(a) Where do fatigue failures usually originate on a metal section? (b) Distinguish between mechanical fatigue, thermomechanical fatigue, thermal fatigue and corrosion fatigue. (c) Differentiate between the S-N curves of plain carbon steels and aluminum alloys. (d) Why is it important to have a smooth surface on a sample used for rotating beam fatigue test? (e) State four ways to increase the fatigue life of a component.