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Elements of Structures | |
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There are three basic types of materials or elements in the structure of an airplane: stiffened shells, stiffened plates, and I-beams. Stiffening means that the plate or shell has oddly shaped pieces of metal welded to the back side to strengthen it. This allows the plate or shell to carry more weight. On an airplane, the fuselage (body) and nacelles (outer covering of the engine) are covered with stiffened shells. The wing itself can be considered an I-beam. Spars are welded to the I-beam, at right angles, to form the wing. The top and bottom surfaces of the wings are covered with stiffened plates. Computing the loads on the different components (parts) of an airplane can be very difficult. Tension loads (pulling molecules apart) on simple parts can be fairly easy to compute. Compressive stresses (pushing molecules together) can be much more difficult to figure. Plates and shells tend to be thin. This means they can buckle or bend (deform) long before they reach the failure point. For this reason, engineers try to design stiffened shells and plates to delay permanent deformation (bending). This means much more stress can be applied before bending occurs.
One way to understand buckling is to think of a thin rod standing on end
on a solid surface. As more and more weight is placed on top of the
rod, it will reach a critical point and bend or buckle. To compute
where that critical point is the engineer must know the strength of the
material (its elasticity), the length, shape and diameter of the rod.
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