Autofrettage :

• Auto frettage is one of the oldest methods of pre-stressing (method of increasing pressure capacity) the cylinders.
• It is overloading the cylinder before it is put into service.
• In this method, a forged cylinder is subjected to a large internal pressure, so as to expand the internal diameter, causing overstrain.
• The internal pressure is applied is such a way that a portion of the cylinder at the inner surface is subjected to stresses in the plastic range, while the outer surface is still in the elastic range.
• When the pressure is released, the portion of the cylinder at the outer surface contracts exerting the pressure on the portion at the inner surface which has undergone permanent deformation.
• As a result, residual compressive stresses are induced at the inner surface and tensile stresses at the outer surface.
• It is commonly used is gun barrels.
• It not only increases the pressure capacity of the cylinder, but also improves the endurance strength.