Avoiding Failures of Metals
Growth of fatigue cracks under cyclic loading by Ewing & Humfrey, 1903.
The most common cause of failure of metals in service is fatigue. The causes of failures due to static overload are normally
detected and corrected prior to the item being placed in service. It is important to note that fatigue failures often occur when the
metal is loaded below its design limits. It has been found, based on extensive experience, that an important cause of fatigue
failures is the geometry of the component rather than its material. However there are other causes of fatigue failures as well.
Many cases of fatigue failures could have been prevented with proper design techniques such as the elimination of stress
risers. Examples of stress risers include notches, holes, cracks, and diameter changes in the material.
Drawing of fatigue failure at a stress concentration (seen at arrow) in a railroad axle by Joseph Glynn, 1843. The failure led to
the deaths of 55 passengers, and spurred serious inquiry into the mechanisms of crack growth at stress risers under cyclic
Another cause of failures is that due to fretting. Fretting is defined as wear at the asperities , or unevennesses of surfaces
in contact. Wear damage is induced by a combination of load and minute relative motion between the surfaces. The
relative motion may be caused by vibration, repetitive load cycles, thermal expansion, or other forces. The motion can also
be quite small, ranging from micrometers to millimeters, often making it difficult to detect.
Material asperities with no loading (top) and with loading (bottom). Image source: Wikimedia commons,
Mechanical wear at the asperities begins a cycle that rapidly decreases the fatigue strength of the materials. Contact at the
surfaces causes material to break loose, leaving freshly exposed metal and debris, which can freely move between the two
surfaces. By oxidation, this debris can become much harder than the material it broke away from and act as an abrasive,
increasing the rate at which further debris particles are loosened. These new particles can then harden and act as further
abrasive, and so on.
Fretting damage due to microscopic movement. Source:
techcan.com troubleshooting analysis.
It is important to note that the type of relative motion that causes fretting can occur where two materials with different thermal
expansion coefficients are mated in an environment that undergoes repeated heating and cooling cycles. Examples may include
cylinder head gaskets of automotive engines, and connecting joints of outdoor mechanical structures which must endure
repeated temperature cycles with weather changes.
Damaging effects of fretting are not limited to failures of material strength in metals, but also may cause a loss of electrical
conductivity in connective components over time. Lost electrical connectivity during operation may lead to machine failure or
worse. Commercial anti-fretting agents are sometimes employed to provide conductive lubrication at connections to reduce risk
of electrical failure due to fretting.
In both the mechanical and electrical scenarios, steps should be taken to identify and prevent any relative motion of the parts in
Causes of failures in metals typically have origins in the design, fabrication, and use of the components in service.
•Avoid points of high stress concentrations
•Avoid sharp changes of section where such as fillets and keyways
•Be aware of the deleterious effects of notches, holes, threads, tool marks, and other stress raisers.
•Prevent relative motion of parts
•Avoid points of stress concentrations during fabrication such as weld defects, tool marks, cracks due to grinding and quenching,
•Incorporate steps to prevent decarburization and hydrogen embrittlement during processing
•Eliminate residual stresses due to press fits or cold-working occurring during assembly
•Control of fabricating techniques to prevent surface discontinuities such as folds or laps during casting or forging
•Limit service conditions to prevent exceeding the operating loads for which the part was designed
•Protect the components from corrosive atmospheres
•Perform inspection of highly stressed parts