The good news for all involved with flap peening is that the SAE have published a new specification, AMS 2590, for this process. This means that all organizations that use or want to use this very useful technique now have a relevant and up-to-date standard to work to. Some of the principal features of the AMS are as follows:

Arc Height Conversion

The magnetic strip holder used for flap peen verification results in higher arc heights than the conventional strip holder. AMS 2590 gives precise charts to convert A-strip and N-strip arc heights to the magnetic holder arc heights when determining and verifying intensity. Intensity requirements with C-strip values have not been encountered so AMS 2590 provides for this by stating that the cognizant engineering authority must provide a conversion, as required.

Procedures

The arc height conversion shall be performed prior to saturation curve plotting and intensity determination because this sequence produces better curve flattening and more accurate intensity determination.

Intensity Verification

Intensity may be determined by either the SAE J443 arc height method or Almen Strip Coverage method. The J443 method is stated as preferred and, in the experience of the author, is much more accurate.

Almen Strip Use

The standard clarifies that only one Almen strip is required for the entire saturation curve. This is because the magnetic holder does not influence the stresses within the strip so, unlike conventional peening, the strip can be reused.

Almen Strip Peening Technique

Peening of the extreme ends of Almen strips is not necessary; this helps in preventing damage to the flap and a loss of the shot. However, uniform coverage of at least the central two inches (50 mm) of length is essential to ensure accurate arc height measurement across the Almen gauge support ball distance

Initial Tool Speed Estimate

To achieve the required peening intensity, the speed necessary for the flap equipment may be estimated by using a graph in the standard. This is provided as guidance only and the tool speed has to be confirmed or adjusted by generation of a saturation curve and comparison to the requirement.

Hole Peening

The procedure for hole peening is much improved from the old standard. Having verified the intensity requirement using a flat Almen strip, AMS 2590 provides charts to factor the tool speed to provide the same intensity in various bore sizes.

Operator Qualification

Very importantly, the standard specifies the requirements for qualifying a flap peen operator. These include, as a minimum requirement, capability in peening technique, intensity determination and peening procedures. Also the practical demonstration of proper tool use, Almen strip and gauge handling, intensity determination from saturation curves and the ability to reproduce an intensity value (single verification) from a saturation coverage curve previously accomplished are required.

Provision of Training and Qualification

MFN provides training and qualification for flap peening and it is FAA-accepted and meets and exceeds the AMS 2590 requirements.

A workshop for flap peen training and qualification will be presented by the author in the UK in June, and MFN can provide followup on-site training for organizations on a world-wide basis.

For questions contact:
paul@mfn.li

Standards Forum
by Paul Huyton,
MFN Course Director World Wide
more information at www.mfn.li/trainers