Mechanical properties and fracture
Which statement about fatigue S-N curves is correct?
- ACarbon steels often show a fatigue limit, a stress amplitude below which they rarely fail however many cycles are applied
- BAluminum alloys show a more distinct fatigue limit than carbon steels, with the S-N curve becoming fully flat from around 10⁶ cycles
- CA larger tensile mean stress keeps cracks from opening and slows their growth, so the fatigue life at the same stress amplitude is longer
Show answer and explanation
Answer: A. Carbon steels often show a fatigue limit, a stress amplitude below which they rarely fail however many cycles are applied
For carbon and low-alloy steels, the S-N curve often flattens out at around 10⁶–10⁷ cycles, showing a fatigue limit below which failure becomes unlikely. FCC metals such as aluminum and copper alloys rarely show a distinct flat portion, so they are rated by the strength at a set number of cycles such as 10⁷ (the fatigue strength). A tensile mean stress acts to open cracks and speeds their growth, so the life is shorter at the same stress amplitude, and corrections are made with tools such as the Goodman diagram. Note that in corrosive environments or in the very-high-cycle regime beyond 10⁸ cycles, even steels may not show a clear fatigue limit.
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