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

Lead Research Organisation: University of Southampton
Department Name: Faculty of Engineering & the Environment


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.

Planned Impact

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.
Description We have developed three new test methods to identify the response of materials to impacts. All methods rely on ultra-high-speed imaging at 5 million frames per second. The first method, called Image-Based Inertial Impact (IBII) uses a gas gun to propel a projectile to hit a piece of material. The response of the material is recorded thanks to the camera and material mathematical models can be infered from this. The second method is called Image-Based Ultrasonic Shaking (IBUS). It uses ultrasonic vibrations to deform a piece of material instead of the gas gun. The data processing is similar. Finally, the Image-Based Inertial Release (IBIR) test relies on a release waves caused by the fracture of a mechanical fuse during quasi-static loading of a test specimen. It has the advantage of simultaneous identification of the quasi-static and high strain rate responses of a material on the same specimen, as well as using a standard tensile test machine.
Exploitation Route All three techniques have the potential to become the future gold-standards to test materials at high rates of deformation, first in academia then in industry. They represent the future alterative to the current gold standard, the Kolsky or Split Hopkinson Bar. There is already fast-growing interest in Defense and Industry as the techniques provide data sometime simply not currently unavailable.
Sectors Aerospace, Defence and Marine,Construction,Energy,Manufacturing, including Industrial Biotechology,Transport

Description The IBII test was used to generate data for an industrial project in the automotive industry through a consultancy contract.
First Year Of Impact 2019
Sector Transport
Impact Types Economic

Description Development of image-based high strain rate tests for adhesively bonded joints
Amount $217,000 (USD)
Funding ID FA8655-20-1-7014 
Organisation US Air Force European Office of Air Force Research and Development 
Sector Public
Country United Kingdom
Start 08/2020 
End 03/2022
Description Leverhulme Early Career Fellowship for Dr Frances Davis
Amount £78,000 (GBP)
Funding ID ECF-2016-456 
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 03/2017 
End 02/2020
Description Leverhulme Early Career Fellowship for Dr Lloyd Fletcher
Amount £78,000 (GBP)
Funding ID ECF-2018-212 
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 08/2018 
End 08/2021
Description Novel imaging procedures for the identification of the high strain rate behaviour of soft materials
Amount $229,962 (USD)
Funding ID FA9550-15-1-0293 
Organisation European Office of Aerospace Research & Development (EOARD) 
Sector Public
Country United Kingdom
Start 08/2015 
End 06/2017
Description Novel photomechanics approaches to determine the intra- and inter-laminar properties of composite materials at high rates of strain
Amount $124,896 (USD)
Funding ID FA9550-17-1-0133 
Organisation European Office of Aerospace Research & Development (EOARD) 
Sector Public
Country United Kingdom
Start 01/2017 
End 12/2018
Description Surgery enabled by ultrasonics
Amount £6,114,693 (GBP)
Funding ID EP/R045291/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 11/2018 
End 10/2023
Title Image-Based Inertial Impact (IBII) test 
Description This is a new spalling-type method to test materials at high strain rate, wat we think has the potential to become a successor to the current gold standard, the Kolsky (or Split Hopinson) Bar apparatus. The idea is to impact a thin rectangular test specimen in a free-free condition using a gas-gun launched impactor. The compressive waves travels along the specimen and reflects off the free surface as a tensile wave, allowing for the material's tensile response to be evaluated. A set of black dots is printed onto the specimen previously covered by a layer of white paint, producing a grid-pattern. The transient response of the specimen is captured by recording images of the deforming specimen (and grid) with an ultra-high speed camera (5 MHz). Grey level images are converted into displacement maps using spatial phase shifting and strain and acceleration are computed. Acceleration provides stress from equilibrium while strain provides deformation information, allowing for constitutive parameters to be identified (stiffness, strength, non-linear behaviour). This method provides data of unprecedented detail and quality. 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? Yes  
Impact This very new method has already attracted significant interest from industry and research/defense labs (see collaborations section). We are hoping to attract further funding on the back of this technique in the coming year. We have published a series of journal papers illustrating the power of the method. In an effort to transfer the technique to the scientific community, we have published an extnsive manual containing all information required to set up the test from scratch (gas gun design including blue prints, test specimens preparation procedure, experimental protocol, software for processing images to obtain material parameters). 
