Influence of Microshrinkage Cavities on the Plastic Deformation and Fracture Under Tensile Loading in Ferritic Ductile I
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pyright 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00546-8
Abstract This work focuses on the study of the influence of microshrinkage cavities on the plastic deformation and fracture of ferritic ductile iron. Cast samples were especially developed to include dispersed microshrinkage. Tensile testing and digital image correlation analysis are employed to assess the influence of dispersed microshrinkage cavities as preferential sites for crack initiation and propagation under uniaxial static load. The results show that small microshrinkage cavities of up to 3.5 times the area of graphite nodules are not linked to the
initiation and propagation of cracks in ductile iron. The methodology developed in this work becomes useful to evaluate the influence of size, distribution, and morphology of different microshrinkage defects on the damage evolution during tensile loading.
Introduction
The microshrinkage usually appears dispersed along the whole volume of the casting. It consists of micron-size cavities that are formed between the eutectic cells or dendrite arms 7. It forms as a result of the combination of lack of feeding and gas rejection from liquid during solidification. For this reason, microshrinkage is difficult to eliminate by means of casting techniques. The influence of larger shrinkage cavities on the mechanical performance of a part is evident, as they act as significant stress concentrators 9. Although there is a general agreement about the potential deleterious influence of microshrinkage, this subject has been less studied. Several works have analyzed the influence of microshrinkage on the fatigue limit of ductile iron (DI) 9–12. Wang et al. (2013) 12 showed that shrinkage cavities act as crack initiators in particular those located close to the surface of the casting. Nadot et al. (2014) 10 analyzed the change in the fatigue limit of DI with microshrinkage defects located at the specimen surface or in the bulk. They concluded that the near surface defects are much more deleterious than internal defects. Similarly, Novy´ et al. (2018) 11 observed that the microdefects in the near subsurface layers significantly accelerate the fatigue crack initiation process. On the contrary, the microscopic casting defects, which are dispersed in the volume of the testing specimens rarely were
One of the major problems affecting the quality of casting parts are shrinkage defects that form during solidification 1–4 . When high strength castings are produced, it becomes critical to ensure that they are structurally sound 5. The classification of shrinkage defects includes three different types of shrinkage cavities 6–8: the concentrated shrinkage, the dispersed macroshrinkage, and the dispersed microshrinkage. Concentrated shrinkage forms because of metal contraction during cooling of the liquid and the phase change. It can be open or closed to the atmosphere. Feeders are usually employed to prevent the formation of these defects in the casting. The dispersed macroshrinkage consists of cavities of
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