A Short introduction to the Mechanical and Physical Behaviour of Materials under Dynamic Loading
Introductory Lectures, DYMAT2018 International Conference
Sunday, 9 September 2018
14:30 pm, Amphitheatre, Palais des Congrès, Archachon, France
This short course is opened to DYMAT members, and consists in a series of three keynote lectures of 1 hour each.
Stephen Walley, Doctor, Research Associate (Retired), Fracture and Shock Physics Group, Department of Physics, University of Cambridge, United Kingdom
Systematic, quantitative, static mechanical testing of materials started in the middle of the 19th century, largely due to the increasing use of iron and steel in critical structures such as bridges.
The frequent explosions of steam boilers and the realisation that there was no understanding of the effect on steel rails and bridges due to the passage over them of trains also led to the desire to develop high rate tests. However, due to the lack of instrumentation with sufficient time resolution, it was a long time before such tests were developed.
Just before the First World War, Bertram Hopkinson developed a technique that for the first time allowed the analysis of mechanical impulse into force and time. This allowed him to explain qualitatively why steel plates spalled when subjected to explosion or impact. His bar method also led to improvements in fuse trains in explosive shells. But it was not until the Second World War that his invention was used by G.I. Taylor, Enrico Volterra and R.M. Davies to measure the dynamic strengths of soft materials such as explosives and plastics. After the war had ended, Kolsky developed a detonator-loaded version that could be used to measure the dynamic stress-strain curves of metals.
During World War 2, G.I. Taylor also analysed the rod-on-anvil technique (developed in France at the beginning of the 20th century) to enable the dynamic compressive strengths of the hard steels used in tank armour to be estimated.
The use of the split Hopkinson (or Kolsky) pressure bar to obtain the dynamic properties of materials began to ‘take off’ in the 1970s with a few tens of papers a year (now there are around 400 per annum). The technique is used for a wide range of materials, both brittle and ductile.
The Taylor impact test is no longer used for its original purpose (Kolsky bars give much more accurate data). But it has experienced a new lease of life in recent years as a severe test of the predictive abilities of constitutive models and numerical methods.
Stephen M. Walley read Natural Sciences at Corpus Christi College, University of Cambridge 1974-1977, specialising in Physics in his final year. He graduated BA(Hons) in 1977. He then attended Bristol University for one year to do an MSc in the Physics of Materials. He started PhD research in 1978 at the Cavendish Laboratory on impact erosion damage to polyethylene. He graduated PhD from the University of Cambridge in the summer of 1983. Since then he has been involved in a number of projects at the Cavendish Laboratory including ballistic impact on glass/polymer laminates, ignition mechanisms of propellants and polymer-bonded explosives, and high strain rate mechanical properties of polymers, metals and energetic materials. He is minutes secretary of the Governing Board of the DYMAT Association. He is author or co-author of about 100 published journal and conference papers. In retirement, he is writing papers and book chapters of a more historical nature as well as writing up for publication studies performed in recent years by former members of the Fracture and Shock Physics Group.
Patricia Verleysen, Professor, MST-DyMaLab Research Group, EEMMECS Department, Ghent University, Belgium
This lecture starts with an outline of the basic principles of dynamic testing of ductile materials. To characterise the dynamic behaviour of ductile materials, split Hopkinson bar (SHB) test setups are commonly used. The Hopkinson test technique allows reaching strain rates ranging from up to . This short course addresses the merits and limits of SHB setups to test ductile materials. Key topics include:
Application full field deformation measurement techniques: By their very nature, Hopkinson tests have a very short duration (~ms) and use fairly small specimens (~mm). The use of high speed cameras and full field deformation measurement techniques such as digital image correlation can provide a vault of very valuable information to complement the traditional measurements.
Prof. Patricia Verleysen is head of DyMaLab, the Ghent University (Belgium) laboratory for Dynamic Materials Research. DyMaLab focuses on research into the mechanical behaviour of materials under high strain rate dynamic loading conditions and the microstructural phenomena lying at the origin of the observed behaviour.
Research at DyMaLab has a strong experimental component. Test techniques are developed to identify the strain rate dependent constitutive response, including damage and fracture. To this purpose DyMaLab disposes of a wide variety of experimental test facilities, all equipped with state-of-the-art measurement devices. Based on the experimental observations, models are developed to describe the strain rate and temperature dependent mechanical material response.
In addition to high speed material test devices, facilities have been designed and built which allow simulating high speed forming processes and dynamic severe plastic deformation at labscale.
Pascal Forquin, full-professor, Laboratory of Soils, Solids, Structures and Risks, Grenoble Alpes University, France
Brittle materials such as ceramics, rocks, glass and concrete, are widely used in many civil and military applications involving dynamic loading, impulse loading, shock or impact: “Explosive compaction of powders”, “blasting of rocks”, “seismological studies”, “ballistic impact against ceramic armour or transparent windshield”, “plastic explosive against concrete structures”… In most of these applications, the brittle material is subjected to intense loading characterized by high or very high strain-rates (hundreds to several tens of thousands 1/s), high pressure (hundreds to thousands of MPa) leading to extreme and specific damage modes such as multiple fragmentation, dynamic cracking, pore collapse, shearing and mode II fracturing or microplasticity mechanisms etc.
Additionally, brittle materials present complex and fascinating features that justify greater efforts to develop research works. Indeed, they are characterized by random failure stresses under static tension or unconfined compression loadings. At low strain-rates they are sensitive to size effects, the larger the sample the lower its mean strength. In addition, the tensile strength of brittle materials, which increases with the strain-rate and the behaviour of brittle materials at high strain-rates, becomes deterministic. Furthermore, brittle materials are strongly sensitive to confining pressure, their behaviour being more and more ductile with the increase of pressure level.
This course aims at describing the principle and data processing of popular experimental techniques (Hopkinson pressure bar testing, planar impacts) used to investigate the behaviour of brittle materials at high-loading rates as well as their limitations and drawbacks. A particular attention is devoted to the relationship between microstructural parameters and the dynamic response of brittle materials.
Pascal Forquin is head of the Risk and Vulnerability Division in the Laboratory of Soils, Solids, Structures and Risks at the Grenoble Alpes University, Grenoble, France. He has extensive experience with dynamic testing of brittle materials (ceramics, rocks, concrete), notably the edge-on impact, the spalling and quasi-oedometric compression techniques on Hopkinson bars as well as with the modelling of fragmentation processes in brittle solids under impact loading. Prof. Forquin is elected member of the DYMAT Governing Board (European association) and is Associate Technical Editor of JDBM (Journal of Dynamic Behaviour of Materials). He organized the 2016-LWAG workshop (Light-Weight Armour for Defence & Security) and the 22nd DYMAT Technical Meeting dedicated to “Experimental testing and modelling of brittle materials at high strain-rates”, both held in Grenoble in October 2016.