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The manufacturing of nickel-base superalloy forging for aero-engine applications requires a number of thermo-mechanical processing steps, which generate significant residual stresses that can result in distortion, uncontrolled deformation of the component during machining, or, when added to the in-service stresses, they will increase the crack initiation probability and hence reduce component life. Ageing treatments are applied to nickel-base superalloys in order to precipitate the strengthening phases that give to the alloys their remarkable strength over a wide temperature range. These isothermal treatments also result in a reduction of the residual stress level by means of thermal stress relaxation. In this work, the focus has been on the study of the thermal residual stress relaxation induced by the ageing treatment of wrought nickel-base superalloys. The aim of this work is to improve the understanding of the residual stress relaxation process and the different deformation mechanisms involved in the stress relaxation during isothermal treatments. This has been achieved by combining neutron diffraction with a new in-situ heating setup in order to track the elastic strain evolution in the centre of quenched disc shaped forgings of Inconel 718 and Udimet 720LI. This research has resulted in the implementation of an induction heating setup which was designed in order to serve in-situ residual stress analysis during isothermal treatments of large components at temperatures up to 1000°C. The in-situ analysis of residual stresses required the development of a standardised method which consists of monitoring the d-spacing evolution in stressed samples and in stress-free samples during isothermal treatments in order to determine the time dependent stress evolution. Stress calculations were performed using temperature dependent diffraction elastic constants which were determined experimentally for Inconel 718 and Udimet 720LI at high temperatures. The in-situ neutron diffraction measurements in water quenched disc shaped forgings of Inconel 718 of different thickness during isothermal treatments at 720°C revealed that the stress relaxation amplitude does not vary significantly with the initial stress distribution. However, the stress relaxation rate is strongly affected by the annealing temperature as stress relaxation through creep was observed to evolve at a diminishing rate during the isothermal treatment of Inconel 718 at 720°C and 750°C while no further stress relaxation occurred at 650°C. Most of the stress relaxation was found to occur during the heating stage as a result of a combination of plasticity and early stage creep relaxation. In-situ and ex-situ measurements exhibited good agreement on the amplitude of residual stress relaxation. It was found that a heat treatment at 750°C for 8h reduced the stresses by approximately 70% in Inconel 718 and only 20% in Udimet 720LI. For all in-situ experiments it was possible to fit the stress relaxation data by using logarithmic functions, which can now be used for validating and/or improving process models.

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A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in ful filment of the requirements for the degree of Master of Science in Engineering. Johannesburg, May 2018 / This work was aimed at the advancement of the Laser Shock Peening (LSP) process for aeronautical applications. This involved developing a robust strategy for residual stress measurements to various depths in LSP treated samples with different thicknesses and then to perform a parametric analysis of the LSP process. For the residual stress measurement part of the study, aluminium alloy 7075 samples with thicknesses of 10, 6, 3 and 1.6 mm were treated with LSP and the residual stresses were measured using various complementary techniques: Incremental Hole Drilling (IHD), Neutron Diffraction (ND), Synchrotron Energy-Dispersive X-Ray Diffraction (ED-XRD), Laboratory XRD (L-XRD) and Synchrotron Angle-Dispersive XRD (AD-XRD). The results compared favourably and it was concluded that to obtain a complete depth profile in the subsequent work, the residual stresses would need to be measured using at least three methods: one for near the surface, one at intermediate depths and one at greater depths. For the parametric analysis phase of the study, aluminium alloy 7075 samples with thicknesses of 10 and 1.6 mm were treated with LSP; the following LSP parameters were varied: Power Intensity (PI), Spot Size (SS) and %Overlap. The residual stresses were measured using L-XRD, IHD and ED-XRD. In addition to this, the following were investigated: the sample deformation, the surface integrity, the microhardness, and the microstructure under a Scanning Electron Microscope (SEM). A clear trend in the residual stress depth profile as well as the additional characterisations was observed as the parameters varied. This work will form part of a database of LSP results for various alloys that can be used for engineering residual stress profiles using optimal parameter selection for specific industrial applications and as benchmark for the development of LSP Finite Element tools. / MT 2018

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The thermal stresses, residual stresses distributions in a vertical cavity surface emitting laser (VCSEL) are investigated in this thesis. The post-weld shifts of the laser light sources in the VCSEL under the re-flow, temperature cycling test and high temperature aging test have also been simulated and studied. The creeping effects of the 63Sn/37Pb solder on the stresses distributions and post-weld shifts are compared and discussed. The finite element package MARC is used for the stresses and deformation simulations of a VCSEL under different load cases. The heat transfer, the non-linear couple elastic-plastic increment and the creep models are employed in the numerical simulations. The temperature dependent material properties of the 63Sn/37Pb and 96.5Sn/3.5Ag solder are used for the temperature cycling and aging test. The effect of the circle and square solder pad designs on the stresses distributions in a VCSEL is also studied in this thesis. Numerical results indicate that the residual stresses introduced in the solidification is significant in stress and post-weld analyses of a VCSEL.

