Cyclic stress: Fatigue

Abstract

Components in engineering applications operating under cyclic loads, commonly known as fatigue, may become unstable and cause catastrophic failure to occur unexpectedly because of structural instability. It is generally thought that over 80% of all service failures are associated with fatigue. Therefore, operation of machines or their components under cyclic loads are of prime concern. To overcome the difficulty in predicting fatigue failure—because of a large spread of statistical results—it is essential to use many test specimens to reach a meaningful average value below which the probability for fatigue fracture is quite low. Fatigue-resistance evaluation is done by plotting applied stress against the number of cycles, usually referred to as the S-N (curve) relation. In some cases a horizontal line is observed in the plot known as the “knee” representing the endurance limit. At this level of stress or below it, ceramics have the ability to endure a large number of stress-cycles without failure. A favored location of failure initiation is the surface; therefore good surface finish (often by polishing) is recommended which significantly improves fatigue resistance. Introducing compressive stress by any of the following methods, namely, laser treatments, sand blasting or shot peening improve greatly the fatigue resistance. Regardless of the origin of stress when cycling is applied, fatigue damage may result. Thus stress cycling associated with temperature changes is of great concern because it can induce fatigue damage known as thermal fatigue with premature failure in components operating at elevated temperatures. Design to overcome fatigue failure and to increase resistance to cyclic deformation is essential. Environmental effects, among them corrosion, are important in design considerations. Corrosive environments may accelerate the growth offatigue cracks, which initiate at the surface and, therefore, reduce overall fatigue performance.