What Is Cyclic Fatigue?
The cyclic fatigue test is a test in which strain-controlled, rather than stress-controlled, repeated loading is applied, and material behaviour is examined generally in the low-cycle-fatigue region. Hysteresis loops are recorded.
Purpose of Cyclic Fatigue
The purpose is to determine life and hardening/softening behaviour under high-amplitude repeated loading where plastic deformation predominates.
Which Products Is Cyclic Fatigue Applied To?
- Turbine components operating at high temperature
- Pressure vessel and pipe connections
- Automotive engine parts
- Welded structural details
In Which Sectors Is Cyclic Fatigue Used?
Standards Related to Cyclic Fatigue
- ASTM E606
- ISO 12106
- ISO 12111
Cyclic Fatigue Conditions
- Control mode
- Total-strain-amplitude controlled
- Waveform
- Generally triangular or sinusoidal, at a constant strain rate
- Strain ratio
- Rε = εmin/εmax; typically −1 (fully reversed)
- Temperature
- Room temperature, or a high temperature per the specification
Equipment for Cyclic Fatigue
- Servo-hydraulic testing machine
- High-precision extensometer
- Induction- or resistance-heated furnace
- Data-acquisition and hysteresis-recording system
Sample Requirements for Cyclic Fatigue
- The specimen must have a geometry suited to axial loading that prevents buckling
- The surface must be polished and free of residual stress
- At least three specimens per strain level are recommended
- The extensometer attachment region must be prepared
Duration of Cyclic Fatigue
Since the number of cycles is low, a single specimen typically takes hours; the total for different strain levels can extend to several weeks.
Cyclic Fatigue Procedure
- 1The specimen is measured and mounted axially in the machine.
- 2The extensometer is fitted and strain control is verified.
- 3If required, the test temperature is reached and stabilised.
- 4Repeated loading is started at the defined strain amplitude.
- 5Hysteresis loops are recorded; the half-life loop is determined.
- 6Loading continues to fracture and the number of cycles is recorded.
Cyclic Fatigue Acceptance Criteria
The acceptance criterion is given in the specification as a minimum number of cycles at a specified strain amplitude.
Evaluation of Results
Stress amplitude and plastic-strain amplitude are extracted from the half-life hysteresis loop. Hardening or softening behaviour is evaluated separately.
Cyclic Fatigue Report
- Standard applied
- Strain amplitude, ratio and rate
- Test temperature
- Number of cycles to fracture
- Hysteresis loops and half-life data
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 standards
Mekanik Testler — What Does This Testing Field Cover?
The section below describes the field that the Cyclic 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 Cyclic Fatigue
What is the difference between the fatigue test and the cyclic fatigue test?
The classic fatigue test is generally stress-controlled and high-cycle. Cyclic fatigue is strain-controlled and low-cycle, representing conditions where plastic deformation predominates.