Prediction of Creep Failure Time for Pressure Vessels

When a pressure vessel is subjected to internal pressure and is made from a material which exhibits creep, the vessel will expand. If the internal pressure is held constant during expansion, the load on the wall will increase. At the same time, the thickness of the wall decreases. The result of these two simultaneous effects is that the expansion of the vessel is continuously accelerated until the wall thickness has decreased and the load increased to such an extent that the strength of the material is no longer sufficient and fracture of the vessel occurs. The time-to-fracture in the case of simple tensile creep was predicted theoretically by Orowan [8] and shown by Hoff [1] to be in good agreement with experimental results, The basis of their approach is to use true stress and true strain. The creep-failure time is then defined as the time at which the true strain reaches infinity. The present paper extends the foregoing concept to the problem of combined stresses. The creep-failure time is determined for thin, thick, and very thick-walled cylindrical vessels of circular cross section with closed ends subjected to constant internal pressure. The theory is based upon the usual assumptions for predicting creep deformation under combined stress [4–7]. A power relation is used to express the creep rate versus stress relation in simple tension.