A rod in the control system of the lift in a mine-shaft is pre-tensioned with a tensile stress of 150 MPa. During the operation of the lift the rod experiences an additional stress due to braking of 200 MPa each operation of the lift.
The rod contains an internal penny-shaped defect with a radius of 2 mm which is perpendicular to the axis of the rod. The steel has a fracture toughness of 50 MPav m. For the fracture mechanics calculation it is possible to assume that the geometrical factor Y is equal to 1.12.
The fatigue crack growth rate, da/dN, obeys the Paris equation:
da/dN = 10 ^-11 (?K)^3
where da/dN is in m/cycle and ?K is in MPav m.
Calculate the expected life of the rod assuming that the lift is used 10 times per day.
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