Fracture Analysis of the NESC-1 Spinning Cylinder Experiment

Abstract

This paper presents finite-element analyses of the cylinder specimen being used in the international Network for Evaluating Steel Components (NESC) large-scale spinning-cylinder project (NESC-1). The objective of the NESC-1 project is to focus on a complete process for assessing the structural integrity of aged reactor pressure vessels. A new cylinder specimen was reconstituted from segments of the previously tested SC-4 and SC-6 specimens because the relatively high fracture toughness of the original specimen might preclude achieving the test objectives. The wall thickness is greater for the reconstituted specimen when compared with the previous specimen geometry (175 versus 150 mm). Also, the initial and coolant temperatures for the proposed thermal shock may be reduced as much as 25°C to increase the probability of achieving cleavage initiation. Analyses were carried out to determine the combined effects of increasing the wall thickness and lowering the initial and coolant temperatures in the experiment. Estimates were made of the change in hoop strain on the clad inner surface directly above a subclad crack due to initiation and axial propagation of the crack. Three-dimensional finite-element models of the cladded cylinder were generated with 6:1 and 2:1 semi-elliptical 70-mm-deep subclad cracks. The cylinder specimen was subjected to thermal-shock and centrifugal loading conditions and analyzed with a thermo-elastic-plastic material model. The analytical results indicate that lowering the initial and coolant temperatures by 25°C will not significantly change the peak driving force, but will shift the stress-intensity factor (KI ) versus temperature curves so that the crack will become critical at an earlier time in the transient. The peak KI value occurs at a lower temperature (after the crack becomes critical), which increases the probability of achieving cleavage initiation. Also, the calculated hoop strains for the two crack aspect ratios (simulation of 2:1 crack propagating axially) provide an estimated change in hoop strain in the range of 3 to 4 percent on the clad inner surface.