Inelastic deformation is a common but often neglected phenomenon in experimental analysis of metal deformation and in contacts. This neglect leads to degraded measurement accuracy of material properties. Therefore a need arises for material models that a priori incorporate inelasticity. These material models must be simple and comprehensive to have the highest impact in society. This thesis addresses three main sources of inelasticity, namely anelasticity and plasticity in metals and viscosity in contacts. Inelasticity is a dissipative mode of deformation that is mechanically recoverable for anelasticity and viscosity, and irrecoverable for plasticity. We connect the fundamental properties and structures of metallic and soft matter constituents with experimentally accessible measures. The presented models will aid in the development of materials with specific properties that meet the needs of industry. Chapter 2 presents an analytical model of the tensile test tangent moduli and yield points for single-crystallite metals with spatially uniform and nonuniform dislocation distributions across slip systems. The moduli and the onset of plastic flow show a notable dependence on initial dislocation character, spatial dislocation distribution, and loading direction with respect to crystallographic orientations. An improved methodology accounts for elastic compressibility and anisotropy, and the geometric structure of crystal lattices when one measures dislocation network geometry in single metallic crystallites. Chapter 3 contains a seamless, unified stress-strain treatment of dislocation-driven deformation. This treatment combines the three deformation mechanisms of elastic bond stretching, stable dislocation glide, and unstable dislocation glide. The model’s yield criterion connects the bowing out of local dislocation links and global dislocation multiplication. A semi-empirical relation is constructed for the evolution of the dislocation network structure with uniaxial loading. Chapter 4 formulates a macromechanical model of the yield point phenomenon under invariant plane conditions. The heterogeneous stress state across the Lüders front and the plastic flow inside the Lüders band are accounted for. The Lüders band orientation with respect to the tensile direction is not unique; the orientation changes with material properties and tensile specimen geometry by the stress concentration at the front. The model serves to approximate constitutive parameters independent of the test conditions. Chapter 5 elucidates the interplay between adhesion and roughness by modelling the retraction of rigid, wavy indenters from viscoelastic substrates. Viscoelasticity governs adhesive hysteresis across all loading rates, and even in the presence of roughness-induced mechanical instabilities. This confirms the central role that viscoelasticity must play in experimental measurements in the presence of adhesive interfaces in soft matter contacts. Chapter 6 examines the static, quasi-static, and dynamic trajectories of a base-excited mass-spring-damper system in the presence of friction. The differences between the dynamic and the quasi-static solution in engineering problems with viscous, static, and dry friction are assessed. The omission of inertial contributions will under-predict dissipation at both low and high excitation frequencies. This chapter is a guide for future (multi-scale) numerical modelling efforts on adhesion and interface friction, and the hysteretic deformation of metals. Chapter 7 is a general discussion on the impact of inelasticity in metals, that follows from Chapters 2 and 3, the measurement of the yield point phenomenon in Chapter 4, and numerical modelling of dissipative contacts in Chapters 5 and 6. The four models as presented in Chapters 2-6 are readily applicable in experimental measurements and future numerical models. The importance of accounting for inelasticity in experimental measurement and modelling of the yield strength in metals, and adhesive dissipation in soft matter contacts is emphasised. Finally, the state of the art in research on the three main sources of inelasticity and potential applications of the presented models are enumerated, which serve as starting points of future research.@en

