| Structural Member Stresses An aircraft at rest or in flight is subject to applied forces throughout its entire structure. At rest, the weight (pull of gravity) of the wings, fuselage, engines, and empennage causes forces to act downward on the wing and stabilizer tips, along the spars and stringers, and on the bulkheads and formers. These forces are passed from member to member causing bending (tension and compression), twisting (torsion), compression, and shearing. The five stresses in an aircraft are tension, compression, shear, bending, and torsion. The first three of these forces are generally called the basic stresses, and the last two are called the combination stresses. Stresses rarely act singly. Their action is usually combined. In airframe repair, the stresses most frequently encountered are bending, torsion, and shear. The paragraphs that follow describe the application of these forces. Tension. Tension in airframe repair is the force that stretches a structural member. Notice in Figure 2-1 the conditions of the metal strap under the applied force. Under tension, the top of the metal strap is being pulled and the underneath side is being pushed together (compressive force). Some of the strap's material was removed by drilling a hole in it to receive the bolt. This reduced its cross-sectional area. Because the load is constant from one end of the strap to the other and the hole cannot carry any of the load, the stress in the reduced area is greatly increased per unit area. In other words, besides carrying its normal share of the load it is also carrying the load that would have been carried by the removed metal. If the force or load is increased until the strap breaks, the failure will occur at or near the hole. 
Figure 2-1. Combined Forces Applied.
A member's strength under tension is determined on the basis of its gross or total area; however, calculations involving tension must include the net area of the member. Net area is defined as the gross area minus that removed by drilling holes or by making other changes in the section. Putting rivets or bolts in the holes makes little or no difference in added strength. The rivets or bolts will not transfer tension force across the holes they are in. Compression. Compressive stress (compression) in aircraft is the force per unit area that shortens or compresses a structural member at any cross section. Under compressive force, an undrilled member is stronger than an identical member that has holes drilled through it. However, if a plug of equivalent or stronger material is fitted tightly into a drilled member it will transfer compressive force across the hole, and the member can carry approximately as great a load as if there were no hole. Therefore, with compressive loads, the gross or total area can be used to determine stress in a member if all holes are tightly plugged with equivalent or stronger material. Shear. Shear is the force per unit area that slides adjacent pieces of material past each other. The term shear is used because it is sideways stress of the type that is put on a piece of paper or a sheet of metal when it is cut with a pair of shears. For example, if two pieces of metal are bolted or riveted together and sufficient force is applied to opposite ends, the metal pieces will shear, cut, the bolt. Bending. Bending is a combination of two forces. Notice in Figure 2-1 that the bending force produces tension on the top of the strap and compression on the bottom portion. The combined stresses produce a shear action at the neutral axis. This occurs because these forces act in opposite directions and are next to each other at the neutral axis. Shear action does not take place at the extreme upper and lower strap surfaces. Torsion. Torsion in airframe repair is the force that twists a structural member. The stresses arising from this action are shear ones. They are caused by adjacent planes rotating past each other and around a common reference axis at right angles to these planes. As an example, assume a rod is fixed solidly at one end and is twisted by a weight placed on a lever arm at the other. This produces the equivalent of two equal and opposite forces acting on the rod at some distance from each other. These forces create a shearing action all along the rod, with the rod's centerline representing the neutral axis.
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