Rotary peening, also known as flapper peening is mainly used to re-peen small areas of components that were blended to remove a scratch or superficial damage. Since blending removes the beneficial compressive residual stress layer of the initial peening, flapper peening is applied to restore the integrity of the part.

Flapper peening has been around since the 1960’s when it was developed by 3M for the US Army.  At the time, the US Army was looking to improve their fleet readiness by re-peening components in-service.  They wanted to quickly re-peen components directly on the aircraft thus saving considerable time and cost.  This includes the operation of removing the component from the aircraft, disassembling related hardware and cables, transportation to the peening shop, masking and preparation before the peening process, the peening process itself as well as re-assembly and installation. Flapper peening provided a quick and clean method for the US Army to re-introducing compressive residual stresses into their fatigue critical components thus helping their fleet spend more time in the air and less time on the ground.  

Need for more control!

The challenge of fleet availability faced by the US Army almost 50 years ago is still very much present for all Airlines and Maintenance, Repair and Overhaul (MR&O) facilities today. 
Many Airlines and MR&O shops use flapper peening to streamline their operations. A survey revealed that the process is much appreciated for its speed, low cost and cleanliness but that it could be improved with better control of the rotational speed of the flap - the main parameter in flapper peening for determining the Almen intensity. This would result in increased reliability, repeatability and a general sense of confidence in the process.

FlapSpeed™ Controller Design

The above requirements became the basis for the design of a controller for flapper peening named FlapSpeed™.  The Engineers at Shockform developed a small and rugged device that continuously monitors and adjusts the rotational speed (RPM) of the flap using a sensor attached directly to the die grinder. This measured speed is sent back to the controller in a closed loop and the controller adjusts the input of compressed air to maintain the requested RPM. 
If the controller is unable to maintain the desired RPM, an alarm will sound and the controller will stop the flap rotation. This is very valuable when several technicians are simultaneously using the compressed air in the shop.  The air pressure might fluctuate significantly during the peening process affecting the intensity of the peening.  The RPM control and the alarm ensure a very reliable and repeatable process in all environments. 

FlapSpeed™ Features

The software and the user interface for the controller were specifically designed for the flapper peening process. Two modes of operation are available; the RPM mode and the Intensity mode. 
In the RPM mode, the operator can directly type in his desired RPM and the controller will adjust the compressed air pressure to reach that speed in seconds.  This makes the flapper peening operator more productive since he spends less time adjusting the pressure.
Furthermore, many flapper peening operators keep a curve of Almen intensity versus RPM in their tool box.  When starting a new project, an operator would refer to his chart to help select the RPM that would provide the Almen intensity required for the job.
The FlapSpeed™ controller allows up to 3 operators to input their own Almen Intensity versus RPM curve into the controller. This includes an Almen A curve, an Almen N curve and an Almen C curve.  When approved internally or by the client, these curves can significantly reduce or even eliminate the need to generate a new saturation curve for every new job. 
In the Intensity mode, the operator can simply type in the intensity and the associated RPM will be displayed by the controller. This makes the flapper peening process faster than ever. To quickly get started, a default curve, based on data generated by 3M, is already included in the controller. The controller also includes a timer for calculating peening times for Almen strips. This is very useful for generating saturation curves.    

Easier for Audits

Having a flapper peening process that is well controlled will make it easier to pass internal and external audits.  The Nadcap organization, representing Boeing, Bombardier and other OEM’s, has published the Audit Criteria AC 7117/4 that requires that the equipment used for flapper peening consistently maintain the RPM to conform to the required Almen intensity.  This is exactly what the FlapSpeed™ controller offers.  It  brings a level of confidence to the flapper peening process that was previously unavailable. 
A new SAE AMS specification on flapper peening should also increase the confidence into the process. This new specification, to be published by the end of 2009, will address some of the weaknesses found in the old military specification.

Study on Operator Variability

Shockform has started a study on variability in the flapper peening process.  The objective of the study is to determine the variation in Almen intensity introduced when several operators perform flapper peening at the same RPM.  Each operator participating in the study is asked to generate a saturation curve at 3000 RPM. The saturation curve will yield an Almen intensity and a saturation time. By using these two parameters for a statistically representative number of operators, it will be possible to determine the amount of variability introduced in the process from one operator to another. 
The second phase of the study will look at the amount of variability found when a single operator, using the FlapSpeed™ controller, repeats the same job a number of times. The results of the studies will be published soon.

Conclusion 

After almost 50 years, flapper peening remains the method of choice to quickly and cleanly re-peen components after removal of superficial damage. 
Now with the patented FlapSpeed™ controller, the flapper peening process is:
More reliable with the continuous monitoring and adjustment of the RPM
Faster with the use of Almen intensity versus RPM curves directly in the controller
Easier with no worries to meet internal and external requirements such as Nadcap

The FlapSpeed™ controller is a welcomed addition to any Airline or MR&O facility wanting to perform the best quality flapper peening.  

For Information :
Shockform Inc., 17 des TAELS
Blainville, QC, J7C 5B6, Canada
Tel. +1.450.430-8000
Fax +1.450.430-1761
E-mail: info@shockform.com
www.shockform.com