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The predict the required autofretage pressure of different levels

The wide applications of
pressurized cylinder in chemical, nuclear, armaments, fluid transmitting
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.

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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 max.allowable internal pressure but
it cannot increase the max.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 max.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 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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