Predictability of Long-Term Creep and Rupture in a Nozzle-to-Sphere Vessel Model

The results of a long-term (approximately 2-1/2 yr), high-temperature failure test of a pressurized nozzle-to-spherical-shell model made of a well-characterized heat of type 304 stainless steel are presented and compared with inelastic deformation and failure predictions. The model, which was tested at 593°C (1100°F), was instrumented with capacitive strain gages in key locations. In addition to recording strains, the surface of the model in the junction region was periodically examined throughout the test for the formation and progression of the creep cracking that ultimately led to a leakage failure. The inelastic analysis predictions were based on guidelines developed and used for design of elevated-temperature nuclear components. Similarly, the failure model used was that upon which the ASME elevated-temperature Code Case N-47 for nuclear components is based. Thus, the reasonable agreement observed between experimental results and predictions adds confidence in the overall design methodology for elevated-temperature components. The results also demonstrated the leak-before-break nature of failure and the fact that pressure-induced stresses do not necessarily relax with time.