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Fretting
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Fretting occurs when two surfaces are clamped together under a normal load with a small relative motion of the surfaces. The fatigue debit from fretting can reduce fatigue strength from one-half to one-third of the normal endurance limit. Bolted joints, dovetail slots, tapered joints, shaft couplings, flanges, etc. are commonly subject to fretting. Fretting is a damage mechanism which combines the formation of oxide debris by corrosion with a reduction in fatigue strength. Because of the oxidation, fretting is often classified with corrosion processes in the literature. The oxide debris is created as oxide layers are repeatedly formed and removed by the abrasive action of one surface sliding against the other. The fatigue debit from fretting is not caused by the visible corrosion, but by small shear stress initiated fatigue cracks at the edges of the contact zones.
If two blocks are clamped together under a normal force, so that they partially overlap to form a contact zone, and then cyclically loaded placing the contact zone in shear, the maximum shear stress will occur at each end of the contact zone. This “edge of contact” is the location of fatigue crack initiation during fretting. The fretting shear stress distribution is quite shallow, typically extending less than .005 inches into the surface with maximum shear stress magnitude at the edge of contact. Shear cracks will initiate at nominally one third of the life anticipated without fretting, and grow in shear (Mode 2) to the shallow limit of the alternating shear stress, only a few thousandths of an inch. Fatigue failure results from the cracks then propagating under the applied normal stresses in (Mode 1) on through the component to failure.
Because shear and normal stresses are inherently orthogonal, it is not possible to stop the formation of the shear-induced micro-cracks by surface enhancement. However, the deep compression produced by LPB™ holds the micro-cracked tips in high compression preventing propagation. The cracks simply cannot grow beyond the shallow fretting shear stress layer.
LPB™ has been demonstrated to completely mitigate fretting damage for structural aluminum alloys such as 7075-T6 (both aluminum-on-steel and aluminum-on-aluminum), for 4340 steel, and for titanium alloys including Ti-6Al-4V. With LPB™ applied prior to fretting, or even as a post-treatment and repair after initial fretting, fatigue failures cannot occur from the surface or the fretting induced micro-cracks. The fatigue strength achieved with LPB™ is that of the subsurface material, essentially the vacuum fatigue strength of the alloy with no surface failures. LPB™ has been transitioned into production to mitigate fretting failures at the edge of bedding of titanium alloy compressor blade dovetails and disk slots, as well as Ti-6-4 medical implants.
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Stress Corrosion Cracking
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Stress corrosion cracking (SCC) causes the relatively rapid failure of components due to either trans- or inter-granular cracking occurring in susceptible alloy-environment combinations when static tensile stresses are present that exceed some threshold for SCC initiation. SCC is a complex phenomenon not as yet fully understood. Mechanisms proposed include rapid crevice corrosion along grain boundaries and and/or the wedging open of the cracks by oxide formation. SCC is more common in high strength materials, such as high strength aluminum aircraft structural alloys and steels. Susceptibility is affected by alloy segregation, as in the sensitized heat-affected zones of 304 stainless steel welds.
Conventional approaches to SCC mitigation are 1) substitution of a less susceptible alloy or 2) isolation of the alloy surface from the aggressive environment with plating or coatings. Unfortunately, the SCC inducing environment may be common seawater or very low concentrations of dissolved oxygen and chlorine, making isolation and control of the environment extremely difficult. Substitution of an “uncrackable” alloy is expensive, and projecting long-term performance with accelerated laboratory testing is difficult. Often the new alloy is again found to be cracked in service, and the substitution cycle is repeated. Mechanical suppression of SCC, by introducing a deep layer of surface compression to eliminate tensile stresses at the surface in contact with the environment, is the one approach which eliminates stress corrosion cracking at minimal cost.
LPB™ has been demonstrated to completely eliminate stress corrosion cracking in high strength steels like 300M and 4340, structural aluminum alloys including 7075-T6, and 304 stainless steel welds with or without sensitization. By introducing a deep layer of stable high compression, the surface of the susceptible alloy that is in contact with the aggressive environment remains entirely in compression, well below the tensile threshold required for SCC. As a result, SCC is completely mitigated. LPB™ mitigation of SCC has been repeatedly demonstrated for static stress corrosion cracking and its cyclic variant corrosion fatigue. General corrosion continues, but the risk of catastrophic failure caused by SCC driven cracks can be completely eliminated. The higher the strength of the alloy, the more beneficial the effects of surface enhancement. It should be noted that the depth of compression and the cold work induced at the surface are important to the effectiveness of mitigating SCC by surface enhancement. Shot peening, although it provides some benefit, is not as effective as LPB™ because of the shallower depth of compression and the much higher cold working which increases the chemical activity of the surface.
