Table of Contents

12.5 Creep

 Creep fundamentals

The temperatures above which creep stress occurs are material-specific (Fig. "Creep behavior of different metals"). Along with temperature, a further parameter is the duration of the stress. These make the material behavior far more complex than at temperatures below the creep range. In addition, almost all design characteristics also change (Fig. "Temperature influence on material properties").
If materials are subjected to stress at high temperatures, their strength (creep strength) and plastic strain behavior (creep) are time-dependent (Fig. "Life limiting 'creep fracture'"). For this reason, creep strength and creep strain are important design parameters. Ref. 12.5-8 defines creep strength as a test stress that causes fractures after a specific test duration at a specific test temperature. Correspondingly, the time yield limit is the test stress which leads to specific plastic strain (creep) after a set duration of time under stress. One can assume that the hot parts in an engine are subjected to this combined stress consisting of thermal and mechanical loads. Even if there are no noticeable external forces, temperature gradients induce different thermal strains and, if these are restricted, they create stress in the engine part (Fig. "Cracks protecting from thermal fatigue"). The creep behavior also plays an important role in Chapters 11.2.2 to 11.2.4, which deal with the engine components - afterburners, turbines, and combustion chambers - but is only dealt with indirectly in the context of the operating behavior of these engine parts.
Creep can take many different forms. For example, relaxation (stress reduction in a strain-controlled system) of residual stresses, stress relief annealing, and thermal adjustment through local heating are all based on creep effects. Relaxation can already occur at room temperature, even though the material is suitable for operating temperatures of several hundred °C. This behavior occurs in shot-peened titanium alloys, even if a large amount of the the induced residual stress remains. This requires regularly repeating regenerative shot peening of blade roots, for example.

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