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​Research Projects

Engineering applications of neutron and synchrotron radiation through FaME38

The work follows the EPSRC funded collaborative research consortium project, led by Plymouth, on the exploitation of the unique potential of FaME 38 at the ILL-ESRF for engineering applications of neutron and synchrotron radiation. These include in-situ studies involving dynamic and thermo-mechanical loading, fast strain scanning, neutron tomography, very high-resolution X-ray nanotomographic imaging, and small angle scattering. Areas of interest also include deformation mechanisms and texture evolution, phase transformations and micromechanics, in-situ studies of welded and bonded joints, processing and joining high temperature materials for the aero-engine and nuclear industry, stress corrosion interactions and cracking, residual stress-crack interactions, and more reliable life prediction for joints and advanced components/materials. For more information please contact Prof Neil James.

  

Mathematical modeling of stress fields associated with fatigue cracks and their plastic enclave

This is on-going long-term collaborative work that links applied mathematicians (Dr Colin Christopher, University of Plymouth), with fatigue and fracture expertise (Professor Neil James, University of Plymouth) and experimental mechanics (Professor EA Patterson, Liverpool University and Professor Francisco Díaz Garrido at Jaen University in Spain). It aims to develop a new multi-parameter mathematical model of a crack tip under biaxial loading that is experiencing crack tip shielding from the plastic enclave associated with a growing fatigue crack and from surface roughness. It builds on initial work done using Muskhelishvili complex stress analysis and a mimetic algorithm under EPSRC Grant GR/L42391. The current state is that a biaxial mathematical model has been developed (termed the Christopher-James-Patterson or CJP) model, which develops a novel three-parameter stress intensity description of the retarding and driving forces on a crack tip [1]. This model has been experimentally verified using photoelasticity on polycarbonate compact tension specimens [2], using digital image correlation on 2024-T3 aluminium specimens [3]. This work is also leading to new techniques for visualising both the shape and size of the plastic enclave associated with a fatigue crack.

[1] M N James, C J Christopher, Yanwei Lu and E A Patterson (2013), Local Crack Plasticity and its Influences on the Global Elastic Stress Field, International Journal of Fatigue, 46 pp.4-15.  doi.org/10.1016/j.ijfatigue.2012.04.015.

[2] M N James, C J Christopher, Y Lu, and E A Patterson (2012), Fatigue crack growth and craze-induced crack tip shielding in polycarbonate, Polymer 53 pp.1558-1570.  doi:10.1016/j.polymer.2012.01.032.

[3] J M Vasco-Olmo, F A Díaz, A García-Collado and R Dorado-Vicente (2013), Experimental evaluation of crack shielding during fatigue crack growth using digital image correlation, Fatigue & Fracture of Engineering Materials & Structures, available online doi:10.1111/ffe.12136.

For more information please contact Prof Neil James. 

 

Residual stresses associated with welds and shot peening their modification by process conditions and applied fatigue cycles

This is another long term collaborative project with core partners being Professor Neil James (University of Plymouth), Professor D Hattingh at Nelson Mandela Metropolitan University (NMMU), South Africa and Dr Mark Newby at ESKOM, South Africa. Collaborative work has also involved Professor John Yates (Sheffield, and Manchester) [1], Dr Dave Asquith (Sheffield Hallam University) [2], Dr Axel Steuwer, Lund University, Sweden [3] and Professor Rolf Laubscher, University of Johannesburg, South Africa [4]. Peer reviewed synchrotron diffraction experiments have been carried out at the ESRF (ME-197, ME-282, ME-748, ME-992, MA-326 and MA-856) using the BM16, ID15A and ID31 instruments, and neutron diffraction experiments have been performed at the ILL (7-01-167, 7-01-196, 1-01-8, 1-01-58, 1-01-73, 1-02-31, 1-02-44, 1-02-83 and 1-02-128) using the SALSA instrument and at ISIS using the ENGIN-X instrument (RB720574, RB810453 and RB910338). The work has been aimed at improved understanding of the microstructural origins of residual stresses and their modification during applied fatigue cycling and identifying the effects of process parameters on the residual stress fields. It is then possible to predict a priori process conditions may then be chosen that lead to lower residual stress levels and more accurate life prediction procedures developed [e.g. 5-7].

