Rapid development and stiff competition in material related industries such as the automotive, demand very high precision in end products in very quick time. The transformation of raw material into an intricate-shaped final product involves various intermediate steps like design, material selection, manufacturing processes, etc. In all these steps, an in-depth understanding of material behavior plays an important role. The available traditional methods such as trial-and-error, especially in the case of die design, become highly inefficient in terms of time and money. This, there is a growing interest in simulation of the final product in order to predict different parameters which are important in design and manufacturing. Currently available simulation techniques are based on existing theories of plasticity or large deformation. These theories have been developed over several decades and many theoretical and practical issues have been debated over the years. Though the theories have great utility in understanding and solving some practical problems, there are ranges of applications for which no acceptable models are available. Most of these theories are either materials or process-specific with oversimplified real physical situations using assumptions and empirical relations. Development of field equations from first principles to stimulate elasto-plastic deformation is one such, still a subject of on-going discussion. Materials and composites exhibit hysteresis even at very low stresses, i.e., inelasticity is always present under all types of loading. This observation shows that the representing constitutive relation cannot treat the elastic and plastic deformations separately. The deformation is due to changes in size and shape, and studies with varying strain rates show considerable material sensitivity to the rate of deformation. Therefore, a generalized field equation is developed from first principles in the Eulerian coordinate system using material resistance to changes in size and shape, and their rates. The formulation uses a unified approach representing continuous effect of elastic and plastic strains and strain rates. The field equation involves eight material parameters, viz. bulk modulus, shear modulus, material shear velocity, material bulk viscosity, and four more constants associated with activation points related to deviatoric and volumetric strains and plastic strain rates. The elastic moduli, bulk and shear, are constants, and so also the material viscosities, while plastic stain rates are functions of elastic strain rates. The field equation redces to Cauchy’s equation in the solid limit and Navier-Stokes equation in the fluid limit. Simple experimental measurements are suggested to obtain the numerical values of the material parameters. Uniaxial tension tests are carried out on commercially available mild steel and aluminium alloy at different strain rates to quantify any variations in the values of material parameters during large deformation. Experimental results and the classical understanding of material deformation reveal the constant nature of elastic moduli during large deformation and, from fluids, the viscosities seem to remain constant. Around the yield region, materials experience a sharp increase in absorbed energy which is modeled to represent the plastic strain rates. The variations and contributions from elastic and plastic strains, both volumetric and deviatoric, and the corresponding stresses are observed. The effects of strain rate on plastic stress and energy absorbed are investigated. The model is checked for different materials and loading conditions to ascertain the proposed changes to earlier theories. Available experimental data in the literature are used for this purpose. The analysis shows that, though the overall stress-strain relations of different materials look similar, their internal responses differ. The internal response of a material depends on various microstructural factors, like alloying elements, impurities, etc. The present model is able to capture those internal differences between various materials. Numerical solution of different plasticity problems have to be undertaken to ascertain the applicability, generality, realism, accuracy and feasibility of the model.

The presence of residual stresses in engineering components may significantly affect damage evolution and progression towards failure. Correct evaluation of residual stress is of crucial importance for assessing mechanical components, predicting response and ensuring reliability. For example, when failure occurs due to cyclic loading, the underlying damage begins at the nano-, and then micro-scale. It is clear that improving engineering reliability at the micro-scale requires the ability to evaluate residual stress and mechanical properties at the appropriate scale. The key objective of the thesis is to advance the understanding and practice of residual stress evaluation at the micro-scale, and to examine the implications and applications that follow. Significant effort was devoted to the evaluation of two aspects of the relatively novel FIB-DIC micro-ring-core experimental technique: assessing the effects of Ga-ion damage and the quantification of uncertainty in stress evaluation due to unknown crystal orientation. FIB-DIC micro-ring-core milling was then used alongside with synchrotron XRD to study residual stress effects on fatigue crack growth propagation rate following the occurrence of overload or underload. The effects of the two principal mechanisms of crack retardation following an overload, residual stress and crack closure, were separated by testing samples at different loading ratios. Whilst, the acceleration after an underload was studied using validated non-linear FEM analyses. Conceptual focus was placed on the macro-micro-nano residual stress decomposition into Type I, II & III according to scale and, detailed examination was conducted experimentally and numerically. In the context of shot-peening surface treatment, residual stresses were modelled using a novel eigenstrain-based modelling procedure for arbitrarily shaped components. Furthermore, a fine scale characterisation was performed of the recast layer produced by EDM, with particular attention paid to the residual stress. The investigations presented in this thesis open new perspectives for the assessment of material reliability. Improved failure prediction models will be elaborated based on the insights obtained in the present study.

Geometrical discontinuities, like sharp notches, appear in constructions and engineering structures and lead to stress concentrations. These technical objects are very dangerous due to the fact that they reduce the structural conformity and can lead to crack initiation. Technical objects are not always designed as homogenous bodies but can consist of two or more materials with sharp notches on the interface of these materials. The influence of free surface on crack initiation conditions is studied and assessed by means of 3D model of sharp and bi-material notches with finite thickness. Stress fields around the singular stress concentrators are calculated with finite element method and the results are evaluated by means of criterion of critical quantity. This approach is easy applicable and can be used in combination with the knowledge of basic material properties and results of finite element analysis of the assessed notches. In order to estimate weather crack will initiate from the middle of the observed notched specimen or from its free surface, the value of averaged critical applied stress was introduced. With this value it’s possible to determine the location of crack initiation thru the sample thickness. Thru the ratio of values of critical applied stress in the middle and on the free surface of the observed specimen it’s possible to quantify the influence of the free surface on the location of crack initiation. With the use of this approach it’s shown, that the location of crack initiation depends on more parameters, loading direction, the notch opening angle and the sample thickness. In case of bi-material notches it depends also on the ratio of young modulus.