History of Fatigue

Fatigue is the structural damage that results from repeated or otherwise varying stress which never reaches a level sufficient to cause failure in a single application. Fatigue is also the initiation and growth of a crack, or growth from a pre-existing defect, which progresses until a critical size is reached.

Today's design engineers in most industries are able to use different software based tools to predict durability with regard to the fatigue life of individual components. The tools from these software providers can help eliminate unexpected problems during product development which is critical, given the growing need for reduced costs and development time accompanied by an increase in quality requirements. What can these different industries do to develop products more quickly and cost efficiently? One possibility for cost reduction is to build lighter components that are less material intensive. To do this, and simultaneously meet all requirements regarding quality and safety, it is almost mandatory to use CAE (Computer Aided Engineering) software for durability and fatigue analysis.

Fatigue analysis as we know it today has come a long way. 170 years ago, in 1837, Wilhelm Albert published the first article on fatigue, establishing a correlation between applied loads and durability. Two years later, in 1839, Jean-Victor Poncelet, designer of cast iron axles for mill wheels, officially used the term "fatigue" for the first time in a book on mechanics.
Albert's fatigue tests of mining chains, sketch

The pioneering work of August Wöhler, summarized in his work on railroad axles in 1870, helped improve testing procedures of axles and thereby increased axle life. In addition, he developed the Rotating-Bending Fatigue test and introduced the concept of fatigue limit. Wöhler initiated the development of design strategies for fatigue and identified the importance of cyclic and mean stresses. By the end of the 19th century, Gerber and Godmann investigated the influence of mean stress and proposed simplified theories. In 1886, Johann Bauschinger wrote the first paper on cyclic stress-strain hysteresis behavior of materials (named later after himself: Bauschinger effect). Based on these theories and procedures, engineers implemented fatigue analysis in product development processes and were able to predict product life better than ever before. Despite these developments, fatigue analysis remained a niche methodology, carried out only by experts for many years to come.


Publication of Wöhler's fatigue experience, 1871

At the beginning of the 20th century, Sir James Alfred Ewing demonstrated the origin of fatigue failure in microscopic cracks and refuted the re-crystallization theory. This theory arose after one of the worst rail disasters of the 19th century occurred near Versailles in 1842, in which the leading locomotive broke an axle. In 1910, O.H. Baskin defined the shape of a typical S-N curve by using Wöhler's test data and proposed a log-log relationship. L.Bairstow simulataneously developed the concepts of cyclic hardening and softening by investigating stress-strain response during cyclic loading. With the work of Alan A. Griffith in 1920, investigating cracks in glass, the birth of fracture mechanics took place.

A first practical design tool came up with the work of A. Palmgren's (1924) and A. M. Miner in 1945. Both developed independent of each other the linear damage hypothesis. In the 1950's and 1960's, substantial progress was achieved in explaining fatigue crack growth, with L. F. Coffin and S. S. Manson and their achievements with plastic strain in the tip of cracks in 1954 and P. C. Paris in 1961. Paris proposed methods for predicting the rate of growth in individual fatigue cracks; this was the first systematic method for handling crack propagation using fracture mechanics.


Life time curve after Manson and Coffin

Particularly with regard to simulation methods used today, Tatsuo Endo and M. Matsuiski set a milestone in fatigue methods in 1968 when they devised a rainflow-counting algorithm, enabling the reliable application of Miner's rule to random loads.

In the 1980's and 1990', more focus was put on investigating multiaxial fatigue and thermo-mechanical fatigue, already being calculated with the help of computer technology. The changes and new possibilities in fatigue design have been significant with the use of simulation technology. It was possible to simulate real loadings under variable amplitude conditions with specimens, components, or full-scale structures. Integrated CAE restricted testing to component durability rather than using it for development. Increased digital prototyping with less testing has become a goal for the 21st century fatigue design.

nCode International is one company that has made significant contributions to the history of fatigue in the last 25 years. Established in 1982, nCode was one of the first suppliers of durability engineering solutions. nCode originally focused on providing expertise in durability, fatigue and materials testing for the British rail industry. Since then, nCode has established itself as a global leader in the field. The companies working with nCode represent manufacturers in markets such as Ground Vehicle, Aerospace, Military, Heavy Equipment, Fleet Operations, Oil and Gas, Power Generation, and Rail.


Workflow process displayed in the user interface of DesignLife

With tools such as GlyphWorks, DesignLife and FE-Fatigue, nCode has contributed to an easier and more widespread use of fatigue analysis during product development. Today there are a wide variety of possibilities for predicting durability. Depending on which data is available, the engineer can use real data (i.e. for loads) or simulated data, manipulate it and use it as boundary conditions for product life predictions. As simulation models become more complex and closer to the real world, it is expected that the emphasis in new product development will be more on simulation, and testing will be used to validate and/or complete those models. With Design Life, nCode now offers a workflow tool with unique features to integrate physical test data into fatigue analysis. This product provides the industry with a workflow tool that eliminates the need for speculation and hypotheses regarding load data, and facilitates establishing and executing the entire durability process within the design function.


Durability workflow process of a typical application in DesignLife

Autor(en): Dr. -Ing. Stephan Vervoort, nCode International; Dipl.-Ing. Gerd Wurmann, Blue Gecko Marketing GmbH

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