4. Surface finish

It has long been known that roughly machined mechanical parts fail faster than perfectly polished ones. This is because fatigue cracks usually initiate on the surface of mechanical parts before leading to failure. It should be noted that, during the machining of a mechanical part, the creation of surface roughness is always accompanied by a change in the structural state of the material's surface layers (work hardening, thermal phenomena) and a change in the state of stresses (residual stresses).

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