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Abstract Grinding introduces unavoidable residual stresses of significant but unknown magnitudes. The effect of residual stresses in surface integrity is related to the nature of the residual stresses which relies purely on the process parameters and the workmaterial properties. It is a well-known fact that the fatigue strength of a ground component is increased by introducing compressive stresses. On the other hand, fatigue cracks may originate at regions of maximum tensile stress and usually at the surface of the material. Moreover, stress corrosion cracking is another consequence of critical surface tensile stress. Added to that, the residual stresses may result in dimension alteration and surface distortion, particularly for thin products such as plates. The beneficial effects of compressive residual stresses have been widely recognized in industry. The wise application of such a principle would bring about improved economical use of parts subjected to fatigue loading and aggressive environmental conditions. Therefore a better understanding of residual stress mechanisms is necessary to increase the dimensional accuracy and improve the surface integrity of ground elements, particularly for parts with high precision and manufactured by automated production lines. Consequently, the development of reliable models for predicting residual stresses is of great value in reducing the amount of measurements and experimental tests of residual stresses. Unfortunately, little effort has been devoted so far to develop appropriate models to take into account grinding conditions, workmaterial properties and boundary conditions. This thesis aims to investigate the residual stress mechanisms induced by grinding in terms of grinding parameters. In order to obtain a full understanding, both the roles of individual factors causing residual stresses (i.e. mechanical, thermal and phase transformation) and their couplings were carefully studied with the aid of the finite element method. The studies include: (1) residual stresses due to thermal grinding conditions, (2) residual stresses due to iso-thermal mechanical grinding conditions, (3) coupling of thermo-mechanical conditions, (4) coupling of thermo-phase transformation, and (5) the full coupling of all the factors. It is found that under sole thermal grinding conditions, the heat flux associated with up-grinding may lead to a higher grinding temperature compared with that of down-grinding. A constant flux introduces the least temperature rise if the total grinding energy is the same. Higher convection heat transfer not only decreases the grinding temperature but also makes the temperature rise occur mainly within a thin surface layer. A similar effect can be achieved by applying higher table speeds. When the grinding temperature is less than the austensing temperature, surface residual stresses are tensile. The heat generated within the grinding zone causes a very non-uniform temperature field in the workpiece. The part of the workmaterial subjected to a higher temperature rise expands more significantly and causes compressive stresses because of the restraint from its surrounding material that expands less. When the surface heat flux moves forward, the material outside the grinding zone contracts under cooling. Since the workmaterial has been plastically deformed during thermal loading, the contraction is restrained and thus a tensile stress field is generated locally. If a workpiece material experiences a critical temperature variation in grinding, phase transformation takes place and a martensite layer appears in the immediate layer underneath the ground surface. It was found that the growth of martensite develops a hardened zone with a higher yield stress that expands with the movement of the heat flux. A tensile surface residual stress is then developed. When the volume growth of material takes place during phase change, compressive residual stresses may also be generated. Under iso-thermal grinding conditions, it was found that plane stress is mainly compressive regardless of the distribution of surface traction and the direction of the tangential grinding force. With up-grinding, the residual stress in the grinding direction is always tensile. However, down-grinding may yield compressive surface residual stresses if the magnitude of the ratio of horizontal to vertical grinding forces is sufficiently large. Moreover, it is noted that discrete surface traction, which is more reasonable in terms of simulating the individual cutting of abrasive grits, would bring about more complex residual stress distribution that is very sensitive to the combined effect of individual cutting grits. If thermal and mechanical grinding conditions are coupled, a state free from residual stresses may be achieved if grinding heat is low and either the convection heat transfer or the table speed is high. However, it is found that the full coupling of the mechanical deformation, the thermal deformation and deformation by phase change results in tensile residual stresses. The effects of cooling and mechanical traction in this case however are minor. In summary, the research of this thesis explored the following: (a) grinding temperature development in terms of a wide range of grinding parameters together with the effect of temperature-dependent material properties, (b) the origin and onset of irreversible deformation due to mechanical loading, thermal loading and phase change under critical grinding conditions, (c) the effects of individual residual stress mechanisms and their partial and full couplings, and (d) the selection of grinding conditions to achieve beneficial residual stresses. Finally, based on the new findings in this research, a more comprehensive methodology is suggested for further study.

Bibliography: leaves 240-257. / xvii, 274 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / The main objective of this research is to develop a fundamentally new fatigue design and assessment procedure which can be used to predict the variation in crack size, strength and inspection periods as well as endurances. Basic fracture mechanics and fatigue procedures are used to illustrate this new technique. / Thesis (Ph.D.)--University of Adelaide, Depts. of Civil & Environmental Engineering and Mechanical Engineering, 1997?