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.
Figure 1-10a.
Examples of Stress and Strain.
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