Fluid Structure Interactions Research Group

1. Fluid Structure Interactions Research Group Synchronised IRT and DIC capture to measure the strain rate dependency of fibre reinforced composites Duncan A. Crump1, Janice M. Dulieu-Barton1 and Steve W. Boyd1 Faculty of Engineering and the Environment, University of Southampton, UK. dac400@soton.ac.uk Motivation Objectives • The military use composite materials in land, sea and air vehicles in high performance applications, to assist with speed and maneuverability. • Collisions or blast loads (Figures 1 and 2) cause high strain rate events to occur, that may cause the composite structure to fail completely or to suffer damage that reduce service life • To reduce the risk of catastrophic failure after a high strain rate event it is important to fully understand the structural performance of polymer composite materials and structures under such loading. • It is known that the behaviour of composite materials is dependent upon the strain rate, and that during the formation of damage a temperature change occurs within the material. • Develop high strain rate testing facilities at University of Southampton. • To develop digital image correlation (DIC) procedures for capturing deformations at high velocity based. • To obtain a full field picture of the temperature evolution during the high strain event using infra-red thermography (IRT). • Develop the use of simultaneous IRT and DIC at high framing rates. • To provide a thermomechanical characterisation of the material performance using experimental data to validate existing measured data and models. • To use full-field techniques to assess the performance and damage tolerance of materials after experiencing high strain rate events. Figure 1: Bird strike on aircraft nose Figure 2: Slamming on RNLI lifeboat Intermediate strain rate – VHS Instron machine High strain rate – Split Hopkinson pressure bar • Tensile coupons up to 20 m/s • Strain rates up to 102 s-1 • Test times between 1 – 15 ms • Load recorded using 80 kN Kistler load cell • Inertia removed using slack adaptor designed in-house • Enlarged protective enclosure allows several optical systems to be used simultaneously • All optical and data acquisition systems triggered by central pulse • Striker velocity up to 30 m/s • Strain rates in the 103 s-1 • Designed around composites • Strain gauge signal captured by Picoscope at 20 MHz. Figure 6: Split Hopkinson pressure bar rig • Five CFRP crossply specimens (10 x 10 mm) tested at ~13.5 m/s • Strain rate of ~ 1000 s-1 achieved • Using the Hopkinson analysis on signals from strain gauges mounted on the incident and transmission bars the stress/strain curves in Figure 7 were obtained. % x- strain Figure 3: VHS Instron test machine • GFRP crossply with 0° and 90° surface plies with wasted section to ensure failure capture • High speed images at 5 kHz over 512 x 256 pixels • IR images at 15 kHz over 64 x 12 pixels 10 0 -10 (a) Figure 4: Example results from crossply GFRP specimen, DIC -green box and IRT - red box Figure 7: CFRP crossply at ~ 1000 s-1, inset is an example of high speed image of speckled surface • Figure 8 demonstrates the ability to trigger high speed camera adequately to obtain DIC aligned temporally with testing pulse. • Further work to improve specimen alignment and hence test consistency • Extend the SHPB to soft materials such as foam with polymeric and hollow transmission bars • Figure 4 shows high speed image of 0° specimen, with DIC and IRT data at 20 ms (14 kN) superimposed. • Figure 5 plots DIC strain and IRT temperature vs stress at the image mid-point. • DIC strain show linear relationship to stress for both specimens. • IRT temperature both indicate an initial drop • Towards failure temperature is higher for 90° specimen (b) Figure 8: Example of incident, reflected and transmission pulses together with timing of DIC data Figure 5: (a) 0° surface, (b) 90° surface ply FSI Away Day 2012 Acknowledgement: This project is supported by funds from the Engineering and Physical Sciences Research Council and Defence Science and Technology Laboratory