Development of fatigue crack growth testing under thermo-mechanical fatigue conditions

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.

Isothermal and thermomechanical fatigue crack growth behavior of 316LN stainless steel under load‐ and strain‐controlled modes

“…Compared with the crack growth test under isothermal temperature conditions, the TMFCG test obviously has higher difficulties, such as how to accurately measure the crack length under variable temperature conditions, which leads to a relatively limited number of related studies. 22 The current TMFCG studies mainly focus on nickel-based alloy materials of aero engine components, 23 and mainly discuss the change of crack growth rate of materials under different phase relations and load holding time, [24][25][26] while the related studies on the TMFCG behavior of austenitic SS materials are seldom reported. Palmert 27 conducted TMFCG tests on single-crystal nickel-based alloys.…”