Table of Contents
12.6.1 Low-Cycle Fatigue (LCF)
Low-cycle fatigue (LCF; Fig. "LCF as lifespan determining") is not defined by a low frequency, although this is the most common type. The determining factor is the load level at which a “noticeable plastic deformation” occurs in the life-determining part zone. These load cycles are, in most cases, induced by very low-frequency mechanical loads such as centrifugal force changes or resulting changes in the gas pressures, and constricted thermal strain. These usually occur during startup/shutdown cycles and changes in output. However, it is also plausible that FOD and/or blade fractures could cause sudden high-frequency plastic deformations that lead to LCF fracture (Volume 1, Ill. 184.108.40.206-4).
Figure "Terminology of LCF fatigue stress": The limits of low-cycle fatigue (LCF) in the range of short-time fatigue are shown in the Woehler diagram in Fig. "LCF as lifespan determining". LCF-stress can be classified into isothermal and non-isothermal fatigue. Isothermal fatigue occurs at a constant temperature and under externally originating cyclical loads. This can occur through inertial forces such as the centrifugal forces of rotor components, external forces such as gas bending forces or clamping forces, or very high dynamic loads (for example, caused by consequential damages or resonance, due to flutter vibrations, etc.).
Non-isothermal fatigue can be further divided depending on whether there is an external constraint (e.g. centrifugal force) or an internal constraint (e.g. constrained thermal expansion).
In the case of an external constraint, this type of fatigue is referred to as thermo-mechanical-fatigue (TMF). A typical example of this is the hub area of a turbine disk in which thermal strain cyclically overlays with powerful loads resulting from centrifugal forces. This combination can occur either in-phase or phase-shifted.
Thermal fatigue (TF) occurs when temperature changes create temperature gradients in the part which cause constrained thermal strain and corresponding stress cycles to accumulate (“internal force”, strain-controlled process, Fig. "Cracks protecting from thermal fatigue"). In this case, external forces play either a minor role or no role at all. A typical example is high-pressure turbine guide vanes, in which TF cracking seems to be unavoidable and is therefore frequently allowed within specified limits.