[chris_seven]

The ads which state 'shot peened to relieve stresses' clearly demonstrate a lack of understanding of this process and I think originates from China.

Shot peening is a process which, if carried out correctly, can significantly improve the fatigue performance of many metallic components

By bombarding the surface of a metal with a smooth round shot in a controlled manner it is possible to introduce a significant level of compressive stress and this is extremely beneficial in terms of fatigue endurance.

Fatigue is responsible for around 95% of mechanical failures and at a basic level is easy to describe.

In simple terms most fatigue failures can be described as a two-stage process, Stage 1 is fatigue crack initiation and Stage 2 is fatigue crack growth.

Fatigue crack initiation generally accounts for 90% of the life of a component.

Fatigue cracks normally develop on the surface of a component and initially form due to what is referred to as 'slip band intrusion and extrusion' which are crystallographic defects that are virtually impossible to detect.

The development of these defects always require the presence of traction vectors which are usually caused by shear stresses and commonly form in specific crystallographic directions.

The presence of residual compressive stresses means that the surface tensile stresses must be of a much greater magnitude to cause the damage that would occur if the surface were unstressed.

Nitriding has a similar influence on fatigue life as it also introduces surface compressive stresses but Shot Peening avoids the need for heat treatment.

If is carried out correctly shot peening is very beneficial to components such as Turbine Blades, Con Rods and many other critical parts. If it is carried out badly it can have a negative impact on fatigue life and can introduce defects.

The quality of the shot, the screening of shot to remove broken particles as well as the velocity and the direction of application all have a significant influence on performance.

With used components it is difficult to make a decision with regard to the amount of fatigue damage that has been accumulated unless the part has been used in very controlled conditions and monitored in terms of the stresses that have been generated in service. Many Aircraft components are monitored in this manner but the costs of producing the base line data to make this approach work is very high.

Once a detectable defect has developed any component affected should be discarded.
With a used con rod it is possible that the part could have used the majority of its 'fatigue initiation' life and may be ready to crack.

By careful shot peening it is possible to remove the accumulated damage and effectively re-life the part.

By using companies that are FAA/CAA Approved it is very likely that excellent results will be obtained at a very reasonable cost.

I would suggest that Con Rods are shot peened in the following manner.

Shot Peen 200% Coverage 6-10 ALMEN "A" WITH #230 Austenitic Stainless Steel Shot.