Thermo-mechanical fatigue

As thermo- mechanical fatigue (English thermo -mechanical fatigue ) the superposition of a mechanical fatigue loading (see material fatigue ) denotes a cyclic thermal loading. In the design and construction of turbines for aircraft gas turbines and thermo- mechanical fatigue is an important point.

Term

For thermo- mechanical fatigue, the abbreviation of the English term thermo- mechanical fatigue (TMF ), is also in the German language, often used. Rarely found in the literature, the German abbreviation TME.

Delimitation of thermal mechanical fatigue

Conventional Fatigue is the cyclic mechanical loading of the material under isothermal conditions ( constant temperature), which can eventually lead to loss of strength and catastrophic material failure by fracture.

Thermal fatigue is the cyclic loading of the material by temperature changes without force. The material failure occurs in this case by the appearance of a Thermospannungsgradienten.

Now, if cyclic thermal and mechanical cyclic loading coupled, then one speaks of thermo-mechanical fatigue, which has a great importance in the design and interpretation of thermally and mechanically loaded components.

Basics

A component as well as a sample under thermo- mechanical load is subject to both a cyclic mechanical strain, such as by centrifugal forces, and the cyclic thermal expansion. The material is therefore subject to the following total strain

Types of thermo- mechanical fatigue

A phase shift exists between the cyclic thermal and mechanical stress which affects the fatigue life, as well as the plastic deformation markedly. Based on the phase shift is different to several cases of a thermo-mechanical test:

  • Phase in Test ( IP): = 0 °, i.e., the sample undergoes the same elongation by a tensile force, as well as for thermal expansion by heating
  • Out-of- phase assay ( OP, sometimes OOP ) = 180 °, i.e., the sample undergoes a simultaneous compression due to a compressive force, as well as for thermal expansion by heating
  • Clockwise Diamond test (CD ) = 90 °, this is the classical CD- test
  • Counter- Clockwise -Diamond - test (CCD): = -90 °, this is the classic CCD Test
  • In general, any TMF test with [0 °, 180 ° ] can be considered as a CD or CCD Test

The largest material stress occurs here on the OP test, so here the fatigue life is lowest in most cases, compared with (C) CD and IP test. The lifetimes of IP, OP or different (C ) CD tests are not comparable with each other or with isothermal and thermal fatigue tests, since the stresses on the material is very complex and unpredictable.

A component (eg turbine blades ) can be in different areas of several types of thermo- mechanical stress be exposed (for example, IP load at the leading edge of the turbine blade, CCD conditions in the blade material ).

Furthermore, for the characterization of a TMF - test, the heating and cooling rates (typically about 10 K / s), the holding time at the maximum temperature, the lower and upper temperature limit of the TMF test and mechanical strain amplitude and the existing medium elongation a non-symmetrical strain application result has (-1). As a result, several TMF tests is then obtained strain Wöhler diagrams in the design of components of importance.

Application

Air turbine materials are tested in TMF attempt to simulate the start- landing cycles. At the start of the turbine, the material quickly from the ambient temperature to the operating temperature (about 1050 ° C) is heated with simultaneous mechanical loading; then reversed during the landing and shutting down the turbines. As today's turbine blades consist mainly of single-crystal nickel-based superalloys, which are vulnerable to high temperatures, corrosion and oxidation, the components are often protected with an anti-oxidation coating or thermal barrier coatings (so-called TBC, Eng. Thermal barrier coating ) before the attack.

For car engines special TMF tests are used, in which the TMF load additionally a high-frequency vibration stress is superimposed to simulate the damage in later use better.

Complexity of thermo-mechanical fatigue

Besides the pure thermo-mechanical fatigue loading acting on a component in use yet further burdens a:

  • HCF fatigue (high cycle fatigue ), for example by vibrations in the engine / in the turbine ( high cycle fatigue )
  • LCF fatigue ( low cycle fatigue ), low cycle number ( cycle fatigue )
  • Kriechbelastung, for example, by the centrifugal force to a turbine blade
  • Fretting fatigue / tribological stress, eg in the Schwalbenschwanzeinspannungen the turbine blade
  • Oxidation, e.g., by the hot ambient temperature
  • Hot gas corrosion, e.g., by gases containing corrosive combustion products
  • Impaktbelastung, eg by bird strike

Since each of these individual loads can already trigger complex reactions in the material, the total load is not simply the sum of the individual loads, but must be examined separately in a component test.

For material research is, however, important to examine the individual damage influences separately, as this in the alloy development and in the determination of failure mechanisms special adaptations can be made.

  • Materials Science
TBC (disambiguation)
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