In several modern key industrial components (high speed train wheels, heart valves ...) cyclic loading occurs at very low stress levels which induce very long fatigue lives (NR larger than 109 cycles - the gigacycle regime). In this regime, component failure is predominantly caused by the initiation and the propagation of internal cracks. Experimental data on the growth of such cracks is still scarce at the moment, mainly due to experimental difficulties [HON][SER][YOS].
In this work, we present a method to produce fatigue samples with controlled internal defects which are used to induce internal crack initiation [JUN]. Diffusion bonding of thin metal sheets containing defects created by femto laser machining is used to produce sharp notches in the bulk of Ti-6Al-4V samples whose geometry and size enable to perform synchrotron tomography in situ fatigue tests. In situ experiments show that internal cracking systematically occurs at the notch. The propagation of those internal cracks is characterised in 3D. After approximately 50% of the fatigue life (for the stress level investigated) the cracks acquire a very regular quasi circular shape, which is typically observed for internal cracks. The da/dN - DK curves obtained are discussed with respect to crack growth data of the same material for tests performed in various environments. Different diffusion bonding environment conditions and thermal treatment are also tested and confirm the crucial role of the environment on internal crack growth.
This manufacturing method can be applied to other metals (cast iron, aluminium, etc.) but this presentation will focus on internal crack initiation in Ti-6Al-4V.