A." "Nishimura

"Nishimura,  A."

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§8. Fracture Toughness of Structural Material for LHD Nishimura, A A cryogenic structural material will support a large electro-magnetic force in LHD system. To evaluate a toughness of the structural material in LHD, a large bend bar of base metal was prepared and a fracture toughness test was carried out in liquid helium under 3-point bending. Results were compared with the data obtained with a conven-tional compact tension (CT) specimen. The tough-ness of LHD structural material will be discussed and a wedge effect model will be presented for addressing the difference of the data of the bend bar and the CT specimen. A test material is SUS 316 used for a support structure in LHD. The bend bar geometry is 87.5 mm thick and 175 mm wide, with a span of 700 1nm. An initial notch was machined in the L-T ori-entation and a fatigue pre-crack was induced at 295 K according to ASTM E 813-89. The bend bar was loaded in 3-point bending using a 10 MN cryogenic mechanical test machine. Ram travel was measured using a cantilever beam extensometer. Ram moved in stroke control at 0.01 mmls, while the outputs of load, stroke and crack opening displacement were continuously recorded. The resistance curve data of the large bend bar at 4 K is shown in Fig.1. The J-i1a plot shows an anomalous "back-up" and measured critical J value is 1528 kJ/m2. Similar to the bend bar, a small back-up region exists at low loads in the CT specimen shown in Fig.2. This data gives the toughness of 592 kJ/m2, and this is much lower 3500 3000 2500 N' 2000 .§ ' c. ' 1000 500 0 0 LBBM-2 at 4.2 K (Side grooves) 2 3 4 Crack extension (mm) Fig.l J-R curve of large bend bar at 4 K. than that obtained with the bend bar. Figure 3 describes a wedge effect model. On a loading process, a small plastic deformation zone forms at the crack tip even at low loads and a crack opening displacement at the notched edge (COD) is increased. During unloading, the plastically de-formed region is compressed and wedged within the surrounding elastic stress field. Consequently, as the applied load is reduced to zero, the perfect unloading is prevented and the change in COD will be limited to an amount less than that obtained during loading process. As the result, a reduced elastic compliance is measured by the COD and re-sults in indications of an apparently shorter crack. Significant differences between CT and bend bar specimens are addressed with this model. The stress distribution beyond the crack tip in the bend bar becomes steeper and it tends to produce a plas-tic zone larger in the vertical direction to the crack growth direction in comparison with the CT speci-men. This vertically enlarged plastic zone will re-duce the unloading compliance more than that in the CT specimen and results in generating a larger region of the back-up. The obvious back-up will push up the critical J value. This work has been performed in collaboration with Hitachi Ltd. 800 C\j 600 ~ -, 6 400 200 0 ~--~~--~--~----~--~ -0.5 0 0.5 1.5 2 Crack extension (mm) Fig.2 J-R curve of CT specimen at 4 K. At minimum load Fig.3 Illustration of wedge effect model. 13 

Fracture Toughness of Structural Material for LHD

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