Case Study: Ultimate Strength and Airplane Bolts


Concepts Shown:

ultimate strength, safety




When constructing an airplane, engineers must consider the forces which will act on the plane during take-off, flight, and landing. The materials with which the plane is constructed must be able to withstand these forces, unanticipated forces, and the environment. Materials must be chosen with properties which will increase the safety factor of the aircraft. For example, steel bolts are used to connect the wing to the plane's cabin. Steel bolts are also used to attach the engines to the wing, but there is a major difference between the bolts. Cabin bolts, which are made out of a 4340 steel, have a higher ultimate strength than engine bolts, which are made out of a 17-4 PH stainless steel. Depending on the heat treatment, cabin bolts have an ultimate strength that ranges from 1500-1900 MPa whereas engine bolts have an ultimate strength ranging from 1200-1400 MPa, meaning cabin bolts can withstand a greater force than engine bolts before failure.

One reason for the higher ultimate strength is that cabin bolts need to support the weight of the wing and engines whereas engine bolts only need to support the weight on the engine. A more important reason is wind forces. When the plane encounters turbulence, large forces act upon the wing and engines. If forces acting on the wing are greater than the ultimate strength of the cabin bolts, the wing will break off and the plane will fall to the ground. However, because the engine bolts have a lower ultimate strength, they will fail before cabin bolts causing the engine to break away from the wing. The cabin bolts can now withstand greater wind forces than they originally had to because they no longer have to support the weight of the engines. Without engines, the plane will be able to glide to the ground, greatly increasing the possibility of a successful emergency landing.


Dave Goodman

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