PTP projects

TWI and Cambridge University have an association going back 50 years. DTI funding has brought about further collaboration. A Postgraduate Training Partnership (PTP) scheme funded 20 PhD students co-supervised by TWI and Cambridge University. Many of the projects created advanced modelling techniques that were adopted by TWI to better serve its member companies.

Case studies:

• Application of low stress no distortion (LSND) techniques to the TIG welding of aerospace structures
• Design of composite adhesive joints
• Micro-mechanical modelling of cleavage failure in structural steel
• The industrial application of ultrasonic wave propogation models to the NDT of surface engineering processes

Application of low stress no distortion (LSND) techniques to the TIG welding of aerospace structures

Mechanical fasteners are currently the primary method of joining aerospace structures. However, the need to reduce weight and cost is driving the development of welded aerospace structures. Many of the most advanced aerospace alloys are considered 'unweldable' by conventional techniques due to their susceptibility to distortion and solidification cracking during welding. Robin Preston's research concentrated on developing finite element models for the 'unweldable' aerospace alloy 2024-T3. The thermally induced residual stresses in the region adjacent to the weld can cause excessive distortion of thin structures. LSND techniques have been suggested as a means of reducing residual stress and hence distortion. The development of LSND from a concept into a commercially viable fabrication technique requires accurate weld models in order to optimise the parameters. As Fig.1 shows, the out-of-plane distortion is considerably reduced when a LSND welding technique is adopted. Finite element results for predicted residual stresses have been validated using neutron diffraction measurements.

Fig.1. Demonstration of reduction in distortion when LSND techniques are employed

Design of composite adhesive joints

Stefanie Feih used finite element analysis to optimise the performance of adhesively bonded joints between composite materials (fibre reinforced plastics). An example is shown in Fig.2. Prediction of initial damage and failure modes of adhesive composite joints cannot yet easily be achieved. Her research to date has concentrated on developing a programme to predict progressive damage for every increment of a non-linear analysis. Common composite failure modes included are fibre breakage, fibre buckling, matrix cracking in tension or compression and the onset of delamination.

Fig.2. Finite element for the design of composite joints

Micro-mechanical modelling of cleavage failure in structural steel

In continuing TWI's work on Local Approach methods, Sylvain Bordet investigated the application of the method to fracture of a modern high strength structural steel (BS Grade 450EMZ). His project involved using FE modelling, low temperature CTOD tests, microstructural and fractographic analyses.

The industrial application of ultrasonic wave propagation models to the NDT of surface engineering processes.

Rachel McCarthy carried out research to extend the capabilities of computer methods developed by TWI for ultrasonic surface wave interaction with various geometries. Computer intensive models are combined with more efficient analytical methods to produce faster analyses, thus resulting in a hybrid method of modelling UT waves.

To find out more about how TWI could benefit your company, please contact us at nmo@twi.co.uk