Short Answer:

Fatigue failure is a type of mechanical failure that occurs in materials due to repeated or fluctuating loads over time, even if the applied stress is much lower than the material’s ultimate strength. It does not happen suddenly but starts with small cracks that slowly grow with each cycle of loading.

Fatigue failure usually occurs in machines or structures that face repeated stress, such as bridges, aircraft, or engine parts. These tiny cracks develop at points of stress concentration and gradually spread, leading to sudden and complete failure of the component, often without any visible warning.

Detailed Explanation:

Fatigue failure

Fatigue failure is a very common issue in mechanical engineering where a material or a component fails after being subjected to repeated or cyclic loading. Unlike simple overload failure, fatigue failure can happen at stress levels much below the yield strength of the material. This makes it especially dangerous because the failure can be sudden and unexpected.

This kind of failure is common in parts of machines that are in constant motion, such as shafts, beams, airplane wings, railway axles, or rotating engine parts. Fatigue failure is responsible for a large percentage of structural and mechanical breakdowns, making it a very important subject in engineering design and analysis.

Let us understand clearly how fatigue failure occurs step-by-step.

How fatigue failure occurs

Fatigue failure generally occurs in three main stages:

1. Crack initiation:
   This is the beginning stage. When a material is repeatedly loaded and unloaded, tiny cracks start forming at weak points. These weak points are usually near surface imperfections, holes, scratches, sharp corners, or welding joints. These are called stress concentrators. The crack begins at these locations after several load cycles.
2. Crack propagation:
   Once the crack starts, each cycle of loading and unloading causes the crack to grow slowly. This crack increases in length with time and moves deeper into the material. This process can take thousands or even millions of cycles depending on the load and material. The crack growth is slow but continuous, and it weakens the component more and more with each load.
3. Final fracture:
   When the crack becomes too large, the remaining uncracked part can no longer bear the load. This causes a sudden and complete break of the material or part. At this stage, failure occurs instantly, and often there is no time to repair or stop the damage.

One dangerous thing about fatigue failure is that the material looks perfectly fine from outside until it breaks. This is why it is also known as “silent failure.”

Factors affecting fatigue failure

Several factors affect how and when fatigue failure will occur. These include:

• Stress level and type: Higher stresses and tensile stress cause faster fatigue.
• Load frequency: More load cycles per second increase fatigue rate.
• Surface finish: Rough surfaces cause cracks to start earlier.
• Material properties: Ductile materials like mild steel handle fatigue better than brittle materials.
• Environmental effects: Corrosion, temperature changes, and humidity can make fatigue worse.
• Size and shape: Large parts or sharp notches increase stress concentration.

Prevention of fatigue failure

Engineers use several methods to prevent or reduce fatigue failure in machine parts:

• Use of smooth surface finish: Polishing and removing scratches reduce crack initiation.
• Avoid sharp corners: Using rounded corners reduces stress concentration.
• Use of better materials: Selecting materials with high fatigue strength helps.
• Proper heat treatment: Improves the resistance of the material.
• Shot peening: A process where the surface is bombarded with small balls to increase surface strength.
• Regular inspection: Nondestructive testing (NDT) like ultrasonic or magnetic tests can detect small cracks early.

Designers also use a “factor of safety” and plan for expected fatigue life, which is called “fatigue life estimation.” Engineers calculate how many cycles a part can handle before failure and replace or maintain it before that limit is reached.

Real-life examples

1. Aircraft wings: Face thousands of cycles due to takeoff and landing. Even small cracks can cause big accidents if not detected.
2. Bridges: Repeated stress from moving vehicles causes fatigue over years.
3. Rotating shafts in engines: Subjected to constant stress and must be designed carefully.
4. Railway wheels and axles: Have failed in the past due to undetected fatigue cracks.

Conclusion

Fatigue failure is a slow and progressive form of material breakdown caused by repeated or cyclic loading. It starts with microscopic cracks at weak spots, grows over time, and ends in sudden failure. Even though the stress may be low, the repetition of loading causes the damage. That is why understanding fatigue is important in mechanical design. Proper material selection, surface treatment, and regular inspections help prevent fatigue-related accidents in machines and structures.