12. Cumulative Damage: Cycle Counting and Life Prediction

Loads so far considered had constant amplitude (CA), σ_a or S. This constitutes a particularity that rarely happens in real life. Usually, cyclic loads have variable amplitudes (VA) that are referred to as the load spectrum or time history. Even apparently simple operating conditions actually result in a more or less frequent variability of load amplitudes with time. Think about a pressure vessel that is heated up and cooled down every working day. During normal functioning at the nominal pressure the vessel can experience some pressure variations that depend on numerous factors, often aleatory. These pressure variations are generally small amplitude fluctuations, but they are applied to the vessel shell with high mean stress provided by the heat up and cool down cycle. In addition, from time to time there might be large temperature and pressure transients that results in major stress excursions that do not occur too often, but in a 20 or 30 years of expected life they can sum up to a consistent number.