In the frame of an irradiation program of American Iron and Steel Institute (AISI) 316H (Unified Numbering System [UNS] S31 400) stainless steel at fast (E > 0.1 MeV) ranging from 0.1 to 2 dpa, the fracture toughness of the steel was measured, using the J parameter, at 0, 0.1, and 0.3 dpa at 350 and 550°C. A dimensional analysis has been used to derive J resistance curves as well as JIc values for the base material and its welds. The specimens used were three-point bend (3PB) specimens, 20 mm high by 15 mm thick by 80 mm effective span. A noteworthy result concerning the base material is the lowering at 550°C and 0.1 dpa of dJ/da by about 50% and of JIc by about 35%. The other dJ/da and JIc values were not significantly altered at 550°C. Apart from this case, generally low degradation of base metal properties is found at both temperatures between the nonirradiated and the irradiated material.
The weld material exhibits significantly lower initiation toughness and tearings moduli in the preirradiation and post-irradiation conditions, but these fracture mechanics (FM) characteristics are practically unaltered at both temperatures and at the above mentioned fluence levels. The heat-affected zone (HAZ) material was also studied. It was found that all J-Δa values were bounded on the upper side by the base material at 350°C and on the lower side by the welded material at 550°C for corresponding fluence levels.
Following tests carried out on compact tension [C(T)] specimens in AISI 316H steel, the engineering approach (EA) of Shih et al. and the key curve method (KC) of Ernst et al. were used to calculate, from the obtained load-displacement records, the crack extensions that took place. A method to define unambiguously the strain hardening exponent entering the EA estimation procedure is outlined. This gave satisfactory predictions of the load-displacement levels encountered in the tests. As a consequence, a calibration function, required in the KC analysis, could be elaborated. Both methods yielded final crack extensions that agree within 15% with the measured ones. The estimated values thus remain within the generally accepted maximum allowable errors.
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