Abstract
Non-isothermal conditions during flight cycles have long led to the requirement for thermo-mechanical fatigue (TMF) evaluation of aerospace materials. However, the increased temperatures within the gas turbine engine have meant that the requirements for TMF testing now extend to disc alloys along with blade materials. As such, fatigue crack growth rates are required to be evaluated under non-isothermal conditions along with the development of a detailed understanding of related failure mechanisms. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct current potential drop techniques for polycrystalline nickel-based superalloys, such as RR1000. Results have shown that in-phase (IP) testing produces accelerated crack growth rates compared with out-of-phase (OOP) due to increased temperature at peak stress and therefore increased time dependent crack growth. The ordering of the crack growth rates is supported by detailed fractographic analysis which shows intergranular crack growth in IP test specimens, and transgranular crack growth in 90° OOP and 180° OOP tests. Isothermal tests have also been carried out for comparison of crack growth rates at the point of peak stress in the TMF cycles.
Highlights
Disc materials for high temperature applications in aero engines are predominantly nickel-based superalloys, whose thermo-mechanical fatigue (TMF) lives have been investigated thoroughly by numerous authors, mainly through strain control tests
A significant challenge in TMF crack growth (TMFCG) is finding a compatible combination of heating and crack monitoring techniques
Waspaloy specimens with a 10 × 10 mm, CC10 (Corner Crack, 10 mm gauge width), cross-section were tested in a conventional split radiant furnace, and compared to tests where heating was provided through the use of an induction coil
Summary
Disc materials for high temperature applications in aero engines are predominantly nickel-based superalloys, whose thermo-mechanical fatigue (TMF) lives have been investigated thoroughly by numerous authors, mainly through strain control tests. TMF testing is best explained in terms of phase angles and loading directions. Since there are two control variables, the phasing between thermal and mechanical cycles usually provides an appropriate description of the TMF condition. Typical cycles usually involve phasing where the stress and temperature increase/decrease concurrently, and the cycle is considered in-phase (IP), or where the stress begins to decrease/increase, the temperature increases/decreases, and the cycle is 180◦ out-of-phase (OOP). An infinite number of phase angles between these two extremes are possible. Marchionni et al [1] carried out such work on Nimonic 90 between 400 and 850 ◦ C, finding that
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