Elastic modulus of ductile irons

Understanding the role of microstructure during deformation is necessary in order to optimize and predict the mechanical properties of ductile irons. While much work has been done on metallurgical studies of ductile irons, the elastic modulus has never been studied in detail for newer ductile irons such as austempered ductile irons (ADI). In ductile irons, both the graphite and the unique matrix microstructure cause an inherent non-linearity in the stress strain curve.; This investigation is the first to deal with the modelling and prediction of effective dynamic elastic modulus of ductile irons using finite element modelling (FEM). Both plane stress and axisymmetry were used to predict elastic moduli. With finite elements, it is possible to include microstructural parameters that are extremely difficult or impossible to model analytically. Assuming that the graphite acts as a void, the effective dynamic elastic modulus (DEM) was modelled by considering the effect of small strains on void variables of volume fraction, shape, size and distribution. No matrix effects have been considered in finite element modelling. It was seen that void volume fraction affects the DEM significantly. For a given volume fraction, it was also seen that shape has a significant effect on the DEM. Some analytical modelling based on existing literature sources was also used to predict effective moduli. The Mori-Tanaka model was found to give reasonable predictions. An experimental investigation was done using the Jominy method to measure DEM as a function of microstructure, independent of the graphite volume fraction. Matrix moduli in ductile iron were found to be tentatively ranked as: {dollar}rm Esb{lcub}martensite{rcub} < Esb{lcub}tempered-martensite{rcub} < Esb{lcub}ferrite-pearlite{rcub}.{dollar}; The elastic modulus under load was investigated by considering the microstructural phenomenon of damage that occurs during deformation. Qualitative experimental studies were conducted on conventional ductile iron to study damage for single and repeated loading cases. On the basis of a continum damage mechanics model, damage progression ASTM classes of ADI was shown to be related to strain and compared with experimental results. For ADI, no evidence of damage was observed in the elastic region. Both the strain at which damage initiates/grows and the non-linearity of the damage-strain curves were found to be related to the yield strength. Preliminary investigations were also undertaken to study strain induced transformation in ADI. The presence of strain induced martensite was found to reduce the elastic modulus.