Short Answer:

Fatigue is caused by the repeated or cyclic loading of a material, which leads to the gradual development of small cracks and eventual failure, even when the applied stress is below the material’s ultimate strength. Over time, these stresses weaken the internal structure of the material.

When a component is subjected to fluctuating or reversing loads, such as bending, vibration, or rotation, it experiences continuous stress variations. This constant repetition of stress cycles produces microscopic cracks that grow progressively, causing fatigue failure in mechanical parts.

Detailed Explanation:

What Causes Fatigue

Fatigue in engineering materials occurs due to the continuous application of cyclic or fluctuating stresses over a long period. Unlike sudden failure caused by a single overload, fatigue develops gradually as a result of repetitive stress that may be much lower than the material’s yield or ultimate strength. It is one of the most common causes of failure in mechanical components like shafts, gears, springs, and aircraft parts.

Fatigue failure is progressive in nature and occurs without much visible warning. It starts at a microscopic level, where tiny cracks form due to repeated stresses. These cracks grow larger with each loading cycle until the material finally breaks.

1. Cyclic Loading and Stress Variation

The main cause of fatigue is cyclic loading — when a material experiences repeated changes in the magnitude or direction of applied stress. For example, in rotating shafts or airplane wings, the stresses vary from tension to compression repeatedly.
Each load cycle creates internal movements of atoms within the metal structure. Over time, these movements weaken the bonds between atoms, leading to small cracks. Even when the applied load is below the elastic limit, the repetition of stress cycles causes damage accumulation.

2. Stress Concentration

Another major cause of fatigue is stress concentration, which occurs at points where the shape of a component changes suddenly — such as holes, notches, grooves, keyways, or sharp corners.
At these points, the stress intensity becomes much higher than the average stress in the rest of the material. The increased local stress encourages the initiation of small cracks, which later expand under cyclic loads. Hence, components with smoother transitions and fillets have better fatigue resistance than those with sharp edges.

3. Surface Roughness and Defects

The surface condition of a material also plays a crucial role in fatigue. A rough surface contains small ridges and valleys that act as stress raisers. These tiny irregularities serve as starting points for cracks.
Processes like machining, grinding, and welding may leave micro-scratches or internal defects, making the material more susceptible to fatigue. Polishing and surface hardening treatments (such as shot peening) can significantly reduce fatigue damage by removing surface defects and inducing compressive residual stresses.

4. Environmental Factors

The surrounding environment can accelerate fatigue failure. When metals are exposed to corrosive environments, such as moisture, saltwater, or chemicals, corrosion weakens the material’s surface.
This phenomenon is called corrosion fatigue. It occurs because the combined action of cyclic stress and corrosion attacks the surface film, allowing cracks to start more easily. Temperature changes, especially high or fluctuating temperatures, can also promote fatigue by altering the metal’s internal structure.

5. Material Properties and Microstructure

The type of material and its internal microstructure greatly influence fatigue resistance. Metals with uniform grain structures generally show better fatigue strength than those with inclusions, impurities, or weak grain boundaries.
Heat treatment and alloying can improve fatigue resistance by refining the grain size and increasing hardness. On the other hand, materials that are brittle or have poor ductility tend to fail more quickly under cyclic loading.

6. Residual Stresses

Residual stresses are stresses that remain inside a component after manufacturing processes like welding, casting, or machining. If these stresses are tensile in nature, they combine with external cyclic stresses and make fatigue cracks form faster.
However, if the residual stresses are compressive, they help reduce fatigue damage. This is why techniques like shot peening or surface rolling are used to introduce compressive stresses and increase fatigue life.

7. Improper Design and Loading Conditions

Poor design is another key cause of fatigue failure. Components that are not properly designed to handle fluctuating loads or that have sudden changes in cross-section are more prone to fatigue.
Incorrect alignment, vibration, and unbalanced rotating parts also increase cyclic stresses. For example, in engines or rotating shafts, even small misalignments can produce alternating stresses that lead to early fatigue failure.

Conclusion

Fatigue is mainly caused by repeated cyclic loading, which gradually weakens the material and leads to the formation and growth of cracks. Factors such as stress concentration, surface roughness, environmental effects, and material properties all contribute to how quickly fatigue develops.
By improving design, surface finish, and material selection — and by controlling environmental conditions — engineers can significantly reduce the risk of fatigue failure in mechanical components.