The wide applications of

pressurized cylinder 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 in the wall of thick-walled cylinder before it is put into usage is

called Autofretage, which it means; a suitable large enough

pressure to cause yielding within the wall, is applied to the inner surface of

the cylinder and then removed. So that a compressive residual

stresses are generated to a certain radial depth at the cylinder wall. Then,

during the subsequent application of an operating pressure, the residual

stresses will reduce the tensile stresses generated as a result of applying

operating pressure 1,3.

The effect of

residual stresses on load-carry capacity of thick-walled cylinders have been

investigate by Amran Ayob and Kabashi Albasheer 4, using both analytical and

numerical techniques. The results of the study reveal three scenarios in the

design of thick-walled cylinders. Amran Ayob and M. Kabashi Elbasheer 5, used

von.mises and Tresca yield criteria to develop a procedure in which the 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 maximum allowable internal pressure

but it cannot increase the maximum internal pressure to case whole thickness of

the cylinder to yield. Noraziah et al. 6 presented an analytical autofretage

procedure to predict the required autofretage pressure of different levels of

allowable pressure and they validate their results with FEM results. They found

three cases of autofretage in design of pressurized thick – walled cylinders.

Ruilin Zhu and

Jinlai Yang 7, using both yield criteria von.mises and Tresca, presented an

analytical equation for optimum radius of elastic-plastic junction in autofretage

cylinder, also they studied the influence of autofretage on stress distribution

and load bearing capacity. They concluded, to achieve optimum radius of elastic

– plastic junction, an autofretage pressure a bit larger than operating

pressure should be applied before a pressure vessel is put into use. Zhong Hu

and Sudhir Puttagunta 8 investigate the residual stresses in the thick-

walled cylinder induced by internal autofretage pressure, also they found the

optimum autofretage pressure and the maximum reduction percentage of the

von.mises stress under elastic-limit working pressure. Md. Tanjin Amin et al.

9 determined the optimum elasto – plastic radius 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 maximum

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 maximum 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.