Question

1. Explain, at the microscale, the process of fatigue, with emphasis on why metals experience fatigue.
2. A pressure vessel is made of aluminum alloy and operates at 15 cycles per minute. It operates 50% of the time at a stress amplitude of 1400 psi; the fatigue life at this stress level is 1,000,000 cycles. The other 50% of the time a stress amplitude of 2100 psi is exerted on the vessel; the fatigue life for the material at this stress level is 225,000 cycles. Find the expected life in hours for this hydraulic pressure unit.

Answer 1

Fatigue in materials is defined as the process of progressive and localized plastic failure occurring in the material resulting due to exposure to cyclic loading. These localized cyclic stresses and strain cumulatively form cracks and lead to material failure.

Fatigue failure is localized and cumulative in nature. It can occur at a significantly lower load than the strength of the material. Fatigue failure begins at microscale caused by localised stress concentration, generally at the locations of geometric irregularities, causing minute plastic deformations in the material structure. The microplastic deformations with time and repetitive loads, progress and combine to form micro-cracks in the metal structure.  Crack initiation is a very complex process affected by many factors. Once the cumulative deformation cause crack initiation in the metal surface the crack propagation starts. It has been experimentally and analytically concluded that the crack propagation occurs at a significantly lower load than crack initiation. The micro-cracks progress quickly through the metal structure as it progressively forms the centre of weakness.  

Generally, points of surface irregularities form the centres of stress concentration leading to crack initiation. In the absence of any geometric irregularities, the crack initiation is directly proportional to the yield strength of the material.

Once the crack initiation begins in the metal surface, the crack also serves as locations of entry of corrosive agents, such as Chlorides and moisture inside the metal structure. The localised cracks in metals also become sites of micro electro-voltaic cells (cation and anion) where corrosion/oxidation of metals starts. The cumulative effect of these external deteriorating agents and internal weakness in metal structure caused due to cracking ultimately leads to material failure.