The Effect of Graphite Fraction and Morphology on the Plastic Deformation Behavior of Cast Irons
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THE mechanical behavior of cast irons is complex due to the occurrence of the precipitated graphite phase. The graphite particles give rise to stress concentrations and yielding of the adjoining matrix where the rate of plastic deformation partly depends on the morphology of the graphite. A higher rate of plastic deformation is observed for lamellar-shaped graphite compared to nodular-shaped graphite. This has been shown by Sjo¨gren and Svensson in an earlier study[1] where acoustic emission measurement was used to register the rate of plastic deformation. This earlier study also showed that plastic deformation occurs during the seemingly linear elastic region of the tensile test, independent of graphite morphology, giving rise to the nonlinear stress-strain curve, which is typical for cast irons. The immediate yielding, which results in the observed nonlinearity on loading, is important to take into consideration in the design work of a cast iron component because it occurs at low stress/strain levels. To be able to calculate and simulate the permanent strain at different stress levels, it is necessary to have TORSTEN SJO¨GREN, R&D Engineer/Postdoctoral Student, is with Daros Piston Rings AB, SE-435 23, Mo¨lnlycke, Sweden, and Division of Component Technology, Jo¨nko¨ping University, SE-551 11, Jo¨nko¨ping, Sweden. Contact e-mail: [email protected] INGVAR L. SVENSSON, Professor, is with the Division of Component Technology, Jo¨nko¨ping University. Manuscript submitted June 21, 2006. Article published online April 20, 2007. 840—VOLUME 38A, APRIL 2007
material parameters describing the plastic deformation. The most widely used mathematical relationship correlating stress-strain data is shown in Eq. [1]. In Eq. [1], often called the Hollomon equation,[2] n is the strain hardening exponent, K is the strength coefficient (MPa), r is the true stress (MPa), and e is the true (plastic) strain: r ¼ KðeÞn
½1
Using Eq. [1], the parameters K and n can be calculated from tensile test data. These parameters are needed to describe the plastic deformation behavior of a material (normally determined from a log-log plot of the true stress–true strain data). Strain hardening, observed as a positive slope in the plastic region of the tensile stress-strain curve, occurs as the dislocations interact with different defects in the fully plastic regime. The defects can be point, line, surface, or volume defects where volume defects refer to, e.g., porosity and inclusions (such as graphite particles). The interactions between dislocations and defects result in hardening; i.e., additional stress must be applied to overcome the restriction of dislocation motion that arises from the defects.[3] The strain hardening exponent is a measure of the resistance to plastic deformation and can be seen as a multiplication factor of the number of dislocations and their entanglement in the metal. In this study, a set of cast irons with different graphite morphologies, covering flake, compacted, and nodular graphite, and a set of gray cast irons w
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