The pressure a bit larger than operating pressure

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 inthe wall of thick_walled cylinder 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 cylinder and then removed. So that acompressive residual
stresses are generated to a certain radial depth at the cylinder 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 cylinders have been
investigate by Amran Ayob and Kabashi Albasheer 4, using both analytical andnumerical
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
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 cylinder 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 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, 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. Zhong Hu
and Sudhir Puttagunta 8 investigate the residual stresses in thick_ walled
cylinder 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. Tanjin 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.