Short Answer:

Surface treatments like shot peening are used to improve the fatigue life of metal parts. Shot peening involves bombarding the surface of a material with small steel or ceramic balls. This creates tiny dents on the surface, which causes the outer layer of the metal to compress. This compression helps prevent small cracks from starting.

By adding compressive stress on the surface, shot peening stops the formation of tension cracks that lead to fatigue failure. As a result, the part becomes stronger and can survive more load cycles without breaking. This method is widely used in automotive, aerospace, and mechanical parts that face repeated stress.

Detailed Explanation:

How surface treatments like shot peening improve fatigue life

Fatigue failure is a serious problem in mechanical engineering, especially in parts that face repeated stress over time. Examples include engine parts, gears, springs, shafts, and aircraft components. These parts often fail not because of one-time overload but because of repeated small stresses that slowly cause cracks and eventually break the part. One very effective method to protect such parts is shot peening—a type of surface treatment that greatly increases their fatigue life.

What is shot peening?

Shot peening is a mechanical surface treatment process. It works by striking the surface of a metal component with small round balls, called “shots.” These shots are usually made of steel, glass, or ceramic and are blasted at high speed onto the surface.

When these shots hit the metal, they create small dents or dimples. This might sound like damage, but it’s actually helpful. These small dents create compressive stresses on the surface of the material. These stresses are opposite to tensile stresses, which normally cause fatigue cracks.

So, in simple terms:

• Shot peening = compressive surface layer = stronger material surface

How shot peening improves fatigue life

Fatigue failure usually starts at the surface of a component because this is where most tensile stress acts. Tiny cracks begin to form due to repeated stress cycles. However, when the surface is already under compressive stress (due to shot peening), it becomes much harder for these cracks to start.

Here’s how it helps step-by-step:

1. Creation of compressive stress
   The shot impacts push the outer surface inwards, which makes the metal try to expand back. But since the inside of the metal resists this expansion, compressive stress remains on the surface.
2. Block crack formation
   Tensile stress opens up cracks. Compressive stress does the opposite. So, when compressive stress is present, crack formation is delayed or completely stopped.
3. Crack growth is slowed
   Even if a tiny crack forms, the compressive stress slows down how fast it grows. This gives more time for detection and maintenance.
4. Increases fatigue strength
   Because cracks are delayed or slowed, the part can survive more loading cycles. This means its fatigue strength and fatigue life are increased.

Other benefits of shot peening

• Better resistance to corrosion fatigue: Small cracks due to corrosion are blocked by the compressive stress.
• Improves wear resistance: The roughened surface becomes more durable.
• Relieves internal stress: Reduces harmful residual stresses from previous machining or heat treatment.

Where shot peening is used

Shot peening is commonly used in:

• Aerospace parts: Turbine blades, landing gear, structural frames
• Automotive parts: Engine valves, springs, connecting rods, crankshafts
• Industrial machinery: Gears, bearings, tools
• Power plants: Steam turbine components, fasteners, etc.

This treatment helps prevent failures in high-stress environments, especially where safety is critical.

Limitations of shot peening

While shot peening is very useful, it also has some limits:

• It works mostly on metal surfaces.
• It needs proper control; too much peening can damage the surface.
• It cannot fix deep cracks—only prevents or delays surface cracks.

Still, when used correctly, it is one of the most cost-effective ways to increase fatigue life.

Conclusion

Shot peening is a valuable surface treatment that improves fatigue life by adding compressive stress to the surface of metal parts. This compressive stress stops small cracks from forming and slows down their growth, protecting the component from fatigue failure. It is widely used in industries where parts experience continuous stress and need to last a long time. By using shot peening, engineers can make machines safer, more reliable, and longer-lasting without changing the base material or design.