[1] M N James, D J Hughes, Z Chen, H Lombard, D G Hattingh, D Asquith, J R Yates and P J Webster (2007), Residual stresses and fatigue performance, Engineering Failure Analysis 14 pp.384-395.  doi:10.1016/j.engfailanal.2006.02.011

[2] D T Asquith, A L Yerokhin, A D Evans, M N James, J R Yates and A Matthews (2013), Evaluation of residual stress development at the interface of plasma electrolytically oxidized and cold worked aluminium, Metallurgical and Materials Transactions A 44 Issue 10 pp.4461-4465.  doi.org/10.1007/s11661-013-1854-0.

[3] A Steuwer, D G Hattingh, M N James , U Singh, and T Buslaps (2012), Residual stresses, microstructure and tensile properties in Ti-6Al-4V friction stir welds, Science and Technology of Welding and Joining 17, No. 7, October 2012 pp. 525-533.  doi.org:10.1179/136217112X13439160184196.

[4] J J Klopper, R F Laubscher, A Steuwer and M N James (2011),  An investigation into the effect of weld technique on the residual stress distribution of 3CR12 (DIN 1.4003) built-up structural sections, Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, 225 No. 3 pp. 123-132.  doi:10.1177/0954420711404326.

[5] M N James, S-P Ting, M Bosi, H Lombard and D G Hattingh (2009), Residual strain and hardness as predictors of the fatigue ranking of steel welds, International Journal of Fatigue, 31 pp.1366-1377.  doi:10.1016/j.ijfatigue.2009.03.006.

[6] D G Hattingh, D L H Bulbring, A Els-Botes, M N James (2011), Process Parameter Influence on Performance of Friction Taper Stud Welds in AISI 4140 Steel, Materials and Design 32 pp.3421-3430.  doi:10.1016/j.matdes.2011.02.001.

[7] C Blignault, D G  Hattingh and M N James (2012), Optimising friction stir welding via statistical design of tool geometry and process parameters, Journal of Materials Engineering and Performance 21 No. 6 pp.927-935.  doi: 10.1007/s11665-011-9984-2.

For more information please contact Prof Neil James

 

Multiaxial fatigue assessment of aluminium friction stir welded tubes

This project is an international network grant, involving Professor Luca Susmel (PI – University of Sheffield), Professor Neil James (Co-Investigator – University of Plymouth, Professor Roberto Tovo (University of Ferrara, Italy) and Professor Danie Hattingh (Nelson Mandela Metropolitan University, Port Elizabeth, South Africa).  This project aims to identify and assemble the resources from an international network of Universities to create and formalise a bespoke approach suitable for designing aluminium friction stir welded joints against multiaxial fatigue.  This requires capability and expertise across metallurgy, technological processes, and structural integrity.  Multiaxial fatigue behaviour of friction stir welded tubular connections has never been investigated systematically before, despite its growing importance in, for example, the ground transportation industry.  Project outcomes will allow the foundations to be laid for new design recommendations specifically tailored to the complex case of multiaxial fatigue assessment of aluminium FS welded tubular joints.  Such joints have high technological impact while efficient design procedures confer weight saving and cost benefits to the industry, to society and to more sustainable use of resources (in this respect, it should be noted that aluminium can also be efficiently and readily recycled, and that what limits its wider use in structural design is uncertainty regarding fatigue lifing and performance on welded joints).  The work is being carried out on 6082-T6 aluminium tubes which are being friction stir welded using a specially developed technique involving pin withdrawal to avoid leaving a hole in the welded joint. For more information please contact Prof Neil James. 

