Cyclic plastic material behavior leading to crack initiation in stainless steel under complex fatigue loading conditions

The improvement of the reliability and of the safety in the design of components belonging to the primary cooling circuit of a light water nuclear reactor is nowadays one of the most important research topics in nuclear industry. One of the most important damage mechanisms leading the crack initiation in this class of components is the low cycle fatigue (LCF) driven by thermal strain fluctuations caused by the complex thermo-mechanical loading conditions typical for the primary circuit (e.g. operating thermal transients, thermal stratification, turbulent mixing of cold and hot water flows, etc.). The cyclic application of the resulting plastic deformation to the steel grades commonly used for the fabrication of piping parts (e.g. austenitic stainless steels) is associated with a continuous evolution of the mechanical response of the material. As an additional complication, the cyclic behavior of stainless steels is influenced by temperature, strain amplitude and cyclic accumulation of inelastic strain (i.e. ratcheting). The accurate prediction of the structural response of components belonging to the primary cooling circuit requires the development of a reliable constitutive model that must be characterized by a reduced complexity to allow its application in an industrial context. In this framework, the main goal of the current dissertation is to formulate, calibrate and implement in a commercial Finite Element code, a constitutive model that is suitable for the stainless stain grade 316L subjected to complex loading conditions. As a first task, a characterization of the mechanical behavior of 316L subjected to uniaxial and multiaxial strain-controlled conditions (including LCF and ratcheting) is carried out performing several tests in the laboratories of the Paul Scherrer Institute (PSI, Villigen, Switzerland) and of Politecnico di Milano (Italy). The uniaxial experiments demonstrate that, prescribing a strain-controlled ratcheting path, a harder material response is induced with respect to the equivalent uniaxial LCF test. An additional hardening is also noticed in multiaxial tests, when a non-proportional loading history is imposed. The experimental results show that this additional hardening is accompanied by a lifetime reduction. Further experiments are carried out to investigate the loading-rate influence on the mechanical response of the 316L under strain- and stress-control and to determine the necessity to implement a time-dependent constitutive model. A set of interrupted tests has been also performed to retrieve samples suitable for the characterization of the microstructural evolution of 316L subjected to ratcheting conditions. The microstructural characterization has been carried out by means of a Transmission Electron Microscope (TEM) in a collaboration with the High Temperature Integrity Group at the Swiss Federal Laboratories for Material Science and Technology (EMPA, Duebendorf, Switzerland). The experimental observations reported in the first part of the current dissertation inspired the formulation of a novel constitutive law consisting in a modification of the well known Chaboche model. In this formulation named '5DChabEP', the model's parameters are not constant but are allowed to vary as a function of 5 internal variables. The proposed constitutive model is implemented in the commercial Finite Element code ABAQUS and an automatized procedure is developed to calibrate the material parameters. The descriptive and predictive capabilities of the constitutive model, coupled with an advanced multiaxial damage criterion, are evaluated under several loading conditions using, as references, experimental data and simulations performed by means of the original Chaboche formulation. In general, the possibility to vary the material parameters as a function of a set of internal variables is found to be an extremely efficient approach to provide accurate stress calculations and lifetime predictions enhancing significantly the performance of the original Chaboche constitutive law. Finally, a sensitivity analysis is carried out to characterize the confidence bounds of the output of the constitutive model and to identify the factors that are mostly responsible for the uncertainty in the calculations. In this framework, the elementary effects (EE) method is found to be the ideal tool to carry out, with a limited computational cost, a non-local sensitivity analyses on three different case studies. (author)