What Is Thermomechanical Fatigue?
The thermomechanical fatigue test is a test in which mechanical strain and a temperature cycle are applied simultaneously, in a controlled phase relationship. It represents the real loading condition of parts that operate through heating and cooling.
Purpose of Thermomechanical Fatigue
The purpose is to determine the effect on life of stresses arising from thermal expansion in components whose operating temperature varies.
Which Products Is Thermomechanical Fatigue Applied To?
- Exhaust manifold and turbocharger components
- Gas-turbine blade and combustor parts
- Engine cylinder head
- High-temperature pipe joints
In Which Sectors Is Thermomechanical Fatigue Used?
Standards Related to Thermomechanical Fatigue
- ISO 12111
- ASTM E2368
Thermomechanical Fatigue Conditions
- Temperature cycle
- Lower and upper temperature and heating/cooling rate defined in the specification
- Phase relationship
- In-phase or out-of-phase (the phase between mechanical strain and temperature)
- Strain control
- Total- or mechanical-strain controlled
- Cycle duration
- Defined to include heating, dwell and cooling periods
Equipment for Thermomechanical Fatigue
- Servo-hydraulic TMF test system
- Induction heating and forced cooling
- High-temperature extensometer
- Multi-channel temperature measurement system
Sample Requirements for Thermomechanical Fatigue
- Hollow or solid cylindrical specimen (for temperature uniformity)
- Thermocouple attachment points must be defined
- Surface finish quality must be recorded
- The number of specimens is set out in the specification
Duration of Thermomechanical Fatigue
Because the cycle duration is long, a single specimen can take days. The whole programme extends over several weeks.
Thermomechanical Fatigue Procedure
- 1The specimen is prepared and thermocouples are attached.
- 2The temperature and strain cycle profile is defined.
- 3The phase relationship is set and the system is stabilised.
- 4Simultaneous temperature and mechanical cycling is started.
- 5Stress–strain and temperature are recorded at each cycle.
- 6It continues to fracture or the defined number of cycles.
Thermomechanical Fatigue Acceptance Criteria
The acceptance criterion is given in the specification as a minimum number of cycles at the defined cycle profile.
Evaluation of Results
The change in stress response with cycling, crack initiation, and the effect of oxidation are evaluated together. Out-of-phase conditions generally give shorter life.
Thermomechanical Fatigue Report
- Standard applied
- Temperature profile and phase relationship
- Strain amplitude and control mode
- Number of cycles and damage location
- Stress–strain and temperature records
Accreditation
EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.
Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.
Scope and authorisationsRelated Regulations
- High-temperature component design specifications
- Automotive OEM exhaust-component standards
Mekanik Testler — What Does This Testing Field Cover?
The section below describes the field that the Thermomechanical Fatigue test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.
Mechanical testing measures how a material or a part behaves under force. The family ranges from static loading (tension, compression, bending) to time-dependent behaviour (fatigue, creep), and from hardness and wear to fracture toughness.
What these tests have in common is that the result is reported as a MATERIAL PROPERTY. Yield strength or hardness is not a conformity decision; it is numerical data against which the design and the product standard will compare. Mechanical test reports are therefore usually an input to an engineering decision rather than to a product approval — material selection, safety factor, life prediction.
The distinction between static and dynamic behaviour is the key to understanding the field. A part may appear entirely unstressed under a static load yet break under repeated loading; fatigue fracture occurs at stresses well below the yield strength and gives no warning. For rotating, vibrating or cyclically loaded parts, a tensile test alone is therefore not sufficient evidence.
Test conditions determine the result, and this is critical for the readability of the report: specimen geometry, loading rate, temperature, conditioning and the gripping arrangement. The same material gives different values under different conditions. In plastics the rate effect is very pronounced, in metals the temperature effect; in composites the specimen orientation changes the result entirely.
Hardness is the most misused measurement in the field. Because it is fast, close to non-destructive and inexpensive, it is often used in place of strength. Conversion tables from hardness to strength exist, but they are APPROXIMATE and depend on the material group; using them as the sole evidence in material acceptance is wrong.
The fracture mechanics side requires a specific competence. Fracture toughness and fatigue crack growth rate measurements make it possible to calculate how long a cracked structure will last; they are the basis of damage-tolerant design in fields such as pressure equipment, aircraft structures and pipelines.
Method groups in this field
Static tests
Tension, compression, bending, shear and torsion. Yield and tensile strength, elongation and modulus of elasticity are obtained.
Impact tests
Charpy and Izod notch impact; the brittle-to-ductile transition behaviour of the material is determined as a function of temperature.
Hardness
Brinell, Vickers, Rockwell, Shore and micro hardness. Valuable for surface treatment depth and local differences in strength.
Fatigue
Force- and strain-controlled axial fatigue, fatigue in bending; S-N curves and the endurance limit.
Creep and stress relaxation
Behaviour under long-term load at high temperature; decisive for boiler, turbine and pressure equipment materials.
Fracture mechanics
Plane-strain fracture toughness, the J integral, CTOD and fatigue crack growth rate.
Wear and friction
Taber and rotating drum abrasion, coefficient of friction measurements.
Commonly confused in this field
- Calculating strength from hardness is approximate; it cannot be used as the sole evidence in material acceptance.
- Adequate static strength does not demonstrate fatigue strength; a cyclically loaded part requires a separate test.
- If test rate and temperature are not reported, the result cannot be compared — especially for polymers and composites.
- A test specimen and the actual part may not give the same value; moulding orientation, weld lines and differences in surface treatment all have an effect.
- A strength value given for a composite without stating the specimen orientation is meaningless.
- ASTM and ISO counterparts measure the same quantity but use different specimen geometries; the results are not treated as directly equivalent.
Which decisions these results support
- Design verification and material selection
- Incoming material control and supplier approval
- Meeting the mechanical requirements of the product standard
- Failure analysis, life prediction and damage tolerance calculation
- Verifying heat treatment and surface treatment processes
Related Tests
Frequently Asked Questions About Thermomechanical Fatigue
According to which standard is Thermomechanical Fatigue made?
Thermomechanical Fatigue is executed in scope ISO 12111 and ASTM E2368. The limit values and acceptance criteria to be applied are defined in the product standard; The version valid for your product is confirmed in writing at the offer stage.
Which products does Thermomechanical Fatigue apply to?
Mainly applied to Exhaust manifold and turbocharger components, Gas-turbine blade and combustor parts, Engine cylinder head and High-temperature pipe joints. Whether your product is included in the scope is determined by examining its technical specifications and intended use.
Under what conditions is Thermomechanical Fatigue done?
The determining parameters are: Temperature cycle — Lower and upper temperature and heating/cooling rate defined in the specification, Phase relationship — In-phase or out-of-phase (the phase between mechanical strain and temperature) and Strain control — Total- or mechanical-strain controlled. The full terms are in the "Test Conditions" section on the page and vary depending on the product standard.
What kind of sample is needed for Thermomechanical Fatigue?
Hollow or solid cylindrical specimen (for temperature uniformity), Thermocouple attachment points must be defined and Surface finish quality must be recorded. The number of samples is determined according to the product standard and the scope to be reported.
How long does Thermomechanical Fatigue take?
Because the cycle duration is long, a single specimen can take days. The whole programme extends over several weeks. This duration belongs to the experiment itself; sample preparation, queuing and reporting are also evaluated. A realistic schedule for your product is given in the offer.
What is the Thermomechanical Fatigue result called "appropriate"?
The acceptance criterion is given in the specification as a minimum number of cycles at the defined cycle profile.