What is shot peening?
Shot Peening is a process in which compressive stress are induced into the material. These stresses are induced with the help of abrasive (mainly shots or cut wire) by bombarding the surface of the component to be shot peened with these abrasives.
What is the objective or what can be achieved by doing this process?
With Shot Peening the fatigue life of a component can be increased i.e. the life cycle of the component, which encounters fatigue, stress/loading can be increased with this process
Apart from fatigue, shot peening enhances life of components, which encounters stress-cracking corrosion and is also used by designers to reduce weight and FOS of any component.
Is shot peening similar to shot blasting?
There is a vast difference between the two. Shot Peening Machine is a highly controlled process. Though the process of propelling the abrasives remains the same. The difference and control in process are related to shot size, coverage, striking angle and intensity.
What is fatigue stress or fatigue failure?
Any components (such as shaft, gears, connecting rods, propeller shaft, knuckle joints etc.) that undergo fluctuating load or stress or power etc, undergoes a stress called as fatigue stress.
Fatigue stress could be a variation in load levels or complete stress-neutral-strain cycle or a stress-neutral or strain-neutral cycle. In short any components that undergo variation in stress experiences fatigue stress.
Why fatigue or fatigue failure needs to be addressed with utmost importance?
Fatigue failure is a sudden failure with minimal indication or signs before the failure occurs. That means detection of such failure can be difficult and expensive.
Mostly designers tend to design components with additional tolerance (particularly one which involves human life with higher FOB) i.e. over-design the components so that it does not fail but Fatigue is a phenomena in which components encountering fatigue load can fail well below its yield stress. Hence any components that are designed way above its loading capacity can fail if the loading is fluctuating in nature. Thus when it comes to design of an aircraft component or Automotive or any components that have direct consequence to human, developers and designers pay at most attention to material/component that encounter fatigue stress/loading.
How does shot peening provide a remedy to fatigue failure?
It’s a common understanding that all material/components consist of micro cracks (even if the components are grinded there are small micro cracks present in it), which are not visible to naked eyes, these can be visible with any micro-scope above 40X Zoom.
Now when material undergoes fatigue loading these micro cracks tends to join with other micro cracks to form macro cracks which can be visible to naked eyes if analyzed with utmost care. Further during service these macro cracks fails the material without any indication in a sudden fashion.
Now what Shot Peening Machine does is when we hit or bombard the material/components with abrasives, a small dent or dimple is formed on the surface of the component. These dimples forces the micro cracks present on the material to close. Thus eliminating the root cause (micro cracks) from the material/component.
Does that mean components which undergoes shot peening process, does not fail when loaded with fatigue stress?
No, eventually the components will fail but the cycle of failure when compared to non-shot peened material will be much higher approximately (200 – 2000%) depending on the component and application. i.e. is if a propeller shaft fails after ‘X’ cycle, by undergoing shot peening process the component life can be increased to 200 or more times the X cycle depending on the process.