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Title: Microstructure dependent damage mechanisms studies and modeling on high silicon spheroidal graphite iron
Authors: Sujakhu, Surendra
Keywords: DRNTU::Engineering::Mechanical engineering
Issue Date: 2018
Source: Sujakhu, S. (2018). Microstructure dependent damage mechanisms studies and modeling on high silicon spheroidal graphite iron. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: Spheroidal Graphite Irons (SGIs) have graphite particle inclusions in an iron matrix. The matrix structure controls overall mechanical properties, and the graphite morphology plays a vital role in crack initiation and propagation behavior. High silicon Solution Strengthened Ferritic (SSF) SGIs are developed to provide higher strength with excellent ductility. In SSF SGIs, graphite nodules shape has a key role in damage micromechanisms. The graphite nodule growth morphology can go through transitions to form degenerated graphite particles other than spheroidal graphite nodules in SGI microstructure. Additional thermal and mechanical processes influence SGI microstructure affecting mechanical properties and damage micromechanisms. Most of the damage mechanism studies on SGI were focused on the role of spheroidal graphite nodules on the stable crack propagation region. In this work, microstructure and properties of different SGI grades were compared, and EN-GJS-500-14 SSF SGI was further deep cold rolled and thermal cycled to study the effect of these processes on the material microstructure. Tensile and fatigue damage mechanisms were studied in detail to understand the role of different forms of graphite particles in SGI microstructure. The microstructure characterization result and damage mechanisms were formulated in a Representative Volume Element (RVE) model, and then to multiscale SGI material microstructure model. Microstructure studies and nanoindentation test results showed that the general microstructure of as-cast, deep cold rolled (DCR) and thermal cycled SGI can be characterized by graphite morphology, matrix composition and its phase properties. In DCR process, plastic hardening of the ferrite matrix was obvious. The large plastic flow of the ferrite matrix caused subsurface graphite particles to appear as a surface crack, which must be considered in graphite particles characterization. In thermal cycling process, the graphite-ferrite interface state was the most susceptible region in the microstructure, which needs to be included in microstructure characterization. In the tensile test, the matrix-nodule interface decohesion and plastic deformation of the ferrite matrix were the dominant damage mechanisms. Less influenced by nodule shape, graphite particles showed decohesion from the ferrite matrix at the overall stress of 400 MPa to 420 MPa, which is close to the yield stress of the material. In a separately performed Fatigue Crack Initiation (FCI) and Fatigue Crack Propagation (FCP) tests, the graphite particle shape plays a decisive role in crack initiation and propagation. In the crack initiation region, degenerated graphite particles dominated cracks initiation, and in the crack propagation region, the spheroidal graphite-matrix decohesion inducing a crack tip blunting effect was the most frequent damage mechanism in the SGI microstructure. FCI tests exhibited that cracks initiation were either by internal graphite cracking or by decohesion or by a combination of decohesion and internal cracking. A quantitative study of graphite nodules damage revealed that internal cracking and combine damage of most of the graphite nodules with Roundness Shape Factor (RSF) less than 0.5, whereas the spheroidal nodules with RSF higher than 0.9 showed the matrix-nodules interface decohesion. In FCP test, the degenerated graphite particles were mostly fractured, and crack branching was often observed either by main crack kinking towards nearby graphite nodules or by secondary cracks growth toward the main crack. The presence of shrinkage cavities of a size comparable to graphite particles size behaved similar to degenerated graphite particles, initiating microcracks in the ferrite matrix. Graphite particles were modeled as voids, unbound elastic particles and surface based cohesive interface bound elastic particles in the RVE model. The RVE model with surface based cohesive interface showed a better representation of the SGI microstructure. It enabled graphite particles to be modeled as bound particles until critical stress was reached, which was later modeled as partially debonded graphite particles. X-FEM crack initiation and propagation in the RVE model illustrated requirement of defining multiple enrichment regions in ABAQUS to allow initiation and growth of multiple cracks. Further, a multiscale SGI material microstructure modeling approach was developed for miniature Compact Tension (CT) specimen. The microstructure submodel generated by FE representation of real SGI micrograph well represented the stress and strain inhomogeneity in the microstructure. The inhomogeneous strain in the microstructure model was validated by DIC result, which showed slightly higher strain magnitude at the similar higher strain locations. X-FEM crack initiation and propagation simulation in the complex microstructure model showed the influence of the graphite particles in the crack initiation and propagation. However, the simulation could not be completely converged mostly due to limitations in available X-FEM formulation in ABAQUS. So, it is suggested to use ABAQUS user subroutines to model X-FEM crack growth in complex models.
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