Metal Fatigue

Updated 23 October 2009.

The term metal fatigue refers to gradual degradation and eventual failure that occur under loads which vary with time, and which are lower than the static strength of the metallic specimen, component or structure concerned. The static strength is the load which causes failure in one application. The loads responsible are called fatigue loads. These loads are cyclic in nature, but the cycles are not necessarily all of the same size or clearly discernible. A fatigue load in which individual cycles can be distinguished is sometimes called a cyclic load.

Metal fatigue is largely a descriptive subject, and as such it has accumulated an enormous literature. Nevertheless, the basic concepts needed for an understanding of the metal fatigue literature are reasonably straightforward. The descriptions can be divided into two groups, metallurgical and mechanical. Metallurgical descriptions are concerned with the state of the metal before, during and after the application of fatigue loads, and are usually taken to include the study of metal fatigue mechanisms. Mechanical descriptions are concerned with the mechanical response to a given set of loading conditions, for example the number of load cycles needed to cause failure. Mechanical descriptions are more useful from an engineering viewpoint, where service behaviour must be predicted.

Rigorous definition of exactly what is meant by metal fatigue is difficult and not particularly helpful for its understanding. An early dictionary definition is: the condition of weakness in metal caused by repeated blows or long-continued strain. A more recent definition is: failure of a metal under a repeated or otherwise varying load which never reaches a level sufficient to cause failure in a single application. A Wikipedia definition of fatigue is: in material science, fatigue is the progressive, localised, and permanent structural  damage that occurs when a material is subjected to cyclic or fluctuating strains at nominal stresses that have maximum values less (often much less than) the static yield strength of the material. The resulting stress may be below the ultimate tensile stress, or even the yield stress of the material, yet still cause catastrophic failure. Metal Fatigue has also been hijacked as the name of a computer game.

From an engineering viewpoint, metal fatigue matters because it is a major potential cause of failure of components and structures, including load bearing consumer items. In a sense, the problem might appear to have been largely solved, in that catastrophic failures due to metal fatigue are now rare. Official inquiries into catastrophic metal fatigue failures, involving loss of life or major financial loss, usually indicate a clear reason for the failure and often indicate apparent human negligence. Lesser metal fatigue failures are still common and cause a great deal of inconvenience and expense. In other words, they are a nuisance rather than catastrophic. These lesser failures are often unrecognised as being due to metal fatigue unless they happen to be seen be an expert.

In essence, the mechanisms involved in the fatigue failure of a plain metallic specimen with a polished surface are simple, although the details may be complex, especially when viewed at smaller scales, and may vary between different metals. Firstly, a fatigue crack is initiated at the specimen surface, secondly the crack propagates slowly across the specimen and finally, when the net cross section has been sufficiently reduced, a static failure takes place on the final load cycle. In the presence of cracks, crack like defects (such as some types of inclusions), and sharp notches the crack initiation phase is largely absent. Because of the nature of metal fatigue mechanisms, the fatigue lives of specimens, components and structures are sometimes dominated by fatigue crack initiation, and sometimes by fatigue crack propagation. From a practical viewpoint, probably the most significant advance in the understanding of metal fatigue behaviour was the general realisation, some four decades ago, that many components and structures are crack propagation dominated. Cracks, or crack like flaws, may be introduced during manufacture, especially if welding or casting is used, or cracks may form early on during service.

The first known, reasonably well documented, metal fatigue failures were in clock mainsprings. The use of uncoiling springs, rather than descending weights, as a driving force was an important factor in the development of clocks for general use, and appears to have started in the early fifteenth century. By the late eighteenth century the technology for the manufacture of durable watch and clock mainsprings was well established; a detailed description of the state of the art of making watch springs was published in 1780. Even so, high quality watches and clocks were designed (and still are) so that a broken mainspring could easily be replaced. This shows that metal fatigue failures were indeed a problem.

The first known catastrophic fatigue failure, involving major loss of life, was the Versailles (France) railway accident in 1842. The train was unusually long, with 17 carriages hauled by two steam engines. The front axle of the leading, four wheeled engine failed due to metal fatigue and the body of the leading engine fell to the ground. The second engine smashed it to pieces. Following carriages passed over the wreck and some were set on fire. This, and numerous other railway axle failures, led to extensive investigations into the nature of metal fatigue.

For more information on metal fatigue see the Publications page.

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