With the economy struggling worldwide, aging aircraft have become a serious issue. Thanks to some harsh lessons (Aloha 243, TWA 800, United 232, ASA 529, etc.), the world has already learned that the effects of corrosion and fatigue can be deadly and need to be dealt with accordingly.
Organizations like the Joint Council on Aging Aircraft (JCAA) combining U.S. DOD and FAA efforts, and CASA (Australia) are doing their best to remedy the situation. So far, the methods of dealing with aging have focused on increased inspections, repairs, and maintenance, with massive costs and a great deal of Aircraft on Ground (AOG), or retiring the aircraft and simply buying another. With people struggling to keep money in their pockets and the current economic climate, it’s becoming less and less likely that anyone will be able to afford either option.
Because of this, aircraft all over the globe are being kept in the air longer and not always maintained as directed by their country’s or even the manufacturer’s requirements. Not only are they endangering themselves, but the rest of the world’s population as well. Fourteen percent of all aviation accidents are due to maintenance problems involving a lack of detection of either fatigue or corrosion damage. Even the U.S. Air Force, with a 23-year average age for the fleet, has issues with detecting these faults. The tiniest crack or weak spot in any aircraft component could be catastrophic and the burden of detection is solely on the shoulders of appointed inspectors and maintenance personnel.
Due to these facts, in the United States the FAA requires every plane to undergo a 14-day, 100-hour maintenance check on the craft’s 14th anniversary of service and every seven years thereafter. Brandon Battles, vice president of the aviation and consulting firm Conklin and de Decker, says, “Regardless of the class or type of aircraft … maintenance costs can range from 10 to 45 percent of the total yearly estimated operating expenses. Ten percent might not seem significant but when total operating costs can be in the hundreds of thousands or millions of dollars, then even 10 percent becomes significant.”
One company involved in the making of the FAA’s Aviation Safety rule estimated that this would cost his business an additional $363 million per year in rescheduling and $285,790,000 in lost revenue. In 2006, the International Air Transport Association (IATA) reported that anywhere between $300 and $1,800 is spent on maintenance per flight hour, the exact average being $870. Of course, the price will vary with a number of factors, one of these being the age of the aircraft. Unfortunately, even all of these costly regulations and precautions won’t necessarily save us from our aging aircraft.
As a result of their complex construction and the fact that they’re under the most stress, engines (particularly turbine), propellers, and retractable landing gear are the most likely components to experience failures due to fatigue or corrosion. Foreign object damage (FOD), such as a small rock striking the propeller, could create a weak spot and the fatigue originating from that impact could eventually force the component to fail. Engines are accountable for 40 percent of all maintenance costs. On more than one occasion, blades have flown off their engines. Manufacturers even place a protective capsule around the blades now to prevent them from shooting into the fuselage should a failure occur. And though it will increase your velocity by 12 to 20 knots, retractable landing gear alone will add four to six hours of maintenance to annual inspection times.
Regrettably, some replacement components required for repair are no longer being manufactured or have excessive lead times, resulting in increased AOG or even early retirement.
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.
Process control during LSP is complicated by variations in the thickness and even turbulence of the water tamp, or debris in the beam path, that affect the power of the shock wave produced. As a result, quality control of the process is difficult, no matter how controlled the laser is. Studies have shown internal cracking on laser-treated components caused by superposition of echoing shock, so treating thin sections like engine blades and vanes can prove to be fairly difficult. Make sure the processed parts are inspected closely to ensure that processing hasn’t inadvertently caused damage to the same part you were trying to improve.
Low plasticity burnishing
LPB, currently available through Delta TechOps and PAS Technologies, uses hydrostatic ball and roller tools to produce the plastic deformation necessary to create the desired compression for the work piece. LPB has even been shown to have the ability to produce through-thickness compression in blades and vanes, greatly increasing their damage tolerance more than 10-fold, effectively mitigating most FOD and reducing inspection requirements.
The LPB process uses either CNC machines or industrial robots in the MRO shop or manufacturing environment along with the patented tools to create a deep layer of residual compression that mitigates surface damage. LPB has been documented to create compression as deep as 12 mm into the component in a single treatment (average is 1-7+ mm), leaving the surface superior to what it was before and actually rendering the component, in most cases, better than new. The cold work produced from this process is less than 5 percent, although it can be adjusted for varying needs, making it widely applicable throughout the aerospace industry. LPB has already been successfully applied to engines, propellers, propeller hubs, and landing gear.
The LPB process can be performed on-site in the shop or insitu 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 percent 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.
Treating the components of your aircraft with any of these surface enhancement fatigue and damage tolerance strengthening techniques can save huge amounts of maintenance and inspection time and money, more than offsetting the initial cost of implementation. With the current weak economy and maintenance-related accident rates going up, surface enhancement processes are invaluable and need to be looked into by everyone. Surface enhancement may not fix all problems, but at least you won’t have to worry about stress corrosion cracking or fatigue failures of critical components, and your aircraft can spend more time on wing rather than in the shop.
This article was submitted by Julia Prevey from Lambda Technologies. For more information visit www.lambdatechs.com.