Cyclic stress is the distribution of forces (aka stresses) that change over time in a repetitive fashion. As an example, consider one of the large wheels used to drive an
aerial lift
An aerial lift, also known as a cable car or ropeway, is a means of cable transport in which ''cabins'', ''cars'', ''gondolas'', or open chairs are hauled above the ground by means of one or more cables. Aerial lift systems are frequently employe ...
such as a
ski lift
A ski lift is a mechanism for transporting skiers up a hill. Ski lifts are typically a paid service at ski resorts. The first ski lift was built in 1908 by German Robert Winterhalder in Schollach/Eisenbach, Hochschwarzwald.
Types
* Aerial ...
. The
wire cable
Steel wire rope (right hand lang lay)
Wire rope is several strands of metal wire twisted into a helix forming a composite
''rope'', in a pattern known as ''laid rope''. Larger diameter wire rope consists of multiple strands of such laid rope in a ...
wrapped around the wheel exerts a downward force on the wheel and the
drive shaft supporting the wheel. Although the shaft, wheel, and cable move, the force remains nearly vertical relative to the ground. Thus a point on the surface of the drive shaft will undergo tension when it is pointing towards the ground and compression when it is pointing to the sky.
Types of cyclic stress
Cyclic stress is frequently encountered in rotating machinery where a
bending moment
In solid mechanics, a bending moment is the reaction induced in a structural element when an external force or moment is applied to the element, causing the element to bend. The most common or simplest structural element subjected to bending mo ...
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.
Cyclic stress and material failure
When cyclic stresses are applied to a material, even though the stresses do not cause
plastic deformation
In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strain ...
, the material may fail due to
fatigue. Fatigue failure is typically modeled by decomposing cyclic stresses into mean and alternating components. Mean stress is the time average of the principal stress. The definition of alternating stress varies between different sources. It is either defined as the difference between the minimum and the maximum stress, or the difference between the mean and maximum stress.
Engineers try to design mechanisms whose parts are subjected to a single type (bending, axial, or torsional) of cyclic stress because this more closely matches experiments used to characterize fatigue failure in different materials.
References
{{Refimprove, date=September 2014
Materials science
Mechanics