Evaluation on fracture toughness of low activation ferritic steel (JLF-1)

Low activation ferritic steels have been developed for a candidate of structural materials in nuclear fusion reactor applications. This paper dealt with the effects of rolling direction and side groove on the fracture toughness of low activation ferritic steel, JLF-1 Steel (9Cr-2W-V-Ta). The fracture toughness of JLF-1 steel showed the anisotropy in the rolling direction. In the case of J-R curve, JQ and slope of the longitudinal JLF-1 steel (BML) for the rolling direction were a slight larger than those of transverse JLF-1 Steel (BMT) at room temperature. In order to obtain the valid fracture toughness of JLF-1 Steel through the UC method, the CT specimen with a side groove of 40% was recommended. As one of low activation ferritic steels for the IEA test program, the JLF-1 (9Cr-2W-V-Ta) steel has been developed and investigated by the active cooperation of Japanese Universities (1,3,4). The low activation ferritic steels mounted in a high magnetic field such as fusion reactor components may be affected by the operating cycle and the mechanical vibration that is associated with the magnetic damping. For the sound design of fusion reactor, it is necessary to investigate the effect of magnetic damping on the mechanical properties of JLF-1 steel. Prior to its widespread, practical use, however, several fracture properties of the JLF-1 steel including tensile property, fatigue property and fracture toughness must be evaluated, preferentially. Especially, the investigation of fracture toughness and J-R curve is essential for practical applications of the JLF-1 steel into fusion areas, since it suffered from the elastic-plastic fracture behavior caused by the history of operation temperature. When the JLF-1 steel is used as the blanket structural material of fusion reactors, the experimental data for tensile properties and fracture toughness are very important to estimate safety life span of the blanket by critical crack length with the irradiation. The tensile properties and fracture toughness characteristics of unirradiated materials are required before the estimation of irradiated materials (2,5). Finally, the correlativity between magnetic field and fracture properties of JLF-1 steel should be established for its active application. Due to the limited amount of materials, the unloading compliance method with single-specimen method (ASTM E813-89), which was the alternative of multi-specimen method, was recommended for the examination of fracture toughness (6,9).