25-01-2013, 02:43 PM
Effect of Autofrettage on Allowable Pressure of Thick-Walled Cylinders
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Abstract.
In optimal design of thick-walled cylinders, there are two main objectives to be achieved:
increasing its strength-weight ratio and extending its fatigue life. This can be achieved by generating a
residual stress field in the cylinder wall prior to use. In the present study, the objective is to present an
analytical autofrettage procedure, with the aim of predicting the required autofrettage pressure for various
levels of allowable pressure. The results could contribute to a better understanding of the role autofrettage
plays in providing strength and performance in pressure vessels. A corresponding FEM validation will be
provided. The results reveal three scenarios in the design of thick-walled cylinders. For maximum load
carrying capacity, non-autofrettage is suitable when, in service, the whole wall thickness is intended to be
yielded. Full autofrettage is suitable when, during subsequent operation, yielding is limited at the inner
surface. Optimum autofrettage of the cylinder is suitable if a minimum equivalent stress is required.
Introduction
Thick-walled cylinders are widely used as critical components in pneumatic and hydraulic systems and
as storage and processing vessels. These components require a strict analysis for optimum design to ensure
reliable and safe operational performance. The increasing scarcity of materials and higher costs has attracted
researchers’ attention to the elastic–plastic approach which offers more efficient use of materials.
Autofrettage is a process of generating residual stresses in the wall of a thick-walled cylinder prior to use.
A pressure, large enough to cause yielding within the wall, is applied to the inner wall of the cylinder and
then removed. Upon release of this pressure, compressive residual circumferential stress is developed to a
certain radial depth at the bore. These residual stresses serve to reduce the tensile stresses developed as a
result of subsequent application of an operating pressure, thus increasing the load bearing capacity. Using the
Tresca yield criteria together, with an autofrettage level parameter, a precise solution for residual stress was
developed. The main objective of the paper is to find an optimum autofrettage pressurizing level when the
cylinder is subjected to a limiting pressure. In this study, an analytical solution was carried out to investigate
the effect of autofrettage on the limit loads of cylinders.
Conclusions
The effects of autofrettage level parameters on the pressure capacity of cylinders were studied. From the
results of this study, the following points may be concluded.
1. The autofrettage process increases the maximum allowable internal pressure.
2. There are three cases of autofrettage in the design of pressurized thick-walled cylinder:
• Non-autofrettage is suitable if yielding is allowed throughout cylinder wall thickness.
• Full autofrettage is suitable if yielding is allowed at the inner surface only.
• Optimum autofrettage case is suitable if the maximum equivalent stress is to be optimized.