No abstract

The increasing performance requirements of gas turbines are driving higher operating temperatures in critical components, leading to greater creep and fatigue interactions. There is thus a requirement for thermomechanical fatigue (TMF) test data, including TMF crack growth rates. The test equipment required to meet this challenge is briefly discussed, along with the development of the test methodology. Thus, the accurate measurement and maintenance of thermal performance during TMF are considered. The determination of fatigue crack growth rates by conventional electrical potential difference measurements has been employed, utilising convenient dwells within the TMF cycles, where both load and temperature are held constant for a brief period. These experimental methods have been demonstrated with trials on the advanced nickel base superalloy RR1000 used for turbine discs. This work has also highlighted significant microstructural effects on the crack growth rates, with the coarser grain size offering a reduced crack growth rate.

No abstract

As the need for the prediction of component life and maintenance interval schedules becomes more demanding, there is an increasing requirement for thermo-mechanical fatigue (TMF) test data including fatigue crack growth rates under such conditions. The test equipment requirements to meet this challenge are discussed and finally a conventional servo-electric load frame is utilised in combination with a radiant lamp furnace to generate the desired thermal cycles. The radiant lamp furnace enables reasonably consistent temperature gradients to be achieved with notched test pieces.The temperature calibration method to achieve the desired thermal cycle will be briefly described, along with some considerations for ensuring reproducibility in the thermal cycles applied during tests. The measurement of crack growth under TMF conditions will also be considered. Such measurements are challenging because of the changing thermal conditions and the effect on conventional potential difference (PD) electrical methods. These effects make it difficult to continuously monitor crack size during the thermal/load cycles. Thus, convenient dwells within the TMF cycles, where both the load and temperature are held constant for a brief period, have been utilised to make time-averaged crack size measurements using PD methods. The performance of these experimental methods has been demonstrated with some trials on an advanced nickel base superalloy, RR1000.