Thermally assisted autofrettage of thick cylinders

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Autofrettage is a classical metal working technique for increasing the pressure bearing capacity, fatigue life, creep and stress-corrosion cracking resistance of thick-walled cylindrical and spherical vessels. Autofrettage is carried out by applying a load to the thick-walled cylinder or sphere that causes a partial or full plastic deformation. When the vessel is subsequently unloaded, compressive residual stresses are induced in the vicinity of the inner wall whereas tensile residual stresses are induced in the vicinity of the outer wall. On the basis of the type of load used for the plastic deformation, autofrettage can be classified into five types such as hydraulic, swage, explosive, thermal and rotational autofrettage.Literature review indicates that hydraulic autofrettage is one of the two most popular and widely accepted type of autofrettage processes, together with swage autofrettage. However, the requirement of very high pressure makes the process costly. On the other hand, the thermal autofrettage is very simple although the achievable maximum increase in the pressure carrying capacity is limited due to the restriction on the maximum allowable temperature difference. Another detrimental feature that is common in any autofrettage processes is that the tensile residual stresses in the vicinity of the outer wall weakens the autofrettaged cylinder or sphere, a condition that is further aggravated in the presence of external surface flaws like cracks or wears as the tensile stresses causes the cracks to open up.
Supervisor: Uday Shanker Dixit