Failure criteria for pressure vessels

Thread Content

There are mainly four commonly used failure criteria for pressure vessels at present: ① Elastic failure criterion: It assumes that plastic deformation occurs when the metal on the inner wall of the vessel reaches the actual yield point of the material, resulting in the loss of the purely elastic state; at this point the vessel has failed. This criterion limits the vessel’s operation to a purely elastic state, as it believes that plastic deformation will alter the properties of the metal and increase the likelihood of corrosion. Therefore, the stress level is kept below the yield limit to prevent plastic deformation from occurring. ② Plastic failure criterion: It is assumed that when plastic deformation occurs in the material of the inner surface of the container, this plastic deformation is confined to a limited range due to the restraint exerted by the external elastic material, and thus the container does not reach a dangerous state. Only when plastic deformation spreads from the inner wall to the outer wall does the volume of the container expand significantly, leading to instability; it is at this point that the load-bearing limit is reached ； This criterion takes the overall yield of the vessel wall as an indicator of container failure. ③ Elastoplastic failure criterion: This criterion is applicable to situations involving repeated loading. It assumes that the stresses in different parts of the vessel play different roles in causing its failure. For example, even when the stress on the vessel walls is much lower than the material’s yield limit, local plastic deformation may have occurred in the areas where the cylinder meets the head or the connections to the nozzles. However, since these areas are surrounded by elastic regions, such local plastic deformation does not necessarily lead to the failure of the vessel under repeated loading. Damage accumulation only occurs once the ‘stability’ limit is exceeded, but destruction does not happen immediately. ④ Blowout failure criterion: For an ideal plastic material, once the container as a whole yields, plastic deformation continues to increase even if the pressure no longer rises, the wall thickness keeps decreasing, ultimately leading to the destruction of the container. The failure due to blasting theory holds that materials are not ideally plastic; due to strain hardening, the container will not break down if the pressure does not continue to increase. It is only when the pressure reaches a certain level that the container will burst and fail. In the design, a safety factor is applied to the blasting pressure based on the operating pressure ； This criterion is generally used in the design of ultra-high pressure vessels. Elastic failure criteria are used in the conventional design of pressure vessels, while plastic and elastoplastic failure criteria are employed in analytical design.