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## Cyclic stress
## Additional recommended knowledge## Types of cyclic stressCyclic stress is frequently encountered in rotating machinery where a bending moment is applied to a rotating part. This is called a 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 moment applied but have a torque that varies significantly over time. ## Cyclic stress and material failureWhen cyclic stresses are applied to a material, even though the stresses do not cause plastic deformation, 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. 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. ## References |

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cyclic_stress". A list of authors is available in Wikipedia. |