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Autofrettage is a metal cold forming technique used to increase resistance to corrosion and cracking while also strengthening the material. It involves subjecting the material to very high pressure causing internal portions of the part to yield plastically, resulting in internal compressive residual stresses once the pressure is released. High-velocity gun barrels – tank and ship guns – are manufactured using this process.
The mechanical autofrettage process requires the manufacture of mandrels that are specially designed and dimensioned for the particular tube, gun barrel, etc. In addition, the use of hydraulic autofrettage requires the use of high-pressure seals to be attached to the tube, gun barrel, etc., and with equipment failure can result in the rapid and/or uncontrolled release of high-pressure fluid. Such a release is dangerous and requires additional safety equipment to be used with hydraulic autofrettage systems.
Addressing these weaknesses, Army scientists have developed a process for creating residual compressive stress at a surface of a structure without resorting to custom mandrels with dangerous high-pressure fluids. The approach utilizes an expander placed within the barrel which exerts pressure on the interior surface of the structure. An axial compressive force applied to the expander is of sufficient magnitude to cause radial expansion of the expander and result in predominantly elastic strain to the outer surface and elastic plus plastic strain to the inner surface of the barrel. When the axially compressive force is removed from the expander, the elastic strain is recovered at the outer surface of the structure, however, the plastic strain at the inner surface results in compressive residual stress.
The process is appropriate for use on gun barrels, pipes, tubes, and other tubular articles, for example, a ¾” pipe, tanks, pressure vessels, containers and so forth.
- The process can be used to apply different magnitudes of residual compressive stress as a function of location within a structure
- US patent 8,056,279 is available for license