| The ultra pressure equipment is widely applied in petroleum chemical industry to stabilize current output of oil-gas filed and improve recovery efficiency of new oil and gas well,when the petroleum and gas as underlying resource is increasing shrinking as well as the development of oil resource is gradually hardship. To increase loading capacity and working life of ultra pressure equipment,enhanced the thickness of pressure vessel below the yielding stress of material is adopted as well as applied autofrettaged technology is indispensable. The autofrettaged process which pressure the internal face to guarantee to happen plastic deformation can decrease the stress level of the wall vessel and improve fatigue strength by the residual stress with the autofrettaged process.The crack caused for surface scratch,metal inclusion,corrosion pit and forging defect is indispensable for any equipment processing. The stress concentration happened to the crack tip is inevitable to influence on the residual stress distribution leading to changing autofrettaged processing efficiency when the ultra pressure equipment apply the autofrettage technology. So based on the fracture mechanic theory,hypothesis that the initial crack exist in the actual part,the distribution law of fracture parameter at crack tip is researched as well as crack propagation behavior is estimated through characterization of stress intensity factor for the impact on residual stress,which expand research field about autofrettaged technology applied to the ultra pressure equipment with crack. The crack formation in practical engineering is main three-dimension issue.For the inner structure of high pressure equipment be generally complex,the fracture parameter cannot be calculated easily by current fracture mechanic theory as well as be difficult to verify theoretical result by test. So considering the view of research feasibility,the crack located in the core complex cavity can be abstraction simplified thick-walled cylinder with crack through analysis the location of crack happened to the inner complex wall. It has guiding significance for practical engineering application to judge the crack at the complex cavity to the autofrettaged processing effort.Considering the view of elastic-plastic mechanic,the formula for calculating residual stress caused for unloading the autofrettaged pressure is established through the ideal elastic-plastic model and bilinear kinematic hardening model. Through the fracture mechanic theory about weight function,the accuracy of weight function about thick-walled cylinder is verified and analyzed as well as the function for calculating stress intensity factor about residual stress caused for autofrettaged process is derived. The fracture parameter at crack tip and stress distribution about thick-walled cylinder under various crack formation is solved by numerical calculation. Meanwhile,through adopted penetrated and elliptical crack formation about thick-walled cylinder with crack,the variation law of residual stress and J-integral after autofrettaged process is researched.Through analysis the penetrated crack formation,the result is shown that:the crack existed in the thick-walled cylinder mainly cause a small scale stress change along the direction of crack growth.However the stress state is basic similar to the non-crack thick-walled cylinder when the diameter distance exceed that small scale.The maximum residual compressive stress occur to crack tip along the direction of crack growth not stress concentration place.That means accelerate to decrease the crack growth rate.The change in diameter ratio K has influence on the maximum residual shear stress at crack front,and residual shear stress at crack tip,and the residual compressive stress field along the direction of crack growth. When the diameter ratio K is less than 1.6,the autofrettaged process is not very effective to decrease initial growth at crack tip. However,when the diameter ratio K is greater than 1.7,the autofrettaged process has gradually effective to decrease crack growth rate with diameter rate increasing.But when the autofrettaged pressure increase,it has less affect the crack growth rate with diameter ratio K increasing.When the diameter ratio K has been in low level,it is more obvious to restrain initial growth at crack tip if the less level of autofrettaged pressure is chosen. For the J-integral along the direction of thickness after autofrettaged process,The value of J-integral formed by residual stress fields is less than zero as well as presented bathrub-curve when the thickness is less than 40mm.It means the residual compressive stress mainly take control of crack tip.But the crack tip near constraint location is mainly taken control of residual tensile stress.Through analysis the elliptic crack formation,the result is shown that:when the ratio of crack depth c to crack length is less than 1,the maximum residual compressive stress at crack front is gradually increasing with the diameter ratio enhancing.However when the diameter ratio is greater than 1,that occur to gradually decrease.The change of the ratio of major axis and minor axis of ellipse has less effective to residual shear stress at crack tip,which the value basically keep constant.Because of the end of each side and center at ellipse curve are located in the surface and the maximum radius of curvature which easily occur to stress concentration,so the residual compressive stress at end of each side and center is greater than other location.The autofrettaged process is more effective to restrain initial crack growth at the crack tip away from inner surface.At the same time,when the ratio of crack depth to crack length is equal to 1.4,the autofrettaged process is most effective to restrain crack growth. If the crack formation remain unchanged,the autofrettaged process is not more effective to crack initial growth with the diameter ratio of thick-walled cylinder increasing.But when the ratio of major axis and minor axis of ellipse is been high level,the autofrettaged process is more effective to restrain the crack initial growth at inner surface with diameter ratio K increasing.So the effect of thickness of thick-walled cylinder change to crack tip has relationship with the structural size of crack. |
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