Process optimization for friction stir welding using in-process monitoring of the tool forces and torque

The work is collaborated with Professor D Hattingh of the NMMU, South Africa and Dr A Steuwer. Part of this work has been orientated towards understanding how the polar plot of ‘force footprint’ measured on a cycle-by-cycle basis during welding may be interpreted to yield information that can be linked to mechanical properties, defects and fatigue performance. There is also a collaborative long-term beam time proposal on stress engineering for friction welding with robotic sample manipulation accepted by the ESRF (experiment ME-1165 on ID15A), which is collaborative with Professor PJ Withers and Dr M Preuss at University of Manchester, and Professor L Edwards at Open University. For more information please contact Prof Neil James. 

 

Surface coating technologies and tribology

Surface coating is an effective way to improve chemical and tribological performance of a component. Various polymer or metal composite coatings containing nanoceramic particles or nanotubes can be made as corrosion barrier or bearing liner. A polyimide based composite coating containing nanoceramic particles is developed to improve wear and erosion resistance for offshore structures, automotive bodies and aeroengine compressors. This material can be applied as a coating onto steel or aluminum components by air spray followed by thermal curing. A composite plating process has been developed to generate coatings containing solid lubricants to control friction and reduce galling in thread tubular connections. Two tribological test rigs with dataloging devices are available for friction and wear testing under reciprocating sliding. This is complemented by a full range of mechanical testing and microstructural analytical apparatus including a laser confocal 3D surface profile meter, digital optical microscopes within the School and access to high resolution SEMs, TEM in Electron Microscope Centre to investigate surface topography and chemistry effects on tribological properties. For more information please contact Dr Huirong Le.

 

Design and simulation of offshore/marine composite structures

There is increasing use of fibre reinforced plastic composites in offshore/marine structures such as wind turbines, marine renewable energy converters due to their lightweight and anti-corrosion characteristics. However, unpredicted failures of these structures under dynamic loading in corrosive environment and the associated maintenance costs are still major concerns in the industry. Current research in this field is aimed to develop a more realistic laboratory test method to study failure mechanisms of fibre reinforced composites under fatigue loading in saline environment. This is complemented by microstructural studies using high resolution SEMs and finite element modeling to understand the failure mechanisms at microstructural scale. The findings will inform the optimization of composite manufacturing and the structural design with composite materials. For more information please contact Dr Huirong Le.

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Advanced nano biomaterials for dental/bone implants

Nanomaterials and composites hold the promise to improve the performance of biomedical implants. Research in this field is carried out in collaboration with Prof Christopher Tredwin at the Peninsula Schools of Medicine and Dentistry and Prof Richard Handy at the School of Biological and Health Sciences. One research topic is the design and fabrication of nanotube/nanoparticle reinforced polymer or ceramic composites. Ceramic cements such as calcium phosphate and/or sulphate are commonly used in dental/bone surgery. They are limited by low strength and low toughness. Current research is intended to evaluate the feasibility of using geoceramics as new cement and the effects of nano-fillers (e.g. carbon nanotubes, nanoceramic particles) on the mechanical and physical properties and to ensure the biocompatibility of these new materials. Another active research topic is to develop bioactive ceramic coatings for titanium implants aimed to reduce the risk of infection and loosening failure. Current research is focusing on anodised titanium oxide coating on the biocompatibility and bioactivity of the implants. For more information please contact Dr Huirong Le.

  

Steel web tapered tee section cantilevers

This project aims to develop a design method for steel web tapered tee-section cantilevers. An analytical study has been carried out to determine the lateral-torsional buckling capacities of such beams. Experimental work of 10 tee-section cantilevers has been carried out to investigate both local and global buckling behaviours. A parametric study using finite element analysis methods is currently being undertaken to predict the structural performance of such beams. More tests will be carried out to validate the finite element analysis models. Finally a simple design method will be developed. For more information please contact Dr Boksun Kim.

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Concrete reinforced using graphene oxide flakes

The aim of this research is to investigate the mechanical properties of mortar and concrete reinforced using graphene oxide flakes. This research will be carried out in two phases. Firstly, the rheology properties of graphene reinforced cement and concrete at early hydration process will be investigated. This will include the dynamic Young's modulus, shrinkage, released heat, and interface problem between cement and grapheme. Secondly, physical laboratory tests will be carried out to determine the mechanical properties of cement and concrete reinforced using graphene oxide flakes, and compare the strength, stiffness and durability of cement/concrete reinforced using graphene oxide flakes with those with ordinary concrete. For more information please contact Dr Boksun Kim.

