Short Answer:

Vibrations cause fatigue by creating repeated or cyclic stresses in a material, which gradually weaken it over time. When a component vibrates continuously, it undergoes alternating tension and compression, leading to the formation of microscopic cracks. These cracks slowly grow with each vibration cycle until the component finally breaks.

Even if the vibration stresses are below the material’s ultimate strength, repeated exposure can cause fatigue failure. This type of failure is common in rotating machinery, engines, bridges, and aircraft parts, where continuous vibration leads to gradual damage and eventual breakdown.

Detailed Explanation :

How Vibrations Cause Fatigue

Vibrations cause fatigue because they produce fluctuating stresses and strains in a material, similar to cyclic loading. Every time a machine or structural component vibrates, its surface and internal layers experience alternating forces — one moment in tension and the next in compression. Although these stresses may be very small individually, their repeated application over thousands or millions of cycles results in microscopic structural changes, leading to fatigue failure.

Fatigue due to vibration is one of the most common causes of failure in rotating and reciprocating machinery. It is particularly dangerous because it occurs progressively and often without any visible warning until the part fails completely. Vibrations in machines may arise due to unbalance, misalignment, looseness, or resonance conditions.

When these vibrations are continuous, they apply alternating stresses that initiate tiny cracks, usually at weak points such as surface defects, weld joints, or corners. Over time, these cracks propagate through the material, reducing its cross-sectional area and load-carrying capacity until final fracture occurs.

Mechanism of Fatigue Due to Vibration

The process of fatigue caused by vibration generally occurs in three stages:

Crack Initiation:
When a material vibrates, it undergoes cyclic stresses. The repeated motion leads to microscopic plastic deformation at the surface or within weak regions of the material. These deformations accumulate with time, forming small cracks. Crack initiation usually occurs at surface irregularities, scratches, sharp corners, or material defects where stress concentration is high.
Crack Propagation:
Once a crack is formed, each vibration cycle causes it to grow slightly deeper into the material. As the vibration continues, the crack progresses rapidly, especially if the vibration amplitude (stress level) is high. The rate of crack growth depends on the material properties, stress amplitude, frequency of vibration, and environmental factors like temperature and corrosion.
Final Fracture:
When the crack becomes large enough, the remaining material cannot support the applied load, and sudden fracture occurs. The fracture surface often shows two distinct regions — a smooth area (where slow crack growth occurred) and a rough area (where rapid failure happened).

This process can happen over a long period, making fatigue failure due to vibration gradual but inevitable if not detected early.

How Vibration Creates Cyclic Stress

When a component vibrates, it experiences alternating stress because its position and load continuously change. For example:

In one half of the vibration cycle, the material is stretched (tension).
In the next half, it is compressed.

This tension–compression cycle repeats many times per second, depending on the vibration frequency. Each repetition weakens the atomic bonds within the material. Over time, even small cyclic stresses below the yield strength can cause microstructural damage, leading to fatigue failure.

In machines operating at high speeds (like turbines, pumps, and engines), the frequency of vibration can reach thousands of cycles per second, accelerating fatigue failure.

Factors Influencing Fatigue Due to Vibration

Amplitude of Vibration:
Higher vibration amplitude means higher stress variation, which leads to faster fatigue crack initiation and growth.
Frequency of Vibration:
The greater the frequency, the more cycles occur in a short period, increasing fatigue damage accumulation.
Resonance:
When vibration frequency matches the natural frequency of a component, large amplitude oscillations occur. This dramatically increases stress and accelerates fatigue failure.
Material Properties:
Ductile materials like steel resist fatigue better than brittle materials like cast iron or aluminum. The surface finish and internal structure also play key roles.
Temperature:
High or fluctuating temperatures change material strength and elasticity, influencing fatigue behavior.
Surface Condition:
Rough surfaces, scratches, and notches increase stress concentration, making them ideal spots for crack initiation.
Corrosion:
Corrosive environments weaken the material surface, promoting quicker crack growth, a condition known as corrosion fatigue.
Lubrication and Maintenance:
Poor lubrication increases friction-induced vibration, which can raise stress levels and reduce fatigue life.

Examples of Vibration-Induced Fatigue

Rotating Shafts:
Shafts in motors or turbines often fail due to continuous bending vibration, leading to surface cracks at high-stress zones.
Aircraft Wings:
Wings experience continuous aerodynamic vibrations, leading to fatigue cracks in joints and riveted areas.
Bridges:
Repeated vehicle loads and wind-induced vibrations cause fatigue in metal joints and supports.
Gear Teeth:
Gear meshing generates cyclic vibrations that lead to pitting and surface fatigue failure.
Bearings:
Rolling elements in bearings experience repetitive contact stresses, resulting in vibration-induced surface fatigue.

Prevention of Fatigue Caused by Vibration

Reducing Vibration Amplitude:
Balance rotating parts properly and maintain alignment to minimize vibration forces.
Avoiding Resonance:
Ensure the machine’s operating speed does not coincide with its natural frequency.
Using Damping Materials:
Rubber, viscoelastic, or composite damping layers help absorb vibration energy and reduce cyclic stresses.
Proper Surface Finishing:
Smooth surfaces reduce stress concentration and delay crack initiation.
Lubrication and Maintenance:
Regular maintenance and proper lubrication lower friction and vibration levels.
Design Modifications:
Avoid sharp corners and sudden section changes that act as stress raisers.
Material Selection:
Use materials with higher fatigue strength and corrosion resistance in high-vibration environments.
Monitoring and Inspection:
Regular vibration analysis and non-destructive testing (like ultrasonic inspection) help detect cracks before they cause failure.

By implementing these preventive measures, the damaging effects of vibration-induced fatigue can be minimized effectively.

Conclusion

Vibrations cause fatigue by generating cyclic stresses that lead to progressive damage and crack formation in materials. These small stresses, when repeated millions of times, result in microscopic crack growth and eventual structural failure. Factors such as vibration amplitude, frequency, material properties, and surface conditions influence the rate of fatigue damage. Preventing vibration-induced fatigue requires proper machine balancing, damping, maintenance, and design improvements. Controlling vibration not only prevents fatigue failure but also ensures longer machine life, higher reliability, and safer operation.