CRACK INITIATION AND GROWTH IN STAINLESS STEEL TUBES UNDER THERMAL SHOCKS

Abstract

The purpose of this experiment was to study crack initiation and propagation in the walls of stainless steel tubular test sections under thermal shock loading conditions in sodium. A special rig was built wherein a hot (up to 540°C) tube was cyclically submerged in cold sodium thus inducing stress-strain distributions in the tube wall susceptible of generating superficial cracks and propagating them. Extensive thermometrical measurements were carried out so that heat-exchange conditions could be evaluated and temperature-time history computed. As far as crack initiation is concerned, a test was run with an AISI 316 tube whose wall during one typical cycle was brought to 540°C uniform temperature and subsequently sustained a thermal shock in sodium at 200°C. Restabilization of the wall temperature at 540°C ended the cycle. The alternating plastic strains existing in the outer fibres of the tube during these thermal shocks have been calculated. The values found were confronted with results gathered from the literature concerning the endurance of the particular stainless steel in low cycle fatigue. The fact that no cracks initiated after 77000 cycles, is discussed in the light of the experimental data, mathematical analysis and l.c.f. predictions. A series of tests were run with axial cracks machined in the outer wall. With a thermal shock from 540°C to 200°C crack growth initiated after roughly 5000 cycles and the crack then grew through the (6 mm thick) ligament in about 5000 cycles. Before complete penetration, a crack coplanar with the machined crack started to grow from the inside. From observations of striation spacings on a scanning electron microscope, the crack growth rate was found to be relatively constant. These results were confronted with FEM calculations. Extensive examinations of the test tubes material have been carried out in order to study the damage due to thermal shocks.