Thermo-mechanical fatigue

The superposition of a mechanical fatigue load with a cyclical thermal load is referred to as thermo-mechanical fatigue (English thermo-mechanical fatigue , abbreviation also in German usage often  TMF , more rarely  TME ) . It is important in the design and construction of thermally and mechanically stressed components such as gas turbines and turbines for aircraft .

Demarcation

Conventional material fatigue is the cyclical, mechanical loading of the material under isothermal conditions (constant temperature), which can ultimately lead to loss of strength and material failure due to breakage .

Thermal fatigue, on the other hand, is the cyclical stress on the material due to temperature changes without the application of force. Here material failure occurs due to thermal stress gradients .

Basics

A component as well as a sample under thermo-mechanical stress is also subject to cyclical mechanical expansion , e.g. B. by centrifugal forces , and a cyclical thermal expansion . Thus the material is subject to the total expansion: ${\displaystyle \varepsilon _{\text{mech}}}$ ${\displaystyle \varepsilon _{\text{therm}}}$

${\displaystyle \varepsilon _{\text{total}}=\varepsilon _{\text{mech}}+\varepsilon _{\text{therm}}}$

Chronological sequence of the thermo-mechanical loading of a sample (here CCD test)

Types

A phase shift can exist between the cyclical thermal and mechanical loads , which noticeably influences the fatigue life and the plastic deformation . A distinction is made between several cases of a thermo-mechanical experiment based on the phase shift: ${\displaystyle \delta }$

• In-phase test (IP): = 0 °, i.e. H. the sample simultaneously experiences an elongation by a tensile force , as well as a thermal expansion by heating ${\displaystyle \delta }$
• Out-Of-Phase-Test (OP, sometimes also OOP): = 180 °, i.e. H. the sample experiences a compression due to a compressive force , as well as a thermal expansion due to heating ${\displaystyle \delta }$
• Clockwise Diamond Test (CD): = + 90 °, this is the classic CD test ${\displaystyle \delta }$
• Counter-Clockwise Diamond Test (CCD): = -90 °, this is the classic CCD test ${\displaystyle \delta }$

In general, every TMF test can also be viewed as a CD or CCD test. ${\displaystyle \delta \notin [0^{\circ },180^{\circ }]}$

The greatest material stress occurs during the surgical test, so that the fatigue life is the lowest in most cases. The lifetimes of IP, OP or various (C) CD tests are not comparable with one another or with isothermal or thermal fatigue tests, as the stresses on the material are very complex and unpredictable.

A component (e.g. turbine blade) can be exposed to different types of thermo-mechanical stress in different areas (e.g. IP stress on the leading edge, CCD conditions in the blade material).

Furthermore, a TMF test is characterized by:

• the heating or cooling rates (usually approx. 10  K / s)
• the holding time at maximum temperature
• its lower and upper temperature limit
• the mechanical expansion amplitude and any mean expansion that may result in non-symmetrical expansion: ${\displaystyle \varepsilon _{\text{mech}}}$ ${\displaystyle \varepsilon _{\text{mean}}}$

${\displaystyle R_{\varepsilon _{\text{mech}}}={\frac {\varepsilon _{\text{min}}}{\varepsilon _{\text{max}}}}={\frac {\varepsilon _{\text{mean}}-\varepsilon _{\text{mech}}}{\varepsilon _{\text{mean}}+\varepsilon _{\text{mech}}}}\neq -1}$.

As a result of several TMF tests, strain Wöhler diagrams are obtained , which are important for component design .

application

Aviation turbine materials are tested in the TMF test in order to simulate the take-off-landing cycles. When a turbine is started, the material is quickly heated from ambient temperature to operating temperature (approx. 1050 ° C) with simultaneous mechanical stress; the other way around when landing and switching off the turbines. As today's turbine blades mainly composed of single-crystal nickel-base superalloys consist, at high temperatures and against corrosion and oxidation are susceptible, the components are often with an oxidation protective layer or thermal barrier coatings (engl. Thermal Barrier Coating , abbreviated as TBC) from attack protected.

Special TMF tests are used for vehicle engines, in which a high-frequency vibration load is also superimposed on the TMF load in order to be able to better simulate the damage in later use.

complexity

In addition to the pure thermo-mechanical fatigue load, other loads also affect a component in use:

• HCF ( high cycle fatigue ) fatigue , e.g. B. by vibrations in the engine / turbine (long-term fatigue)
• LCF fatigue ( Low Cycle Fatigue ), with low number of load cycles (short-term fatigue )
• Creep load , e.g. B. by centrifugal force on a turbine blade
• Friction fatigue / tribological stress, e.g. B. in the dovetail clamps of the turbine blade
• Oxidation, e.g. B. by the hot ambient temperature
• Hot gas corrosion, e.g. B. from exhaust gases containing corrosive combustion products
• Impact load , e.g. B. by bird strikes .

Since each of these individual loads can 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.

On the other hand, it is important for materials research to examine the individual damaging influences separately, as special adaptations can be made in this way during the alloy development and the determination of the damage mechanisms.

literature

• Ralf Bürgel: Handbook high temperature materials technology. Basics, 3rd edition, Friedrich Vieweg & Sohn Verlag, Wiesbaden 2006, ISBN 978-3-528-23107-1 .
• Horst Biermann, Lutz Krüger (Hrsg.): Modern methods of material testing. Wiley-VCH Verlag GmbH, Weinheim 2015, ISBN 978-3-527-33413-1 .

Web links

• Thermomechanical Fatigue (accessed October 18, 2019)
• Innovative testing methodology (accessed on October 18, 2019)
• Use of materials at high temperatures (accessed on October 18, 2019)
• Influence of the temperature-strain phase position on the thermomechanical fatigue behavior of Ni-based alloys (accessed on October 18, 2019)
• Temperature change behavior of single and multi-phase metallic materials in the crack initiation and crack advancement phase (accessed on October 18, 2019)