In a pressure vessel that undergoes cyclic thermal operation with the simultaneous imposition of loading beyond the elastic limit at peak temperature (e.g. a boiler, or a rocket engine), ratcheting occurs when plastic deformation occurs with each load/thermal cycle. In some cases, only the inner wall is beyond the yield point at high temperature, but upon cooling the plastic strain accumulated in the inner wall is enough to force yielding of a small area of the outer wall, and this accumulation of plastic strain increment occurs for every pressure/temperature cycle of operation. The pressure vessel then has a design cycle limit, and surpassing that limit will rupture the vessel.
