Exam Question

(b) Premature fatigue failure is prevented by careful attention to detail at the design stage to ensure that cyclic stresses are sufficiently low to achieve the required endurance. Stress concentrations should be avoided where possible; a design with smooth 'flowing' lines is usually the optimum. From the graph, it can be seen that if the material is subjected to high stress, it will last for less number of cycles. If the stress is within the limit, it will last for an infinite number of cycles.
(c) Fatigue failure is when the surface of a material begins to crack or fracture, causing the part to weaken. This occurs when structures are subjected to high stress over an extended period of time. The main factors contributing to fatigue failures include high tensile stress with a large amount of variation or fluctuation in the way it is applied, particularly when this is repeated over a large number of cycles. Other factors include stress concentration, corrosion, temperature, overload and weaknesses in the structure of a metal.
Mechanical Fatigue damage develops as a result of exposure to off-cyclic stresses for an extended period of time. Components that are designed for alternating mechanical stresses are not usually subject to fatigue failure unless damaged by some other mechanism (e.g. foreign object impact damage).
Vibration Fatigue is a type of mechanical fatigue caused by vibration of equipment or piping during operation. This could occur as a result of operating equipment beyond designated integrity operating windows. Vibration-induced fatigue damage is typically caused by poor design, lack of support (or dampeners), or excessive support or stiffness. The amplitude and frequency of vibration are critical factors for vibration fatigue damage that leads to crack initiation and crack propagation.