The wide applications of pressurized sylinder  in chemical, nuclear, armaments, fluid transmitting plants,power plants and military equipment, in addition to the increasing scarcity and high cost of materials lead the designers to concentrate their attentions to the elastic – plastic approach which offers more efficient use of materials 1, 2.The process of producing residual stresses inthe wall of thick_walled sylinder  before it is put into usage is called autofretage, which it means; asuitable large enough pressure to cause yielding within thewall, is applied to the inner surface of the sylinder  and then removed. So that a compressive residual stresses are generated to a certain radial depth at the sylinder  wall. Then, duringthe subsequent application of an operating pressure, the residual stresses will reduce the tensile stresses generated asa result of applying operating pressure 1,3.The effect of residual stresses onload-carry capacity of thick_walled sylinders have been investigate by Ayob and Albasheer 4, using both analytical andnumerical techniques. The results of the study reveal three scenarios in the design of thick_walled sylinders. Ayob and Elbasheer 5, used von.mises and Tresca yieldcriteria to develop a procedure in whichthe autofretage pressure determined analytically resulting in a reduced stress concentration. Then they compared the analytical results with FEM results. They concluded that, the autofretage process increase the max.allowable internal pressure but it cannot increase the max.internal pressure to case whole thickness of the sylinder  to yield. Noraziah et al. 6 presented an analytical autofretage procedure topredict the required autofretage pressure of different levels of allowable pressure andthey validate their results with FEM results. They found three cases of autofretage in design of pressurized thick_ walled sylinders.Zhu and Yang 7, using both yield criteria von.mises and Tresca, presented an analytical equation for optimum radius of elastic-plastic junction in autofretage sylinder , alsothey studied the influence of autofretage on stress distribution and load bearing capacity. They concluded, to achieve optimum radius ofelastic – plastic junction, an autofretage pressure a bit larger than operating pressure should be applied before a pressure vessel is put into use. Hu and Puttagunta 8 investigate the residual stresses in thick_ walled sylinder  induced by internal autofretage pressure, also they found the optimum autofretage pressure andthe max.reduction percentage of the von.mises stress under elastic-limit working pressure. Md. Amin et al. 9 determined the optimum elasto_plasticradius and optimum autofretage pressure using von.mises yield criterion , then they have been compared with Zhu and Yang’s model 8. Also they observed that the percentage of max.von.mises stress reduction increases as value of radius ratio (K) and working pressure increases. F. Trieb et al. 10 discussed practical application of autofretage on components for waterjet cutting. They reported that the life time of high pressure components is improved by increasing autofretage depth due to reduction of tangential stress at inner diameter, on other hand too high pressure on outside diameter should be avoided to prevent cracks generate. In addition to determine the optimum autofretage pressure and the optimum radius of elastic-plastic junction , Abu Rayhan Md. et al.11 evaluated the effect of autofretage process in strain hardened thick_ walled pressure vessels using equivalent von.mises stress as yield criterion. They found, the number of autofretage stages has no effect on max.von.mises stress and pressure capacity. Also, they concluded that, optimum autofretage pressure depends on the working pressure and on the ratio of outer to inner radius.

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