Types of Cyclic Stress
Cyclic stress is frequently encountered in rotating machinery where a bending moment is applied to a rotating part. This is called a cyclic bending stress and the aerial lift above is a good example. However, cyclic axial stresses and cyclic torsional stresses also exist. An example of cyclic axial stress would be a bungee cord (see bungee jumping), which must support the mass of people as they jump off structures such as bridges. When a person reaches the end of a cord, the cord deflects elastically and stops the person's descent. This creates a large axial stress in the cord. A fraction of the elastic potential energy stored in the cord is typically transferred back to the person, throwing the person upwards some fraction of the distance he or she fell. The person then falls on the cord again, inducing stress in the cord. This happens multiple times per jump. The same cord is used for several jumps, creating cyclical stresses in the cord that could eventually cause failure if not replaced.
Cyclical torsional stresses are stresses repetitively applied tangent to an axis. As an example, consider a compact disc drive. Each time a disc is inserted into the drive, a motor applies torque to the disc via a drive shaft. Once disc reaches the desired rotational velocity, relatively little torque is required to maintain the speed. Thus the torque varies over time as the drive spins up a disc and slows it down. Unlike the ski lift example above where the torque is relatively constant but the load due to cable tension created a bending moment, compact disc drive shafts have little to no bending movement applied, but have a torque that varies significantly over time.
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