The Static Recrystallization (SRX) phenomenon of austenite in C-Mn steels have a large impact on the final mechanical properties of the steel. However, the austenite recrystallization kinetics are difficult to investigate experimentally, due to solid-state phase transformations during cooling to room temperature. The best model alloys to examine SRX are those austenitic alloys such as, Ni-30Fe alloy, that do not experience undesirable phase transformation and that have similar stacking fault energy (SFE) as austenite in C-Mn steels. The hot-working of the Ni-30Fe alloy has been conducted at high deformation temperatures, followed by an uniaxial compression test at a low strain condition (ε = 0.2) with a strain rate of 1s-1, and an annealing temperature of 900°C (above recrystallization start temperature). Thereafter, the alloy is experimentally investigated by Electron Backscatter Diffraction (EBSD), to characterize the austenite microstructure and to measure the crystallographic orientations. The experimental measurements from the EBSD technique are subsequently analyzed using MTEX software to study the SRX phenomenon that occurs during hot rolling of steels.The technique to separate recrystallized and deformed grains in this research has adopted the Kernal Average Misorientation (KAM) and Grain Average Misorientation (GAM) technique, rather than the previously utilized Grain Oriented Spread (GOS) technique which incidentally produced slower and inaccurate recrystallization kinetics. The initial and final grain size calculations have been conducted by consideration of the annealing twins, and the reported results are in close agreement with earlier conducted work of Sellars model. The EBSD scan which captures the Geometrically Necessary Dislocations (GND) have been experimentally measured after deformation to be approximately 4.5x1014m-2, by utilizing the approach of KAM, curvature tensor and the Burgers vector with an estimated 13% error. Additionally, the effect of recovery has been captured with the continuous decrease in stored energy as a function of time, where the GND density is estimated to be 7.3x1012m-2 after the annealing treatment.The GND density present in the deformed microstructure is significant to provide the net driving force necessary for nucleation, based on the validated strain-induced-boundary-migration (SIBM) mechanism. The initial nucleation site is observed at triple junctions, and the selected nucleation criterion factor for the captured GND densities, estimates the driving force for nucleation to favour single sub-grain SIBM. The minimum critical sub-grain size required for bulging is calculated to be 3.40 ± 0.44 μm after deformation, which is an increasing parameter with time, as a result of the recovery process. The number of recrystallized grains captured at different time intervals using the validated GAM technique assists in experimental nucleation rate calculations, that have been subsequently converted for a 3D microstructure, in order to compare with the nucleation framework of Rehman's model. The recrystallization nucleation process is experimentally observed to obtain peak values at 2s and continues until all the GNDs are annealed out of the specimen. There is a large discrepancy in the nucleation rate predicted by the model, which is much higher with respect to the experimental observations reported. Finally, the experimental recrystallization kinetics is observed to be in close agreement with the JMAK model during the initial phase of the annealing treatment, but deviates in the later phase of the recrystallization process.

The present thesis investigates recrystallization and related phenomena in interstitial free (IF) and low carbon (LC) microstructures. Emphasis is given mostly on the early stages of recrystallization, i.e. nucleation stage. The investigations are performed with experimental measurements and computer simulations. In all studies, recrystallization is observed with close coupling to the deformation substructure. Crystallographic texture analysis is used as a means to: (a) confirm trends between the simulated and experimental microstructure and (b) interpret the evolution of recrystallization in terms of selective subgrain growth. The goal of this thesis is to obtain insight into recrystallization initiation and evolution in low alloyed cold rolled steel sheets. @en

Dual-phase (DP) steels are an important class of advanced high-strength steels (AHSS) and constitute a major share of steels for the automotive industry. A microstructure consisting of hard martensite embedded in a soft ferritic matrix gives them a good combination of strength and ductility. The martensite formation in the microstructure from austenite involves a shape and volume change, which is accommodated by the deformation of the surrounding ferritic matrix. This accommodation is known to impart typical characteristics in DP steels such as the absence of a yield point, continuous yielding and high initial work hardening rate. This thesis is an attempt to understand and model the aforementioned accommodation process in the ferritic matrix of DP steels. Traditionally, in predictive modelling of DP steel mechanical behaviour, the region of ferrite which undergoes deformation to accommodate martensitic transformation is taken into consideration as a constant thin layer of strain-hardened ferrite at the ferrite/martensite interface. This approach is shown to be inadequate for capturing local variations in ferrite deformation. Hence, electron backscatter diffraction (EBSD) experiments were carried out to study in detail the influence of various microstructural features on local variations in the transformation-induced deformation of ferrite. It was found that the crystallographic orientation of ferrite grains, martensite variant and its prior austenite grain (PAG) play an important role in determining the extent of transformation-induced deformation of ferrite. Taking a cue from this, a novel methodology comprising sequential experimental and numerical research on DP steels is developed which combines the results of PAG reconstruction, phenomenological theory of martensite crystallography (PTMC) and EBSD orientation data to estimate ferrite deformation due to every martensitic variant formed, via full-field micromechanical calculations on a virtual DP steel microstructure. Furthermore, the influence of self-accommodation during martensite variant formation on transformation-induced deformation of ferrite was also investigated. It is shown that the higher the number of variants which form from a PAG, the less the deformation caused by that PAG in the surrounding ferritic matrix. This is because of a decrease in the effective magnitude of the shear component of martensitic transformation during multi-variant transformation. The scientific findings presented in this work can be used for developing predictive models for the mechanical behaviour of not only DP steels but any multiphase steels which exhibit plastic accommodation and residual stresses in their microstructure due to martensitic phase transformation.@en