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Stress Concentrations
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Failures, whether by fatigue or stress corrosion cracking, will always occur first at the locations of highest stress, which may be highly localized. The highest stressed points are usually stress concentrations in fillets, boltholes, key ways and similar structural features where the stress is inherently higher than in the uniform section of the component by as much as factor of two or more. A zone of residual compression can often be designed to cancel the effect of the physical stress concentration. The depth and magnitude of the compressive zone can be designed using the fatigue design analysis method developed by Lambda Research to calculate the mean stress distribution required at each point throughout a finite element model of the high stress region to overcome the reduced strength caused by the stress concentration.
Low Plasticity Burnishing (LPB™) has been effectively applied to eliminate stress concentrations caused by surface damage including FOD and corrosion pitting. This same technology can be extended to mechanical features of the component. Knowing the mean and alternating stress distributions in service, appropriate CNC processing code and pressure files can be created to introduce just the amount of residual compression necessary to mitigate the stress concentration. The precise CNC reproduceability of LPB™ allows the designed compressive field to be introduced reliably and repeatably during manufacture or during repair and overhaul. It should be noted that only localized stress concentrations can be addressed in this manner. A layer of deep surface compression added to the surface of a uniformly loaded section will simply increase the interior tension, moving failure subsurface.
Introduction of designed compression by LPB™ to overcome component weaknesses discovered in service is a common application of the new technology. Thousands of aircraft engine compressor vanes treated with LPB™ are now flying with the stress concentration at the airfoil-platform fillet radii effectively eliminated. A fatigue life limiting feature discovered in service can be eliminated without changing either the material or component design.
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Corrosion Pitting
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Corrosion Pitting is a primary source of fatigue site nucleation throughout virtually all industrial and military applications of metallic components other than stainless and titanium alloys. In airframe, automotive, naval, and piping applications corrosion pits produce stress concentrations reducing the fatigue strength. Even without the further complication of stress corrosion cracking or its cyclic variant corrosion fatigue, the corrosion pit can produce a stress concentration with a kt equal to at least 3, reducing the effective fatigue strength to one-third of the undamaged value. It is common practice in aircraft overhaul to remove the pitted layer by grinding followed by shot peening, producing a new surface. The pitting process then begins again, but now with a reduced section thickness. The structure is weakened with each iteration and eventually must be retired.
The introduction of a layer of stable compression on the order of a millimeter deep by Low Plasticity Burnishing (LPB™) can completely mitigate fatigue failures from corrosion pitting. Extensive work on laboratory coupons has eliminated surface failures entirely from severely pitted 7075-T6 aluminum, a high strength aircraft structural alloy, and 4340 steel, 50 HRC, commonly used for landing gear. LPB™ either before or after pitting resulted in the same benefit, allowing surface enhancement to be used effectively either for initial manufacture or during repair and overhaul. Because the compressive layer exceeds the depths of the pits, fatigue initiation is completely eliminated in what would otherwise be a high stress concentration. Application of LPB™ to fatigue critical areas can reduce inspection frequency and maintenance costs while providing component life extension.
The nature of the pitting process in aluminum alloys offers a special opportunity to virtually eliminate corrosion pitting induced fatigue failures in aircraft structures. The depth of aluminum alloy pits increases rapidly initially, but asymptotically approaches a maximum depth over time. Extensive literature shows this is true for saltwater corrosion in laboratory specimens as well as in service. As the pit depth increases, the ionic diffusion to the bottom of the pit needed to support growth is retarded, and the pit eventually shuts down through passivation. New pits then start again on the surface at another location. Thus a series of pits are generated in the surface over time, with depth never exceeding some maximum determined by the alloy and solution. 7075-T6 aluminum exposed to alternate immersion in 3.5% sodium chloride solution reached a maximum pit depth of nominally .019 inches. Because shot peening may only introduce .010 inches of compression, the pits may exceed the depth of the shot peening compression in only a few years of service. Any benefit of shot peening is then eliminated and the structure is left weakened by the pit stress concentrations. If an LPB™ layer of compression over .040 inches deep is introduced, corrosion pits will never reach even half the depth of compression. Fatigue failure from corrosion pits can be permanently eliminated without the need to even remove the corroded material and reduce section thickness for the life of the aircraft. Limited life supported by continuing maintenance is converted to safe-life, the service life is extended, and maintenance costs are reduced without changing either the material or design of the component.
