Cyclic stress in engineering refers is an internal distribution of forces (a stress) that changes over time in a repetitive fashion. As an example, consider one of the large wheels used to drive an
aerial liftAn aerial lift is a means of transport in which gondolas or open chairs are hauled above the ground by means of a cable.Types of aerial lifts include:...
such as a
ski liftThe term ski lift can refer to many different types of uphill transport, in three main classes:-Lift systems and networks:The network lifts at a ski resort can be arranged in several patterns depending on the size and topography of the area. Several lifts may be arranged radially outward from a...
. The
wire cablethumb|Steel wire rope Wire rope consists of several strands laid together like a helix. Each strand is likewise made of metal wires laid together like a helix...
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.
Cyclic stress in engineering refers is an internal distribution of forces (a stress) that changes over time in a repetitive fashion. As an example, consider one of the large wheels used to drive an
aerial liftAn aerial lift is a means of transport in which gondolas or open chairs are hauled above the ground by means of a cable.Types of aerial lifts include:...
such as a
ski liftThe term ski lift can refer to many different types of uphill transport, in three main classes:-Lift systems and networks:The network lifts at a ski resort can be arranged in several patterns depending on the size and topography of the area. Several lifts may be arranged radially outward from a...
. The
wire cablethumb|Steel wire rope Wire rope consists of several strands laid together like a helix. Each strand is likewise made of metal wires laid together like a helix...
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. Because the wheel rotates many times during the use of the machine, this cycle of tension and
compressionCompressive stress is the stress that, when applied, acts towards the center of that material. When a material is subjected to compressive stress, then this material is under compression. Usually, compressive stress applied to bars, columns, etc. leads to shortening.Loading a structural element or...
is repeated many times — hence the name
cyclic stress.
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 jumpingBungee jumping is an activity that involves jumping from a tall structure while connected to a large elastic cord. The tall structure is usually a fixed object, such as a building, bridge or crane; but it is also possible to jump from a movable object, such as a hot-air-balloon or helicopter,...
), 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 discA Compact Disc is an optical disc used to store digital data. It was originally developed to store sound recordings exclusively, but later it also allowed the preservation of other types of data. Audio CDs have been commercially available since October 1982...
drive. Each time a disc is inserted into the drive, a motor applies
torqueTorque, also called moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
to the disc via a drive shaft. Once disc reaches the desired
rotational velocityRotational speed indicates, for example, how fast a motor is running. Rotational speed is equivalent to angular speed, but with different units. Rotational speed tells how many complete rotations there are per time unit...
, 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 momentA bending moment exists in a structural element when a moment is applied to the element so that the element bends. Moments and torques are measured as a force multiplied by a distance so they have as unit newton-meters , or foot-pounds force...
, compact disc drive shafts have little to no bending movement applied, but have a torque that varies significantly over time.
Cyclic stress and material failure
When cyclic stresses are applied to a material, even though the stresses do not cause plastic deformation, the material may fail due to
fatigue'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading...
. Fatigue failure is typically modeled by decomposing cyclic stresses into mean and alternating components. Mean stress is the time average of the principal stress. Alternating stress is the difference between the mean and the maximum or the mean and the minimum value the principal stress takes on. 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.