Volume 8, Issue 6 (June 2011)
Creep-Fatigue at High Temperature of Notched Single Crystal Superalloys
The effect of hold time on the high temperature notched fatigue strength of the CMSX-4 single crystal superalloy has been experimentally investigated. Notched specimens with orientation 〈001〉 have been tested at 950°C in load control in the low-cycle fatigue (LCF) life regime both without and with dwell. For the same applied stress range (net-section stress), the effect of hold time results in an appreciable reduction of fatigue life. Several scanning electron microscope (SEM) observations of the failed specimens have been made in order to associate the actual failure (crack initiation) location to the correct crystallographic orientation and to the critical location in the notch as given by suitable multiaxial fatigue parameters. Moreover, SEM observations reveal the change of the failure mode from pure fatigue to fatigue/creep conditions. In order to model the material behavior in the notch, stress-strain analysis of the notched specimens was performed by an elastic-plastic finite-element method (FEM) procedure; the parameters describing the material behavior of the single crystal superalloy have been identified by comparison with experimentally determined stress-strain fatigue behavior and creep data of smooth specimens. A suitable model for the assessment of the reduction of fatigue life in creep-fatigue conditions is proposed.