Dr S Hallett
Reader in Composite Structures
Dr Hallett obtained a BSc in mechanical engineering from the University of Cape Town in 1993. After this he was awarded a Rhodes Scholarship to complete his D.Phil at the University of Oxford where he studied impact damage in composite materials on a Rolls Royce supported project.
Before joining the University of Bristol as a research associate in 2000 he worked at Ove Arup and Partners. There he gained experience of vehicle crash worthiness and safety as well as finite element analysis techniques. He was appointed as a lecturer in Aerospace Structures in 2002, then promoted to Senior Lecturer in 2007 and Reader in Composite Structures in 2011. He is currently Technical Director for the Rolls-Royce University Technology Centre (UTC) for composites based in the Advanced Composites Centre for Innovation and Science (ACCIS) at the University of Bristol.
Research
Dr Hallett is a member of the Aerospace Composites research group (ACCIS). His principal research interests are listed below.
Notched Damage in Composite Materials
Damage development and ultimate failure loads are being studied using four different lay-ups, each of which is scaled in the in-plane dimensions. Finite element analysis is being used to model the damage development prior to ultimate failure. This has been done using an interface element whose failure is based on critical fracture energies. Good correlation in terms of damage extent and location has been obtained.
Impact Damage Mechanisms
One major disadvantage of composite materials is that they are very susceptible to impact damage both at low and high velocity. The damage which occurs is complex and is made up of a combination of surface indentation, fibre breakage, matrix cracking, fibre pull-out and de lamination.
3D Reinforced Composites
3D woven composites offer potential benefits in terms of cost saving due to the ability to produce near-net-shape pre-forms. There is additional potential benefit in terms of the reinforcement gained from the 3rd through-thickness fibres. Research is focusing on quantifying this benefit and investigating strategies for numerical modelling that take account of the specific architechtures of the different weave styles
Cohesive zone formulations for finite element analysis
Cohesive zone elements are being formulated for use in finite element analysis to model the intra-ply splitting and inter-ply de lamination. Composite failure is a complex and often progressive process and in order to capture overall structural behaviour it is important to capture the details of the damage development process. This approach captures this well and has shown to give extremely good correlation across a range of loading scenarios.
Fatigue of Composites
In many respects composites are considered superior to metals with regard to fatigue behaviour. Often fatigue performance is not considered important in design of aerospace structures since strain allowables are relatively low. In the case of rotating components however large numbers of cycles can be accumulated in a relatively short time. Research is being undertake to understand fatigue damage growth in composites through use of acoustic emission monitoring techniques and new damage accumulation models are being developed to predict initiation and propagation of damage.