Core research at TWI in design and engineering

TWI Bulletin, July - August 2006

The Core Research Programme (CRP) at TWI is one of the most valuable benefits of Industrial Membership.

John Wintle is the Programme Manager of the Design and Engineering Core Research Suite. He is Consultant Engineer for Structural Integrity, and is active in promoting new research projects. You can contact him on john.wintle@twi.co.uk

It provides the leading edge technology and innovation that gives Members competitive advantage, both from the direct use of the research themselves and from the enhanced capability that TWI can deliver to them through commercial projects. As John Wintle explains when research is presented at conferences, published in journals and used in codes and standards, the CRP builds on TWI's reputation as the world centre of welding and materials joining technology.

The CRP is a co-ordinated programme of projects selected for their relevance to our Members. The wide range of our Members' interests is reflected in the diversity of projects in the programme, and there is normally something that will interest every Member. The programme is funded principally by Member subscriptions and the current budget is around £1.8m per year.

In addition to funding from Members, TWI also seeks ways of increasing the value of core research from linkages with other projects and funding regimes, and from information and contributions-in-kind from Members. This is called gearing. CRP projects are geared to collaborative projects under the European Framework and DTI Technology Programmes, where joint funding increases the amount of research that can be done. The CRP also benefits from material donations and loans of equipment from Member companies.

Not only does the CRP provide the results of projects that can be of immediate use to Members, it also creates knowledge and information relevant to Members' future needs. At the same time the projects help to develop new capabilities and facilities at TWI that can be used to solve Members' problems. In short, it maintains TWI expertise and laboratories, and gives younger members of staff the opportunity to develop their knowledge and skills and exploit their creativity.

Organisation

The CRP is run as a series of projects planned to take place over a period of three years. The projects cover topics in technical fields across the spectrum of materials joining technology. The programme is constructed to contain both high priority projects, and projects that address more innovative technologies that Members and TWI consider as being of high potential to impact industry in a three to 10 year vision. The projects are therefore a mixture of industry pull and technology pushing the boundaries.

The programme is controlled through a Research Board comprising representatives of the Industrial Membership and independent advisers, and TWI's Research Management Team. The Board approves the projects to be undertaken, monitors progress and ensures the quality of reports is high. For convenience, the Board is divided into four Research Committees dealing with specific suites of technology: design and engineering, joining and fabrication, metals and weldability, and plastics, adhesives, ceramics and electronics. The Members on the Board have an important role to guide and advise TWI on industrial trends and future requirements.

The TWI Director of Research and Technology leads the CRP Management Team. At the working level, there is a Programme Manager for each technology suite who monitors progress and encourages the Project Leaders, helping them where necessary to deliver reports.

Core research is still a major activity at TWI making up approximately 20% of its technical work at any time. There is core research in all Technology Groups, and around half the technical staff are involved at some stage over the three year programme cycle. This is good news for a research and technology organisation that wishes to stay at the leading edge of its field, and for the individuals who want an opportunity to push forward the boundaries of joining technology.

Design and engineering

The remainder of this article features the work being done under the Design and Engineering Suite of projects. The research is concerned with the impact and performance of joints within the context of real engineering structures and components. For this reason the suite mostly contains projects relating to joint properties, degradation, structural integrity assessment methods, non-destructive testing technologies and welded plant inspection.

In 2006 TWI is in the final year of the current three-year programme. Although work started in 2004, planning started in 2003 when ideas for projects were solicited from TWI staff and Industrial Members, and selected by the Research Board. From the ideas submitted, a total of 18 projects were chosen in design and engineering which engineers from the Structural Integrity and NDT Groups would lead.

It is not possible in this article to describe every one of the 18 projects in the current suite in detail, but the following may give an impression of the diversity and technical range. The projects can be grouped into the four main technology areas of fracture, fatigue, modelling and NDT, but the boundaries are not clear-cut. Several projects link technology areas, something that TWI encourages.

Fracture and fatigue projects

The usefulness of fracture toughness data can be greatly increased through fitting the data to a so-called Master Curve that can allow extrapolation to different thickness and temperatures. While this curve has been developed and validated for parent material, its application to welded regions is more uncertain as these are by their nature less homogenous. Special methods of treating weld toughness data are being tested to see if the Master Curve approach remains valid.

Another project is addressing the issues of deriving acceptance criteria for embedded flaws in pipe girth welds. This is of great interest to our Members in the offshore industry where susbsea pipes are unreeled or welded and laid from lay barges. The project aims to produce more realistic assessment methods to allow safer pipe installation.

