Residual stresses in thick-walled cylinders resulting from mechanically induced overstrain

Autofrettage is a process for inducing elastic response in thick-walled cylinders subjected to internal pressures which otherwise cause plastic strains. To extend the use of autofrettage to higher pressure applications and to elminate many of the problems encountered in the use of the conventional process based on the use of direct internal hydrostatic pressure, a new technique has been developed which utilizes the mechanical advantage of a sliding wedge to produce the desired bore enlargement. Since the use of a sliding wedge or mandrel will induce shearing forces at the mandrel-cylinder interface, the resultant residual-stress distribution will differ from that theoretically predicted as characteristic of the direct hydrostatic process. It is the purpose of this work to determine the residual-stress distribution as a function of magnitude of overstrain and diameter ratio, and how it affects the reyielding characteristics of cylinders autofrettaged by this technique. Residual-stress distributions, determined by the Sachs boring-out technique for diameter ratios ranging from 1.5 to 2.3 and for several different magnitudes of overstrain, are shown. The shearing force associated with this technique induces substantial longitudinal residual stresses. The increase in the magnitude of this longitudinal residual stress with overstrain and the resultant decrease in the tangential residual stress are shown and discussed. Hydrostatic reyielding tests of autofrettaged cylinders are used to substantiate the decrease of tangential residual stress with increased overstrain. The substantially lower optimum overstrain as compared to the direct hydrostatic technique is shown and discussed. For optimum overstrain, the elastic strength of cylinders autofrettaged by swaging is comparable to that characteristic of the conventional process.