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Basic Loads | |
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Airplanes are made up of parts or components. These components experience stress or loads in flight or even while standing still. Stresses have effects both outside and inside various airplane components. It is important to understand the effects of these stresses. If a long wire were hung from the ceiling with a weight on one end, the weight would place a force on the wire. Inside the wire, the force would be evenly distributed along the wire. Since the weight is hanging at the end of the wire, the stress is referred to as a tensile (tension) stress. The atoms of the wire are being pulled apart by the force distributed along the wire. If the wire were a thin metal rod on a firm surface with a weight on the top, it would still experience the force evenly distributed throughout the rod. In this case, it is called compressive stress. Here the atoms of the rod are being pushed together. In both cases, the force is lined up with the wire or rod. Tension and compression stresses occur when the atoms of a component are being pulled apart or pushed together. Another type of stress occurs when layers of atoms try to slide across each other sideways. This is called shear stress. The shear stress can be illustrated by using two blocks of wood nailed together. If one block of wood was pushed to the right and the other block of wood was pushed to the left, the nails would experience a shear stress at the point where the two blocks of wood meet. When a component experiences any of the stresses described above, it may deform because of the force applied. For example, the wire hanging from the ceiling with a weight on the end. The wire may stretch a little bit because of the weight pulling down on it. If the original wire were 12 inches long, and enough weight was applied to it so that it stretched an additional inch, then the strain ratio would be 1/12 inch/inch. This is a measure of the deformation of a component. Engineers use a diagram called a stress-strain diagram to predict how much loading (or stress) a component can experience before it fails or breaks. The "Hook's Law" plots the relationship between the stress on a component and the amount of deformation. A string of bubble gum can be used for illustration of the stress-strain interaction. When a small weight is placed (a coin perhaps) at the bottom of the string, it will stretch a little bit. When the weight is removed the bubble gum will go back to its original length. Heavier weight can be used, and each time the bubble gum will go back to the original length. Eventually, however, the weight will get so heavy that the bubble gum string will not return to its original length, it will permanently deform. The point at which the gum becomes permanently deformed is called the elastic limit. If even heavier weights are attached to the bubble gum string, it will reach the point of the yield strength of the gum. This means the gum will no longer shrink back towards its original form, when the weight is removed. The gum string will stay fully stretched out. When this happens, the gum is close to failure (breaking!). This region is called the small plastic region. If more weight is added the gum string will break. Engineers say that the gum has been pushed past its ultimate strength. This last part of the curve (on the diagram) is called the large plastic region.
The components in an aircraft are usually designed for loads that remain
in the elastic region. They are not intended to fail or become
deformed! The engineer looks at the estimated load (stress) and chooses
the proper type and size of material needed. Most engineers prefer to
design a part with a safety factor built in, so that unexpected loadings
(stresses) from wind gusts, turbulence, or downdrafts will not exceed
the yield strength of the part.
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