Table of Contents
12.5.2 Creep Damage
Creep damage is observed on all hot parts, and is also dealt with in the chapters on combustion chambers, turbines, and afterburners. Therefore, the following is merely an overview. The damage is usually plastic deformations caused by creep. Typical examples include the folding of the rear edges of turbine stator vanes (Fig. "Creep strain influenced by hot gases"). This is caused by the thermal strain that is resisted by the shrouds. Centrifugal force can cause rotor blades to deform ( ) and also cause blade leaves or shrouds to fracture (Fig. "Typical damages to rotor blade shrouds").
Engine parts also suffer damages that cannot be immediately connected to creep damage. These parts include titanium rotor disks and rotor blades. Dwell time, i.e. the holding time at high stress levels, plays an important role in the cyclical loading of titanium alloy rotor disks. It can considerably reduce the number of tolerable startup/shutdown cycles, shortening part life (Fig. "Lifespan verification by cyclic spin test "). The blade roots of titanium alloy compressor rotor blades must undergo repeated shot peening when creep has greatly reduced the protective compressive residual stresses (Volume 2, Ill. 6.2-4).
Figure "Creep deformation at overheated turbine vanes" (Refs. 12.5.1-4 and 12.5-5): Integral-cast turbine stators, such as this one from a small gas turbine used as a helicopter power plant, are especially susceptible to folding-over of the rear blade edges (Fig. "Creep deformations in turbine blades"). This is due to the temperature difference between the hot blade and the relatively stiff and cool inner and outer shrouds. This creates powerful compressive stresses in the blade and leads to folding-over due to creep deformation.
Figure "Creep resistance by coarse grain structure" (Ref. 12.5-5): Creep in the middle blade section can lead to considerable constriction, especially in uncooled turbine rotor blades (those in older engine types made from forged materials are especially sensitive) in the case of overheating and/or insufficiently creep-resistant materials or structures. If only one blade in a set is affected, then the cause can most likely be found in the material. The depicted case involved this type of single blade. An investigation revealed that the structure had overly fine grains outside of the material specifications. The grain size was typical for that before growth during heat treatment. Larger grains increase the creep strength. This damage is evidently due to a lack of heat treatment, i.e. a manufacturing flaw.
Note: There are frequent efforts to treat cast parts, which have typical large grains and good creep strength but poor dynamic strength at the surface, in a way that creates a fine-grained zone at the surface. This is desirable because this area experiences the greatest dynamic HCF loads during flexural vibrations. In order to create this type of fine-grained zone, it seems possible to cause critically large surface deformation followed by recrystallization annealing. This should result in a fine-grained surface zone on a coarse-grained cross-section. However, there has been no reported solution to this task that would be suitable to serial production.