Low plasticity burnishing
Encyclopedia
Low plasticity burnishing (LPB) is a method of metal improvement that provides deep, stable surface compressive residual stress
es with little cold work for improved damage tolerance
and metal fatigue
life extension. Improved fretting
fatigue and stress corrosion performance has been documented, even at elevated temperatures where the compression from other metal improvement processes relaxes. The resulting deep layer of compressive residual stress has also been shown to improve high cycle fatigue (HCF) and low cycle fatigue (LCF) performance.
LPB was developed and patented by Lambda Technologies, a small family-owned company from Cincinnati, Ohio, in 1996. Since then, LPB has been developed to produce compression in a wide array of materials to mitigate surface damage, including fretting, corrosion pitting, stress corrosion cracking
(SCC), and foreign object damage
(FOD), and is being employed by such companies as Delta TechOps and PAS Technologies, as well as NAVAIR, to aid in their daily MRO operations. To this day, LPB is the only metal improvement method applied under continuous closed-loop process control and has been successfully applied to turbine engines, piston engines, propellers, aging aircraft structures, landing gear, nuclear waste material containers, biomedical implants and welded joints. The applications involved titanium, iron, nickel and steel-based components and showed improved damage tolerance as well as high and low cycle fatigue performance by an order of magnitude.
The ball rolls across the surface of a component in a pattern defined in the CNC code, as in any machining operation. The tool path and normal pressure applied are designed to create a distribution of compressive residual stress. The form of the distribution is designed to counter applied stresses and optimize fatigue and stress corrosion performance. Since there is no shear being applied to the ball, it is free to roll in any direction. As the ball rolls over the component, the pressure from the ball causes plastic deformation to occur in the surface of the material under the ball. Since the bulk of the material constrains the deformed area, the deformed zone is left in compression after the ball passes.
failures. Lambda designs a new tool for each component to provide the best results possible and to ensure that the apparatus reaches every inch on the component. With this practice of constantly redesigning, along with the closed-loop process control system, LPB has been shown to produce a maximum compression of 12mm, although the average is around 1-7+mm. LPB has even been shown to have the ability to produce through-thickness compression in blades and vanes, greatly increasing their damage tolerance over 10-fold, effectively mitigating most FOD and reducing inspection requirements. No material is removed during this process, even when correcting corrosion damage. LPB smooths surface asperities during machining, leaving an improved, almost mirror-like surface finish that is vastly better looking and better protected than even a newly manufactured component.
, laser peening
, gravity peening or deep rolling. Cold work is particularly important because the higher the cold work at the surface of a component, the more vulnerable to elevated temperatures and mechanical overload that component will be and the easier the beneficial surface residual compression will relax, rendering the treatment pointless. In other words, a component that has been highly cold worked will not hold the compression if it comes into contact with extreme heat, like an engine, and will be just as vulnerable as it was to start. The reason LPB produces such low percentages of cold work is because of the aforementioned closed-loop process control. Other processes have some guesswork involved and are not exact at all, causing the procedure to have to be performed multiple times on one component. For example, shot peening, in order to make sure every spot on the component is treated, typically specifies coverage of between 200% (2T) and 400% (4T). This means that at 200% coverage (2T), 5 or more impacts occur at 84% of locations and at 400% coverage (4T), it is significantly more. The problem is that one area will be hit several times while the area next to it is hit less times, leaving uneven compression at the surface. This uneven compression results in the whole process being easily "undone", as was mentioned above. LPB requires only one pass with the tool and leaves a deep, even, beneficial compressive stress.
The LPB process can be performed on-site in the shop or in situ on aircraft using robots, making it easy to incorporate into everyday maintenance and manufacturing procedures. The method is applied under continuous closed loop process control (CLPC), creating accuracy within 0.1% and alerting the operator and QA immediately if the processing bounds are exceeded. The limitation of this process is that different CNC processing codes need to be developed for each application, just like any other machining task. The other issue is that because of dimensional restrictions, it may not be possible to create the tools necessary to work on certain geometries, although that has yet to be a problem.
