Atube made of an aluminum alloy with ultimate strength u = 476 mpa has a diameter of 120 mm and a thickness of 4 mm. the tube is subjected to a constant bending moment of 2 kn·m and an internal pressure that repeatedly cycles between zero and a maximum of 15 mpa. determine: (a) the cycles that can be applied before failure nf, if the material follows the fatigue relationship a =a nf m, where a = 839 mpa and m = –0.102.