The wide applications ofpressurized sylinder in chemical,nuclear, armaments, fluid transmitting plants,power plants and military equipment, in addition to the increasing scarcity andhigh cost of materials lead the designers toconcentrate their attentions to the elastic – plastic approach which offersmore efficient use of materials 1, 2.The process of producing residualstresses inthe wall of thick_walled sylinder before it is put in to usage is called autofretage, which it means; asuitable large enoughpressure to cause yielding within thewall, is applied to the inner surface ofthe sylinder and then removed.So that a compressive residual stresses are generated to a certain radial depthat the sylinder wall. Then, duringthesubsequent application of an operating pressure, the residual stresses willreduce the tensile stresses generated asa result of applying operating pressure1,3.
The effect ofresidual stresses onload-carry capacity of thick_walled sylinders have beeninvestigate by Ayob and Albasheer 4, using both analytical andnumericaltechniques. The results of the study reveal three scenarios in the design of thick_walledsylinders. Ayob and Elbasheer 5, used von.mises and Tresca yieldcriteria todevelop a procedure in whichthe autofretage pressure determined analyticallyresulting in a reduced stress concentration. Then they compared the analyticalresults with FEM results. They concluded that, the autofretage process increasethe max.allowable internal pressure but it cannot increase the max.
internalpressure to case whole thickness of the sylinder to yield. Noraziah et al. 6 presented ananalytical autofretage procedure topredict the required autofretage pressure ofdifferent levels of allowable pressure andthey validate their results with FEMresults. They found three cases of autofretage in design of pressurized thick_walled sylinders.Zhu and Yang 7, usingboth yield criteria von.mises and Tresca, presented an analytical equation foroptimum radius of elastic-plastic junction in autofretage sylinder , alsotheystudied the influence of autofretage on stress distribution and load bearingcapacity.
They concluded, to achieve optimum radius ofelastic – plasticjunction, an autofretage pressure a bit larger than operating pressure shouldbe applied before a pressure vessel is put in to use. Hu and Puttagunta 8investigate the residual stresses in thick_ walled sylinder induced by internal autofretage pressure, alsothey found the optimum autofretage pressure andthe max.reduction percentage ofthe von.
mises stress under elastic-limit working pressure. Md. Amin et al. 9determined the optimum elasto_plasticradius and optimum autofretage pressure usingvon.mises yield criterion , then they have been compared with Zhu and Yang’smodel 8. Also they observed that the percentage of max.von.
mises stressreduction increases as value of radius ratio (K) and working pressureincreases. F. Trieb et al. 10 discussed practical application of autofretageon components for waterjet cutting. They reported that the life time of highpressure components is improved by increasing autofretage depth due toreduction of tangential stress at inner diameter, on other hand too highpressure on outside diameter should be avoided to prevent cracks generate. Inaddition to determine the optimum autofretage pressure and the optimum radiusof elastic-plastic junction , Abu Rayhan Md. et al.11 evaluated the effect ofautofretage process in strain hardened thick_ walled pressure vessels usingequivalent von.
mises stress as yield criterion. They found, the number of autofretagestages has no effect on max.von.mises stress and pressure capacity. Also, theyconcluded that, optimum autofretage pressure depends on the working pressureand on the ratio of outer to inner radius.