An innovative procedure for establishing lifing criteria for turbine disk bores under multiaxial states of stress

An innovative procedure is presented here for determining the minimum low cycle fatigue (LCF) life of turbine disk bores under multiaxial states of stress. The procedure statistically combines multiaxial data from spin pit tests with the data from uniaxial LCF tests. The connecting link between the two data sets is a new probability density function which can be derived from the uniaxial data and subsequently applied to the probability of failure calculated for a set of mini disks. The overall objective was to establish which of the criteria, principal stress-based versus effective stress-based, is valid for life estimation of the turbine disk bores made of DP-718. The minidisks, designed to simulate the same biaxial stress ratio as in an actual turbine disk bore, were tested at 589 K by cycling between 5000 and 52,000 rpm in a spin pit. The testing was interrupted to check for fatigue crack initiation in the bore by means of a proprietary vibration monitor. Only one of the disks showed a valid crack initiation. The initiation life is defined as the number of cycles required to initiate a crack of 381 μm (0.015 in.) depth, and was derived experimentally through subsequent striation counting. The approach established principal stress range with its associated stress ratio as the proper lifing parameter for DP-718 mini disks. Fractographic observations of similarity in the crack initiation modes observed for the alloy support the methodology.