چكيده به لاتين
Abstract:
Ductile cast iron (DCI) due to sufficient strength, easy casting and low price is widely used in various industries. Up to now, extensive researches have been done on the properties of different grades of ductile iron. In this thesis, the micromechanical analysis of the fracture of ferritic ductile iron based on ductile fracture mechanism and the micromechanical fracture of pearlitic ductile iron based on brittle fracture mechanism have been investigated. Microstructures of ferritic and pearlitic ductile irons have been fully studied and the dominant parameters describing the material microstructure have been extracted. These parameters play a main role in the results of the micromechanical analyzes and fracture mechanism of DCI. Then, the appropriate unit cells that have the highest compatibility with the parameters of ductile iron microstructure have been determined.
Using nanoindentation and nanoscratch tests, elastic-plastic properties of different phases of DCI have been extracted. The modified Oliver-Pharr method has been used to extract Young's modulus of different phases. Using the results of finite element (FE) simulation of nanoindentation and doing the optimization process, the strain hardening curve (power law) for different phases of DCI has been extracted. Also by doing the nanoindentation test and FE analysis, the properties at the interface between the graphite nodules and ferrite matrix have been studied. The results show that the thickness of interface layer between these two phases is very small and properties of graphite and ferrite phases near the interface are similar to the main properties of these phases.
A new method to simultaneously extract the stress-strain curve of ferrite and pearlite phases and also the parameters of Gurson material model (GTN) has been presented. By doing micromechanical analyses of an axisymmetric unit cell in tensile and compressive states, and performing the numerical optimization, the properties of ferrite and pearlite phases have been obtained.
By performing experimental tests on simple and notched samples of plane stress, plane strain, axisymmetric and fracture samples, the damage function of ferritic DCI has been extracted. Also, by performing the micromechanical analysis of a three-dimensional unit cell, the damage function of DCI has been analytically derived. The fracture strain of ferritic DCI for different values of stress triaxiality and Lode parameter has been extracted. Also the effect of presence of graphite nodule in stress state with small triaxiality has been studied. The presence of graphite nodule causes the reduction of fracture strain in such stress states. Cohesive laws of the ferrite phase and ferritic DCI have been extracted for various stress states. Micromechanical fracture analysis of ferritic DCI has been done by explicit modeling of voids for two different arrangements of graphite nodules. Weakest link probability theory using Weibull stress distribution has been used to predict the brittle fracture behavior of pearlitic DCI and the results have been compared with experimental and numerical values. Fracture probability distribution in pearlite DCI is directly related to the graphite nodules size distribution. This relationship has been determined using the Weibull distribution relationship.
Keywords: Ductile cast iron, Ductile fracture, Micromechanics, Nanoindentation and nanoscratch, Brittle fracture, Weakest link method.
