The University of Southampton
Engineering and the Environment

Research project: Imaging materials deforming really fast

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New ultra-high speed cameras enable to record a great number of images at very high frame rates. What is meant by ultra-high speed is illustrated by Videos 1 and 2 in the media section. (videos forthcoming)

Project Overview

Measuring impact forces is extremely difficult during fast deformation events because of ‘inertial effects’. Indeed, acceleration levels can be extremely high during such tests. Acceleration can be measured relative to g, the acceleration suffered by a falling object on Earth (g=9.8 m/s2). Human beings cannot usually resist more than 3 g. In Video 2 above, the acceleration levels nearly reach 1 million g (!) even though the impact speed is only 30 m/s.
As a consequence of this situation, researchers have developed clever ways to measure impact loads, like the split Hopkinson bar fixture. However, this procedure suffers from strong limitations which have hindered the development of more refined and accurate mechanical models for materials deforming at high rates.

The research that I am developing here aims at designing novel test procedures removing the need for external impact force measurement, replacing it by a load cell based on acceleration measured through the imaging technique (as seen on Video 2). Acceleration multiplied by density is force per unit volume, so we have a volume distributed load cell which data are embedded in the recorded images. The stress-strain curve below (Figure 1) was obtained from the data in Video 2 using the procedure in http://eprints.soton.ac.uk/id/eprint/346985. I believe that this is a world-premiere to obtain such impressive quality data for strain rates above 2000 /s in the elastic response domain.

Click for larger image
Fig. 1

Sponsor: European Office of Aerospace Research and Development (EOARD, Dr Matt Snyder)

 

Video 1: slow (2000 images per second) - Balloons filled in water exploding after performation by a projectile (www.photron.com)

 

 

 Video 2: VERY fast (5 million images per second!) – Deformation (right) and acceleration (left) during an impact test on a composite coupon with a projectile at 30m/s. Ultra-high speed camera, Shimadzu HPV-X.

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