An aircraft structure must be designed so that it accepts all of the stresses imposed upon it by the flight and ground loads without any permanent deformation. Any repair must accept these stresses, carry them across the repaired area, and transfer them back into the original structure.
These stresses are considered to flow through the structure, so there must be a continuous path for them, with no abrupt changes in cross-sectional areas along the way. Abrupt changes in cross-sectional areas of aircraft structure that are subject to cycle loading or stresses result in a stress concentration that may induce fatigue cracking and eventual failure. A scratch or gouge in the surface of a highly stressed metal component creates a stress concentration at the point of damage and may eventually lead to part failure.
Forces acting on an aircraft, whether it is on the ground or in flight, introduce pulling, pushing, or twisting forces within the various members of the aircraft structure. While the aircraft is on the ground, the weight 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, twisting, pulling, compression, and shearing forces.
As the aircraft takes off, most of the forces in the fuselage continue to act in the same direction. Because of the aircraft's motion, these forces increase in intensity. The forces on the wingtips and the wing surfaces, however, reverse direction; instead of being downward forces of weight, they become upward forces of lift.
Lift forces act first on the skin and stringers, then pass to the ribs, and finally are transmitted through the spars into the fuselage. The wings bend upward at their ends and may flutter slightly during flight. This wing bending cannot be ignored by the manufacturer in the original design and construction and cannot be ignored during maintenance.
Although an aircraft structure is composed of rigidly riveted or bolted structural members and skin, components such as wings are designed to flex under aerodynamic loads much like a leaf spring.
The six primary types of stress acting on an aircraft structure are tension, compression, shear, bearing, bending, and torsion (twisting). The first four are commonly called basic stresses; the last two, combination stresses. Stresses usually act in combinations rather than singly. [Figure 1]
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| Figure 1. Stresses in aircraft structures |
Tension
Tension is the stress that resists a force that tends to pull apart. Engine thrust pulls the aircraft forward, while aerodynamic drag resists its motion. The resulting forces create tension that tends to stretch structural members. The tensile strength of a material is measured in pounds per square inch (psi) and is calculated by dividing the load (in pounds) required to pull the material apart by its cross-sectional area (in square inches).
The strength of a member in tension is determined on the basis of its gross area (or total area), but calculations involving tension must take into consideration 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. Installing rivets or bolts in the holes does not significantly restore tensile strength because the fasteners do not effectively transfer tensile loads across the drilled holes.
Compression
Compression, the stress that resists a crushing force, tends to shorten or squeeze aircraft parts. The compressive strength of a material is also measured in psi. Under a compressive load, an undrilled member is stronger than an identical member with holes drilled through it.
However, if a plug of equivalent or stronger material is fitted tightly in a drilled member, it transfers compressive loads across the hole, and the member carries approximately as large a load as if the hole were not there. Thus, for compressive loads, the gross or total area may be used in determining the stress in a member if all holes are tightly plugged with equivalent or stronger material.
Shear
Shear is the stress that resists the force tending to cause one layer of a material to slide over an adjacent layer. Two riveted plates in tension subject the rivets to a shearing force. Usually, the shear strength of a material is either equal to or less than its tensile or compressive strength.
Shear stress concerns the aviation technician chiefly from the standpoint of the rivet and bolt applications, particularly when attaching sheet metal, because if a rivet used in a shear application gives way, the riveted or bolted parts are pushed sideways.
Bearing
Bearing stress resists the force that the rivet or bolt places on the hole. As a rule, the strength of the fastener should be such that its total shear strength is approximately equal to the total bearing strength of the sheet material. [Figure 2]
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| Figure 2. Bearing stress |
Torsion
Torsion is the stress that produces twisting. While producing thrust, the engine also generates torque that tends to twist the aircraft about its longitudinal axis. This twisting tendency is resisted by aerodynamic forces and the aircraft structure, creating torsional stress.
The torsional strength of a material is its resistance to twisting or torque (twisting stress). The stresses arising from this action are shear stresses caused by the rotation of adjacent planes past each other around a common reference axis at right angles to these planes.
This action may be illustrated by a rod fixed solidly at one end and twisted by a weight placed on a lever arm at the other, producing the equivalent of two equal and opposite forces acting on the rod at some distance from each other. A shearing action is set up all along the rod, with the center line of the rod representing the neutral axis.
Bending
Bending (or beam stress) is a combination of compression and tension. The rod in Figure 1E has been shortened (compressed) on the inside of the bend and stretched on the outside of the bend. Bending produces tensile stress on one side of the neutral axis and compressive stress on the opposite side. The side under tension or compression depends on the direction of the applied load.
These stresses act in opposition on the two sides of the center line of the member, which is called the neutral axis. Since these forces acting in opposite directions are next to each other at the neutral axis, the greatest shear stress occurs along this line, and none exists at the extreme upper or lower surfaces of the beam.