Abstract: The rate of impact between the peening elements and an internal surface of a hollow part is a function of the vibration frequency, and there is a cut-off frequency at which a hollow part can vibrate and induce repeated impact between its internal surface and the peening elements because the rate of impact becomes erratic and loses its cyclical nature as the vibration frequency deviates from the cut-off frequency. The present invention provides a method for determining the cut-off frequency at which a hollow part can vibrate and maintain the repetitive nature of the impact between its internal surface and the peening elements. Such a method requires a peening element speed limit ratio, which is the ratio of the velocity of the hollow part compared to the velocity of the peening element above which the rate of impact begins to become erratic and lose its cyclical nature.

Abstract: The present invention is a method and apparatus for peening the internal surface of non-ferromagnetic hollow parts. Ferromagnetic peening elements are inserted into the non-ferromagnetic hollow part, and a magnetic dipole creates a magnetic field through the non-ferromagnetic hollow part attracting the peening elements which contact a portion of the internal surface of the non-ferromagnetic hollow part. The magnetic field is disengaged and a second magnetic dipole creates a second magnetic field, thereby causing the peening elements to contact a second portion of the internal surface. The magnetic dipoles are repeatedly turned on and off, thereby causing repeated impact within the interior of the hollow part until the internal surface attains a predetermined stress level.