The FEM results. They concluded that, the

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 acompressive 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.

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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.