The into use. Zhong Hu and Sudhir Puttagunta [8]

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 inthe wall of thick_walled cylinder before it is put into usage iscalled Autofretage, which it means; asuitable large enoughpressure to cause yielding within thewall, is applied to the inner surface ofthe cylinder and then removed.

So that acompressive residualstresses are generated to a certain radial depth at the cylinder wall. Then,duringthe subsequent application of an operating pressure, the residualstresses will reduce the tensile stresses generated asa result of applyingoperating pressure 1,3.The effect ofresidual stresses onload-carry capacity of thick_walled cylinders have beeninvestigate by Amran Ayob and Kabashi Albasheer 4, using both analytical andnumericaltechniques. The results of the study reveal three scenarios in the design of thick_walledcylinders. Amran Ayob and M. Kabashi Elbasheer 5, used von.

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mises and Trescayieldcriteria to develop a procedure in whichthe autofretage pressuredetermined analytically resulting in a reduced stress concentration. Then theycompared the analytical results with FEM results. They concluded that, the autofretageprocess increase the max.allowable internal pressure but it cannot increase themax.

internal pressure to case whole thickness of the cylinder to yield.Noraziah et al. 6 presented an analytical autofretage procedure topredict therequired autofretage pressure of different levels of allowable pressure andtheyvalidate their results with FEM results. They found three cases of autofretagein 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, alsothey studied the influence of autofretage on stress distributionand load bearing capacity.

They concluded, to achieve optimum radius ofelastic- 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 thick_ walledcylinder induced by internal autofretage pressure, also they found the optimum autofretagepressure andthe max.reduction percentage of the von.

mises stress underelastic-limit working pressure. Md. Tanjin Amin et al.

9 determined theoptimum elasto_plasticradius and optimum autofretage pressure using von.misesyield criterion , then they have been compared with Zhu and Yang’s model 8.Also they observed that the percentage of max.von.

mises stress reductionincreases as value of radius ratio (K) and working pressure increases. F. Triebet al. 10 discussed practical application of autofretage on components forwaterjet cutting.

They reported that the life time of high pressure componentsis improved by increasing autofretage depth due to reduction of tangentialstress at inner diameter, on other hand too high pressure on outside diametershould be avoided to prevent cracks generate. In addition to determine theoptimum autofretage pressure and the optimum radius of elastic-plastic junction, Abu Rayhan Md. et al.11 evaluated the effect of autofretage process instrain hardened thick – walled pressure vessels using equivalent von.misesstress as yield criterion. They found, the number of autofretage stages has noeffect on max.von.

mises stress and pressure capacity. Also, they concludedthat, optimum autofretage pressure depends on the working pressure and on theratio of outer to inner radius.