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
Metal and Metallurgy Testing — 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.
Metal and metallurgical testing determines the load a metal or a welded joint can carry, how it fails and what its internal structure looks like. The output of this family is often not about the product itself but about the identity of the MATERIAL: is the steel really of the declared grade, does the weld seam deliver the strength the design assumes, does the casting contain an unacceptable internal defect?
The practical importance of the field comes from the fact that a metal part rarely fails gradually. A plastic part deforms, ages, cracks and usually gives warning. A weld seam or a fastener, by contrast, often breaks without warning — especially where fatigue and brittle fracture are involved. That is why in fields such as pressure equipment, lifting gear, structural steel and pipelines, material testing is a requirement of REGULATION rather than of design.
The family brings together three groups of methods, and each answers a different question. Mechanical tests quantify how the material behaves under load. Non-destructive examination looks for defects without damaging the part. Metallography and chemical analysis explain why the material behaves the way it does — through grain structure, phase distribution, inclusion content and alloy composition.
The acceptance criterion for these tests almost never comes from the test standard, and this is the key to understanding the field. The test standard says how the measurement is to be made; the acceptable values come from the product standard, the material grade or the fabrication code. ISO 6892-1 tells you how to run a tensile test; EN 10025 tells you what yield strength that steel must deliver. When reading a report, the product standard it was assessed against is essential.
Sampling is an inseparable part of the result in this field. Two specimens taken from the same heat may give different values depending on the direction and location they were taken from; in rolled products, specimens parallel and perpendicular to the rolling direction produce different results. Product standards therefore also govern sampling location and orientation, and the report must document this.
Method groups in this field
Mechanical tests
Tensile, compression, bending, notch impact (Charpy), hardness (Brinell, Vickers, Rockwell), fatigue, creep and fracture toughness. These give the behaviour of the material under load in numerical form.
Non-destructive examination
Radiography, ultrasonic examination, penetrant, magnetic particle and visual examination. It looks for surface and internal defects without damaging the part and is an indispensable step in welded fabrication.
Metallography
Specimen preparation, etching and microscopic examination. Grain size, phase distribution, inclusion content, case depth and the weld heat-affected zone are assessed.
Chemical analysis
Alloy composition is verified by optical emission spectrometry and wet chemistry; confirmation of the material grade runs through this step.
Corrosion behaviour
Salt spray, pitting and crevice corrosion and intergranular corrosion tests. In stainless steels these can indicate the quality of the heat treatment.
Commonly confused in this field
- A material certificate (such as EN 10204 3.1) is not a test report; it is the manufacturer's declaration. In critical applications independent verification is carried out, and the two must not be confused.
- ASTM and ISO tensile tests measure the same quantities but use different specimen geometries; the results are not treated as directly equivalent, and whichever the specification calls for is the one applied.
- Conversion tables between hardness and strength are APPROXIMATE. Calculating yield strength from hardness is not sufficient evidence for material acceptance.
- In non-destructive examination, "no defect found" does not mean there is no defect; defects below the detection limit of the method may be present. The report must state the method applied and the sensitivity level.
- A weld seam passing does not show that the welder or the procedure is qualified; these are separate processes.
Which decisions these results support
- Verifying the material certificate and accepting incoming material
- Approving welded fabrication; welder and procedure qualification
- Manufacturing control of pressure equipment and structural components
- Failure analysis and determining the root cause of field damage
- 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.