Shot Peening Process Controls Ensure Repeatable Results

Author:  John S. Eckersley
Source:  The Shot Peener magazine, Vol 15 / Issue 2, Summer 2001
Doc ID:  2001023
Year of Publication:  2001
Abstract:  
Reprinted with permission from Shot Peening Theory and Application. Edited by Dr. A. Niku-Lari. First published by IITT-International www.iitt.com INTRODUCTION Shot peening is a cold working process applied to the surface of metal parts to increase their resistance to fatigue, fretting fatigue, stress corrosion cracking, corrosion fatigue and various other tensile stress related metal failures. Shot peening has also been used effectively to form aerodynamic curvatures into large sculptured panels such as aircraft wing skins, some over 90 feet (30 m) long. In all cases, the effectiveness of the process is due to the introduction of residual compressive stresses into the surface of the parts. When a metal surface is indented, a residual compressive stress is induced as the subsurface material attempts to push out the indentation and restore the surface to its former condition. Shot peening, by bombarding the surface with millions of tiny spheres of steel, glass or ceramic, totally covers the surface with indentations, creating an even layer of compressive stress that is at its maximum, just below the surface and is at least equal to 50% of the ultimate tensile strength of the metal. It becomes immediately apparent that, for shot peening to be consistently effective and reproducible, a number of parameters must be controlled. Among the most important: 1. The peening media. 2. The energy of the shot stream. 3. The percentage of indentation of the part surface. 4. The repeatability of the peening machine.