WEB Prediction of process induced microstructure evolution, transformation plasticity and residual stresses in steel reinforcing bars
Modern architectural engineering commonly relies on reinforced concrete as a main building material. In this composite, steel reinforcement bars (rebars) are embedded into a concrete matrix to counteract the concrete's relatively low tensile strength and thus improve the overall mechanical performance. One of the factors in ensuring operational safety and long service life of the structure under dynamic loading is the fatigue behaviour of the rebars. Previously, their design was mainly concerned with manufacturing and concrete-rebar interface constraints. Our research addresses the optimization of fatigue life within these constraints by developing integrated numerical process and constitutive models that allow us to study the interaction of geometrical properties, parameters of the manufacturing process, microstructure evolution and transformation plasticity in order to predict residual stresses on the component scale.
Regarding the production process, we focus on the heat treatment of the hot rebars after profile rolling as predominant source of the process induced microstructural and residual stress state. This implies prior analysis of the inverse heat conduction problem to identify suitable boundary conditions for our finite element simulations based on in-process measurements. We propose a semi-analytical method for this identification problem and discuss the underlying simplifications.
Given the requirement to apply our constitutive model to the complex geometry of rebars, we adopt a component-level continuum formulation where the microstructural composition and the constituents' state variables are incorporated through volume-averaged quantities instead of spatially resolving the grain structure. This approach imposes the challenge to specify on a purely macroscopic scale suitable equations for both the description of microstructural changes as well as the special thermoelastoplastic component behaviour encountered during phase transformations. To this end we couple kinetic models for diffusive and martensitic transformations and implement a plasticity formulation based on the model of Leblond to account for transformation plasticity. We propose an implicit numerical solution scheme and discuss associated challenges and simulation studies to compare this method with common J2-plasticity based approaches.
Finally, we provide an overview of the experimental approaches for material and process parameter characterization and model validation.