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In context of machine design, what is mean fatigue strength? (as I am thinking I am mixing it up with midrange strength)

In my class we deal a lot with steels and this is a low strength steel, but I can't find those three words "mean fatigue strength" put together followed by a definition and/or formula...

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Fatigue is failure cause by cycling loads which do not exceed the yield stress of the material, typically over tens of thousands of cycles. Not to be confused with work hardening which occurs over relatively few cycles at stresses exceeding the yield stress of the material.

Fatigue strength is usually defined by the stress at which a material will exhibit no fatigue however many load cycles it is exposed to (aka endurance limit) or the load at which a sample will fail after N cycles. Also fatigue properties are a statistical distribution rather than an absolute value hence the qualification with 'mean', although this is often omitted as it is implicit in the definition.

In many cases low alloy steels have a fatigue strength which is close enough to their yield stress that once factors of safety have been taken into account it becomes a non-issue.

Fatigue limts can also be affected by the surface condition of the sample eg surface roughness, notches etc.

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Steel has an endurance limit, other metals do not. It is about half of the tensile strength.So steels cyclically stressed in a range between zero and one half of their tensile strength, will have very long life ( I hate to say infinite). Otherwise , fatigue strength for other metals and steel stressed above half of tensile is a specific number of cycles at a specific stress level.

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