Lately we have seen an increase in the intensity readings of qualified (and fixed!) shot peen processes. None of the machine parameters have been changed and the shot size distribution hasn't changed either. When investigating this, we noticed that the hardness of the Almen strips that we use now is different than of the Almen strips that were used during qualification, although both are within tolerances. Also the strip thickness is a little bit less than it was during qualification. The numbers are:
Code:
              Qualification      Now
hardness      47.9 HRC           46.0 HRC
thickness     1.32 mm            1.30 mm
Arc height    0.122 mm A         0.148 mm A
I think these figures can explain the increased intensity:
- Thinner strips will have a larger deflection.
- A lower hardness indicates that the strip material has a lower yield strength. A lower yield strength means that more plastic deformation occurs under the same load. More plastic deformation results in a larger strip curvature.

However some literature about this state the influence of strip hardness should be opposite to what we are seeing. For example in "Factors That Influence Almen Strip Arc Height" by Peter Bailey and Jack Champaigne (ICSP-09) it is stated that harder strip result in a higher arc height. On the other hand the paper "Effect Of Work-piece Hardness On Peening Intensity Under Local Peening" by Sharma, M. C. (ICSP-3) shows a different conclusion. Figures 2 to 4 show that the arc height decreases when strip or work piece hardness increases.

So my question is twofold:
1) Has anybody experienced this influence of strip hardness (and thickness) before? If so, what was the experienced influence of strip hardness?
2) What could be the reason that the first paper comes to a different conclusion than the second paper? As said we experience behavior similar to that discussed in the second paper, and we even have an explanation for it, but the first paper contradicts this. The question is of course which conclusion is actually correct?