  

Structural performance of castellated members under static and dynamical loads

Castellated beams have been increasingly used as structural members in buildings. A castellated beam is usually fabricated from a standard universal beam by the process of a profiled flame cut along its length, then shafting into the profile, followed by the welding of the two halves together. The theory behind the castellated beam is to increase the beam’s depth and strength without adding additional material. The process could increase the depth of the beam by approximately 50% and thus greatly increases the beam’s stiffness and strength while bending about its major axis. Therefore, castellated beams are most often used in long span applications with light or moderate loadings such as primary beams in floors and roofs. A further advantage of castellated beams is the holes in the web which provide a route for services. Since castellated beams are made usually from I-beams or H-columns, they tend to be deep and slender and have reduced torsional stiffness of the web due to the openings in the web. Hence they are more susceptible to web distortional and/or lateral-torsional buckling. This project is to investigate the structural performance of castellated members when they are subjected to static and/or dynamic loads. The study involves the axially compressed buckling, lateral torsional buckling and web shear and web distortional buckling of castellated members. The objective of the research is to develop design curves for both castellated columns and beams with various web opening configurations by using both experimental and finite element numerical analysis methods. For more information please contact Dr Boksun Kim or Prof Long-yuan Li.

 
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Fire safety engineering design of cold-formed steel members

The cold-formed steel members are widely used as the intermediate members between the main structural frame and the corrugated roof or wall sheeting in buildings. The popularity of such members has dramatically increased in recent years due to the advantages such as consistency and accuracy of profile, ease of fabrication, high strength-to-weight ratios, lightness, and flexibility, which can result in more cost-effective designs and more sustainable construction. The drawback of the cold-formed steel members is the weak buckling resistance due to thin thickness. This drawback becomes even more serious when they are at elevated temperatures. This project is to investigate the thermal buckling behaviour of cold-formed steel members subjected to various different loading conditions when they are in a fire scenario. The aim of the project is to develop calculation methods which can be used to predict the fire resistance of cold-formed steel members. For more information please contact Dr Boksun Kim or Prof Long-yuan Li.

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Development of sustainable electrochemical corrosion protection systems for reinforced concrete structures (DOSECOPS)

This is an EU-funded project under FP7-PEOPLE-2011-IRSES. The aim of this project is to develop new electrochemical treatment methods for both new and old reinforced concrete structures to minimize both repairing and monitoring costs, and improve the structures’ long-term safety. The project involves research groups from five universities, three in Europe (University of Plymouth, Politecnico di Milano, and Chalmers University of Technology) and two in China (Zhejiang University and Shenzhen University). The research work covers: 

  • To investigate the ingress behaviour of nano-particles in concrete materials and the mechanism that they react with cement paste to produce a cementing reaction in the pore space to form a new composite.
  • To develop performance-based novel, smart cathodic prevention systems for new marine RC structures using modern solar power equipment, in which the power source of the cathodic prevention system will be supplied by solar power and the system will be operated and controlled by the system itself.
  • To develop finite element analysis models to simulate the transport of various ionic and molecular species in the pore electrolyte of concrete under the influence of electrochemical treatment with internal reinforcing steel cathodes of varied configurations and external surface anodes placed in electrolytes containing positively charged nano-particles, and apply the model to simulate the electrochemical chloride removal, electrochemical realkalisation, and electrochemical nano-particle injection processes for carbonated concrete, chloride contaminated concrete and the cathodic prevention systems to examine the effectiveness of the processes.

For more information please contact Prof Long-yuan Li.

