Pressure Vessels - Primary and Secondary Stress

Primary stress results in immediate catasrophic failure. For instance, if the wall thickness of a vessel is not sufficient to withstand the pressure of the hydrotest, the vessel will rupture. Secondary stress will result in fatigue failure after enough cycles have occurred. For example, if the vessel is heated up to operating temperature, it will expand, and some parts may even yield, but there won't be a failure until a certain number of thermal cycles have occurred. The vessels I typically work with run at operating temperature for a year or more between shutdowns, so the number of cycles is low.