How can I determine the endurance limit of steel and how does it get measured properly?

Fatigue is defined as the damage that occurs in a material due to the repetitive application of loads that may be substantially below its yield point. Most engineering materials contain defects at a microstructure level. These defects serve as regions of stress concentrations, thus amplifying the applied stress and encouraging fatigue crack initiation and propagation. (Learn more in the article The Effects of Stress Concentration on Crack Propagation.)

The value of the stress below which a material can endure an infinite number of repeated load application cycles without failure is known as the fatigue limit, or endurance limit. 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 indefinite number of repetitive cycles from that specific loading. Endurance or fatigue limit should not be confused with fatigue strength, which is the maximum stress a material can withstand for a given number of loading cycles.

The endurance limit of steel can be measured in several ways. Specimens are usually tested using various types of fatigue testing machines depending on the mode of fatigue and kind of loading under consideration. Once the sample is loaded in the test device, it is subjected to a specific alternating stress and tested to failure. The load is gradually reduced until the sample is able to withstand a sufficiently large number of cycles (i.e., around 10⁷ to 10⁸) without breaking. This value of this load is termed the endurance limit.

Some of the most common machines used to determine the endurance limit of steel components include:

• Axial (direct-stress) testing machines – The test specimen is subjected to a uniform alternating axial stress or strain throughout its cross-section.
• Bending fatigue machines – These are the most common type of fatigue machines. Bending fatigue machines can be classified as:
  • Cantilever beam machines – A tapered cantilevered specimen (one fixed and one free end) is subjected to a cyclic load at the free end, resulting in alternating tension and compression on the top and bottom faces of the sample. (Some ways to measure compression and other forces are examined in the article 6 Tests to Measure a Material's Strength.)
  • Rotating beam machines – 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 of cyclic stresses.
• Torsional fatigue testing machines – Torsional fatigue testing machines apply an alternating clockwise and counterclockwise rotational stress on the specimen. As with all endurance limit tests, the rotational stress is reduced until the sample remains intact after a sufficiently large number of load cycles.