Fatigue crack growth of tungsten in the semi-ductile temperature regime

Tungsten will play an indispensable role for novel fusion reactors due to its exceptional physical properties, however, its low damage tolerance involves risks. Therefore, fracture toughness has been extensively investigated over decades, but the understanding on the resistance against fatigue crack growth remains scarce. In this study, fatigue crack growth and fracture toughness experiments were performed on pure tungsten (ITER-grade) in the semi-ductile temperature regime at 773 K in high vacuum. To account for microstructural anisotropy, different crack plane orientations were investigated, and the impact of shielding effects was examined by varying the load ratio R between 0.1 and 0.7. A central outcome is the unexpected behavior in the threshold regime compared to common metals. Doubling the stress intensity factor barely affected the propagation rate, while in the Paris regime, the rate increased by two orders of magnitude. This behavior is attributed to the changing slip reversibility of dislocations and the rewelding of the crack wake in vacuum. Further findings are the insensitiveness of the crack growth rate against changes of mean stress up to R = 0.5 and the early onset of delamination in the case of fatigue loading, caused by the reversible loading in the cyclic plastic zone. This study constitutes an important step towards elucidating the intrinsic and extrinsic fatigue crack growth mechanisms in tungsten and represents a key milestone in developing safe, damage-tolerant structural tungsten-based components.