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Basic Loads | |
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In addition to studying the effects of the outside forces on the components of an aircraft, it is important also to study the effects of the force inside the components. If a long metal 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 experienced evenly throughout the wire. If the force was divided by the cross-sectional area of the wire (the area of a cut across the wire), the resultant value is called the stress. In this case, with the weight hanging at the end of the wire, the stress is referred to as a tensile stress. The atoms of the wire are being pulled apart by the force distributed in 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. The stress would still be computed as the force divided by the cross-sectional area of the rod, but in this case, it would be called a 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 a stack of three blocks. If the three blocks were stacked together and the middle block was pulled to one side (by using a sideways force!), the middle block would slide between the top and bottom blocks. The two surfaces where the blocks slide are called the shear surfaces. If the three blocks were nailed together and the middle block was pulled to the side, the nail would experience a shear stress at the points where the three blocks met. The nail would try to resist the pull, but the cross-sections at the two shear surfaces would feel a shear stress. The value of the shear stress would be computed using the sideways force divided by the cross-sectional area of the nail at the two shear surfaces. When a component is placed in a situation where it experiences a force distributed throughout, another interesting feature may occur. The component may deform because of the force loading. In a tensile test, for example, such as the wire hanging from the ceiling with a weight on the end, the wire may stretch a little bit because of the weight on the end. Most of the time, the stretching may be difficult to see, because it may be a very small increase if the wire is strong. On the other hand, if something weak were used for the wire, such as bubble gum, and a weight were attached to the end, the stretching would be very visible. The strain that a component experiences under stress is a measure of the deformation of the component. In the tensile test case, the strain would be the ratio of the additional length due to stretching divided by the original length. If the original wire were 12 inches long, and enough weight was applied to the end so that it stretched an additional inch, then the strain would be 1/12 in/in. Compressive stresses and shear stresses may also deform a component, and in each situation, a strain would be computed as a measure of the deformation. Engineers use a diagram called a stress-strain diagram to predict how much loading a component can experience before it fails or breaks. It plots the relationship between the stress on a component and the amount of deformation. A very famous relation is used for the relationship: HookeUs Law. The string of bubble gum can be used for illustration of the stress-strain interaction. When a small weight (a coin perhaps) is applied to the end of the bubble gum, it will stretch a little bit. When the weight is removed, the bubble gum will go back to its original length. Heavier weights 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 be permanently deformed; it will shrink back, but not back to its original length. The point at which the gum becomes permanently deformed is called the elastic limit, and the part of the curve leading up to that point is called the elastic region. If even heavier weights are attached to the bubble gum, it will reach the point of the yield strength of the gum. This means the gum will no longer shrink when the weight is removed; it 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 strain will grow much more quickly than the stress, and the gum will stretch farther and farther. Eventually, enough weight will be added to cause the gum to break. Engineers say that the gum has been pushed past its ultimate strength. This last part of the curve 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 to become deformed! The engineer chooses the proper
type and size of material for a component based on the
estimated loading. He or she will design the part for
a design stress that is less than the yield strength.
The safety factor of a design is the ratio of the
yield strength to the design stress. Most engineers prefer to
design a part with a safety factor on the order of
3 to 5 so that unexpected loadings from wind gusts,
turbulence, or downdrafts will still not exceed the yield strength
of the part.
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