstract : Monitoring pressurized water reactors casted duplex stainless steel parts is of interest in long-term operation of nuclear power plants. It has been shown that these components are subjected to ageing when held at their operating temperature. In the long run, thermal ageing results in embrittlement due to a microstructural evolution of one of the two phases constituting duplex steels. Kinetics and impact of ageing on fracture mechanics have been studied to a great extent. However, deformation and damage mechanisms leading to fracture are not yet fully understood owing to the complexity of duplex steels microstructure. Resorting to traditionally used global energy balance methods (J-Δa curves) is not sufficient to understand the primordial role of microstructure in the evolution of mechanical properties with thermal ageing. In this context, a microstructure characterization is performed jointly to in-situ mechanical characterization to build a consistent micromechanical model taking the multi scale aspect of the microstructure into account. Eventually, the model must provide a fine description of the material mechanical behaviour taking into account the aged state In order to relate crystal plasticity finite element computations to experimental observations, digital images correlation (DIC) is used to provide full-field measurements. They must be consistent with the deformation mechanisms of the studied steels and be accurate enough to capture small deformation mechanisms occurring in each phase (at a micrometre scale) and the resulting displacements at the larger scale of the microstructure (several millimetres). Duplex steels natural texture is not sufficient to carry out digital images correlation. Thus, an adequate speckle pattern was designed and deposited through electron beam lithography.