Dr Foroogh Hosseinzadeh

The manufacturing processes used to make high performance parts for structures and systems inherently introduce locked-in residual stresses at both the macro-scale and micro-scale. In particular surface machining and modification operations create near-to-surface residual stress distributions that have high gradients associated with intense work hardening and/or local phase transformation effects. The aim of the proposed PhD project is to develop improved measurement systems that can quantify near-to-surface (up to 100 microns) residual stresses in manufactured components with high certainty.

An existing CDT led by the Centre for Nuclear Engineering (CNE) at Imperial College in collaboration with multi-disciplinary nuclear groups at Cambridge (Nuclear Energy Centre, CNEC) and The Open Universities, is now joined by the University of Bristol and Bangor University to form a new Nuclear CDT, bringing together over 50 academics with internationally-leading research across nuclear materials (e.g. primary circuit structures, fuels, structural materials and wasteforms), joining/manufacturing technologies, structural integrity assessment methods and standards, thermal hydraulics, geological storage and disposal (e.g. rock mechanics), safety and security (e.g. accident modelling and structural integrity), nuclear technology policy and public engagement, plant decommissioning and clean up, regulation, new nuclear build, advanced reactor technology, the fuel cycle, plant life extension, and naval nuclear propulsion.

Manufacturing processes often introduce locked-in stresses, namely residual stresses, in the fabricated parts. They could cause distortion and cracking, influence performance, reduce products lifetime leading to premature failure and result in lower productivity. In high value manufacturing it is of paramount importance to characterise the state of residual stress in manufactured parts. However, residual stresses are notoriously difficult to characterise. Prediction of residual stresses requires sophisticated advanced computer modelling that are often time consuming and the reliability of predictions depends on the expertise of the modeller and input data. Experimental characterisation of residual stresses require access to measurement techniques, expertise or central facilities. Often multiple residual stress measurement techniques are required to unlock useful information. They can be costly, time consuming, difficult to implement due to the nature of the component or limitation of available techniques. A neural network model was developed at the Open University and applied to predict residual stress profiles in austenitic stainless steel pipe girth welds. This project proposes to further develop the neural network approach and implement it to other manufacturing processes. It will provide the manufacturing catapults and industrial end users a readily accessible tool for prediction of residual stresses in manufactured parts without the need to get access to central facilities, measurement equipment, expertise or expensive computational modelling. It can be used as a user-friendly decision making tool to design manufacturing process parameters with a view to introduced controlled level of residual stress. This will be of great value for high value manufacturing.

The contour method is one of the most recently developed techniques for measurement of residual stress. The contour method is performed in four stages: (i) the component is sectioned in two halves along a plane of interest using electric discharge machining, (ii) the resulting out of plane displacement of the created cut surfaces caused by the relaxation of residual stresses is measured often using a Coordinate Measuring Machine (CMM) fitted with a touch probe or an optical sensor, (iii) the measured out of plane displacements are processed to remove the effect of shear stress and artefacts introduced due to the wire EDM process or surface measurement, (iv) the processed data are used as surface boundary conditions to a Finite Element (FE) model of one of the cut parts and fully elastic stress analysis is performed to back calculate the original and pre-cut state of residual stress. An outstanding advantage of the contour method compared to other strain relief and diffraction based techniques is that it can provide a cross-sectional map of residual stress using readily available equipment in most workshops. In theory the ability of the technique in measuring through thickness residual stresses is not limited by the size or geometrical complexity of components. Although only one component of the stress tensor, perpendicular to the cut surface, can be measured using one single contour cut, the technique has been expanded to measure multiple components of the stress tensor when combined with other techniques or multiple cuts used. In addition unlike diffraction based techniques the contour method is not sensitive to microstructural variations that is often observed in weldments. The contour method, however, has been mainly applied to measure residual stresses in metals, homogeneous and isotropic materials. The challenge of this research project is to develop the technique to measure residual stresses in inhomogeneous, anisotropic and electrically non-conductive materials such as composites.

This is a Horizon 2020 project proposal. The consortium comprises 19 EU participants: 9 represent research institutions, 5 represent academia, 4 represent industry and one represents a regulatory authority. A systematic ageing management procedure is the basis for justifying the safe long term operation of nuclear power plants. One fundamental part in this process is to demonstrate the integrity of the nuclear power plant components. The required safety margins are determined by considering various degradation and ageing mechanisms and postulated defects. This project focuses on identified open technology gaps related to piping components. OU Contributions relate to WP2 which is concerned with the simulation, measurement and treatment of residual stress.

The safe operation of engineering structures is vital in safety critical industries. Structural integrity failures can have catastrophic consequences in terms of loss of life and financial circumstances. Meanwhile there is a strong interest in reducing costs, light-weighting, increasing design life and life extension. To this end reliable structural assessments are essential at the design stage and through in-service life to ensure continuous profitable operation of assets. Residual stresses are inevitably introduced in engineering structures during manufacturing processes. Their presence can have adverse effects on the behaviour of structures and contribute to driving force promoting degradation mechanisms including crack initiation, crack growth, stress corrosion cracking, fatigue and fracture. Therefore it is of paramount importance that the state of residual stresses in engineering structures is carefully and reliably characterised so that remedial actions could be taken to enhance the lifetime of current materials or novel designs and manufacturing methods developed and optimised. For the first time, this project proposes novel advances that will radically bring new capabilities for the contour method of residual stress measurement in several ways: it will unlock maps of residual stress in multiple directions simultaneously. Of a true step change is extending the application of the technique to measure 3D maps of residual stress. The application of novel advances enables the technique to be implemented on complex engineering structures. The proposed research has the potential to provide far more complete residual stress information about safety critical components of high interest to engineers in the aerospace, petrochemical, power generation and nuclear industries.