Is There an “Ultimate” Autofrettage Process?

To increase the elastic-carrying capacity of a gun barrel, beneficial residual stresses are introduced to the barrel’s wall, commonly by the autofrettage process. There are two major autofrettage processes: the hydrostatic and the swage. While the theoretical solution for hydraulic autofrettage has been available and accessible for a long time, the available models for swage autofrettage have been quite limited. The issue of hydraulic versus swage autofrettage was intensively investigated pointing to the clear advantages of swage autofrettage in both the level of the residual stresses created and the total fatigue lifetime obtained. Nevertheless, it is generally accepted that overstraining a barrel to the same level of autofrettage by either the swage or the hydraulic processes produces the same safe maximum pressure (SMP) for firing. In the present analysis, the recently developed 3D code, which finally enables a realistic simulation of both swage and hydraulic autofrettage, is validated against experimental findings for several gun barrels. All the numerical results are found in excellent agreement with the test results in terms of the permanent bore enlargement (PBE) and the safe maximum pressure for these barrels. In order to compare the two autofrettage processes, the code is applied to two hypothetic 120 mm gun barrels simulating both swage and hydraulic autofrettage. The detailed numerical comparison between the two different autofrettage processes points to the fact that the swage autofrettage process is superior to the hydraulic autofrettage process. These results are very encouraging and call for continuing the pursuit of finding an “ultimate” autofrettage process that will yield the “optimal gun barrel.”