Autofrettage of Cylinders: Prediction of Pressure/External Expansion Curves and Calculation of Residual Stresses

Abstract

The shear stress-strain relationship for an initially stress-free 2 1/2 per cent nickel-chromium-molybdenum steel under reversed stress has been determined by twisting thin tubes. The effect of a low-temperature heat treatment given to this material after various amounts of overstrain has been similarly determined. The information so obtained has been used to calculate by methods outlined in Appendixes I and II the behaviour of thick cylinders of the same material when subjected to one or two cycles of internal pressure, with or without an intervening low-temperature heat treatment, and to calculate the stresses throughout the material of these cylinders at any stage in the processes. Such cylinders have been made and subjected to the treatment described, the pressure and dimensional changes being measured at all stages. They have then been bored out in steps until all the overstrained material has been removed, the corresponding dimensional changes being again measured. One of the assumptions used in the calculations has an important effect on the values of the residual axial stresses; moreover the axial dimensional changes during the boring-out process are so small that great accuracy cannot be claimed for the values measured. To this extent the results are inconclusive. It is, however, shown that both the dimensional changes and the shear stress distribution (from which the radial and hoop stress distribution can, of course, be deduced) at any stage in the process can be calculated with satisfactory accuracy by the relatively simple approximate methods described.