Shot Peening Theory, Application, & Controls
David Breuer - Metal Improvement Co., Inc.
Shot Peening is primarily used for fatigue life improvement. A particle of shot impacting a surface causes the surface to crater and yield upon impact. When the impacting energy is expended and the particle bounces off, a zone of residual compressive stress developes around, within, and beneath the deformed crater. This compressive stress often reaches a maximum of about 60% of the ultimate tensile of the material and this maximum is reached slightly below the surface. At the surface the compressive stress is usually about half of the maximum, but it is still very effective in preventing the initiation of fatigue cracks on the surface. As depths increase beyond that of the maximum compressive stress, the residual stress in the material gradually becomes less compressive and ultimately becomes tensile. The depth at which the residual stress becomes tensile marks the "depth of the compressive layer" and is usually several thousandths of an inch below the surface.
Because surface cracks can only initiate in tension, any residual compressive stress present at a surface can be subtracted from any tensile load the surface is subjected to, thus either increasing the loading capacity of the part; or more commonly increasing the life (measured in cycles) of part life before failure in a high cycle fatigue situation. Fatigue life is typically improved from 300% to 1000% (measured in cycles to failure) by proper shot peening.
It is the nature of the S/N (fatigue curve) that at low cycle counts a steep slope exists (high stress change for cycle count) so peening does not dramatically increase fatigue life in the low cycle realm. However as the S/N curve gets to high cycle counts, the curve flattens (modest change in stress for major changes in cycle count), so the residual surface stresses created by shot peening greatly increase fatigue life in the high cycle realm.
Shot peening can help to overcome notch sensitivity. X-ray diffraction is the method used to directly measure the stresses set up by shot peening, however given a part geometry, and its material properties the effects of peening can be fairly accurately predicted, together with the potential benefits in fatigue. Generally speaking, the harder the part, the harder the shot that is used except when surface finish requirements dictate otherwise.
Some application where shot peening has greatly contributed to the life and suitability of products include aircraft landing gear (where increased operational strength is achieved), the ID of compression springs where (if life is no problem) loads can be increased by 10 to 30%, extension springs at the root of the hook ID, and the OD of torsion springs. Spring rates decrease form 1 to 3% as a result of peening.
Gears present special problems. Root failures are effectively prevented by peening, but peening of the flanks can promote failure (due to roughness and friction) unless a "super finish" is applied. Super finishing involves peening with finer shot and is sometimes chemically assisted to bring the surface to 10 - 20 micro-inch (.5 micron).
Weldments which are subjected to vibration often benefit greatly from peening or stress relief followed by peening.
The heaviest use of peening (50% or more) is by the aerospace industry for such parts as landing gear, propellar hubs, and impellers.
Shot peening can also help mitigate stress corrosion cracking, and is often used for this purpose on stainless steel or aluminum parts. Essentially, stress corrosion cracking requires three components to initiate - 1) corrosive environment, 2) susceptable alloy, 3) tensile stresss. By elimitating tensile stress, peening can eliminate this problem.
For threads, small roots, and difficult geometries small glass beeds (down to 0.002" dia.) can be used as media. Glass is also used when surface contamination from steel shot may be a problem.
Control of the peening process is achieved in three ways.
1) The media (shot) itself is controlled for size and geometry. It is inspected after each 8 hours of use. A classifier selects the proper size, and then a concentric flight channel ensures that escaping media is spherical.
2) Intensity Control. Almen strips are controlled thin strips of metal, which will "arc" due to the compressive residual stresses present on the peened side. The height of the arc is a very repeatable function of the intensity of the peening operation. Intensity is influenced by such variables as media mass, velocity, and impingement angle. The Almen however measures the intensity and effectiveness of the peening indepentent of these variables. Almen strips are placed on parts at or near the critical areas of potential failure and left in place during the peening process. Measuring the arc of these strips, ensures that intensity was properly controlled.
3) Coverage Control. Direct inspection is the best method of measuring coverage. Sometimes this is assisted by alowing the peening operation to remove tracer die. Remaining die indicates misses in coverage.
There are also some specialized uses of peening such as peen forming aerodynamic countours. Using the same principle as the Almen strips, metal sheet is formed into contours by controlling peening intensity. Still more applications include glare reduction of a surface, decorative (textured) surfacing, and friction reduction.
An excellent book "Shot Peening Applications" is available from Metal Improvement Company, Inc. in soft cover or CD-ROM. The e-mail is: METALIMP@ix.netcom.com and web site is www.metalimprovement.com.