The study started with a phenomenological analysis that was conducted by simulating a blade-off event in LS-DYNA. We were interested in evaluating how the blade could be loaded during this event. The most loaded areas of the blade were analyzed in terms of strain, strain rate and loading type (called triaxility). This first step helped designing the experiments.
These experiments were conducted on Split Hopkinson bars. They have highlighted its ductile behavior of TiAl in compression and brittle behavior in tension.Tests were also carried out to investigate the effect of loading and temperature on the behavior up to failure.
A material model in LS-DYNA was then chosen as the best candidate to reproduce the behavior observed experimentally. It has the ability to consider different behavior in tension and compression. A failure model was calibrated to reproduce the failure strain on different loading. A validation has finally been done by simulating the experiment and confirming that this material model could fit the experiments.
The next step was to design impact experiments to reveal the main failure mechanisms of TiAl. A specific impact configuration was designed to shot spherical projectile on heated flat TiAl specimen. The projectile was launched with the help of a gas gun at velocities up to 300 m/s. The target was heated at temperatures up to 800°C with an induction heating machine. High speed camera was set to capture the failure at the back face of the target. Different temperatures, impact velocities and projectile diameters were considered. These tests gave additional information on the failure of TiAl.
Numerical simulations were then done. It showed that failure was mainly driven by a shear state in the impact area. This observation helped adjusting the failure model in ranges that were not investigated before. It therefore confirms that a failure model that depends on triaxiality was a good choice. Most relevant impact tests were simulated and the failure crack morphology changes with the impact velocity that was experimentally observed is quite well captured. This step has also highlighted some limitation of the model: it is not possible to introduce a temperature dependency, then making mandatory to simulate the target with a homogeneous temperature.
The following experimental step aimed at performing impact validation tests. It was decided to design three impact experiments : fragments on leading edges (LE) and trailing edges (TE), fragments on real blades and blade on blade. The TiAl targets were all heated at 800°C. The first two configuration showed that LE could withstand impact velocities of Ø3 mm steel spheres at around 130 m/s while TE was much weaker due to its low thickness. Nickel blades did not show any failure on LE at 210 m/s but failed on the TE at 200 m/s with Ø5 mm spheres. For last configuration, TiAl blade was launched with a controlled orientation at 120 m/s. Impact conditions were estimated by the simulation of a blade-off in LS-DYNA. No TiAl blade has withstand this kind of impact contrary to Ni blades.
All these test cases were simulated and results are very conclusive. Comparable projectile residual velocities were obtained. The model is also able to reproduce the major cracks encountered in all cases. On heated target, some secondary cracks are not reproduced leading to an underestimation of the number of generated fragments. At its current version, the model can be used to investigate the blade behaviours under additional not experimentally-tested configurations and help engineers to improve the design.
