Computational and Experimental Assessment of the Mechanical Behavior of Ductile Cast Iron at a Microstructural ScaleWednesday (26.09.2018) 10:45 - 11:00 S1/01 - A02 Part of:
This study focuses on the characterization of the mechanical properties of the metallic matrix of Ductile Iron (DI) by the use of nanoindentation and Atomic Force Microscopy. The heterogeneity present in the matrix is accounted for, as local properties are measured. In a second part of the study, the properties measured are introduced as data for the numerical assessment of the deformation and fracture of this material under uniaxial stresses. Advanced numerical techniques are implemented and used for this purpose. The numerical analyses encompass the linear elastic and the early damage stages. The so-called early damage stage comprises the nodule debonding and distributed plasticity in the matrix. In addition, the deformation and damage of DI under uniaxial tensile loading was examined at the microscopic scale by in-situ optical microscopy.
The results showed that two different zones could be differentiated along the ferritic DI matrix, fist-to-freeze zones (FTF) and last-to-freeze zones (LTF). These zones have different chemical composition resulting from the partition of alloying elements during solidification. Both zones have similar elastic behavior but different plastic behavior, with the LTF showing greater strength values. The experimental observation of the micro-scale damage shows that crack nucleation takes place preferentially along the nodule-matrix interface (debonding mechanism) and the FTF zones. The micro-strain measurements verified that the FTF zones show higher plastic strain values than the LTF zone, in agreement with the measurements by nanoindentation and AFM. In addition, the tests showed that the ferritic matrix suffers a generalized damage before the final fracture, that involves the nucleation and growth of multiple cracks, not just the main crack leading to fracture. The numerical simulations are found effective for capturing the sequence and extent of the damage mechanisms in the microstructural scale and to estimate, via inverse analyses, parameters of the matrix-nodule debonding law. The model implemented is used to predict the changes in the behaviour of DI resulting from variations in the nodule count (number of graphite nodules per unit area) and in the mechanical properties of the matrix. The results of this study allow for a better understanding of the micromechanics of DI. The methodology is currently applied to characterize and model the mechanical behaviour of other varieties of DI.