Abstract
This report examines tribology, the science of erosion and wear. More precisely focuses on the effect of erosion and wear on aircraft structures, as it is an important and costly aspect of the aeronautics industry. It looks at the three main areas that make up tribology: friction, wear, and lubrication, and the effect these have on aircraft structures. It examines the mechanisms of erosion and wear; that is the analytic science behind tribology. It looks at how tribology affects the main parties involved the aeronautics industry. It examines some solutions used to prevent erosion and wear. Finally there is a section on how the science of tribology can be used to our advantage when dealing with aircraft, particularly with regard to paint stripping methods. Several sources were used to compile this report including a talk from Paul Butler, which focused on paint removal methods. It found that tribology is an important and costly factor in the aircraft industry, and that great effort is expended to prevent wear and erosion on aircraft. It also found that the science of tribology is used to some advantage in aircraft maintenance. ?
Introduction
Tribology is the science of surfaces interacting with each other in relative motion. More exactly it studies how this surface interaction affects the materials involved. It consists of three main areas: the friction forces involved when the surfaces interact, the wear that is a result of this friction, and the effect of lubrication on this interaction. Friction is the lateral force encountered when two surfaces come into contact, caused by the unsmooth nature of their skins. The force is determined by a number of factors including the weight acting on the surfaces, the kinetic force involved and the friction coefficient of the surfaces. This friction force is often responsible for wear of either of the surfaces involved. Wear is a process whereby material is lost during the interaction of a surface with other surfaces or its environment. Wear can be due to a number of factors including abrasion (two surfaces rubbing, scuffing or scratching), erosion (repeated localized mechanical impact, for example, sand particles impacting on metals) or corrosion (wear due to chemical interaction, for example, oxidation). Lubrication is the process of reducing friction, and therefore wear, between two surfaces by the application of a lubricant. Lubricants are usually in liquid form (hydrodynamic), but can be solids (graphite for example) or occasionally gasses. Or methods include the introduction of a boundary.
Tribology In Aircraft
Aircraft operate under some of the most extreme conditions possible in any environment. An average passenger aircraft might travel from sea level to 30,000ft and experience a temperature range of over 90?C. This presents an opportunity for several types of wear on several areas of the aircraft. One of the main areas of wear on an aircraft are the engines. This is because they operate under the most extreme conditions of any part of the aircraft. A jet engine may experience temperatures from subzero to in excess of 1000C. Jet engines also consist of many different moving parts, some of which can reach rotational speeds of up to 15,000 revs/min. This causes abrasive wear and also fretting. Fretting is when two surfaces rub together at high frequency and low amplitude in the presence of a corrosive environment. Engines are also prone to erosion from elements mixed in with the air intake. Particles such as sand and dirt, when impacted at high velocity and dragged along the surface of the blades can cause significant damage over time. Another area which suffers significantly from wear is the aircraft skin. Rain erosion is a major factor, especially on the nose and leading edges. At the high speeds jet aircraft fly at, the impact of raindrops sometimes containing corrosive chemicals can create wear called pitting. This is where the impact of the rain drop creates a small indent on the surface of the plane. This can be a problem on the control surfaces of the aircraft. Another problem experienced on the skin is crevice corrosion, where a corrosive liquid gets between joints, borderlines or under a coating. Both these types of corrosion can lead to exfoliation, where some of the skin separates in a leaf-like fashion. Oxidation is another form of corrosion which affects aircraft skin, however with materials like aluminum, which is used extensively in aircraft, oxidation can form a hardened layer which actually protects the metal and prevents further corrosion.
Another area where wear is prominent is the undercarriage/brakes. Aircraft brakes are much more complicated than automotive brakes, containing several pistons and are designed to absorb up to 135 MJ of energy. They convert a large amount of kinetic energy to heat energy in a short period of time, involving large friction forces. As stated already, friction is one of the main causes of wear so wear is an obvious consideration when designing aircraft landing gear. ?
Conclusions
From this report a number of conclusions can be drawn:
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