What distinguishes shakedown from ratcheting in pressure vessels?

Shakedown is characterized by the idea that the material behaves in a stable manner under repeated loading—once a structure reaches a stable state, it can accommodate cyclic loads without experiencing further plastic deformation. Essentially, in shakedown, the material yields once and then establishes new load paths or internal stresses that allow it to handle future loads within its elastic limit, maintaining structural integrity without further changes.

On the other hand, ratcheting refers to a progressive accumulation of plastic deformation with each application of load. In this scenario, even though the overall load may not exceed the material's yield strength, the repetitive nature of the loads leads to a gradual shift in the structure's shape and stiffness, resulting in permanent deformation.

The distinction lies inherently in the nature of the responses: shakedown leads to a stable state where no additional deformation occurs under cyclical loads, whereas ratcheting causes ongoing, cumulative changes within the material. Understanding this difference is crucial in the design and analysis of pressure vessels, as it impacts the longevity and safety of the vessel under repeated loading conditions.