Due to the fairly recent development of new surface enhancement and metal strengthening techniques, an alternative is available. Lengthy inspections and maintenance procedures won’t need to be as frequent, and component availability issues may no longer lead to aircraft grounding. Surface enhancement introduces a layer of compressive residual stress that retards fatigue and stress corrosion crack initiation and growth. Surface enhancement is now being used to extend component life without the need to change either the material or the design of the part.
When choosing a surface enhancement technique, several criteria should be considered before making a final decision: the speed of the treatment; how many times you need to process the component; how deep your component will be protected from or treated for corrosion, pitting, fretting, etc.; and the level of cold work the process produces. 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.
Currently, the available surface treatments include shot peening, controlled coverage peening (CCP), laser shock peening (LSP), deep rolling (roller burnishing), low plasticity burnishing (LPB), and controlled plasticity burnishing (CPB). The most commonly used and best known of these are shot peening, deep rolling, LSP, and LPB. LSP and LPB are fairly new, LPB being as recently developed as 1996, and are currently being explored by engineers worldwide for their effectiveness.
Shot peening, however, was developed in the 1920s and is used all over the world in almost every industry that deals with metal fatigue. Most commonly used today with CNC machines, shot peening repeatedly impacts the component surface, creating small indents or dimples all over the exterior. The component’s resistance to the forced expansion creates a shallow layer of residual compression, thereby improving the fatigue strength.
Coverage for shot peening is calculated as a percent of the surface impacted. Usually 200 to 400 percent is used due to the fact that shot peening is not exact, but a random process. This means one spot may have already been hit four times before the spot next to it experiences the first strike. Because of this, exposure is not uniform and the surface is very highly cold-worked, making it a poor candidate for the majority of aviation needs.
Roller burnishing, widely used in Europe, produces an extremely high cold-worked surface. In this process, a roller tool is repeatedly applied to the piece being worked with sufficient force to yield the surface, creating a layer (about 1 mm+ average) of compression. The level of cold work generated from this treatment can be beneficial for applications where the treated component won’t be exposed to extreme stress, but is detrimental at elevated temperatures. Roller burnishing is limited to rotationally symmetrical components and lathe operations.
Laser shock peening
LSP uses high-speed, high-powered lasers to shock the component with enough force to yield the surface with minimal cold work. A coating, usually black tape or paint, is applied to absorb the energy. Short energy pulses are then focused to explode the ablative coating, producing a shock wave. The beam is then repositioned and the process is repeated, creating an array of slight indents of compression and depth with about 5 to 7 percent cold work. A translucent layer, usually consisting of water, is required over the coating and acts as a tamp, directing the shock wave into the treated material. This computer-controlled process is then repeated, often as many as three times, until the desired compression level is reached, producing a compressive layer as deep as 1-2 mm average.
LSP is already in use on a number of aircraft engines, frames, and other components. The fact that the treated components will hold their compression in service makes LSP a great candidate for the aviation industry. Unfortunately, there are also several negative aspects to the process that may affect some aircraft owners. LSP is the most expensive of all metal strengthening processes, costing anywhere from 10 to 100 times as much as the other surface treatments. The process requires repeated coating of the material to get the desired strength, as well as expensive equipment, and is not adaptable to a shop environment.
LPB uses residual compression by design to increase damage tolerance in metallic components, greatly extending their service life
Pacific Propeller International has begun robotically processing aircraft components for the US Navy, using Lambda Technologies' patented low plasticity burnishing (LPB).