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• 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. Fracture toughness of the divertor armor in ITER needs to be elucidated for prediction of the response of armor materials to intense heat loading. The present study has been performed to establish a method for inserting a well, defined fatigue precrack into the test specimens and measure the reliable fracture toughness values of tungsten (W) materials that exhibit low temperature brittleness. Sub-sized, single edge notched bend (SENB) specimens with two orientations, L -T and T-L, taken from a commercially available pure W plate manufactured in accordance with the ITER material specifications (ITER specification W, ALMT-grade), were precracked in two-step fatigue loading: fully uniaxial compression fatigue to provoke a crack from the notch tip, and subsequent 3-point bend (3 PB) fatigue to extend the crack length for the minimization of influence of the residual tensile stresses generated by compression fatigue. During compression and 3 PB fatigue loading the fatigue crack was closely monitored by continuous in-situ observations on the notch tip. Continuous tracing of the crack tip position has enabled capture of the fatigue cracking behavior as a kinematic response in a rolled W material, disclosing a marked difference in crack inducement and extension between the two orientations. 3 PB fracture testing of the precracked specimens at room temperature (RT) yielded ~10 MPa√m for the T-L and 12~13 MPa√m for the L -T orientation as fracture toughness: anisotropic variation in fracture toughness of the W plate is comparatively small. Fracture surface observations reveal that a crack path is entirely transgranular for the L -T, while crack propagation involves intergranular fracture to a large extent for the T-L orientation. These results obtained are discussed and compared with the literature data. [ABSTRACT FROM AUTHOR]

Fatigue pre-cracking performance and fracture toughness in polycrystalline tungsten (W) and molybdenum (Mo) have been investigated in relation to grain boundary (GB) configuration with respect to the crack advance direction. Sub-sized, single edge notched bend (SENB) specimens with three different orientations, R-L (ASTM notation) for a forged Mo rod and L-S and T-S for a rolled W plate, were pre-cracked in two steps: fully uniaxial compression fatigue loading to provoke crack initiation and its stable growth from the notch root, and subsequent 3-point bend (3PB) fatigue loading to extend the crack. The latter step intends to minimize the influence of the residual tensile stresses generated during compression fatigue by moving the crack tip away from the plastic zone. It is shown that fatigue pre-cracking performance, especially pre-crack extension behavior, is significantly affected by the specimen orientation. The R-L orientation, giving the easiest cracking path, permitted crack extension completely beyond the plastic zone, while the L-S and T-S orientations with the thickness cracking direction of the rolled plate sustained the crack lengths around or possibly within the plastic zone size due to difficulty in crack advance through an aligned grain structure. Room temperature fracture toughness tests revealed that the 3PB fatigued specimens exhibited appreciably higher fracture toughness by about 30% for R-L, 40% for L-S and 60% for T-S than the specimens of each orientation pre-cracked by compression fatigue only. This indicates that 3PB fatigue provides the crack tip front out of the residual tensile stress zone by crack extension or leads to reduction in the residual stresses at the crack tip front. Strong dependence of fracture toughness on GB configuration was evident. The obtained fracture toughness values are compared with those in the literature and its strong GB configuration dependence is discussed in connection with the appearance of pop-in. [ABSTRACT FROM AUTHOR]