Dear Jack,
How much material can be removed after peening (honing, lapping, polishing etc) without affecting the fatigue life?

According to SAE practices:
From AMS 2430:
3.5.3.1 The amount of metal removed by honing, lapping, or polishing shall be not more than 10%
of the specified minimum arc height for "A" and "C" intensities and not more than 3% of the
specified minimum arc height for "N" intensities

From AMS 2432:
3.6.2.2 For other parts, no more than the equivalent of 10% of the specified
minimum "A" intensity or equivalent "N" or "C" intensity (see 8.5) shall
be removed from the surfaces.

From AMS-S-13165:
I didn't find anything in this spec regarding post peening surface removal.

I'm not sure I comprehend what is meant in AMS 2432. For example, if you used an intensity of .010-.014A then you would be limited to 10% of the lower limit or 10% of .010 or 0.001 inch could be removed. But, if the intensity were in the "N" scale - I'm not sure if you could still remove .001 inch or if you could remove .003 inch since the relation between "N" and "A" intensity is approximately 3:1.

Hi,

Although the level of compressive residual stresses is very dependable on the type of material, heat treatment (if any) and shot intensity, the shallow depth of a 20-40 micrometers is very important. The maximum compressive residual stresses are tend to be in this range and the more material that you cut away from the surface the more that you minimizing this beneficial layer.

Regards.

The amount of material that can be removed without affecting fatigue life depends primarily on the depth of the compressed layer. This depth is rarely measured, whereas the peening intensity - which is directly related to that depth - is readily available. AMS2432 makes a brave attempt to offer guidance. Harder materials have a shallower compressed depth than have softer materials (3.6.2.1 cf 3.6.2.2)for a given peening intensity. Hence only 5% (of relevant minimum peening intensity) is allowed for hard over 1517 MPa U.T.S.) materials and 10% for softer materials. The "relevant minimum peening intensity arc height" depends on the strip type. AMS2432 is clear that the recommended height is based on the use of A strips. If N strips are being used then the allowable removal is either 5%/3 or 10%/3 - depending on the U.T.S. of the component. If C strips are being used then the allowable removal is either 5% times 3.5 or 10% times 3.5 - again depending on the U.T.S. of the component material. In all cases the percentage removal is to be based on the minimum of the customer's allowable intensity range.

Pretty much useful guidance offered by AMS2432. But some of the mechanical layer removing process introduce some negative stresses in the material, does the AMS2432 count that?
It is for sure that one can use this guide as a quick reference but in case of saving money, saving weight and sensative components for example, i see no harm of measuring some points and deciding the level of stresses before and after the layer removing process.

Ams2432B does not take account of the potentially-beneficial effects of most fine finishing processes. It does require that evidence of shot peening must remain after fine finishing. That means that only the high points of the surface are removed. These contain lower levels of compressive stress than deeper material. Additionally most of the stress-raisers are polished off.

If money is no object, measurements of residual stress would, of course, be useful.