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Framework for modeling poro-electro-chemo-mechanical interactions in biological tissues

It is well known that the interaction between the mechanical deformation of solid and the flow of fluid in a fluid-saturated porous material can be described by poromechanics. If the fluid is an ionic solution then the flow will lead to the change of chemical potential as well as the transport of ionic species, which in turn will change the mechanical properties of the porous material. Understanding of the interaction between mechanical and ionic transport through porous media is essential for a number of engineering applications including saline transport and subsequent shrinkage/swelling of soils, chloride induced reinforcing steel corrosion in concrete, and transport of nutrients through cell membranes in biomaterials. Currently, this kind of multi-phase problems has been described by using macroscopic Poisson-Nernst-Plank equations. The phenomenological equations, however, require a number of pre-known material parameters, which, although not impossible, are very difficult to determine. This is particularly so in biological materials, as the material constants are largely dependent on the local organization of the material itself, not only in terms of the mechanical properties, but also in terms of their electro, chemical and biological properties. The aim of this research is to develop a theoretical framework for describing the interactions between solid, fluid, and chemical phases in charged porous materials by using different scale models with an application to the human cornea – a tissue that has some structural similarity with the old clay-straw composite but has much sophisticated chemical and biological features. For more information please contact Prof Long-yuan Li.

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Impact Case Study Example: An Innovative Friction Welding Platform for Creep Damage Assessment and Repair of Thermal Power Plant Components

 
 
This case study deals with research undertaken by Prof Neil James at Plymouth University leading to the development of an innovative friction stir welding process (friction hydro-taper pillar processing, FHPP) and a bespoke welding platform that improves the assessment and repair methodology for creep damaged thermal power station components. This technology, developed in collaboration with Nelson Mandela Metropolitan University and with industry investment, enables power station engineers to extend the life of power generating plant leading to multi-million pound cost savings (over £66M in direct financial savings are demonstrated in this case) plus significant safety and societal impacts. It has been patented in South Africa and a spin-off company has been formed.
  
 
Friction stir welding (FSW) is a solid-state welding process with major advantages in cost and performance, compared with fusion welding. However, process parameters are chosen empirically and no direct measurement of weld process parameters such as force and torque was available on commercial welding platforms until recently. It has therefore been difficult to assess optimum welding parameters in terms of residual stresses, defects and fatigue performance or to make a priori process-property-performance predictions. In order to introduce a new repair technique into the power station industry a detailed research understanding of the process-property-performance relationships is required, and individual weld repair techniques require full certification.
 
Process-property-performance relationships in FSW have been one of the core research programmes of Prof. James at Plymouth University over the last 14 years, involving a substantial collaboration with Hattingh at Nelson Mandela Metropolitan University, who led the FSW platform development, and with industrial collaboration and funding from the Dutch steel and aluminium-making firm Hoogovens, subsequently continued through Corus R&D, Rotherham when Hoogovens was acquired by Corus.  Latterly, partners from ESKOM, the South African power utility, have been actively involved in the FSW work and in associated neutron diffraction residual stress experiments led by Prof James.
 
This research, in which Plymouth has made a major contribution to the understanding of defects, process optimisation and residual stress (PhD projects with Lombard and Bradley), has enabled moving away from an empirical approach to choice of FS welding parameters (James et al 2003). The research has resulted in a thorough understanding of the influences of tool speed, feed rate and geometry on residual stresses, microstructure and defects, and hence on mechanical and fatigue properties, which was developed in collaboration with Hattingh over the period 2003-2012 [2-5]. One significant outcome from this work, from a jointly supervised PhD (Blignault), was a unique technique for assessing optimum process parameters via a graphical FSW interface, the force footprint diagram [2] and the development of an instrumented FSW platform measuring forces, torque and temperature. Extensive research into the primary influential parameters on weld output properties as a function of tool geometry (2005-2008 – Blignault, Lombard) further demonstrated that maximum force on a tool during its rotation (the force footprint apogee) and its angular rotation during welding captured aspects of the plastic deformation in the stir zone which were fundamental to achieving a high performance, defect-free weld (Blignault). This research showed that fatigue performance and defect population in FS welds could be correlated with frictional power and heat input into the welds (Lombard). This allowed a priori prediction of optimised regimes of tool feed and rotational speed in FS welding (Bradley, Lombard).
 
