Table of Contents
12.6 Dynamic Fatigue
Dynamic fatigue is strength loss in a part caused by crack initiation and growth under dynamic loads. If the dynamic loads (Fig. "Examples for thermomechanical loads") remain below a material-specific threshold value (fatigue limit), the number of tolerable load cycles will be sufficient to attain the prescribed life span. Some materials, such as steels, can tolerate any number of load cycles below the fatigue limit (Fig. "Fundamentals of LCF damage mechanism").
Dynamic fatigue is the most important life-determining load in turbine engines (Fig. "Examples for thermomechanical loads"). In rotors, the low-frequency load cycles during the start-up/shut-down phases play an important role (LCF). In this case, centrifugal force and restricted thermal strain act in combination (Fig. "Turbine disk loads during operation cycles"). Compressor blading is primarily stressed by high-frequency vibrations (Chapter 12.6.3), i.e. resonance. The hot parts, such as turbine blades ( ) and combustion chambers (Fig. "Thermal fatigue in combustion chambers") are subjected to powerful low-frequency cyclical loads from restricted thermal strain (thermal fatigue, Chapter 12.6.2). Of course, virtually all other engine parts are also under dynamic loads. For example, running tracks in roller bearings and tooth flanks on gears are subjected to dynamic loads from the cyclically changing pressure during force transmission. Dynamic fatigue is compounded by accompanying effects such as corrosion (Volume 1, Chapter 5.4.3), fretting, and the influence of the surrounding atmosphere on the fracture-mechanical behavior (crack growth, Chapter 12.2).
The load/time graph of the vibrations during operation can vary widely and be very complex. This is true of the stress peaks, rising and falling slopes, and also any dwell times. Depending on the temperatures, materials with different dynamic strengths react to these influences with varying degrees of intensity. Titanium alloys can tolerate far fewer load cycles in the LCF range if there is a dwell time of several minutes at maximum stress level (high temperature low cycle fatigue=HTLCF; Ills. 12.6.1-16 and 12.6.1-17). Creep and fatigue damages evidently occur during the dwell time.
In addition to titanium alloys, many other materials also exhibit dwell effects (Ref. 12.6.1-19). For example, solder, low- and high-alloy steels, and Ni alloys are also affected. Materials react in specific ways depending on any compressive or tensile stresses acting during the dwell time. Both tensile stress-sensitive (e.g. CrNi18/8, IN 100, Rene 95) and compressive stress-sensitive (e.g. Ti-6Al-4V, Waspaloy) materials have been identified. In LCF-stressed parts, such as rotor disks, certain zones are plastically deformed during acceleration due to tensile stress. When the force is removed (shut-down), compressive residual stresses are created in these zones. This results in dwell times occurring in these parts both during the tension phase as well as during the compression phase (Ill. 12.6.1-21).
Engine parts have typical zones in which dynamic fatigue can be expected, and these correspond to the operating loads (Ill. 12.5.1-3). These loads can be LCF loads that are a fundamental part of life span calculations. However, it is also possible that “unexpected” high-frequency vibrations and damages in the HCF load range may occur.
Dynamic fatigue through LCF loads is generally considered in the design process and the verification of part life span. Therefore, quantities such as the typical cyclical load combination of centrifugal force and thermal strain that occur during the start-up/shut-down process, as well as the number of expected load cycles over the required life span are usually sufficiently well known.
However, the situation is completely different in the case of high-frequency vibrations with loads in the HCF range. These are usually vibrations whose incitement force and possible occurrence are not sufficiently understood. The high frequency leads to extremely rapid crack growth after initial cracking, and can lead to a fracture within seconds. This makes it impossible to satisfactorily determine the time of possible initial cracking, nor can the crack be “caught” in an inspection. When dimensioning parts in order to prevent HCF failure, if resonances cannot be prevented by design, only design-specific experience will suffice. This requires proven design data. Ultimately, “the engine will tell us.”
Figure "Examples for thermomechanical loads": This table provides an overview of the mechanical-thermal loads in turbine engines with the aid of typical examples (Ref.12.6.1-15). The examples are certain part-specific phases of operation. When one considers the entire load spectrum of an engine part, it is almost always the temporal consequence and/or combination of several load types.
