Response of split Hopkinson bar apparatus signal to end-surface damage, numerical and experimental studies

A Split Hopkinson bar apparatus is a widely used method to obtain material properties at high strain rates. These properties are essential in the development of new materials as well as their associated constitutive models. During routine tests, the surfaces of the bars at the specimen/bars interface were damaged. To check if the damage influenced the signal response, control tests were done using the well characterized Al 6061-T6. Results showed that artefacts were added to the signal. This paper presents the experimental and numerical approaches developed to understand the effects of surface damage. The approach used consists of introducing series of known gaps between input and output bar to simulate a variation of surface damage. The numerical simulations, performed using a hydrocode, were done to confirm that signal response could not be associated with other several types of error in the system

Using a shock tube to predict the response of polymeric foam to a blast loading

Understanding the behaviour of polymeric foam materials under blast wave loading is of great importance for the design of efficient personnel protective equipment against explosive devices. Material response to a shock wave generated from a shock tube is quite different from the real blast wave response. However, shock tube experiments are more practicable and efficient than free-field blast trials when it comes to the characterization of high-rate material behaviour. The objective of this study is to find a correlation in the responses of polymeric foams to these two different wave shapes. Shock tube experiments as well as free-field blast trials have been conducted on three polymeric foams of varying thickness and density. Using shock impulse, a correlation between the two responses was found. Regimes of overpressure amplification and attenuation caused by the foam material were clearly identified in the case of a blast loading

Development of a multi-section striker for split Hopkinson bar experiments and numerical simulations

High strain rate testing is essential in the development of new materials under high dynamic loading conditions. Often, these new materials are very expensive and of limited quantities. For high dynamic loading using the split Hopkinson apparatus, the idea of using a striker that produces multiple strain rates in one test was investigated. The concept consists of using a multi-section striker with two different diameters to produce two strain rates using a single test sample. To test the concept, a first striker having two equal-length sections of different diameters was used and the results looked promising. A second striker made of several equal-length sections of two different diameters was tried. Results obtained confirm the usefulness of the approach, but preliminary results needs to be analyzed carefully to optimize the striker design. Numerical simulation using the hydrodynamic finite element code LS-DYNA was used to model the split Hopkinson bar and striker and contribute in the understanding of results

An assessment of blast modelling techniques for injury biomechanics research

Blast-induced Traumatic Brain Injury (TBI) has been affecting combatants and civilians. The blast pressure wave is thought to have a significant contribution to blast related TBI. Due to the limitations and difficulties of conducting blast tests on surrogates, computational modelling has been used as a key method for exploring this field. However, the blast wave modelling methods reported in current literature have drawbacks. They either cannot generate the desirable blast pressure wave history, or they are unable to accurately simulate the blast wave/structure interaction. In addition, boundary conditions, which can have significant effects on model predictions, have not been described adequately. Here, we critically assess the commonly used methods for simulating blast wave propagation in air (open-field blast) and its interaction with the human body. We investigate the predicted blast wave time history, blast wave transmission and the effects of various boundary conditions in 3 dimensional (3D) models of blast prediction. We propose a suitable meshing topology, which enables accurate prediction of blast wave propagation and interaction with the human head and significantly decreases the computational cost in 3D simulations. Finally, we predict strain and strain rate in the human brain during blast wave exposure and show the influence of the blast wave modelling methods on the brain response. The findings presented here can serve as guidelines for accurately modelling blast wave generation and interaction with the human body for injury biomechanics studies and design of prevention systems