The complementary range of expertise contributed by the three partners in this project was fundamental to taking research into platform development for industry. James has driven the fundamental research insights, Hattingh the platform design and development, and Newby/Doubell have provided a direct link into the South African power utility, ESKOM, in the highly important areas of stress analysis and welding (Doubell – Chief Welding Engineer, ESKOM, Newby – Stress Consultant). Support was provided by ESKOM to manufacture the prototype FTPP platform and to make the internal business case for qualifying the machines for power station use.
 
 
The following publications have all appeared in high quality journals and have been through a peer review process.
 
[1] M N James, D G Hattingh and G R Bradley (2003), Weld tool travel speed effects on fatigue life of friction stir welds in 5083 aluminium, International Journal of Fatigue, 25 pp.1389-1398.  51 citations in Scopus at 31/7/13; journal impact factor in 2012 1.976.
[2] D G Hattingh, T I van Niekerk, C Blignault, G Kruger and M N James (2004), Analysis of the FSW force footprint and its relationship with process parameters to optimise weld performance and tool design, Invited Paper (INVITED-2004-01), IIW Journal Welding in the World, 48 No. 1-2 pp.50-58.  Journal of the International Institute of Welding; non-members papers by invitation only.
[3] H Lombard, D G Hattingh, A Steuwer and M N James (2008), Optimising FSW process parameters to minimise defects and maximise fatigue life in 5083-H321 aluminium alloy, Engineering Fracture Mechanics 75 pp.341-354.  38 citations in Scopus at 31/7/13; journal impact factor in 2012 1.413.
[4] D G Hattingh, D L H Bulbring, A Els-Botes, M N James (2011), Process Parameter Influence on Performance of Friction Taper Stud Welds in AISI 4140 Steel, Materials and Design 32 pp.3421-3430.  4 citations in Scopus at 31/7/13; journal impact factor in 2012 2.913.
[5] C Blignault, D G Hattingh and M N James (2011), Optimising friction stir welding via statistical design of tool geometry and process parameters, Journal of Materials Engineering and Performance 21 No. 6 pp.927-935.  5 citations at 31/7/13; journal impact factor in 2012 0.915.
[6] H Lombard, D G Hattingh, A Steuwer, M N James (2009), Effect of process parameters on the residual stresses in AA5083-H321 friction stir welds, Materials Science and Engineering A, 501 pp.119-124.  23 citations in Scopus at 31/7/13; journal impact factor in 2012 2.108.
 
 
This case study describes the impact of Prof James’ fundamental research into welding and residual stresses which enabled development of fundamental insights into FSW, resultantly to development of the automated Friction Hydro Pillar Processing (FHPP) by James’ long-standing collaborator, Hattingh (in conjunction with James), the technology’s development and patenting as WeldCore and early industrial application in collaboration with the South African Power Untility, ESKOM. A spin-off company has been formed to further develop the technology and apply across the globe. Savings of more than £66M, in addition to significant process and societal impacts have already been achieved. 
 
This technology has been piloted in providing power station engineers with evidence that secures confidence in life extension of the current power generating plant. It has impacted on business performance by allowing the postponement of major capital expenditure and a multi-million pound cost saving. The underlying research provides the necessary direct link between FHPP welding conditions, the service performance and residual stresses; this enables welding to be performed on safety-critical power plant components using an automated platform. Automated FHPP has been termed Weldcore and provides structural information that was previously unobtainable, which resultantly leads to longer service life of critical structures due to improved monitoring; deferment of capital expenditure; lower risk of catastrophic failure; and increased plant uptime, hence an increased widespread operational profits [5.1].
 
Weldcore allows cost-effective assessment and repair of creep exhaustion in steam power plant components that would otherwise be difficult or impossible to repair and to certify for continued safe operation. The technology and the impact thereof has only been possible because of a long-standing collaboration between Hattingh and James, instantiated by sabbaticals, shorter professional visits, collaborative research projects and joint publishing, allowing James’ fundamental insights to be applied. WeldCore was developed at NMMU and was awarded first prize in the South African National Innovation Competition in August 2010. The process was also awarded the prize for “research leading to innovation by a group” at the South African National Science and Technology Forum awards in May 2011.
 