Residual stress
Residual stresses are stresses that remain after the original cause of the stresses has been removed. They remain along a cross section of the component, even without the external cause. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment...
es with little cold work for improved damage tolerance
Damage tolerance
Damage tolerance is a property of a structure relating to its ability to sustain defects safely until repair can be effected. The approach to engineering design to account for damage tolerance is based on the assumption that flaws can exist in any structure and such flaws propagate with usage...
and metal fatigue
Metal Fatigue
Metal Fatigue , is a futuristic science fiction, real-time strategy computer game developed by Zono Incorporated and published by Psygnosis and TalonSoft .-Plot:...
life extension. Improved fretting
Fretting
Fretting refers to wear and sometimes corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration...
fatigue and stress corrosion performance has been documented, even at elevated temperatures where the compression from other metal improvement processes relaxes. The resulting deep layer of compressive residual stress has also been shown to improve high cycle fatigue (HCF) and low cycle fatigue (LCF) performance.
History
Unlike LPB, traditional burnishing tools consist of a hard wheel or fixed lubricated ball pressed into the surface of an asymmetrical work piece with sufficient force to deform the surface layers, usually in a lathe. The process does multiple passes over the work pieces, usually under increasing load, to improve surface finish and deliberately cold work the surface. Roller and ball burnishing have been studied in Russia and Japan, and were applied most extensively in the USSR in the 1970s. Various burnishing methods are used, particularly in Eastern Europe, to improve fatigue life. Improvements in HCF, corrosion fatigue and SCC are documented, with fatigue strength enhancement attributed to improved finish, the development of a compressive surface layer, and the increased yield strength of the cold worked surface.LPB was developed and patented by Lambda Technologies, a small family-owned company from Cincinnati, Ohio, in 1996. Since then, LPB has been developed to produce compression in a wide array of materials to mitigate surface damage, including fretting, corrosion pitting, stress corrosion cracking
Stress corrosion cracking
Stress corrosion cracking is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when...
(SCC), and foreign object damage
Foreign object damage
Foreign Object Debris is a substance, debris or article alien to a vehicle or system which would potentially cause damage.Foreign Object Damage is any damage attributed to a foreign object that can be expressed in physical or economic terms that may or may not degrade the product's required...
(FOD), and is being employed by such companies as Delta TechOps and PAS Technologies, as well as NAVAIR, to aid in their daily MRO operations. To this day, LPB is the only metal improvement method applied under continuous closed-loop process control and has been successfully applied to turbine engines, piston engines, propellers, aging aircraft structures, landing gear, nuclear waste material containers, biomedical implants and welded joints. The applications involved titanium, iron, nickel and steel-based components and showed improved damage tolerance as well as high and low cycle fatigue performance by an order of magnitude.
How it works
The basic LPB tool is a ball that is supported in a spherical hydrostatic bearing. The tool can be held in any CNC machine or by industrial robots, depending on the application. The machine tool coolant is used to pressurize the bearing with a continuous flow of fluid to support the ball. The ball does not contact the mechanical bearing seat, even under load. The ball is loaded at a normal state to the surface of a component with a hydraulic cylinder that is in the body of the tool. LPB can be performed in conjunction with chip forming machining operations in the same CNC machining tool.The ball rolls across the surface of a component in a pattern defined in the CNC code, as in any machining operation. The tool path and normal pressure applied are designed to create a distribution of compressive residual stress. The form of the distribution is designed to counter applied stresses and optimize fatigue and stress corrosion performance. Since there is no shear being applied to the ball, it is free to roll in any direction. As the ball rolls over the component, the pressure from the ball causes plastic deformation to occur in the surface of the material under the ball. Since the bulk of the material constrains the deformed area, the deformed zone is left in compression after the ball passes.
Benefits
The LPB process includes a unique and patented way of analyzing, designing, and testing metallic components in order to develop the unique metal treatment necessary to improve performance and reduce metal fatigue, SCC, and corrosion fatigueCorrosion fatigue
Corrosion fatigue is fatigue in a corrosive environment. It is the mechanical degradation of a material under the joint action of corrosion and cyclic loading. Nearly all engineering structures experience some form of alternating stress, and are exposed to harmful environments during their service...
failures. Lambda designs a new tool for each component to provide the best results possible and to ensure that the apparatus reaches every inch on the component. With this practice of constantly redesigning, along with the closed-loop process control system, LPB has been shown to produce a maximum compression of 12mm, although the average is around 1-7+mm. LPB has even been shown to have the ability to produce through-thickness compression in blades and vanes, greatly increasing their damage tolerance over 10-fold, effectively mitigating most FOD and reducing inspection requirements. No material is removed during this process, even when correcting corrosion damage. LPB smooths surface asperities during machining, leaving an improved, almost mirror-like surface finish that is vastly better looking and better protected than even a newly manufactured component.
