Austenitic stainless steel exhibits excellent ductility and high strain hardening. When the steel is loaded in tension to a stress σk above its proof strength and then unloaded a permanent plastic elongation will result. If the steel is loaded again it will remain elastic up to this higher stress. This characteristic of the strain hardening offers the space to increase the allowable stress of the steel by pressure strengthening. In practice the finished vessel was loaded by internal pressure to the strengthening stress of σk, which is well above the proof strength of the material and greatly improves the bearing capacity of the vessel. The strain energy function-based principle accompanied with a constraint condition was presented according to the true stress-strain curve of material to determine the strengthening stress of σk. Using this principle, the finite element analysis of pressure strengthening was performed where material and geometrical nonlinearity were considered in the large deformation computation of pressure vessel. Also, the strain under the load steps of the vessel was studied in strengthening operation process, which was coincident with numerical results. For the strengthened pressure vessels, the strength margin was evaluated by the comparison of the measured plastic collapse pressure with the design pressure.

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