The underpinning research carried out by Prof James on welding and residual stresses facilitated focussed development of fundamental insights into FSW and led to a number of collaborative strain scanning experiments with James as PI. Accurate knowledge of weld-induced residual stress distributions and their modification by heat treatment is fundamental to the all-important certification of new welding processes in the power generation industry.  Prof James has taken a leading role applying neutron and synchrotron diffraction techniques to steam power plant via peer reviewed experiments [5.2]. Making the weld certification case for incorporation of the FHPP into power plant repair would not have been possible without the detailed knowledge of residual stress fields afforded by neutron and synchrotron diffraction experiments [5.3, 5.4]. Equally, the process has to be controlled to deliver specific and reliable outcomes in terms of microstructure, defects and residual stresses, which would not have been possible without the type of in-depth knowledge and understanding of process-property-performance linkages provided by the research.
 
One example concerns blade attachment holes in the steam turbine rotor discs of Hendrina Power Station in South Africa where original equipment manufacturers (OEM) life calculations led to a replacement recommendation. Turbine component design is complex and historically the industry follows OEM replacement recommendations without testing true life exhaustion of components with complicated geometries. 
 
Testing the WeldCore FHPP platform for creep assessment and repair on Unit 6 at Hendrina Power Station in 2011, showed that the creep life of the high pressure turbine was less than 50% exhausted. This was in contrast to the OEM recommendations to urgently replace the turbines on all ten units after their calculations indicated creep exhaustion levels of >>100% at the unit life (270–300,000h of operation). Unit 6 was returned to service without further outage delay or operation with a reduced output. To meet the OEM’s recommendations the alternative would be to remove two stages of blades and run with reduced output until a replacement turbine could be manufactured (2 years) and then enter into a long replacement outage again (an additional 80 days [5.3]). The work on unit 6 demonstrated that the scheduled replacement of the turbines for all ten Units at Hendrina Power Station (with a cost of over £6.5M per unit) could therefore be delayed until the decommissioning date of the Power Station. This condition monitoring and life extension of the discs saved the power utility some £65M in direct replacement costs and an extended outage period [5.5]. Aside from the significant cost savings made, the avoidance of any outage is particularly pertinent for ESKOM as whilst “the international norm for spinning reserve is 15% … Eskom currently has on average 3% ... Any loss of generating capacity increases the risk of load shedding (blackouts)” [5.3].  The extended outage period avoided has been estimated as at least 8 weeks [5.3]. The work by Plymouth on the performance-processing-weld parameters in FHPP was fundamental to the certification and to the parametric design of the welding platform. 
 
To maximise the impact of the research, Prof James has also provided ESKOM with CPD short courses on failure analysis. During the most recent course in 2011, 33 mechanical and materials engineers from ESKOM’s Research, Testing and Development department attended. Prof James delivered this training at below market rates (R40000 paid rather than estimated market value of R132000 [5.3, 5.4]) as part of the technology transfer process. ESKOM clearly regard the training as important stating: “failure analysis knowledge is critical for engineers operating in our environment”  [5.3].
 
Since initial use on the turbine blades WeldCore has also been applied to two main steam pipework applications at Lethoabo Power Station and a main steam valve inlet pipe at Kendal Power Station. At Lethoabo, application of the technique (in 2012) proved that the components had to be replaced (total cost = R318m (~£19.8m)) in order to prevent a major safety incident of these safety critical systems. ESKOM views safe operation as extremely important [5.3] and in early 2013 at Kendal, WeldCore proved that the serviceable life of the steam valve inlet pipe could be extended, thus saving ESKOM a further R16m (~£1m) in parts/down time/etc. costs. [5.3]
 
Now that initial technology transfer and development work has been completed a spin-off company (MantaCor (Pty) Ltd) was registered on 28 March 2011 and has been assigned the rights to conduct commercial activities to develop and market the machines on a commercial scale. As a result of the early commercial work two further commercial projects with a combined value of  £100k have been completed outside the scope of that taken on for ESKOM and resultantly, a systems engineer, a process engineer and two technicians are employed in South Africa [5.4. 5.6, 5.7].
 
