Fatigue crack growth of ITER-grade tungsten in the brittle to ductile transition regime

Due to the excellent properties of tungsten for fusion applications, research in this field is experiencing a renaissance. In particular, the damage tolerance and the endurance of the divertor and monoblocks of Tokamak reactors, which must withstand extremely harsh conditions, are subjects of concern. A first step towards closing the knowledge gap on the fatigue crack growth behaviour of components manufactured from ITER-grade tungsten in the ductile to brittle temperature regime is addressed in this study. Fatigue crack growth tests were performed in a custom-made high vacuum furnace in combination with sensitive electrical crack length measurements. Generally steep crack growth curves (Paris exponent ∼ 7) with an extended Paris-like behaviour down to the 10-12 m/cycle range for temperatures up to 773 K were measured, which is attributed to vacuum effects. The proposed crack growth mechanism in the investigated temperature range is identified as a superposition of quasi-static fracture and fatigue crack growth, leading to characteristic brittle cleavage fracture features and large fluctuations in the fatigue crack growth rate up to 673 K. In addition, load rate effects could shift the ductile to brittle transition during fatigue loading by more than 150 K.