Close-up tracing of fatigue precrack evolution and reliable fracture toughness evaluation by the precracked specimens in an ITER specification W plate

•A method for inserting a well defined fatigue precrack in W is established.•Continuous in-situ observations on crack formation and extension during fatigue precracking are performed.•Reliable fracture toughness values of ITER specification W (ALMT-grade) are assessed 4. The RT fracture toughness values are ~10MPa√m for the T-L and 12~13MPa√m for the L-T orientation.•Orientational variation in RT fracture toughness is comparably small in the ALTM-grade plate.