What is Primary Stress?
Primary stress is developed by imposed mechanical loading (forces) most frequently due to weight and pressure. Excessive primary stress causes gross plastic deformation and rupture. Primary stresses are not self-limiting.
Once plastic deformation begins it continues unabated until force equilibrium is achieved, or until failure of the cross section results. Failure can be prevented only by removal of the loading or strain hardening in the material. Allowable limits for sustained stresses are related to the material yield stress, the ultimate strength, or time-dependant stress rupture properties, depending on temperature.
Primary stresses may be further categorized as general primary membrane stress, local primary membrane stress, and primary bending stress. These three categories are of interest because the pipe will not fail until the entire cross section has reached the yield strength.
Local primary stresses may exceed yielding; however under this stress state they will behave as secondary stresses and redistribute themselves as the local pipe wall distortion occurs. The failing moment would be that required to put the entire cross section of pipe in plastic behaviour, not just the extreme fiber.
What is Secondary Stress?
Secondary stress is developed in a piping system owing to constraint of the system against displacements, whether thermal expansion or imposed anchor and restraint movements. Distortion of the piping system as well as local yielding tends to relieve the developed stresses due to imposed displacements, so these stresses are said to be self-limiting. On the other hand, primary stresses are not self-limiting because, as they increase, local yielding does not reduce them. For example, as the internal pressure increases, the pipe wall stresses will keep increasing until the pipe ruptures.
Secondary stresses can result in catastrophic failure often after some time (usually high) number of applications of load. Just because a system has been running for many years does not mean that the system has been properly designed for fatigue.