Multiscale Study of Ductile Cast Iron by Numerical Method

Ductile Cast Irons (DCIs), because of their material characteristics, are increasingly being considered as an alternative to cast, forged, and welded steels for structural applications. In this thesis, an integrated multiscale numerical study is performed on different DCIs to obtain material properties at the higher scale, the continuum scale where the material can be seen as homogeneous, and the lower scale, in this case, the microscale where the dispersed nodules can be distinguished from the matrix. Numerical analysis is performed using the finite element method. To make up for the lack of information on the physical-thermomechanical properties of the constituents, the available literature data were integrated with the results obtained from the CALPHAD methodology applied to both the cast iron and steel that make up the matrix. For the macroscale study, a pressure-dependent material model with a non-associated flow rule is proposed for DCI. The model is calibrated to provide satisfactory results with both uniaxial tensile test and cylinder compression experiments. The local stress and strain tensors at the integration points and the gradient of deformation are recorded to calibrate the matrix and nodule materials. The micromechanical approach, which takes advantage of the composite nature of DCIs, has established itself over the years as a tool for analyzing their behavior. This approach is based on the definition of a representative volume element (RVE), which corresponds to the smallest volume of material that can reproduce the properties of DCI. For the definition of RVE to be effective, all microstructural aspects that influence the overall behavior must be modeled appropriately. These certainly include the volume fraction, the constitutive response of the components, and the behavior of the nodule-matrix interface. In the local residual stress calculations, the behavior of the nodule was assumed to be linear elastic because of the low stresses to which it is subjected during cooling. For the matrix, on the other hand, elasto-plastic-viscous behavior was assumed considering both the primary and secondary creep regimes. To fully exploit the potential of the micromechanical approach, progressively more refined models have been developed to consider the various mechanisms involved in the solidification process. In this context, the effect of viscous creep, which occurs in the early part of the solidification process, on the state of residual stresses, accumulated inelastic deformations, and the macroscopic stress-strain response is analyzed. This thesis is mainly devoted to characterizing the mechanical properties of the nodule, especially at high strain levels. To calibrate the RVE, local loading conditions of the macroscale model are selected where, in the presence of large deformations, the stress state is complex. To impose these conditions on the micro model, displacements of RVE boundaries are calculated based on the deformation gradient of the macro model. In this way, the same strain state is created at both scales. Numerical analysis shows that nodule behavior under compressive conditions is a function of pressure. Therefore, a pressure-dependent material model is assumed for the nodule. Through optimization, unknown material constants are calculated to provide a satisfactory agreement between the stress field of the RVE and that of the macro medium. In addition, the morphology of the nodule in various strain states is investigated. To verify the functionality of the proposed constitutive models and the validity of the assumptions, two different DCI materials are considered. The Comparison between the predictions of the numerical model and experimental results in the literature demonstrates the effectiveness of the applied methodology.