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Foreign Object Damage (FOD)
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Foreign object damage (FOD) is a common occurrence in many applications, but is the nemesis of aircraft engines and structures. Because these components are designed for minimum weight and section thickness, they are particularly vulnerable to any surface damage that will initiate fatigue failure. The formation of any nick or dent in the surface, provided that it is not accompanied by high residual compression, will produce a stress concentration factor. This factor can be as high as 5.0, depending upon the nature of the damage and the shape of the feature produced. FOD is produced by flying debris in the fan and compressor sections of aircraft engines and ground-based turbines, on landing gear and other structural components exposed outside the aircraft. Even maintenance and general operation of an aircraft can result in damage from loose tools, cargo, and so on during maintenance and use. FOD occurring in high stress fatigue critical areas can lead to catastrophic failure. The introduction of a layer of compression deeper than the depth of the damage using single point LPB™ tools on thick sections can completely mitigate FOD. This has been demonstrated in titanium alloys such as Ti-6-4, 7075-T6 aluminum for structures, and 300M and 4340 structural high strength steels. Applications include landing gear, aircraft wing and hinge structures, and fan and compressor sections of engines. The use of caliper tools to apply the LPB™ force to two sides of the component simultaneously can produce through-thickness compression in thin sections such as the leading edge or trailing edge of blades and vanes. The zone of high compression can extend back a significant distance from the edge, providing spectacular damage tolerance to the impact on the edge of the blades. Thousands of LPB™ processed airfoils are currently flying to mitigate FOD and extend component life in aircraft engines without changing either the alloy or the design of the component.
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Weld Induced Tension
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Virtually any method of welding metallic components, be it conventional multi-pass TIG, MIG, electron beam, inertia or friction-stir, involves heating the fusion zone to a high temperature relative to the adjacent parent metal, followed by cooling of the weld back to ambient temperature. Fusion occurring at elevated temperature invariably results in tension as the hot fused metal contracts and is constrained by the surrounding colder material. Although welding with minimal constraint can reduce the residual tensile stresses, tension is difficult to avoid entirely, and in certain geometries can be severe.
Tensile stresses from welding are notorious sources of both fatigue and stress corrosion cracking failures, often occurring well into the useful life of the component. Phase transformations may accompany the welding thermal cycle as in sensitization of austenitic alloys. In fusion welds, the coarse cast-like grain structure of the fusion zones will generally have material properties that are reduced relative to the wrought and perhaps heat-treated parent metal. For all of the above reasons, weld zones typically have lower overall fatigue strength and stress corrosion resistance than the surrounding parent material.
The introduction of a designed residual stress distribution can improve the properties of the weld fusion zone to even exceed that of the surrounding parent metal. Introduction of a layer of compression on the order of a millimeter deep by LPB™ has been found to completely eliminate the fatigue and corrosion fatigue debit in friction stir welded aluminum panels*1. Whether sensitized to separate chromium carbides to the grain boundaries or welded in an as mill-annealed condition, introduction of an LPB™ compressive layer completely mitigates both corrosion fatigue and stress corrosion cracking. LPB™ can be an effective enabling technology to allow greatly improved fatigue and SCC performance of a wide variety of welds, and lends itself through CNC robotic processing to be performed concurrently with the welding operation.
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Corrosion Fatigue
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Corrosion fatigue is the combination of dynamic loading fatigue crack propagation exacerbated by a stress corrosion cracking (SCC) component which accelerates the cracking process. Corrosion fatigue is common in aluminum and steels in contact with salt water, and is a common occurrence in aircraft structures. Because the chlorine ions act as catalysts in the corrosion process, intermittent exposure to even trace amounts of salt can rapidly accelerate fatigue cracking. Corrosion fatigue can occur in combinations of material and environmental stress wherever stress corrosion cracking would also be a possibility, such as austenitic nuclear system components and welds. One of the most damaging aspects of corrosion fatigue is that the corrosive component eliminates the fatigue endurance limit. The corrosive damage component depends upon time rather than stress cycles, and effectively eliminates any safe stress level for infinite life. The S/N curve for an alloy in corrosion fatigue diminishes continuously with increasing cycles.
Introducing a deep stable layer of compression by LPB™ eliminates the stress corrosion component in all alloys investigated to date, including structural aluminums such as 7075-T6, high strength aircraft steels including 300M, and austenitic AISI 304 stainless steel widely used in nuclear applications. A millimeter of high compression introduced with low cold work holds the surface layer below the tensile threshold for stress corrosion cracking, thereby eliminating the cyclic stress corrosion cracking component that accelerates fatigue failures in the aggressive environment.
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