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Mechanical Properties of Gray Iron - Fatigue Properties

Metals which are subjected to repeated or fluctuating loads, such as alternating between tension and compression, can break after a large number of loading cycles even though the maximum stress was well below the static strength of the metal. This type of fracture is called a fatigue failure, although the rate of load application or the length of time over which the cycles occur are not significant. The occurrence of a fatigue crack is directly influenced by the maximum unit stress and the cumulative number of times it is applied.

A fatigue crack starts in an area of high stress concentration after a large number of loading cycles. It is always a brittle type of fracture even when occurring in ductile metals. As the crack progresses, it increases the stress concentration, and the rate of propagation under the cyclic loading increases. When the cross section of the remaining metal becomes insufficient to support the maximum load, complete failure occurs as it would under an excessive steady stress.

The number of stress applications that will induce a fatigue failure is less at higher maximum stress values, and conversely a larger number of stress cycles can occur at a lower maximum stress level before a fatigue crack is initiated. When the number of cycles without failure exceeds 10 million, the endurance life is considered infinite for body-centered-cubic ferrous metals. The maximum stress that will allow this number of cycles is established as the endurance limit, or the fatigue strength or fatigue limit.

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