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.
