ACCEPTABILITY OF DEFECTS UNDER COMBINED CYCLE FATIGUE FOR A PRECIPITATION HARDENED STEEL

Compressor blades can be subjected to Combined Cycle Fatigue (CCF) conditions considering the superposition of the service cycles characterized by a low cycle fatigue cycle controlled by the centrifugal, pressure and thermal load and the blade vibrations. Simple assessment concepts are available in the literature based on the mean minus three sigma concept for life and strength, but these do not generally deliver acceptance maps for defects, in particular for CCF conditions.The present work is devoted to use an analytical approach for crack growth rate for life and strength in CCF conditions aimed to determine acceptance maps for defects focusing on the differences with simple deterministic assessments. The experimental results have demonstrated that, for the specific precipitation hardening steel studied, the small cracks under CCF are able to propagate at growth rates comparable to the absence of crack closure. By using these results, in this paper the crack growth rate variability was used to determine maps for the acceptability of surface damage on components subjected to CCF.