What Is Fatigue?
The fatigue test is a test in which repeated stresses below the static strength are applied to a specimen while the number of cycles to failure is determined. Results are presented as a stress–cycle (S-N) curve.
Purpose of Fatigue
The purpose is to predict the life of parts operating under repeated load and to determine the fatigue-strength limit for design.
Which Products Is Fatigue Applied To?
- Shafts, axles and drivetrain components
- Springs and fasteners
- Welded structural joints
- Aerospace and automotive components
- Pressure vessels and pipe
In Which Sectors Is Fatigue Used?
Standards Related to Fatigue
- ISO 1099
- ISO 1143
- ASTM E466
- ASTM E606
- ISO 12107
Fatigue Conditions
- Loading type
- Axial tension–compression, rotating bending, or torsion
- Stress ratio
- R = σmin/σmax; defined in the specification
- Frequency
- Chosen so as to prevent heating; typically 5–100 Hz
- Termination
- Fracture, or reaching the defined number of cycles (e.g. 10⁷)
Equipment for Fatigue
- Servo-hydraulic or resonance fatigue machine
- Rotating-bending fatigue machine
- Cycle counter and automatic shutdown
- Temperature monitoring
Sample Requirements for Fatigue
- The specimen surface must be machined to the roughness defined in the standard
- The surface must be flawless if no notch or discontinuity is intended
- Specimens at multiple stress levels are required for an S-N curve
- At least three specimens per level are recommended
Duration of Fatigue
This is a long-duration test. A 10⁷-cycle test can take from several days to several weeks, depending on frequency.
Fatigue Procedure
- 1Specimens are machined and their dimensions are measured.
- 2Stress levels and the stress ratio are determined.
- 3The specimen is mounted axially in the machine.
- 4Repeated loading is started at the defined frequency.
- 5Loading continues to fracture or the termination cycle.
- 6The number of cycles for each level is recorded and the S-N curve is plotted.
Fatigue Acceptance Criteria
The acceptance criterion is given in the specification as a minimum number of cycles at a specified stress level, or a minimum fatigue strength.
Evaluation of Results
The S-N curve and scatter are evaluated together. Fracture at the grip or at a discontinuity can invalidate the result; the fracture surface is examined to determine the initiation point.
Fatigue Report
- Applied standard and loading type
- Stress levels and stress ratio
- Frequency and ambient conditions
- Number of cycles for each specimen
- S-N curve and fracture-surface assessment
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
- Aerospace and automotive component specifications
- Pressure-vessel design standards
Mekanik Testler — What Does This Testing Field Cover?
The section below describes the field that the 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 Fatigue
How long does the fatigue test take?
It depends on the target number of cycles and the frequency. High-cycle (10⁷) tests can take weeks; scheduling is therefore clarified at the quote stage.