Elastic Fatigue and Permanent Deformation of Springs

When a spring is subjected to prolonged use, it undergoes various types of stress and strain that can lead to permanent deformation. One of the main reasons for this deformation is elastic fatigue, which is the gradual degradation of a material's elasticity over time.

1. Elastic Fatigue
Elastic fatigue occurs when a material, such as a spring, is subjected to repeated cyclic loading and unloading. Each time the spring is compressed or stretched, it experiences stress and strain. While most materials can handle a certain amount of stress and strain without any permanent deformation, repeated loading and unloading can cause microscopic changes in the material's structure.

2. Cyclic Stress-Strain Relationship
When a spring is subjected to cyclic loading, its stress-strain relationship changes over time. Initially, the spring behaves elastically, meaning it returns to its original shape after being compressed or stretched. However, as the cycles continue, the spring's elasticity gradually decreases.

3. Microscopic Changes in the Material
During cyclic loading, the repeated stress and strain cause the spring's material to undergo microscopic changes. These changes can include dislocation movements, grain boundary movement, and the formation of small cracks. These microstructural changes accumulate over time and result in a decrease in the spring's elasticity.

4. Permanent Deformation
As the spring undergoes elastic fatigue, it reaches a point where the microscopic changes in its material structure become significant. At this stage, the spring loses its ability to return to its original shape and experiences permanent deformation. The amount of permanent deformation depends on factors such as the magnitude and frequency of the cyclic loading, as well as the material properties of the spring.

Conclusion
In conclusion, springs deform permanently after prolonged use due to elastic fatigue. This occurs as a result of repeated cyclic loading and unloading, which gradually degrades the spring's elasticity. The microscopic changes in the material's structure accumulate over time, leading to permanent deformation. It is important to consider elastic fatigue when designing and using springs to ensure their longevity and optimal performance.