A technique is developed which can be used to distinguish the primary and secondary stresses in pressure vessels. The general definition of the two types of stresses stated by the ASME Code is used; a simple viscoelastic model is proposed for each stress category. The proposed model can be extended to elastic as well as plastic regions of strain-hardening materials and can include the mechanical as well as thermal loads. The proposed viscoelastic models are used to judge the nature of elastic stresses and the effective stress-strain curve is used to simulate the state of stress at any stage of loading and the percentage of primary to secondary stresses at any radius of the vessel. It is found that thermal stresses cannot always be categorized as secondary stress, and in the case of thermoplastically loaded vessels they can contribute partly to the primary stress in the vessel.

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