How can I determine and measure steel's endurance limit?
Steel's endurance limit can be measured in several ways. A polished sample -- wherein potential surface initiation sites, such as machining flaws, have been removed, is tested under constant amplitude-alternating loads. Material fatigue limits can be measured by testing small samples or full-sized structural components. Specimens are usually tested using various types of fatigue testing machines, depending on the mode of fatigue and the type ofloadingunder consideration.
Once the sample is loaded into the test device, it is subjected to a specific alternating stress and tested to failure. The load applied for different test specimens is reduced until a sample is able to withstand a sufficiently large number of cycles (i.e., around 107to 108for conventional steels) without breaking. This load, or stress, is termed the endurance limit (also known as "fatigue limit.")
Some of the most common machines used to determine steel components' endurance limit include:
- Axial (direct-stress) testing machines.Here, the test specimen is subjected to a uniform alternating axial stress (tensile compression) or strain throughout its cross-section.
- Bending fatiguemachines.These are the most common type of fatigue machines. Bending fatigue machines can be classified as:
- 悬臂梁的机器。These tapered cantilevered specimens, with one fixed and one free end, are subjected to a cyclic load at the free end, resulting in alternating tension andcompressionon the top and bottom faces of the sample.(For more ways to measure compression, read:6 Tests to Measure a Material's Strength.)
- Rotating beam machines.In this case, a two-point load is applied to a simply supported rotating specimen. As the test sample rotates, the load acting on the specimen induces fluctuating bending stresses. The test is repeated with gradually reduced loading until a condition is reached where the specimen can resist high amounts ofcyclic stresses.
- Torsional fatigue testing machines.Torsional fatigue-testing machines apply an alternating clockwise and counterclockwise rotational stress on the specimen.
What is Endurance Limit?
Fatigue limit, also known asendurance limit, is the level of stress below which a material can endure an infinite number of repeated load application cycles without failure. In other words, once the material is subjected to a stress value that is below the endurance limit, it should theoretically be able to withstand an infinite number of repetitive cycles from that specific loading. Fatigue limit should not be confused withfatigue strength, which is the maximum stress a material can withstand for a given number of loading cycles. Some metals, such asferrousand titanium alloys, have a distinct limit. Others, such as copper and aluminum alloys, do not and will fail under even small vibratory stress amplitudes.(For more on fatigue limits, read:之间的关系Corrosion Fatigue and Stress Corrosion Cracking.)
For a design engineer, a construction material'sfatigue or endurance limitis a critical parameter for avoiding fatigue failures in long-life structural applications or components performing repetitive actions. Unfortunately, fatigue is still one of the most commonfailure modesfor many engineering applications often due topoor design considerations.
For alloys with a distinct limit, endurance limit can be defined as the maximum value of the reversebending stressthe alloy can withstand for a specified number of cycles without failing by fatigue. For conventional steels, fatigue limit can be obtained after about 107cycles with the crack initiating from the surface. A rule of thumb for these alloys is that fatigue limit occurs at about one-half of theultimate tensile strength. This is not the case for high-strength steels, which do not show a fatigue limit after 109cycles. For these alloys, the crack initiates in the interior -- such as at aninclusion.
It should be noted that a material's endurance limit is not an absolute or fully repeatable number and is subject to the test piece's test method and surface finish. Statistical analysis is required to calculate a given material's endurance limit. Note, too, that aweld jointmay have a different fatigue limit than the base metal even though they have similar compositions. Factors that can impact endurance limit include:
- Cold working.
- Heat treating.
- Environmental exposure.
- Surface notches.
- Stress risers.
What is Fatigue?
Fatigueis defined as the damage that occurs in a material due to the repetitive application of loads that may be substantially below itsyield point. Fatigue fractures are a result of the simultaneous action ofcyclic stress,tensile stressesand plastic strain.
Cyclic stress can be either mechanically or thermally applied and will initiate the crack. Tensile stress, on the other hand, will propagate the crack. Most engineering materials contain defects at amicrostructurelevel. These defects, or surface notches, serve as regions ofstress concentrations, thus amplifying the appliedstressand encouraging fatigue crack initiation andpropagation.(For more on stress concentration, read:The Effects of Stress Concentration on Crack Propagation.)
Testing Endurance Limit: Standards and Equipment
Fatigue instruments are typically universal servo-hydraulic or electromechanical testing machines. For very high cycle fatigue, ultrasonic testing instruments can be used. Many specimens are required and testing times can be quite long. Thus, databases are usually consulted by design engineers.
ASME Boiler and Pressure Vessel Codes, such as ASME Section VIII, Division 2, Part 5.5 Protection Against Failure from Cyclic Loading, provides robust fatigue curves for assuring fatigue predictions will be conservative and specifies explicit guidelines for treating welds.
ASTM standards associated with fatigue testing include:
- ASTM E466 (Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials).
- ASTM 606 (Standard Test Method for Strain-Controlled Fatigue Testing) and ASTM E1823 – Standard Terminology Relating to Fatigue and Fracture Testing).
Fatigue limit testing is often performed at ambient temperature. But, if conducted at elevated temperature, the temperature in the gage section must be constantly maintained. Typical atmospheres for the test tend to be inert, dry air or vacuum for elevated temperature tests.
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Written bySteven Bradley| Principal Consultant, Bradley Consulting Services
![Profile Picture of Steven Bradley](http://m.crimeawalks.com/images/uploads/bradleysteve5rweb.jpeg?width=400&height=400&mode=crop&format=jpg&quality=70&scale=both)
Steven Bradley is currently the principal consultant atBradley Consulting Serviceshaving retired after 44 years from UOP-Honeywell as Senior Research Fellow.
Steven is also membership chair of the ASM Failure Analysis Society and is a registered Professional Engineer in the state of Illinois. He has presented and authored/co-authored over 60 technical papers involving failure analyses of complex systems, materials characterization and advanced electron microscopy of materials and catalysts and holds 16 patents. Steven completed his BSSE and PhD in Materials Science and Engineering from Northwestern University.
In addition to being a member of ASM and ASTM, he is also a member of NACE and ACS.
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Related Terms
- Endurance Limit
- Fatigue Strength
- Fatigue
- Fatigue Failure
- 疲劳断裂
- Fatigue Wear
- Fatigue Crack Growth Testing
- Fatigue Loading
- Material Fatigue
- Fatigue Corrosion
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