Cold working
The cold work produced from this process is typically between 2-5%, a great deal less than shot peeningShot peening
Shot peening is a cold working process used to produce a compressive residual stress layer and modify mechanical properties of metals. It entails impacting a surface with shot with force sufficient to create plastic deformation...
, laser peening
Laser peening
Laser peening, or laser shock peening , is the process of hardening or peening metal using a powerful laser. Laser peening can impart a layer of residual compressive stress on a surface that is four times deeper than that attainable from conventional shot peening treatments. A coating, usually...
, gravity peening or deep rolling. Cold work is particularly important because the higher the cold work at the surface of a component, the more vulnerable to elevated temperatures and mechanical overload that component will be and the easier the beneficial surface residual compression will relax, rendering the treatment pointless. In other words, a component that has been highly cold worked will not hold the compression if it comes into contact with extreme heat, like an engine, and will be just as vulnerable as it was to start. The reason LPB produces such low percentages of cold work is because of the aforementioned closed-loop process control. Other processes have some guesswork involved and are not exact at all, causing the procedure to have to be performed multiple times on one component. For example, shot peening, in order to make sure every spot on the component is treated, typically specifies coverage of between 200% (2T) and 400% (4T). This means that at 200% coverage (2T), 5 or more impacts occur at 84% of locations and at 400% coverage (4T), it is significantly more. The problem is that one area will be hit several times while the area next to it is hit less times, leaving uneven compression at the surface. This uneven compression results in the whole process being easily "undone", as was mentioned above. LPB requires only one pass with the tool and leaves a deep, even, beneficial compressive stress.
The LPB process can be performed on-site in the shop or in situ on aircraft using robots, making it easy to incorporate into everyday maintenance and manufacturing procedures. The method is applied under continuous closed loop process control (CLPC), creating accuracy within 0.1% and alerting the operator and QA immediately if the processing bounds are exceeded. The limitation of this process is that different CNC processing codes need to be developed for each application, just like any other machining task. The other issue is that because of dimensional restrictions, it may not be possible to create the tools necessary to work on certain geometries, although that has yet to be a problem.
See also
- Corrosion fatigueCorrosion fatigueCorrosion fatigue is fatigue in a corrosive environment. It is the mechanical degradation of a material under the joint action of corrosion and cyclic loading. Nearly all engineering structures experience some form of alternating stress, and are exposed to harmful environments during their service...
- Damage toleranceDamage toleranceDamage tolerance is a property of a structure relating to its ability to sustain defects safely until repair can be effected. The approach to engineering design to account for damage tolerance is based on the assumption that flaws can exist in any structure and such flaws propagate with usage...
- FODForeign object damageForeign Object Debris is a substance, debris or article alien to a vehicle or system which would potentially cause damage.Foreign Object Damage is any damage attributed to a foreign object that can be expressed in physical or economic terms that may or may not degrade the product's required...
- FrettingFrettingFretting refers to wear and sometimes corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration...
- Laser peeningLaser peeningLaser peening, or laser shock peening , is the process of hardening or peening metal using a powerful laser. Laser peening can impart a layer of residual compressive stress on a surface that is four times deeper than that attainable from conventional shot peening treatments. A coating, usually...
- Metal fatigueMetal FatigueMetal Fatigue , is a futuristic science fiction, real-time strategy computer game developed by Zono Incorporated and published by Psygnosis and TalonSoft .-Plot:...
- PeeningPeeningPeening is the process of working a metal's surface to improve its material properties, usually by mechanical means such as hammer blows or by blasting with shot . Peening is normally a cold work process...
- Residual stressResidual stressResidual stresses are stresses that remain after the original cause of the stresses has been removed. They remain along a cross section of the component, even without the external cause. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment...
- Shot peeningShot peeningShot peening is a cold working process used to produce a compressive residual stress layer and modify mechanical properties of metals. It entails impacting a surface with shot with force sufficient to create plastic deformation...
- Stress corrosion crackingStress corrosion crackingStress corrosion cracking is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when...
- Ultrasonic impact treatmentUltrasonic impact treatmentUltrasonic impact treatment is a method of metal improvement that utilizes ultrasonic energy. The physical characteristics: controlled residual compressive stress, grain refinement and grain size reduction...