Industrial Engineering

Understanding a fatigue limit of a material

The stress level below which an unlimited number of loading cycles can be given to material without inducing fatigue failure is known as the fatigue limit or endurance limit. Aluminum and copper, on the other hand, do not, and eventually fail even from small stress amplitudes. Some metals, such as ferrous alloys and titanium alloys, have a clear limit.

The phrase “fatigue strength” or “endurance strength” is used when a material doesn’t have a clear limit and is defined as the highest amount of entirely reversed bending stress that a material can bear for a predetermined number of cycles before failing from exhaustion.

fatigue material

Cycled stresses, residual stresses, material characteristics, internal flaws, grain size, temperature, design geometry, surface quality, oxidation, corrosion, etc. all have an impact on fatigue life. There is a theoretical stress amplitude value for some materials, most notably steel and titanium, below which the material will not fail for any number of cycles. This value is known as a fatigue limit, endurance limit, or fatigue strength.

In this article, the following questions will be discussed:

  • What is a fatigue limit?
  • Who discovers the fatigue limit?
  • Definitions of fatigue limit
  • What are the typical values for the fatigue limit?

 

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What is a fatigue limit?

The stress level below which an unlimited number of loading cycles can be given to material without inducing fatigue failure is known as the fatigue limit or endurance limit.

To calculate a material’s fatigue life, engineers employ a variety of techniques. The stress-life approach, which is among the most useful, is frequently characterized by an S-N curve, also known as a Wöhler curve. The figure displays this technique. Plotted against component life or the number of cycles to failure is the applied stress (S) (N).

Component life grows slowly at first and then quite quickly as the stress drops from a high value. The data used to plot the curve will be handled statistically because fatigue, like brittle fracture, has such a variable nature. The results scatter is a result of the fatigue sensitivity to several tests and material parameters that are difficult to properly regulate.

Cycled stresses, residual stresses, material characteristics, internal flaws, grain size, temperature, design geometry, surface quality, oxidation, corrosion, etc. all have an impact on fatigue life. There is a theoretical stress amplitude value for some materials, most notably steel and titanium, below which the material will not fail for any number of cycles. This value is known as a fatigue limit, endurance limit, or fatigue strength.

Read more: Understanding compressive strength

Who discovers the fatigue limit?

August Wöhler first proposed the idea of an endurance limit in 1870. Recent studies, however, contend that there are no endurance limits for metallic materials and that, given enough stress cycles, even the lowest stress will eventually result in fatigue failure.

Definitions of fatigue limit

The following terms are defined for the S-N curve:

Fatigue limit

The stress level below which fatigue failure does not occur is known as the fatigue limit (sometimes referred to as the endurance limit). Only some titanium and ferrous (iron-based) alloys can reach this limit because the S–N curve for these materials becomes horizontal at higher N values. Other structural metals, like aluminum and copper, lack a clear failure point and gradually give out even under minor stresses. Standard limits for steels range from 290 MPa to 1/2 the ultimate tensile strength (42 ksi).

Fatigue Strength

According to the ASTM, fatigue strength, or SNf, is the stress level at which failure occurs after a predetermined number of cycles (e.g., 107 cycles) For instance, the annealed Ti-6Al-4V titanium alloy has a fatigue strength of around 240 MPa at 107 cycles and a stress concentration factor = 3.3.

Fatigue life

The fatigue behavior of a material is defined by its fatigue life. According to the S–N plot, it is the number of cycles necessary for failure to occur at a given stress level.

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There are three distinct steps that make up the fatigue failure process:

Fracture initiation occurs when a minor crack develops at a location where there is a concentration of high tension. Crack propagation, in which each stress cycle causes the crack to move forward a little bit. The phase of crack growth often consumes the majority of the fatigue life. Once the expanding crack reaches a crucial size, ultimate failure happens extremely quickly.

At some point of stress concentration on a component’s surface, cracks linked to fatigue failure nearly invariably start (or “nucleate”). Any factor that increases stress concentration and the occurrence of cracks will shorten fatigue life. As a result, fatigue life is improved by polishing rather than grinding to a higher degree of surface finish. The fatigue life of metal components will also be improved by strengthening and hardening the surface layers.

Watch the video below to learn more about a fatigue limit:

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What are the typical values for a fatigue limit?

The limit (Se) for steels typically ranges from 290 MPa to half of the ultimate tensile strength (42 ksi). (Se) is typically 0.4 times the ultimate tensile strength for alloys made of iron, aluminum, and copper.

The maximum usual values for copper are 97 MPa, aluminum 130 MPa (19 ksi), and iron 170 MPa (24 ksi) (14 ksi). Note that these values apply to test specimens that were smooth and “un-notched.” For specimens with notches, the endurance limit is much lower.

The fatigue limit for polymeric materials has been demonstrated to represent the inherent toughness of the covalent bonds that must be broken in order to extend a crack. When loads are kept below the inherent strength, a polymer can run indefinitely without fracture formation as long as other thermochemical processes do not disrupt the polymer chain.

Read more: Understanding Brittleness of materials

In summary

To calculate a material’s fatigue life, engineers employ a variety of techniques. The stress-life approach, which is among the most useful, is frequently characterized by an S-N curve, also known as a Wöhler curve. The figure displays this technique. Plotted against component life or the number of cycles to failure is the applied stress (S) (N).

The stress level below which an unlimited number of loading cycles can be given to material without inducing fatigue failure is known as the fatigue limit or endurance limit. That is all for this article, where the following questions are being answered:

  • What is a fatigue limit?
  • Who discovers a fatigue limit?
  • Definitions of fatigue limit
  • What are the typical values for a fatigue limit?

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