Define endurance strength mechanical11/13/2023 In such cases a fatigue strength S f for a given number of cycles must be specified. These materials instead display a continuously decreasing S-N response, similar to Curve B in Figure 1. Many non-ferrous metals and alloys, such as aluminum, magnesium, and copper alloys, do not exhibit well-defined endurance limits. This is characteristic of steel and titanium in benign environmental conditions.Ī typical S-N curve corresponding to this type of material is shown Curve A in Figure 1. If the applied stress level is below the endurance limit of the material, the structure is said to have an infinite life. Most materials, however, exhibit a continually falling curve as in (b) and the usual indicator of fatigue strength is to quote the stress below which failure will not be expected in less than a given number of cycles which is referred to as the endurance limit.Ĭertain materials have a fatigue limit or endurance limit which represents a stress level below which the material does not fail and can be cycled infinitely. The difficulty is a localized stress concentration may be present or introduced during service which leads to initiation, despite the design stress being normally below the 'safe' limit. In principle, components designed so that the applied stresses do not exceed this level should not fail in service. Some materials, notably low-carbon steels, exhibit a flattening off at a particular stress level as at (a) in Figure 1 which is referred to as the fatigue limit.Īs a rough guide, the fatigue limit is usually about 40% of the tensile strength. Owing to the statistical nature of the failure, several specimens have to be tested at each stress level. S-N test data are usually displayed on a log-log plot, with the actual S-N line representing the mean of the data from several tests. The data are plotted in the form of a stress-number of cycles to failure (S-N) curve. rotating bend, cantilever bend, axial push-pull and torsion. Standard laboratory test use various methods for applying the cyclic load, e.g. Fatigue strength is determined by applying different levels of cyclic stress to individual test specimens and measuring the number of cycles to failure.
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