Title Image-Based Inertial Release (IBIR) test 
Description This is a new test method consisting in loading a specimen in tension with a stardard test machine, and using a notched fused to create a stress release when fracture occurs. By recording full-field deformation during the quasi-static loading and the high strain rate stress release wave, simultaneous quasi-static and high rate response can be recorded on the same specimen. The validation was performed on PMMA but it is thought that this protocol would be extremely useful on biological tissues as they have intrinsic variability. Testing on Human bone samples is currently being envisaged. 
Type Of Material Improvements to research infrastructure 
Year Produced 2020 
Provided To Others? Yes  
Impact No impact yet as this is very recent. 
Title Image-Based Ultrasonic Shaking (IBUS) test 
Description The method uses an off-the-shelf 20 kHz ultrasonic horn to excite a thin rectangular test coupon onto its first longitudinal resonance frequency. One end is bonded onto the tip of the horn while the other is free (inertial loading) This establishes a standing wave with maximum strain in the middle and zero strains at the edges. A set of black dots are printed onto the specimen using a flatbed printer, the specimen having been previously covered with white paint to maximise contrast. An ultra-high speed camera (5 MHz) is used to record pictures of the deforming specimen (and grid) during the test. Spatial phase-shifting is employed to extract full-field time-resolved displacements, from which strain and acceleration are derived. Acceleration is used in conjunction with dynamic equilibrium to provide stress, while strains describe the deformation. Both are used to identify constitutive behaviour (stiffness, damping, even strength for low strength materials like glass or 90° UD polymer matrix composites. Because of the high frequency vibration, small amounts of material damping cause significant adiabatic heating in the specimen centre while little at the edges. Heterogeneous states of stress, strain , strain rate and temperature allows for exploring the material response over a wide range of conditions with a single test. It is similar to a DMTA test, except that it contains much richer information. The test has also the potential to e used to investigate ageing and fatigue. This will be explored in the future. 
Type Of Material Improvements to research infrastructure 
Year Produced 2018 
Provided To Others? Yes  
Impact This has attracted the attention of researchers in Oxford and an exciting campaign at ESRF has taken place in September 2019 to couple this test with X-ray diffraction for the first in-situ assessment of microstructural changes during high rate deformation of materials. Copper, tin, zirconium as well as PMMA have been investigated. This test is also at the source of my involment in a recently awarded EPSRC programme grant (EP/R045291/1) where it sill be used to study the deformation of biological cells and tissues cased by ultrasonic excitation. 
Description ARL Aberdeen carbide 
Organisation US Army Research Lab
Country United States 
Sector Public 
PI Contribution Testing of boron and silicon carbide specimens in high strain rate tension using the new IBII test developed under the grant.
Collaborator Contribution Providing boron and silicon carbide specimens for test.
Impact A presentation of the first test results has been accepted at the 2019 SEM conference in Reno, NV, June 2019. A journal publication is being put together following presentation at the SEM conference.
Start Year 2017
Description Colorado School of Mines - Dr Leslie Lamberson 
Organisation Colorado School of Mines
Country United States 
Sector Academic/University 
PI Contribution This is a long-term collaboration with a young academic in the US, Dr Leslie LAmberson. Dr Lamberson is specialist of dynamic fracture and high strain rate testing of materials. She has developed a keen interest in the Virtual Fields Method and inertial tests as developed in the grant. This is part of the impact of the Fellowship to convince the community that the new paradigm that the project champions is useful. We have been involved in two main specific topics: * Dynamic fracture mechanics: use full-field strain and acceleration to directly obtain the energy associated with crack growth. A journal paper is in preparation. * Measuring the tensile strength of tungsten carbide with spalling tests: this has led to conference and journal papers. Contribution of Southampton: provide the seminal ideas, design and run inertial impact tests, exploit data with the Virtual Fields Method
Collaborator Contribution Contribution of Drexel (then Colorado School of Mines): * Two PhD students working on dynamic fracture: simulations and experiments * One jointly supervised PhD student (Dr Lloyd Fletcher) to apply the IBII test to viscoelastic identification * Provision of tunsgten carbide (tested) and basalt (to be tested) specimens
