The wide applications ofpressurized cylinder in chemical, nuclear, armaments, fluid transmittingplants,power plants and military equipment, in addition to the increasing scarcity andhigh cost of materials leadthe designers toconcentrate their attentions to the elastic – plastic approach which offersmore efficient use of materials 1, 2.The process of producing residualstresses in the wall of thick-walled cylinder before it is put into usage iscalled Autofretage, which it means; a suitable large enoughpressure to cause yielding within the wall, is applied to the inner surface ofthe cylinder and then removed. So that a compressive residualstresses are generated to a certain radial depth at the cylinder wall.
Then,during the subsequent application of an operating pressure, the residualstresses will reduce the tensile stresses generated as a result of applyingoperating pressure 1,3.The effect ofresidual stresses on load-carry capacity of thick-walled cylinders have beeninvestigate by Amran Ayob and Kabashi Albasheer 4, using both analytical andnumerical techniques. The results of the study reveal three scenarios in thedesign of thick-walled cylinders. Amran Ayob and M.
Kabashi Elbasheer 5, usedvon.mises and Tresca yield criteria to develop a procedure in which the autofretagepressure determined analytically resulting in a reduced stress concentration.Then they compared the analytical results with FEM results. They concludedthat, the autofretage process increase the maximum allowable internal pressurebut it cannot increase the maximum internal pressure to case whole thickness ofthe cylinder to yield. Noraziah et al. 6 presented an analytical autofretageprocedure to predict the required autofretage pressure of different levels ofallowable pressure and they validate their results with FEM results.
They foundthree cases of autofretage in design of pressurized thick – walled cylinders.Ruilin Zhu andJinlai Yang 7, using both yield criteria von.mises and Tresca, presented ananalytical equation for optimum radius of elastic-plastic junction in autofretagecylinder, also they studied the influence of autofretage on stress distributionand load bearing capacity. They concluded, to achieve optimum radius of elastic- plastic junction, an autofretage pressure a bit larger than operatingpressure should be applied before a pressure vessel is put into use.
Zhong Huand Sudhir Puttagunta 8 investigate the residual stresses in the thick-walled cylinder induced by internal autofretage pressure, also they found theoptimum autofretage pressure and the maximum reduction percentage of thevon.mises stress under elastic-limit working pressure. Md. Tanjin Amin et al.9 determined the optimum elasto – plastic radius and optimum autofretagepressure using von.mises yield criterion , then they have been compared withZhu and Yang’s model 8. Also they observed that the percentage of maximumvon.mises stress reduction increases as value of radius ratio (K) and workingpressure increases.
F. Trieb et al. 10 discussed practical application of autofretageon components for waterjet cutting. They reported that the life time of highpressure components is improved by increasing autofretage depth due toreduction of tangential stress at inner diameter, on other hand too highpressure on outside diameter should be avoided to prevent cracks generate. Inaddition to determine the optimum autofretage pressure and the optimum radiusof elastic-plastic junction , Abu Rayhan Md.
et al.11 evaluated the effect ofautofretage process in strain hardened thick – walled pressure vessels usingequivalent von.mises stress as yield criterion. They found, the number of autofretagestages has no effect on maximum von.mises stress and pressure capacity. Also,they concluded that, optimum autofretage pressure depends on the workingpressure and on the ratio of outer to inner radius.