The subsequent application of an operating pressure, 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 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

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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.

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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