Impact Six conference papers Pagano S., Lamberson L., Pierron F., Image-based dynamic fracture analysis, Annual SEM Conference (Society for Experimental Mechanics), 6-9 June in Orlando, FL, USA, 2016. Fletcher L., Lamberson L., Pierron F., Inertial impact method for the tensile strength of tungsten carbide at high strain rates, Annual SEM Conference (Society for Experimental Mechanics), 12-15 June in Indianapolis, IN, USA, 2017. Pagano S., Fletcher L., Pierron F., Lamberson L., Image Based Dynamic Fracture Analysis, Annual SEM Conference (Society for Experimental Mechanics), 12-15 June in Indianapolis, IN, USA, 2017. Fletcher L., Pagano S., Pierron F., Lamberson L., A novel photomechanical approach for measuring dynamic fracture toughness, BSSM annual conference, 29-31 August 2017, Sheffield, UK Fletcher L., Lamberson L., Pierron F., An image-based approach for measuring dynamic fracture toughness, Annual SEM Conference (Society for Experimental Mechanics), 4-7 June in Greenville, SC, USA, 2018. Cadiot X., Fletcher L., Pierron F., Lamberson L., Identification of Dynamic Damage Evolution in Composites using the Virtual Fields Method, ASC 33rd Annual Technical Conference, 18th US-Japan Conference on Composite Materials, ASTM D30, 24-26 September in Seattle, USA, 2018. One journal paper in preparation. One published: Fletcher L., Pierron F., An image-based inertial impact (IBII) test for tungsten carbide cermets, Journal of Dynamic Behaviour of Materials, vol. 4, n° 4, pp. 481-504, 2018.
Start Year 2015
Description DSTL composites and sapphire 
Organisation Defence Science & Technology Laboratory (DSTL)
Country United Kingdom 
Sector Public 
PI Contribution We will test polymer-matrix composite and sapphire specimens with our new IBII test methodology developed under the Fellowship to demonstrate potential to DSTL.
Collaborator Contribution DSTL will provide test specimens to explore the feasibility of the new IBII test methodology developed under the Fellowship for polymer-matrix composites and sapphire.
Impact No outputs, this collaboration was stalled by lack of engagement from DSTL (samples were never reveived).
Start Year 2017
Description Loughborough University, Dr Pablo Ruiz 
Organisation Loughborough University
Department Wolfson School of Mechanical and Manufacturing Engineering
Country United Kingdom 
Sector Academic/University 
PI Contribution Performed tests with our ultra-high speed camera to record a release stress wave caused by material fracture.
Collaborator Contribution Provided the PMMA samples and came up with the initial problem of imaging 'solitary waves' released from material fracture.
Impact This has provided us with a very interesting and complementary test methods to measure the low and high strain rate properties of materials on the same sample, and a journal paper has been recently accepted: Fletcher L., Pierron F., The image-based inertial release (IBIR) test: a new high strain rate test for stiffness identification, Experimental Mechanics, available online, 2020. The results were also communicated at a conference: Hooper C., Ruiz P.D., Huntley J.M., Khusnutdinova K.R., Pierron F. and Fletcher L.C., Experimental and theoretical study of longitudinal bulk strain waves generated by fracture, 10-12 September in Belfast, UK, 2019.
Start Year 2018
Description Materials Sciences LLC 
Organisation Materials Sciences LLC
Country United States 
Sector Private 
PI Contribution Tested through-thickness properties of AS4-145/MTM45-1 carbon/epoxy composite at high strain rates, in shear and transverse tension.
Collaborator Contribution Provided an 18 mm thick panel of AS4-145/MTM45-1 carbon/epoxy composite
Impact One journal paper was published, two have been submitted.
Start Year 2017
Description NOVA University, Lisbon, Portugal - High strain rate properties of wood 
Organisation New University of Lisbon
Department Foundation for Science and Technology
Country Portugal 
Sector Academic/University 
PI Contribution Testing of wood samples (Pinus Pinaster Ait) in the RT and RL directions with the Image-Based Inertial Impact (IBII) test developed as part of the award.
Collaborator Contribution Sourcing, cutting, conditioning and posting 20 samples. Visiting Southampton for the test campaign, processing the results. Measuring density. Performing quasi-static tests for reference stiffness and strength.
Impact A journal paper is being drafted. A Master student has been recruted in Lisbon to pursue the work and we are applying for PhD funding in Portugal to further the collaboration. An application to internal funds at Nova University was made in January to fund a visit from the student and Nova OI (Dr José Xavier).
