E. Yamada

J. Yamamoto

K. Horiguchi

S. Ueda

Y. Shindo

National Institute for Fusion Science (NIFS-Repository)

§34. Cryomechanics of Structural Materials for Large Superconducting Magnets Yamada, E., Shindo, Y. , Ueda, S., Horiguchi, K. (Dept. of Mater. Processing, Tohoku U niv.) Yamamoto, J. i) Purpose Fusion energy reactors with superconducting magnets will require cryogenic structural alloys with improved combination of yield strength and fracture toughness at 4K. For austenitic stainless steels used in structural components of superconducting magnets, elastic-plastic frac-ture toughness ( lrc) evaluations at 4K are usu-ally required for quantification of base metals. In structural alloy tensile specimens undergo-ing mechanical work in a liquid helium bath, internal temperature increases are favored by their low specific heat and thermal conductivity at this temperature. The possible occurrence of such heating must be considered in testing structural materials for liquid helium tempera-ture applications. In this study, we consider the cryogenic frac-ture behavior of an elastic-plastic material and the energy dissipation associated with crack ex-tension. Fracture toughness and fractgraphic data are presented for an austenitic stainless steel plate tested at liquid helium temperature. Temperatures of a specimen were also measuted during straining at 4K. The specimen behav-ior is simulated numerically using the finite el-ement technique. We estimate the associated temperature field. These estimates provide a useful guide for interpreting experimental re-sults. ii) Experiment and analysis In order to evaluate the fracture toughness and the temperature rise, lrc tests were per-formed with 1 T compact tension specimens at liquid helium temperature. These tests were conducted in accordance with ASTM E813-81 and E813-87 for determining Jrc using the un-loading compliance technique to monitor crack growth. Adiabatic heating was detected by measuring the internal temperature at a dis-tance X 0 from the load line at the specimen rnidthichness using thermocouples embedded in the ligament along the fracture plane. Two po-sitions of X 0 , 33(No.1) and 37(No.2)mm, are studied. For comparison, the specimen fracture be-havior was calculated using the Dugdale model and the incremental theory. The dissipated en-ergy can be calculated from the change in to-tal elastic strain energy of the specimen during crack extension. We assume that 100% of the dissipated energy is converted into heat. We an-alyze the transient temperature state near the propagating heat source and include the effects of temperature-dependent material properties. iii) Results For the single specimen lrc tests according to ASTM E813-87, we need the side grooved specimen to obtain the Jrc value. The pre-dicted load-displacement response and J val-ues matched experimental result very well. At time td, the load drop corresponding to the pop-in occurred. The experimental result for the temperature at the position of the thermocou-ple (No.1) is shown in Fig.1 as continuous line. The predicted temperature in crack extension using the incremental theory is also shown as dashed line. 80.-----------------------~ FEM(Incremental theory ) Experiment 2 4 Time t-td (sec) Fig. 1. Temperature rise at the position of the thermocouple (No.1). 39 

Cryomechanics of Structural Materials for Large Superconducting Magnets

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