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Dennis Offline OP
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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?

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The figures quoted cannot be explained simply in terms of strip thickness and hardness - because the observed arc height difference is 21%. Strip thickness affects deflection as the square of the thickness difference ratio. This factor accounts then for 3% of the observed difference. Hardness affects indent diameter as the fourth root of the hardness ratio. Assuming that an increase in indent diameter gives a direct increase in strip deflection hardness ratio would predict a 1% increase. Hence only 4% of the observed increase can be attributed to the supposed factors.
The published works of the late Prof. Sharma and that of Bailey and Champaigne are not directly comparable. Sharma used a 5mm nozzle at either 20 or 30mm from samples that, generally, were not Almen strips and had enormous hardness differences. Bailey and Champaigne used actual Almen strips of slightly-different hardnesses and normal peening parameters.
Hardness influences several factors that affect deflection of identical-thickness Almen strips. Summarising: softer strips have thicker deformed layers but the average residual stress is lower. The bending force of thicker layers is closer to the 'neutral axis' which reduces the bending moment. It follows that the overall effect of hardness is difficult to predict.
Finally, it would be very useful if someone could be persuaded to carry out a definitive study of the affect of hardness on deflection of Almen strips.

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A few years ago we experienced a problem where the hardness of the strip(s) varied by approximately five points from the centerline of the test strip. This caused a considerable amount of havoc for a few days until we realized the problem lied with the test strips themselves and not the process. SAE J442 states “The average of at least four readings in each region should be used to make the comparison” Perhaps this could use some further definition.

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Quote:
Originally posted by Socrates:
Strip thickness affects deflection as the square of the thickness difference ratio. This factor accounts then for 3% of the observed difference.
Could you explain the factor you mention?

For the influence of the strip thickness I'd assume that this is for the most part due to the difference in area moment of inertia, which is proportional to the thickness to the power of three. In this case this would account for a difference of 4.7%

Quote:
Originally posted by Socrates:
Hardness affects indent diameter as the fourth root of the hardness ratio. Assuming that an increase in indent diameter gives a direct increase in strip deflection hardness ratio would predict a 1% increase.
In my opinion the indent diameter approach is not correct. The plastic region underneath each indent is much larger than the indent itself and will be larger at lower hardness.
As you have already argued, the influence of strip hardness is complex and difficult to explain with a simple model. However our own small scale test did show a distinct influence. We have continued to monitor it after I posted my first message and again and again we see that (N) strips with lower hardness values have larger deflections. This is opposite to the influence published by Bailey and Champaigne, but they used A strips. Perhaps this gives different results.

As both the work by Sharma and by Bailey and Champaigne are based on experiments conducted more than 17 years ago, a new (and hopefully definitive) study based on the latest insights would indeed be welcome.

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The factor "Strip thickness affects deflection as the square of the thickness difference ratio" is well-established. J442 allows conversion of, for example, A to C values using a factor of 3.5. Comparing the squares of the mean thicknesses (2.385^2/1.295^2) yields 3.4 - close to 3.5. The topic was analyzed in a TSP article Volume 16, issue 4, 2003 "Relationship between Almen Strip Thickness and Arc Heights". You are correct in terms of the strip's bending resistance being proportion to the cube of the thickness. However the bending moment generated by the induced force in the surface is proportional to strip thickness so the deflection effect is reduced by one power - to two. The predicted effect for the maximum allowed thickness range of A strips is 1.32^2/1.27^2 which equals 1.08 - equivalent to a range of plus and minus 4%.

The depth of the deformed layer is directly proportion to the depth of the indents which in turn is directly proportional to the indent diameter. The volume of the indent is proportional to the fourth power of the indent diameter. Hence the reason for the "fourth root of the hardness ratio" effect. You are correct in saying that the plastic region is much larger than the indent itself. The depth of this region is, however, directly proportional to the indent diameter. Another TSP article "Prediction and Control of Indent Diameter", Volume 18, issue 2, 2004 gives the theoretical analysis.

Just one more comment: your original qualification used strips that were right at the limit of allowed thickness. Perhaps it would be better to select strips near the middle of the allowed range for qualification. Subsequent variations in strip thickness would then have less effect.


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