High Strain Rate Characterisation of Composite Materials

The high strain rate characterisation of FRP materials present the experimenter with a new set of challenges in obtaining valid experimental data. These challenges were addressed in this work with basis in classic wave theory. The stress equilibrium process for linear elastic materials, as fibre-reinforced polymers, were considered, and it was first shown that the loading history controls equilibrium process. Then the High-speed servo-hydraulic test machine was analysed in terms its ability to create a state of constant strain rate in the specimen. The invertible inertial forces in the load train prevented a linear elastic specimen to reach a state of constant strain rate before fracture. This was in contrast to ductile materials, which are widely tested with for the High-speed servohydraulic test machine. The development of the analysis and the interpretation of the results, were based on the experience from designing and constructing a high-speed servo-hydraulic test machine and by performing a comprehensive test series. The difficulties encountered in the test work could be addressed with the developed analysis. The conclusion was that the High-speed servo-hydraulic test machine is less suited for testing fibre-reinforced polymers due to their elastic behaviour and low strain to failure. This is problematic as the High-speed servo-hydraulic test machine closes the gap between quasi-static tests rates and lower strain rates, which are achievable with the Split Hopkinson Pressure Bar.
The Split Hopkinson Pressure Bar was addressed in terms of a new wave mechanics model for a linear elastic specimen the Split Hopkinson Pressure Bar. The model was formulated without any assumption of stress equilibrium, constant strain rate, or equal bars and thus provided a useful tool to analyse the equilibrium process. The model showed that whichever stress equilibrium of constant strain rate happen first, depended on the combination of impedance mismatch between the specimen and the bars. The model was compared to test series, and the model correctly indicated when a test set-up was problematic in terms of reaching stress equilibrium and constant strain rate. As shown in literature the incident wave should be linear rising pulse to facilitate stress equilibrium and constant strain rate. The common pulse shaping technique with copper disc’s between the Striker bar and Incident bar were addresses and was concluded the method could create the required Incident waves. However, there was an upper limit in the generated stress rates due to frictional problems and this limited the maximum achievable strain rates. The maximum strain rate was also found to be independent of the specimen gage length, which only controlled the time to maximum strain rate.
The Split Hopkinson Pressure Bar proved able to reach a state of stress equilibrium and constant strain rate, but the key to valid data was found in the control of the Incident wave.