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 of loading under 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 10⁷ to 10⁸ for 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 fatigue machines. These are the most common type of fatigue machines. Bending fatigue machines can be classified as:
  • Cantilever beam machines. 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 and compression on 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 of cyclic 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 as endurance 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 with fatigue strength, which is the maximum stress a material can withstand for a given number of loading cycles. Some metals, such as ferrous and 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: The Relationship Between Corrosion Fatigue and Stress Corrosion Cracking.)

For a design engineer, a construction material's fatigue or endurance limit is 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 common failure modes for many engineering applications often due to poor design considerations.

For alloys with a distinct limit, endurance limit can be defined as the maximum value of the reverse bending stress the 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 the ultimate tensile strength. This is not the case for high-strength steels, which do not show a fatigue limit after 10⁹ cycles. For these alloys, the crack initiates in the interior -- such as at an inclusion.

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 a weld joint may 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?

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. Fatigue fractures are a result of the simultaneous action of cyclic stress, tensile stresses and 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 a microstructure level. These defects, or surface notches, serve as regions of stress concentrations, thus amplifying the applied stress and encouraging fatigue crack initiation and propagation. (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.