The authors carried out a reversed bending fatigue test of flat specimens shot-peened under the most fundamental conditions; that is peened with almost perfect sphere shots made of very rigid metal and as uniformly as possible.
Results obtained are as follows:
1) Excess of impact force causes the increase of dent's size, resulting in the drop of fatigue strength.
2) Number of dents per unit area or density of peening has the critical value beyond which the fatigue strength will begin to decrease. This critical value is larger for smaller dents than for larger dents.
3) The most desirable result is obtained for a combination of smaller dent and larger density of peening.
4) Results described in 1) & 2) may be regarded as the so-called over-peening.