Innovative photomechanical approaches in identification of the dynamic mechanical behaviour of materials

In many areas of engineering, materials suffer deformation at high rates. This is the case when structures undergo impact, crash, blast, etc. but also in material forming like stamping or machining for instance. Therefore, it is essential for design engineers to have reliable mechanical models to predict the behaviour of the materials in such applications. This is enhanced by the spectacular progress in numerical simulation which now enables to perform detailed computations of very complex situations. However, robust experimental identification of refined high strain rate deformation models is lagging behind and hinders the delivery of the full potential of numerical simulations for the benefit of society: safer infrastructures (buildings, bridges, dams), safer means of transportation (crashworthiness of vehicles) etc.
Indeed, in order to perform experimental identification of high strain rate material models, engineers only have a very limited toolbox based on test procedures developed decades ago. The best example is the so-called Split Hopkinson Pressure Bar (SHPB) which has proved extremely useful but has important intrinsic limitations due to the stringent assumptions required to process the test data. These assumptions are the consequence of the very limited instrumentation for which such tests were developed, usually a few strain gauge readings for the standard SHPB set-up. The recent advent of full-field deformation measurements using imaging techniques has allowed novel approaches to be developed and exciting new testing procedures to be imagined for the first time.
The objective of the present project is to lay the foundations of a new era in dynamic testing of materials based on the availability of digital imaging technology to provide full-field deformation measurements at very high speeds. One can then use this information in conjunction with efficient numerical inverse identification tools such as the Virtual Fields Method to design novel test procedures to identify material parameters at high rates. The underpinning novelty is to exploit the inertial effects developed in high strain rate load. These have hitherto been regarded as undesirable in conventional testing. However, in the identification process they can play the role of a volume distributed load cell for which readings are embedded in the full-field deformation measurements. The idea is ground breaking as it has the potential to lift the current major limitations of high strain rate test, i.e. small specimen and constant velocity. The present proposal aims at providing a platform for the applicant to develop this methodology for many different types of situations in terms of materials, loading configuration and strain rate range. The project has the potential to revolutionize high strain rate testing of materials and hence enhance our knowledge of material behaviour. This will in turn benefit many sectors of engineering and society in the long term.

From a societal point of view, the understanding of the dynamic behaviour of materials is a crucial issue in many areas of our lives. Some of the most important are briefly evoked in the following, but there are many others.
* The first one is clearly related to general public safety and mainly concerns infrastructures and means of transportation. Critical infrastructures such as power plants, large buildings, dams, bridges etc. must be designed to withstand natural (earthquakes, storms...), accidental (gas explosion, plane crash...) or voluntary (terrorist attack...) hazards. This requires detailed knowledge of the behaviour of concrete, steel and glass among others. As for transportation, it is clear that the progress of car design for crashworthiness within the last twenty years has saved many lives on the roads but there is still significant need for better data for materials such as composites or new alloys. Also, as the crashworthiness optimization progresses, more and more detailed representation of the behaviour of materials is required to reach further improvements. For instance, the dynamic behaviour of welds is still very much an open problem, as well as the development of more representative human body models to understand and prevent injuries in road accidents.
* The second one concerns manufacturing processes. In order to improve the efficiency of many manufacturing processes, and therefore reduce cost and improve quality, the high strain rate behaviour of both tooling and formed materials is required. Important areas are machining, cutting, stamping. This is currently seen as a strategic area for the future of the UK economy.
* Finally, many military applications involve high strain rate behaviour of materials. The design of effective armour, the behaviour of infrastructures and vehicles submitted to blast, the understanding of the behaviour of explosives etc. all require detailed knowledge of high strain rate behaviour of materials.

The contents of this Fellowship application underpins most of the engineering situations where materials deform at high speed. There are two main categories of end users. The users at the end of the chain are numerical simulation engineers and researchers requiring adequate models and parameters to feed into their simulations. There is an intermediate category of beneficiaries constituted by test engineers performing high strain rate tests within large companies or government defence labs. The usual impact route to establish new mechanical characterization procedures is through academia. These new methodologies need to gain credibility through the scientific community at large before diffusing to industry and then possibly making it to standards, as these are industry driven. The upstream and 'game-changing' nature of this proposal (TRL 1) clearly precludes immediate diffusion through industry. The only exception concerns the defence sector where more long-term research is being developed.

As a consequence, the impact activities for this Fellowship will concentrate both on academia and defence-related institutions. This is reflected by the project partners covering these two areas. While the project unfolds, it is expected that a few key players in the aerospace industry like Rolls-Royce and Airbus will join the project through an informal ad-hoc industrial advisory group.