Cryogenic Fracture Toughness Determination of a Structural Alloy Weldment by Notch Tensile Measurement and Finite Element Analysis

This paper investigates the use of circumferentially notched bar specimens for the measurement of elastic-plastic fracture toughness (J 1C ) of structural alloys and weldments for superconducting magnets in fusion energy systems. Notch tensile tests were performed with small cylindrical notched bar specimens at liquid helium temperature (4 K) using crosshead rates of 0.2 mm/min and 20 mm/s. Test specimens were 5 mm in diameter with a 30-mm reduced section. In the mid-section of the specimen a 1 mm deep electro discharge machined (EDM) notch of 0.2 mm width was machined around the girth, thus maintaining a starting diameter of 3 mm. Correlations between notch tensile strength (σ NTS ), failure energy absorption (E NT ), and J 1C were assessed. The best data fit was found between the ratio σ NTS /J 1C and J 1C . A finite element analysis was also performed to compute directly the J-values. Comparisons of the predicted J 1C with results obtained from conventional JIS Z 2284 standard tests were made. Results of structural alloy weldment with a representatively wide range of J 1C indicate that fracture toughness prediction accuracy is roughly equal to ±25 percent.