A further project of interest to the pipeline industry is addressing the problem of long unstable running ductile fractures. TWI is working on developing new criteria by which pipe materials could be selected for their resistance to, and arrest of, ductile fracture. It involves an advanced fracture methodology known as the cohesive zone model.

In yet another project, guidance on the assessment of flaws under high strain rates as may occur under dynamic loading conditions is being developed.

TWI Members gain a lot of value through the networks and collaborative projects with which TWI is involved. An example is the FITNET Network, which is seeking to develop common procedures for fitness-for-service assessment for use in Europe. Information on fracture case studies, stress intensity factors and fatigue assessment procedures that TWI and other members of the network have generated, is being made available to Members in core research reports.

The fatigue life of welded components under variable amplitude loading remains a difficult area to assess. A project in progress is considering modifications to the traditional Miner's Rule approach that will take the previous stress history into account. The work will be calibrated against test data generated in previous CRP projects.

It is now common practice to undertake finite element stress analysis of welded joints to obtain a so-called 'hot spot stress' at the weld toe that can be used with fatigue design curves. TWI has been investigating the different ways in which the hot spot stress can be calculated and has found a good reliable method. Hot spot stresses calculated from this method can be used with fatigue design curves recommended by the International Institute of Welding.

Monitoring of the strain in welded structures is sometimes the only way to determine the hot spot stress when the loading is not known. While strain gauges are well established, there is now new technology based on diffraction gratings within optical fibres that can determine the strain at several locations with just one fibre. TWI is evaluating the capability of this technology to monitor the strain close to welded joints.

As part of a European collaboration, TWI has been studying the fracture behaviour of welded aluminium extrusions in dynamic crush tests. The aim has been to replicate and understand the running fractures seen at train crashes. Thework was able to do this and recommend improved weld design for aluminium structures.

Modelling and NDT projects

In the modelling area, there is work on low-stress no-distortion welding of aluminium. This welding technique is designed for the welding of thin plates where buckling can occur adjacent to the weld due to weld contraction. The susceptibility to buckling can be reduced by means of a heat sink of carbon dioxide trailing behind the welding head. This project aims to provide a validated approach by which the optimum parameters for a successful weld can be determined without the need for extensive welding trials.

Finite element modelling is also being used to focus guided ultrasonic waves in pipe structures for long range ultrasonic testing. This research supports the development of Teletest, TWI's long range UT system to locate flaws remotely.

A different kind of modelling using a code called CIVA is being used to investigate the response of ultrasonic beams to defects in clad pipe. The theoretical predictions are being compared against experimental measurements to validate the modelling. This research will also enable Members to optimise the parameters for the inspection of one-sided welds using focused beams without need for tests.

At a more practical level, TWI has participated in a European project aimed at introducing time-of-flight diffraction for inspection of pressure vessels after fabrication. TWI has helped its partners to develop defect acceptance criteria comparable with conventional NDT standards, and has set training and certification requirements for operators of the technique. TOFD is an automated UT inspection technique that offers the advantages over conventional manual techniques of time savings and the creation of a permanent record of the condition of the material.

Aside from ultrasonic testing the CRP is investigating using digital radiography to detect faults in flexible riser pipes. These have a complex composite construction of steel webbed armour and polyurethane and are used in the offshore oil and gas production industry to carry oil from the seabed to the surface. The production of a digital image of the structure of the pipe has enabled automated defect recognition algorithms to be developed.

Elsewhere in the NDT area, there has been an innovative project to detect damage in storage tanks through inducing low frequency structural vibrations by means of piezo-electric actuators. This is an extension of the rail tapping principle, where the aim is to detect differences in the frequency spectrum between a defect free tank and one where there is structural damage. The project established the feasibility of the approach, although significant work is needed before it can become a viable practical technique.

What next?

When projects are complete and reports written, they are subject to a stringent peer review process within TWI and by the relevant Research Committee, and extra insight and value is often added at this stage. Where work is judged to have particular commercial value, either to Members or to TWI, the Research Committee can place an embargo on publication for a limited period of time, to allow time for patenting or exploitation by Members. All CRP research reports are available for Members to download from the TWI web site. Alternatively, they can be obtained in paper form from the TWI Library.

Ideas for the next CRP are currently being solicited from Industrial Members, the Research Board and TWI staff. As a TWI Member you are welcome to submit a suggestion for a project, either via the Chairman of the relevant Research Committee or directly to TWI. When all the ideas are collected, they will be sorted according to technology areas, duplicates eliminated and similar ideas combined.

Like the seasons, the TWI CRP is enduring and cyclical. It provides valuable resources through which original research and development can be done, in times when such resources in industry are becoming ever more scarce. Core research is important, and TWI is committed to ensuring that the programme continues to deliver real value and quality to Members.