No abstract

No abstract

Using the concept of the tensile-instability mechanism which determines the ultimate strength of most of the ductile materials under uniaxial or biaxial tension conditions, two new analyses are carried out. In these are considered an effect previously neglected: Variation of Poisson’s ratio (based on total strain) with strain in the plastic range. One analysis treats a uniaxial tensile specimen, and the predicted results are generally in better agreement with experimental values for seven alloys of aerospace structural importance than the results of classical theory, in which Poisson’s ratio is assumed to remain constant at a value of 1/2 in the plastic range. The other considers a thin-walled spherical shell subject to internal pressure, and the predicted result is again closer to the experimental value than that predicted by classical theory.

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] 1 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][5][6][7]. A power relation is used to express the creep rate versus stress relation in simple tension.H IASED upon a large strain theory [2] for a pressure vessel with closed ends under internal pressure, an expression can be obtained relating the significant creep rate to the internal pressure in the vessel and the significant stress.An internal-pressure versus significant-creep-rate relation can be obtained from a significant-creep-rate versus stress relation, based upon a simple tension-creep relation. In this manner, an expression is obtained which relates the creep rate to the creep strain. This equation can be integrated and results in a relation giving the creep strain as a function of time.The creep-failure time is now obtained by letting the true creep strain approach infinity. It so happens that the time necessary to reach infinite strain is a finite quantity. The time to reach infinite strain, called creep-failure time, can be used to predict the time of actual fracture, called creep-fracture time. In reality, the vessel will fracture before it has expanded to infinity. However, the expansion of the vessel (accompanied by a reduction of the 1 Numbers in brackets designate References at end of paper. wall thickness) proceeds very slowly at the beginning and very rapidly toward the end of the life of the vessel. For this reason, the actual fracture time and the time for infinite theoretical strain occur at essentially the same time. This means that the creepfailure time can be used in place of the creep-fracture time for the prediction of fracture of a vessel. Basis of TheoryThis paper assumes times of such values that the influence of the primary creep can be neglected. The creep relation used here expresses the behavior of materials during secondary cree...

No abstract