Reasons to Shot Peen
Shot peening is a cold working process in which the surface of a part is bombarded with small spherical media called shot. Each piece of shot striking the material acts as a tiny peening hammer, imparting to the surface a small indentation or dimple. In order for the dimple to be created, the surface of the material must be yielded in tension. Below the surface, the material tries to restore its original shape, thereby producing below the dimple, a hemisphere of cold-worked material highly stressed in compression.
Nearly all fatigue and stress corrosion failures originate at the surface of a part. Further, it has been well established that cracks will not initiate or propagate in a compressively stressed zone. Since the overlapping dimples from shot peening create a uniform layer of compressive stress at metal surfaces, the process provides considerable increases in part life. Compressive stresses are beneficial in increasing resistance to fatigue failures, corrosion fatigue, stress corrosion cracking, hydrogen assisted cracking, fretting, galling and erosion caused by cavitation. The maximum compressive residual stress produced just below the surface of a part by shot peening is at least as great as one half the yield strength of the material being peened.
In most modes of long-term failure, the common denominator is tensile stress. Tensile stresses attempt to stretch or pull the surface apart and may eventually lead to crack initiation. Because crack growth is slowed significantly in a compressive layer, increasing the depth of this layer increases crack resistance. Shot peening is the most economical and practical method of ensuring surface residual compressive stresses. For applications that require deeper residual compressive stresses than those provided by shot peening, Metal Improvement Company's laser peening process imparts a layer of beneficial compressive stress that is four times deeper than that attainable from conventional shot peening treatments.
Shot peening also can induce the aerodynamic curvature in metallic wing skins used in advanced aircraft designs. Additional applications for shot peening include work hardening through cold work to improve wear characteristics, closing of porosity, improving resistance to intergranular corrosion, straightening of distorted parts, surface texturing and testing the bond strength of coatings.
The graph below compares metal fatigue strength with ultimate tensile strength for both smooth and notched specimens. Without shot peening, optimal metal fatigue properties for machined steel components are obtained at approximately 30 HRc (700 MPa). At higher strength/hardness levels, materials lose fatigue strength due to increased notch sensitivity and brittleness. With the addition of compressive stresses from shot peening, however, metal fatigue strength increases proportionately to increasing strength/hardness. For example, at a 52 HRc (1240 MPa), the metal fatigue strength of the shot peened specimen is 144 ksi (988 MPa), more than twice the metal fatigue strength of the unpeened, smooth specimen.
Manufacturing Processes - Effect on Fatigue Life
Manufacturing processes are known to have a significant effect on meal fatigue properties of parts. These effects can be either detrimental or beneficial, as represented below:
DETRIMENTAL BENEFICIAL Hardening Carburizing Grinding Honing Machining Polishing Plating Burnishing Welding Rolling EDM and ECM Shot Peening
On the detrimental side grinding, machining and welding all can leave the surface of the part in tension, a seedbed for metal fatigue cracks. Hardening, plating and EDM can leave a hard brittle surface. ECM can damage or weaken surface grain boundaries.
On the beneficial side all the listed processes improve metal fatigue life by virtue of the compressive stresses they induce. Shot peening is the most versatile of the list because it provides the highest magnitude of compressive stress in the greatest variety of materials and part configurations.
The graph below presents "s/n" (stress vs. number of cycles to metal failure) curves for different types of grinding. The base line curve is that for "gentle grind" specimens and shows metal fatigue strength of 60,000 psi. The following "severe grind" graph represents that condition produced from faster cutting speeds and/or the taking larger cuts. In this case large amounts of surface tensile stress, the seedbed of tensile metal fatigue cracks, are generated. As shown, metal fatigue strength decreases to 45,000 psi. The last graph presents the metal fatigue strength of "severe grind plus shot peened" specimens. As shown, these specimens increased well beyond even the baseline "gentle grind", providing metal fatigue strength of over 80,000 psi. The compressive stresses generated by shot peening overcame the tensile stresses from severe grinding.
There are several ways of considering these benefits.
