Fatigue Basics - CAESAR II - Help

CAESAR II Users Guide

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Piping and vessels have been known to suffer from sudden failure following years of successful service. Research done during the 1940s and 1950s, primarily advanced by A. R. C. Markl’s "Piping Flexibility Analysis," published in 1955, provided an explanation for this phenomenon, as well as design criteria aimed at avoiding failures of this type. The explanation was that materials were failing due to fatigue, a process leading to the propagation of cracks, and subsequent fracture, following repeated cyclic loading.

Steels and other metals are made up of organized patterns of molecules, known as crystal structures. However, these patterns are not maintained throughout the steel producing an ideal homogeneous material but are found in microscopic isolated island-like areas called grains. Inside each grain a pattern of molecules is preserved. From one grain boundary to the next the molecular pattern is the same, but the orientations differ. As a result, grain boundaries are high energy borders. Plastic deformation begins within a grain that is subject to both a high stress and oriented such that the stress causes a slippage between adjacent layers in the same pattern. The incremental slippages, called dislocations, cause local cold-working. On the first application of the stress, dislocations can move through many of the grains that are in the local area of high stress. As the stress is repeated, more dislocations move through their respective grains. Dislocation movement is impeded by the grain boundaries. After multiple stress applications, the dislocations tend to accumulate at grain boundaries. Eventually they become so dense that the grains "lock up" causing a loss of ductility and thus preventing further dislocation movement. Subsequent applications of the stress cause the grain to tear, forming cracks. Repeated stress applications cause the cracks to grow. Unless abated, the cracks propagate with additional stress applications until sufficient cross sectional strength is lost to cause a catastrophic failure of the material.

You can estimate the fatigue capacity of a material through the application of cyclic tensile/compressive displacement loads with a uniaxial test machine. A plot of the cyclic stress capacity of a material is called a fatigue or endurance curve. These curves are generated through multiple cyclic tests at different stress levels. The number of cycles to failure usually increases as the applied cyclic stress decreases, often until a threshold stress, known as the endurance limit, is reached below which no fatigue failure occurs, regardless of the number of applied cycles. An endurance curve for carbon and low alloy steels, taken from the ASME Section VIII Division 2 Pressure Vessel Code displays below: