Crack propagation under combined cycle fatigue for a precipitation hardened steel

Compressor blades are generally subjected to Combined Cycle Fatigue (CCF) conditions, where the service cycles are characterized by a low cycle fatigue (LCF) cycle controlled by the centrifugal, pressure and thermal loads superimposed to high frequency loadings at positive load ratios R > 0.5 as generated, for example, by the blade vibrations. The recent increasing adoption of renewable energy sources leads to the request to higher level of flexibility for the energy production supplied by gas turbines. The required number of cycles under LCF conditions is increasing leading to lower allowable design stresses. It is thus foundamental to reduce the convervatism adopted with the classical design philosophies. According to a damage tolerance approach, in this activity we have analysed the propagation of small cracks under CCF conditions on a precipitation hardened martensitic steel. The experimental results have mainly shown that the small cracks under CCF are able to propagate at growth rates comparable to the the absence of crack closure. This result, together with the Kitagawa diagram, allowed to determine less conservative criteria for the acceptability of surface damages on components subjected to CCF.