These loads occur not only during operation, but also during production and overhauls. They lead to grinding cracks, cracking through spark erosion, or welding cracks.
Figure "Terms and examples of dynamic loads": In order to understand specifications for material characteristic data, especially for dynamic loads (e.g. Haigh diagram), it is necessary to know the definition of temporal stress progression (Ref. 12.6.1-16). The maximum stress (so = smax ) is the highest amplitude of a sinusoidal load (left diagram). In the same manner, the minimum stress (su = smin ) is the lowest load. Half of the difference between the maximum and minimum stress is the stress amplitude sa . The stress amplitude is around the mean stress (sm).
Depending on the height of the mean stress, certain load ranges are defined and can be identified by the stress relationship R=k = smin / smax . In this formula, smin corresponds to the sum of the smallest value, and smax corresponds to the sum of the greatest value. The results are as follows:
R= -1 pure alternating stress,
R< -1 alternating range,
R<+1 pulsating stress range,
R=0 pure pulsating stress
The top right diagram shows dynamic strength limits as they are often used in English-language materials. The dynamic strength is plotted as the tolerable stress amplitude sa relative to the mean stress sm. Dynamic loads below the Gerber parabola or Goodman line can be tolerated without damage. At the bottom, a typical example is provided for each of the different dynamic loads.
Figure "Lifespan limiting by fatigue cracking" (Ref. 12.6.1-1):
An understanding of engine part-specific crack types and their locations is especially important for both the testing personnel in the shop and at the manufacturer, as well as for boroscope findings on location, inasmuch as these concern the blading. Cracks in disks are extremely rare. The consequences of such cracks (disk failure) can be much more serious than blade failures. Engines are generally not designed to be able to contain disk fragments (Volume 2, Chapter 8). Therefore, disk failures must be prevented. To ensure this, especially high safety requirements are used in disk design. Crack detection during overhauls is extremely important if the disks are given multiple overhauls during their normal life span. The following numbers, which correspond to the crack locations in the adjacent diagrams, each refer to a specific load type that is most dominant in each area.
Integral turbine disk (top left) of the type commonly found in small gas turbines:
(1) Cracks due to rubbing,
(2) Cracks due to blade vibrations in the HCF zone,
(3) Annulus cracks due to thermal fatigue,
(4) Labyrinth cracks due to rubbing and cyclical loads in the LCF and/or HCF zones,
(5 and 6) Fatigue cracks due to LCF caused by changes in centrifugal force and temperature, especially during startup,
(7 and 8) Cracking due to blade vibrations in the HCF zone and/or LCF cracks due to changes in centrifugal force and temperature,
(9) Cracking due to thermal fatigue and/or overheating and oxidation.
Typical disk of an axial compressor (top right):
(1) Fatigue cracks in the dovetail slots due to blade vibrations in the HCF zone or centrifugal force changes in the LCF zone. Cracks often occur on the contact surfaces in combination with fretting damage (Volume 2, Chapter 6),
(2) Cracking in the bolt bores due to centrifugal force changes in the LCF zone,
(3) Cracks in the labyrinth due to rubbing damage,
(4) LCF cracks in the hub area due to changes in centrifugal force and, to a considerably lesser degree than in turbine disks, thermal stress. These cracks usually originate in weak points in the material or those resulting from the manufacturing process.
Radial compressor disks (bottom left):
(1) Cracking due to blade vibrations or disk vibrations in the HCF range,
(2) Fatigue cracks due to LCF from startup/shutdown procedures; may be worsened by disk vibrations,
(3) Crack initiation during rubbing due to thermal stress and overheating damages. Crack growth in the hub area due to centrifugal force changes; at the blade edge, due to blade vibrations,
(4) Cracking due to heavy rubbing of the blade and overloading of the blade root,
(5) Cyclical fatigue due to bending-open of the disk and flexural overload of the head running blade.
Compressor rotor disk:
(1) Fatigue crack formation due to blade vibrations,
(2) Crack initiation in the contact surface due to RPM changes,
(3) Crack initiation due to rubbing,
(4) Crack initiation due to blade vibrations, usually in combination with fretting wear.