DEDUCING CRACK HISTORY IN AN AGED BOILER TUBE FROM FRACTURE SURFACE-TOPOGRAPHY

A new fractographic technique, FRActure Surface Topography Analysis (F RASTA), was applied to a crack in a boiler tube of a fossil-fired powe r plant in an attempt to determine when in its 22-year service history the crack initiated and the rate at which it grew. By comparing the t opographies of the conjugate crack surfaces, we estimated that the cra ck nucleated about 68 000 h after onset of service and experienced sev eral periods of accelerated and decelerated growth with rates ranging from 5 x 10(-5) to 9.5 x 10(-5) mm/h. These findings correlated roughl y with chemical cleanings and startup/shutdown cycles. A similar analy sis of a corrosion fatigue specimen tested under controlled conditions in the laboratory provided results consistent with measured values. T hese results suggest that fractography can be used to extract valuable information on cracking kinetics in aged systems and, thereby, lead t o new ways to predict, monitor, and extend component life.