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Vol. 13
May Issue
Year 2012
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Standards Forum


in Vol. 13 - May Issue - Year 2012
A Review of Flap Peening Control methods



Francois-Xavier Abadie


Paul Huyton

The process of flap peening was developed in the 1970’s as a technological "spin-off" from the Vietnam War. The United States Department of Defence needed a means of peening repair areas on helicopters damaged by small-arms fire. Hence, the process was controlled by a Military Specification MIL-R-81841. From December 2010, a completely revised specification has been issued by the SAE, AMS2590 Rotary Flap Peening of Metal Parts. This new specification has clarified and revised a number of process control aspects from the earlier specification MIL-R-81841.
One of the reasons that flap peening is not controlled with a conventional shot peening specification is that the equipment used for intensity determination differs from conventional equipment and results in different verification values. The Almen strip holder used for flap peening locates the strip with constrained magnets, which move sufficiently to permit the strip to bend. As the strip is not tightly held down by screws, as with the conventional holder, the strip arc heights are greater with a less well-defined flattening-out of the saturation curve. To compensate for the higher values and flatter curve, the saturation ratio used to locate the saturation point and intensity was set at 15% instead of the 10% used with a conventional shot peen holder. The arc heights measured with the magnetic holder have a well-documented relationship to arc heights derived with the conventional holder. The relative values are published graphically in MIL/AMS 81841 and are tabulated in detail in AMS 2590. So a flap peening intensity determined with a magnetic holder can be compared with a conventional shot peen intensity requirement by appropriate conversion of the values. For example, if the conventional intensity requirement is .006 – 0.010 inch A, the magnetic intensity range is .0067 - .0134 inch A.
The AMS 2590 procedure for intensity determination differs from MIL-R-81841 in that the magnetic arc height values are individually converted to conventional readings before the saturation curve is plotted. The saturation point and intensity are then determined using the conventional 10% saturation ratio. This results in a better-defined saturation point and more correlation with the conventional peening process.
My interest, as a specialist in these processes, is in the comparison of intensity values determined by the two methods. Therefore, I contacted my MFN trainer colleague, Francois-Xavier Abadie. In addition to his specialist shot peening knowledge, Francois also has excellent computer programming skills. Francois was able to produce a new version of the PA² in which the saturation ratio is optional between 10% and 15%, enabling me to undertake my study.
I now had to decide the acceptable variation in the intensity result in order that the two methods could be considered equivalent. SAE J2597 is the published standard for computer generated saturation curves and includes arc height datasets with a target intensity based upon a 10% saturation ratio. The intensity values achieved by the programme must be within ± .001 inch of the target value stated in the standard. The PA² programme achieves this, so I then converted the datasets to magnetic holder readings using AMS 2590 tables. I then reset the PA² programme to 15% saturation ratio and calculated the intensity based on magnetic arc heights and then converted it back to conventional readings. This is the original MIL-R-81841 method and so I had a direct comparison of the new AMS2590 method and the original method. To determine the two methods as equivalent, I set a criterion that the resultant intensity values should be less than .0005 inch difference; this is half the J2597 criterion and less than the requirement in any specification that I have reviewed.
There are ten datasets in J2597 and I used seven, which are based on exposure time and not machine speed; the readings are expressed in 0.001 inch.

Dataset

J2597 range required

MIL 81841 method

AMS 2590 method

Both in range?

Less than .0005”difference?

1

5.4 – 7.4

6.3

6.4

YES

YES

2

3.0 – 5.0

3.5

3.9

YES

YES

3

7.0 – 9.0

7.8

7.9

YES

YES

4

3.8 – 5.8

4.8

5.1

YES

YES

5

5.6 – 6.6

6.5

6.5

YES

YES

6

8.8 – 10.8

10.4

10.7

YES

YES

7

5.3 – 7.3

6.4

6.4

YES

YES


Conclusions:
1. The two intensity determination methods give results within 0.0004 inch and so can be considered equivalent by the criteria set.
2. Suitable software, such as PA², can enable users to work easily with either method.

For questions contact paul@mfn.li



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