- First, shot peening allows an increased amount of stress to achieve the same component metal fatigue life.
- Second, shot peening extends the life of any part if the existing stress level is maintained.
- Thirdly, shot peening permits a greater range of acceptable manufacturing operations by providing a consistent surface compressive stress for combating metal fatigue.
Stress Corrosion Cracking
Stress corrosion cracking (SCC) is a progressive fracture mechanism in metals that is a result of the simultaneous interaction of a corrodent and a sustained tensile stress. Structural failure due to SCC is often sudden and unpredictable, occurring after as little as a few hours of exposure, or after months or even years of satisfactory service. Metal components frequently experience SCC in the absence of any other obvious kinds of corrosive attack. Virtually all alloy systems are susceptible to SCC by a specific corrodent under a specific set of conditions.
The tensile stresses necessary for SCC are "static", and they may be residual and/or applied (see chart below).
Sources of Stress for SCC RESIDUAL APPLIED Welding Quenching Shearing, Punching, Cutting Thermal Cycling Bending, Crimping, Riveting Thermal Expansion Machining Lathe-Mill-Drill) Vibration Heat Treating Rotation EDM, Laser/Wire Cutting Bolting Grinding Pressure
Progressive cracking due to "cyclic" stresses in a corrosive environment is termed "corrosion-fatigue". The boundary between SCC and corrosion-fatigue is sometimes vague. However, because the environments that cause SCC and corrosion-fatigue are not the same, the two are treated as separate and distinct metal fracture mechanisms. Compressive residual stresses, such as those induced in the surface layers of a structure by controlled shot peening, could prevent or delay SCC and corrosion-fatigue.
Photomicrographs of peened and unpeened type 304 stainless steel plate surfaces (sensitized at 1200°F - 1 hour and tested for Intergranular Corrosion in NH) 3-HF peened with ceramic beads).
It was discovered at Atomics International that intergranular corrosion can be prevented in austenitic stainless steels by shot peening prior to exposure to sensitizing temperatures. For this purpose, the surfaces must be severely cold worked by the shot peening to break up surface grains and grain boundaries. When exposed to sensitizing temperatures, carbides will precipitate on the multitude of nucleation sites (i.e., slip planes, dislocations) formed within grains rather than preferentially along continuous grain boundaries to support intergranular attack in a corrosive medium.
LEFT - Peened ~~ RIGHT - Unpeened
Exfoliation corrosion is a more severe form of intergranular corrosion that can occur along aluminum grain boundaries in the fuselage empennage and wing skins of aircraft. These grain boundaries in both aluminum sheet and plate are oriented in layers parallel to the surface of the material, due to the rolling process. The delamination of these thin layers of the aluminum, with white corrosion products between the layers, characterizes exfoliation corrosion.
Exfoliation corrosion is often found next to fasteners where an electrically insulating sealant or a sacrificial cadmium plating has broken down, permitting a galvanic action between the dissimilar metals. Where fasteners are involved, exfoliation corrosion extends outward from the fastener hole, either from the entire circumference of the hole, or in one direction from a segment of the hole. In severe cases, the surface bulges outward, but in less severe cases, there may be no telltale blisters, and you can only detect the exfoliation corrosion by nondestructive inspection methods that are not always very effective.
Controlled shot peening can be very effective in the process of both identifying and repairing exfoliation corrosion damage. Service manuals normally call for the removal of the fasteners and then for the use of rotary discs to sand away the corroded material, followed by blending the area and polishing out the tool marks. Aircraft structural engineers have used Metal Improvement Company's controlled shot peening after removal of visible exfoliation corrosion to compensate for the lower fatigue strength of the newly reduced cross-section. The action of peening, however, will cause the surface to blister again, where deeper exfoliation corrosion is present. The surface can then be redressed and repeened until no further blistering occurs. Metal Improvement Company calls this process Search Peeningsm. The process provides both a reliable nondestructive testing of the exfoliated material and a fatigue strength compensation for any reduced cross section.
Metal Improvement Company can perform its Search Peening process on-site at aircraft repair hangers to address exfoliation corrosion.
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