What Is Low Cycle Fatigue?
In the low-cycle fatigue test, repeated strain-controlled loading involving plastic strain is applied to the specimen. The stress-strain loop (hysteresis) is recorded each cycle. Life is usually on the order of tens of thousands of cycles or fewer.
Purpose of Low Cycle Fatigue
The purpose is to determine the material's behaviour under repeated loading in the plastic region (cyclic hardening or softening) and its life. It is used for parts subjected to a small number of severe load cycles, such as thermal cycling, take-off-landing cycles and heavy load cycles.
Which Products Is Low Cycle Fatigue Applied To?
- Turbine discs and blades
- Pressure vessel and pipe connections
- Engine components
- Nuclear and power plant equipment
- Welded structural details
In Which Sectors Is Low Cycle Fatigue Used?
Standards Related to Low Cycle Fatigue
- ISO 12106 (metallic materials — axial strain-controlled fatigue testing)
- ASTM E606 (strain-controlled fatigue testing)
- ASTM E2207 (axial-torsional fatigue)
- ISO 12111 (strain-controlled thermomechanical fatigue)
Low Cycle Fatigue Conditions
- Control
- Total strain amplitude controlled
- Strain ratio
- From the specification; usually fully reversed
- Waveform and rate
- Defined in the standard; stated separately if temperature effects apply
- Quantities measured
- Hysteresis loops, cyclic stress response, cycles to failure
Equipment for Low Cycle Fatigue
- Servohydraulic closed-loop test system
- High-resolution extensometer
- Heating furnace or induction system (for hot tests)
- High-speed data acquisition system
- Alignment verification fixture
Sample Requirements for Low Cycle Fatigue
- Specimen geometry and surface quality must comply with the standard.
- A specimen design preventing buckling in compression must be chosen.
- It must be ensured that the extensometer knife-edges do not mark the specimen.
- A separate specimen is needed for each strain level.
- In hot tests, specimen temperature must be measured directly on the specimen.
Duration of Low Cycle Fatigue
Because life is short, a single specimen's test is on the order of hours. Since many specimens at different strain levels are needed, the overall study spans days; heating times add to this at high temperature.
Low Cycle Fatigue Procedure
- 1The specimen is machined, measured and mounted aligned in the machine.
- 2The extensometer is placed on the specimen's gauge length.
- 3Strain amplitude and waveform are set.
- 4Cyclic loading is started and hysteresis loops are recorded.
- 5Change in the cyclic stress response is monitored.
- 6The test is terminated at the defined load drop or fracture.
- 7The cyclic stress-strain curve and life data are derived.
Low Cycle Fatigue Acceptance Criteria
The method standard defines how the test is performed, not what the result should be. The acceptance limit comes from the material standard, the product specification or the customer's technical drawing; this page therefore gives no numerical limit.
Evaluation of Results
Results are assessed as the strain amplitude-life relationship and usually modelled by separating elastic and plastic components. Cyclic hardening or softening behaviour is reported separately and used directly in design calculations.
Low Cycle Fatigue Report
- Specimen identification, material type and cast/batch number, if any
- Specimen geometry, dimensions and preparation method
- Standard and test conditions applied
- Test temperature and ambient conditions
- Measured values and calculated quantities
- Fracture surface or indentation photographs (where applicable)
- Measurement uncertainty (if requested)
- A statement that the result applies only to the specimen submitted for testing
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
- EN 10204 (inspection documents for metallic products)
- EU 305/2011 Construction Products Regulation (for products used in construction)
- Harmonised standards applied according to the product family
Mekanik Testler — What Does This Testing Field Cover?
The section below describes the field that the Low Cycle 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 Low Cycle Fatigue
Why is it strain-controlled?
When plastic strain dominates, stress-controlled loading causes cumulative elongation in the specimen and the test becomes uncontrolled. Strain control keeps the cycle stable.
Can the result be combined with high-cycle data?
The two regimes rely on different mechanisms, but life models exist that use both together in design. Combining them requires both regimes to have been measured with their own respective methods.