Mechanism of Damage of Ferritic Ductile Iron, Influence of Matrix Heterogeneity

Abstract

Abstract. Ferritic ductile cast iron (FDI) microstructure is composed by graphite nodules embedded in a ferritic matrix. It is usual to assume that the ferritic matrix is homogeneous. However, the experimental analysis shows impurities and in some cases a high degree of heterogeneity. It is necessary to investigate the influence of these heterogeneities on the mechanical properties of FDI. This work focusses on the characterization of the elastoplastic properties of different zones of the ferritic matrix of FDI and the identification of the sequence and extent of the damage mechanisms at the micro-scale under uniaxial tensile loading.The methodologies for the characterization of the material micro constituents and microsegregated zones involve nano-indentation and atomic force microscopy techniques in combination with computational modelling. The analysis is performed by applying inverse analysis algorithms proposed in the literature. The microsegregated zones are identified by using color etching. The assessment of the micro-scale damage mechanisms was performed by in-situ optical microscopy observation of tensile tests of very small specimens.The results led to the quantification of the differences in mechanical properties along the metallic matrix as a result of the existing heterogeneities and allow for a better understanding of the ductile iron damage mechanism.

Small specimen techniques for estimation of tensile, fatigue, fracture and crack propagation material model parameters

“…125 For the ferritic ductile cast iron, cracks prefer to propagate along the same areas as they tend to initiate. 133 Irradiation was discovered to retard crack propagation similarly to how it retarded crack initiation. 128 Kovarik developed a method for measuring the crack propagation rate using mini cantilever specimens and a resonance fatigue test method with micrometric resolution, measuring the crack propagation using a compliance technique.…”

“…125 In a ferritic ductile cast iron the cracks tend to nucleate along the nodule-matrix interface and the first to freeze zones. 133 It was discovered that for 316 stainless steel irradiation retarded crack initiation. 128 Sharp crack tip was identified to be better than rounded notch for pre-cracking a small punch specimen, as it more closely resembled the crack in a CT specimen.…”

Microstrain measurements and damage analysis during tensile loading of intercritical austempered ductile iron

“…In this context, microindentation and nanoindentation tests, in situ microstructural damage observation, digital image correlation (DIC) techniques and computational mechanics, among other methodologies, have been used to investigate the relation between microstructural features and the macroscopic mechanical response of DI materials 7–22 . Microindentation and nanoindentation testings were performed on both the metallic matrix 10,16,18 and the graphite nodules 10,17,18 to assess the mechanical properties of each microconstituent of the DI microstructure. In situ observations during mechanical testing were also carried out with the aim of correlating the damage mechanisms observed during fracture evolution with both the microstructure and the macroscopic mechanical responses 11–16 .…”

“…Microindentation and nanoindentation testings were performed on both the metallic matrix 10,16,18 and the graphite nodules 10,17,18 to assess the mechanical properties of each microconstituent of the DI microstructure. In situ observations during mechanical testing were also carried out with the aim of correlating the damage mechanisms observed during fracture evolution with both the microstructure and the macroscopic mechanical responses 11–16 . Other methodology, combining numerical and experimental analyses in the macro and micro scales, was also applied to estimate the values of different parameters that could not be measured via experiments 10,12,22 .…”

“…The relation between microstructural heterogeneities and damage mechanisms during loading on DI has been recently studied by the authors for both ferritic DI 8,13–16 and IADI 7 . Particularly in Fernandino et al, 7 the analysis involved in situ observation of tensile test specimens of IADI with a metallic matrix composed by 60% ferrite and 40% ausferrite using optical and scanning electron microscopy (SEM).…”