Complex Contour Method

The safe operation of engineering structures is vital in safety-critical industries such as power generation, nuclear, aerospace and oil and gas. Structural 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 integrity 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 various degradation mechanisms. 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.

The contour method, first presented in 2000, is emerging as a powerful technique for the measurement of residual stresses in bulky parts. The method involves making a straight cut in the component of interest along a nominally flat plane where residual stresses are desired to be determined. The created cut surfaces deform due to the relaxation of residual stresses. The deformation of the cut surfaces are measured and then used to back-calculate 2D distribution of residual stress that was present along the flat plane prior to the cut.

Nevertheless, there are still several limitations associated with application of the contour method: a) only a straight cut over a flat plane is used to section components for contour measurements; b) the standard method can only measure 2D distribution of one component of the residual stress tensor over a flat plane; c) the method is limited to symmetric sectioning of the cut parts, d) like other mechanical strain relief techniques, the contour method is prone to plasticity-induced errors where the magnitude of stresses or level of triaxiality is very high and e) most historical measurements using the contour method have concerned simple geometries such as welded rectilinear plates.

For the first time, the "Complex Contour Method" proposes to develop the use of complex cutting paths, for example non-planar and closed complex cutting paths instead of cutting along a flat plane. This innovative approach will radically bring new capabilities for the contour method in several ways: it will unlock map 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. Enabling the technique to deal with complex cutting paths will inherently deal with limitations of the standard method regarding symmetry of the cut parts. Moreover, removing the constraint of a symmetric planar cut opens the potential to mitigate plasticity-induced errors that can accompany standard contour method cuts.

Of another radical step change of the application of the complex cutting paths is that it enables the technique to be implemented on complex engineering structures. For example, the conventional contour method confined to symmetric planar cuts cannot be applied to complex components such as tube penetration welds for pressure vessel heads.

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. In addition for industrial applications, a single complex contour cut offers a far more cost effective tool compared to the cumbersome and time consuming conventional contour method using multiple-method and multiple-cut approaches.

The outcome of this project will contribute to enhancing capabilities within the UK manufacturing, power generation, aerospace and petrochemical industries especially with regard to accounting for residual stresses in designing optimised manufacturing routes and safety assessments.

For example, transforming the aerospace industry to a low carbon industry demands for more fuel-efficient and lightweight aircrafts. This requires the use of new high strength, damage tolerance and lightweight materials and optimised manufacturing and assembly technologies. The proposed Complex Contour Method could be utilised to advance the state-of-the-art in accounting for residual stress in design, manufacture and assessment of components in aircraft structures. For example one of the important inputs to the aging aircraft research programme conducted by the US Federal Aviation Administration (DOT/FAA/AR-07/56,V1) was accounting for the influence of residual stress in the life-prediction methodology by developing safety codes for two- and three-dimensional cracked bodies.

The proposed research also targets the power generation industry at national and international levels. For example, implementation of the proposed technique on complex welded components will provide far more complete understanding of residual stress state in plant components whilst validating weld modelling analysis tools. The use of these advanced measurement and analysis tools will lead to developing strategies to mitigate detrimental weld residual stresses and distortion in design and manufacture of plant components and underpinning safety case assessment methods for plant life extensions leading to £Billions savings (STFC Impact Report 2012).

Direct beneficiaries of the proposed research are Rolls-Royce, EDF Energy and Hill Engineering. Rolls-Royce Submarines business is the design, manufacture and provision of technical support to ensure the safe through-life operation of Pressurised Water Reactor plant on Royal Navy Submarines. The Rolls-Royce Civil Nuclear business is currently building capability for the UK manufacture of high integrity large components for Generation III reactors. The Roll-Royce Submarines (directly supporting the proposed research) and Civil Nuclear businesses would directly benefit from the proposed project. The application of the proposed research has also been identified in Rolls-Royce Aero-Engine components.

Reliable measurement of residual stresses in complex components will be of increasing importance for EDF Energy in plant life extension safety cases for their aging fleet of Advanced Gas Cooled Reactors. EDF Energy is supporting the proposed research to help them understand and manage potential mechanisms of degradation for safety critical applications.

Hill Engineering provides leading edge engineering solutions to complex structural problems for a wide rage of industries in the US and worldwide. The resulting developments from the Complex contour Method is of significant interest to Hill Engineering and the founder of Hill Engineering will provide his support and expertise during the course of the project.

In particular, the potential application of the developed technique will be widely disseminated to the High Value Manufacturing Catapult and EPSRC Innovative Manufacturing Research Centres. Strong interest in the proposed project from these research centres and their associated industrial partners is envisaged. The Complex Contour Method will be utilised to address the residual stress challenges associated with the additive manufacturing enabling the UK to compete in this rapidly growing global market. Another example is its application in the Advanced Forming Research Centre, which in early 2014 announced a strategic programme to establish large-scale collaborative research activity related to manufacturing induced residual stress within the centre.