 
References:
 
[5.1] Eskom internal intelligence brief “INTELLIGENCE BRIEF – QUANTIFICATION OF CREEP EXHAUSTION IN TURBINE ROTORS” RTD/MAT/13/172.
 
[5.2] Experiments: 1-01-8, 1-01-58, 1-01-73, 1-02-83, RB720574, RB910338 (2008-2011)
 
[5.3] Stress Consultant, ESKOM, Research Testing and Development, Lower Germiston Road Private Bag 40175, Cleveland, 2022 SA.
 
[5.4] Director of MantaCor/Professor of Mechanical Engineering/Director of eNtsa, Nelson Mandela Metropolitan University, Summerstrand Campus (North), P.O. Box 77000, Port Elizabeth, 6031, Tel: 041 504 3608, Fax: 041 504 9123
 
[5.5] Technology Strategy and Research Manager (acting), ESKOM, Sustanability Division, Research Testing and Development, Lower Germiston Road Private Bag 40175, Cleveland, 2022 SA.
 
[5.6] Director: Innovation and Technology Transfer, Summerstrand Campus South, NMMU.
 
[5.7] Director: Research Management, Summerstrand Campus South, NMMU.
 
 

Using model updating techniques to predict the behaviour of reinforced concrete beams strengthened with carbon fibre reinforced polymer

Carbon Fibre Reinforced Polymer (CFRP) composites are now widely used in strengthening and repairing concrete structures damaged by natural or human caused disasters. The highly complex nature of the Reinforced Concrete (RC) structural elements strengthened with CFRP has made it difficult to find reliable analytical models that closely represent the true behaviour of these structural elements. This research has demonstrated that using model updating techniques it was possible to successfully address some of the modelling issues with RC beams strengthened with CFRP. In this research the IVCGA has been used as a model updating tool to tune some of the parameters of the analytical model to find solutions that are a good match with their laboratory experimental results. Good agreement with experimental results was achieved using this method.

 
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For more information please contact Dr Yaqub Rafiq.

 

Response of reinforced concrete beams externally strengthened with CFRP under impact loading

Many reinforced concrete (RC) structures including buildings and bridges have suffered damage as a result of earthquakes, tornados, war, terrorist attacks, explosion, falling debris, ocean waves, and other unforeseen circumstances. Evidence has shown that some of the structural elements, such as beams, columns and girders, although damaged but not totally failed, can be strengthened using Carbon Fibber Reinforced Polymer (CFRP) sheets or strips, to make them suitable and safe for use. This is a PhD student project, which is to investigate the behaviour of RC beams externally strengthened by the CFRP. Literature search revealed external bonding technique (EBR-CFRP) and near surface mounting (NSM-CFRP) are two common strengthening techniques. However EBR-CFRP technique has shown to suffer from concrete-CFRP interface debonding failure while NSM-CFRP technique has been proven to overcome the debonding problem.

 

In this research a series of RC beams with and without CFRP have been tested using single point impact loading to induce different degrees of damage to simulate real life situations. These damaged beams were then strengthened with CFRP using both EBR-CFRP and NSM-CFRP techniques. A drop weight impact test machine was manufactured in the Heavy Structure Laboratory of Plymouth University for testing these beams. Details of this impact machine can be seen in Figure 1. Different instruments such as accelerometers, force sensors, high speed camera and deflection measurement devices have been used for data collection. Figure 2 shows a sample of the initial test results comparing non-strengthened RC-beams with RC-beams strengthened using both EBR-CFRP and NSM-CFRP techniques. It was discovered that the NSM-CFRP strengthening technique was much more effective than the EBR-CFRP technique.

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Figure 1. Impact machine and RC-Beam testing arrangements.
 
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Figure 2. Comparison of the different strengthening techniques
 

The next stage of the research would be to use the CFRP strips to repair beams with different percentages of damages induced by the impact loading. Finite element analysis and evolutionary computation techniques will be used to simulate the experimentally obtained information for a reliably practical use.

 

For more information please contact Dr Yaqub Rafiq or Dr John Summerscales.