This paragraph discusses the stresses associated with the qualities or characteristics of metals. Stress is a force placed upon a body and is measured in terms of units of force per unit of area. The force is usually expressed in pounds and the unit of area in square inches. More simply, the expression is stated in pounds per square inch (psi). Stress can be in the form of compression, tension, torsion, bending, shearing, or a combination of two or more of these. All parts of an aircraft are subject to stresses. The various stresses acting on the aircraft parts while in flight have an important bearing on the choice of metals used. The paragraphs that follow describe the stresses. Refer to Figure 1-10a for examples.
Compression. Compression is the decrease of volume of a compressible substance because of pressure being applied. Compressive strength is the resistance to applied pressure. Examples of compression are the pressure applied to the fuel-air mixture in an engine cylinder and the pressure applied to an airplane's landing gear during landing.
Tension. Tension is the force or combination of forces that pulls or stretches a material. The measurement of a material's resistance to stretching or tearing is the material's tensile strength.
Torsion. Torsion is the force that causes a twisting motion. Torsional force is produced when an engine turns a crankshaft; and in that application, the force is called torque.
Bending. Bending is a combination of tension and compression forces. The inside curve of a bend is under compression, while the outside curve is under tension. Main rotor blades on helicopters and wings on airplanes are subjected to bending during flight. Main rotor blades also bend at rest, whether tied down or drooping, because of their weight and flexibility.
Shear. Shear is a stress applied to a body in the plane of one of its faces. The stress exerts a cutting force much the same as the two blades of a large scissors. Two layers of aircraft skin fastened together by a rivet can shear, or cut, the head off the rivet if one layer of skin is allowed to shift sufficiently.
