Failure by fatigue is a possibility at elevated temperatures (less than melting point). This type of failure can occur with less plastic deformation and will have the char­acteristics of a fatigue failure. However, it is generally observed that both fatigue and static strength properties get reduced with increasing temperature. Figure 5.51 illustrates the stress versus fatigue life curve at various temperatures, including room temperature for N-155 alloy (a high temperature alloy of Fe-21Cr-20Ni-20Co — 3Mo-2.5W-1.5Mn-1Nb, wt.%). As already discussed, application of a load particu­larly at higher homologous temperatures would produce time-dependent plastic deformation or creep. The creep-rupture strength decreases with increasing tem­perature. Usually, the alloys that are creep resistant are also found to be fatigue resistant. However, it does not mean that a material with best creep strength will also provide the best fatigue strength. Therefore, it becomes necessary to design against both creep and fatigue, and testing needs to be carried out in a state where both fatigue and creep loading are applied. The main method of investigating creep-fatigue properties is to conduct strain-controlled fatigue tests with variable frequencies with and without intermittent holding period (hold time) during the test. A lower frequency (<104 cycles s-1) and hold times allow creep effects to take place. At higher frequencies and short hold times, the fatigue mode predom­inates and failures just as pure fatigue failure (cracks start at surface and

propagates transgranularly inside the bulk material). At longer hold times or decreased frequencies, the creep effect starts playing a growing role and creep — fatigue interaction becomes important. In this regime, the mixed mode fracture is observed, that is, both fatigue cracking and creep cavitation. At another extreme, when the holding time is extended considerably with cyclic loads occurring only sparsely, the situation resembles pure creep deformation. However, where oxida­tion effects are present, the creep-fatigue interaction becomes much more com­plex. The ASTM E2714-09 standard (Standard Test Method for Creep-Fatigue Testing) describes the specifics of a creep-fatigue test.

Note that creep-fatigue and thermomechanical fatigue are not the same phe­nomena. Creep-fatigue is carried out at constant nominal temperatures, whereas thermomechanical fatigue involves thermal cycling (i. e., fluctuating temperatures).

5.2