3D characterization of internal fatigue cracks initiation and propagation in a Ti alloy during gigacycle fatigue

When submitted to a very large number of mechanical cycles (~ 1e9 cycles) at very low load levels, structural components generally fail because of the apparition of internal fatigue cracks. Experimental data on their growth rate is scarce but X-ray tomography appears to be the adequate technique to detect their initiation and to follow their propagation in metals. For that reason, a new in situ ultrasonic fatigue machine has been developed to detect and monitor the evolution of internal cracks in a cast-aluminum thanks to X-ray microtomography and infrared thermography. However, because of the material large grain size, the observed fatigue cracks possessed complex 3D shapes which were extremely challenging to model. Consequently, because this type of experiment remains the only one that can provide reliable experimental data on internal fatigue crack propagation, this proposal aims at using the same setup on Ti alloy specimens with a larger grain size to obtain smoother crack fronts.