ENGINES
Corrosion – Pitted Steel and Gear Tooth Corrosion and Wear - Photos Courtesy of Vector Aerospace
Fracture A fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid almost always occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displacement develops, in this case perpendicular to the surface of displacement, it is called a normal tensile crack or simply a crack. If a displacement develops tan- gentially to the surface of displacement, it is called a shear crack, slip band or dislocation. Fracture strength, also known as breaking strength, is the stress at which a specimen fails via fracture. This is usually determined for a given specimen by a tensile test, which charts the stress-strain curve. The final recorded point is the fracture strength.
Thermal Shock Thermal shock occurs when a thermal gradient causes dif- ferent parts of an object to expand by different amounts. This differential expansion can be understood in terms of stress or of strain equivalently. At some point, this stress can exceed the strength of the material, causing a crack to form. If nothing stops this crack from propagating through the material, it will cause the object’s structure to fail.
Wear In materials science, wear is erosion or sideways displacement of material from its “derivative” and original position on a solid surface performed by the action of another surface. Wear is related to interactions between surfaces, and
more specifically, the removal and deformation of material on a surface as a result of mechanical action of the op- posite surface. The need for relative motion between two surfaces and initial mechanical contact between asperities is an important distinction of mechanical wear as com- pared to other processes with similar outcomes.
Corrosion Corrosion is the gradual destruction of materials (usually metals) by chemical reaction with its environment. In the most common use of the word, this means elec-
trochemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type
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HelicopterMaintenanceMagazine.com October | November 2013
of damage typically produces oxide(s) or salt(s) of the original metal. Corrosion can also occur in ceramics and polymers — although in this context, the term degradation is more common. Corrosion degrades the useful properties of materi- als and structures, including strength, appearance and perme- ability to liquids and gases. Many structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly af- fected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly, corroding the surface. Because corrosion is a diffusion-controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface can increase a material’s corrosion resistance.
Welding Welding is a fabrication or sculptural process that joins ma- terials, usually metals or thermoplastics, by causing coales- cence. This is often done by melting the work pieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint, with pressure sometimes used in conjunction with heat or by itself, to produce the weld. This is in contrast to soldering and brazing, which involve melting a lower-melting-point material between the work pieces to form a bond between them, without melting the work pieces. As you can see, gremlins come in all shapes and sizes and
rarely, if ever, will they be present alone. They usually start out singularly, but soon reinforcements arrive. The one gremlin we have not discussed is you. As people,
we are very complex beings, and our state of mind at 11:00 a.m. on a Wednesday is probably quite different than what it is at 5:00 p.m. on a Friday. Just when an unscheduled mainte- nance event will present itself cannot be known in advance. How you respond to it can mean the difference between a $3,000 fix and a $30,000 fix, depending on what you did or did not do at the time the event presented itself. Intimate understanding of your helicopter’s engines and recognizing when some small thing is not quite right can be the start- ing point to keeping those pesky gremlins from eating your engines.
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