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. 2017 Jan 4;10(1):34.
doi: 10.3390/ma10010034.

Thermo-Mechanical Fatigue Crack Growth of RR1000

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Free PMC article

Thermo-Mechanical Fatigue Crack Growth of RR1000

Christopher John Pretty et al. Materials (Basel). .
Free PMC article

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.

Keywords: RR1000; TMF; crack growth; induction coil; potential drop.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Corner crack specimen designed for thermo-mechanical fatigue (TMF) crack propagation in induction coil.
Figure 2
Figure 2
Waspaloy crack growth rates vs. stress intensity range: furnace and induction coil comparisons at 650 °C, 450 MPa and R = 0.1.
Figure 3
Figure 3
Thermocouple locations for thermal profiling (a) side view and (b) plan view.
Figure 4
Figure 4
Thermocouple responses during the 80 s cycle.
Figure 5
Figure 5
Thermocouple deviation over the 80-s thermal cycle.
Figure 6
Figure 6
Stress response for ‘fast’ and ‘slow’ isothermal fatigue (IF) tests.
Figure 7
Figure 7
Normalised crack growth rates at a stress intensity range of (a) 20 MPa√m and (b) 40 MPa√m for RR1000 under isothermal conditions.
Figure 7
Figure 7
Normalised crack growth rates at a stress intensity range of (a) 20 MPa√m and (b) 40 MPa√m for RR1000 under isothermal conditions.
Figure 8
Figure 8
Fractography of slow cycles at ∆K 20 MPa√m for (a) 700 °C and (b) 300 °C.
Figure 9
Figure 9
Stress cycles vs. temperature for all TMF conditions. IP: in-phase; OOP: out-of-phase; CW: clockwise; ACW: anticlockwise.
Figure 10
Figure 10
Turbine conditions during a civil engine flight cycle based on Marchand et al. [27].
Figure 11
Figure 11
Normalised crack growth rates at a stress intensity range of (a) 20 MPa√m and (b) 40 MPa√m for RR1000 under TMF and isothermal conditions.
Figure 11
Figure 11
Normalised crack growth rates at a stress intensity range of (a) 20 MPa√m and (b) 40 MPa√m for RR1000 under TMF and isothermal conditions.
Figure 12
Figure 12
Notch, pre-cracking and test recognition from the two types of post-test analysis, (a) specimen stressed to failure and (b) unloaded for sectioning.
Figure 13
Figure 13
Intergranular cracking from IP testing shown by (a) an SEM image presenting crack branching and (b) an EBSD image of the crack tip.
Figure 13
Figure 13
Intergranular cracking from IP testing shown by (a) an SEM image presenting crack branching and (b) an EBSD image of the crack tip.
Figure 14
Figure 14
Transgranular dominant failure for 180° OOP tests showing the fracture surface at ∆K = 20 MPa√m.
Figure 15
Figure 15
Fracture surfaces of 90° OOP at ∆K = 20 MPa√m test in (a) ACW and (b) CW loading directions.
Figure 16
Figure 16
Stress plotted against temperature for 90° OOP cycles in the ACW and CW loading directions.
Figure 17
Figure 17
Crack progression of the 80 s 90° OOP ACW test.
Figure 18
Figure 18
Crack progression of the 500 s 90° OOP ACW test.
Figure 19
Figure 19
Crack width plotted against crack length for both 90° OOP ACW tests during stage 4 of the crack (TMF test).
Figure 20
Figure 20
Normalised crack growth rates at a stress intensity range of (a) 20 MPa√m and (b) 40 MPa√m for RR1000 under TMF conditions.
Figure 21
Figure 21
Energy dispersive X-ray spectroscopy (EDX) line scans taken near the start of the test and near the crack tip for (a) ACW 80-s cycle; (b) CW 80-s cycle and (c) ACW 500-s cycle.
Figure 21
Figure 21
Energy dispersive X-ray spectroscopy (EDX) line scans taken near the start of the test and near the crack tip for (a) ACW 80-s cycle; (b) CW 80-s cycle and (c) ACW 500-s cycle.

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