Start Year 2018
Description ONERA - PhD thesis P. Bouda 
Organisation National Office for Aerospace Studies and Research
Country France 
Sector Public 
PI Contribution Jointly supervised PhD on inertial impact tests for visco-plastic behaviour identification with the VFM. PhD student, Mr Pascal Bouda, is registered in France and Prof. F. Pierron is an official co-supervisor. PhD started in October 2015, will last 3 years. Main contribution: supervision, writing of papers and hosting the student for a few months during the course of the PhD. Performing Image-Based Inertial Impact tests in titanium samples (full, notched and holed).
Collaborator Contribution Financial support, PhD registration, main supervisor and host.
Impact Two conference papers have been published: Bouda P., Notta-Cuvier D., Langrand B., Markiewicz E., Pierron F., Annual SEM Conference (Society for Experimental Mechanics), 4-7 June in Greenville, SC, USA, 2018. Bouda P., Notta-Cuvier D., Langrand B., Pierron F., Markiewicz E., Optimization of an image-based impact test for viscoplasticity identification, 2nd International Conference on Impact Loading of Structures and Materials (ICILSM 2018), 7-11 May in Xi'an, China, 2018. Two journal papers have been published: Fourest T., Bouda P., Fletcher L., Notta-Cuvier D., Markiewicz E., Pierron F., Langrand B., Image Based Inertial Impact test for characterisation of strain rate dependency of Ti6Al4V titanium alloy, Experimental Mechanics, vol. 60, n° 2, pp. 235-248, 2020. Bouda P., Langrand B., Notta-Cuvier D., Markiewicz E., Pierron F., A computational approach to design new tests for viscoplasticity characterization at high strain-rates, Computational Mechanics, vol. 64, pp. 1639-1654, 2019. The PhD viva took take place on March 11th 2019 in Lille, France.
Start Year 2013
Description Oxford - Clive Siviour 
Organisation University of Oxford
Department Department of Engineering Science
Country United Kingdom 
Sector Academic/University 
PI Contribution 5 days of use of gas gun and ultra-high speed camera to impact aluminium and PMMA samples. Provided Shimadzu HPV-X camera, infrared camera and ultrasonic system to perform Image-Based Ultrasonic Shaking (IBUS) tests at ID19 line at ESRF in Grenoble in September 2018. Coupling high speed strain, temperature and x-ray diffraction for different metal and polymer samples.
Collaborator Contribution Making equipment (gas gun, camera) available, and operating it. Reviewing the results and helping with the preparation of papers. Submitting the ESRF proposal, provided experience in X-ray diffraction.
Impact Two conference papers. Dreuilhe S., Davis F., Siviour C., Pierron F., Inverse identification of the elasto-plastic response of metals at high strain rates. Second iDICs conference (International DIC Society), 7-10 November in Philadelphia, PA, USA, 2016. Pierron F., Fletcher L., Siviour C.R., Zhu H., New opportunities in high strain rate testing of composite materials, 8th International Conference on Composites Testing and Model Identification (CompTest), 5-7 April 2017 in Leuven, Belgium. Invited Keynote Lecture. Two journal papers in preparation.
Start Year 2011
Description Solvay composites 
Organisation SOLVAY SA (Commercial Partner)
Country Belgium 
Sector Private 
PI Contribution Tested several composites specimens from UD thermoplastic and thermoset panels with our new IBII test (developed under the Fellowship).
Collaborator Contribution Provision of UD thermoplastic and thermoset carbon fibre composite specimens for testing. Provision of another composite panel for Image-Based Ultrasonic Shaking (IBUS) testing (test also developed under this grant).
Impact Presentation of the results at a Solvay symposium on damage & fracture mechanics in polymers and composites in Brussels on November 28th 2017.
Start Year 2017
Description PhotoDyn Industrial Engagement Event 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact This workshop was aimed at disseminating the findings of the project to industry. The event attracted participants from Airbus, DSTL, QinetiQ, US AFOSR, US ARL, BAe Systems, Solvay Belgium, among others.
Year(s) Of Engagement Activity 2017
Description SOTSEF (Southampton Science Festival) 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Together with Dr Lloyd Fletcher, PDRA on this award, we organize an event on high-speed imaging. This was composed of an interactive presentation on high speed imaging ('guess how fast one needs to record to see this phenomenon') and a demonstration of our gas gun impact facility.
Year(s) Of Engagement Activity 2019