The more accurate term is "strain peening". Tensile strain is applied to the face of the leaf springs that is to be shot peened. This strain should be a large fraction of the yield strength but should not exceed it - otherwise you will be imposing a permanent "pre-set" to the springs. When the applied strain is released (after shot peening)the relaxation increases the favorable compressive residual stress on the peened face.
Calculation of the requisite applied strain is based on knowing the yield strength and elastic modulus of the unpeened leaf spring material. For example: a strain of 0.005 (0.5%) applied to a steel with an elastic modulus of 200GPa will induce a surface stress of 1GPa (0.005 x 200GPa). If this exceeds the known yield strength then a 0.5% pre-strain is too high. A rough target would be to apply a pre-strain that induced half of the yoield strength of the leaf spring steel.
There is an excellent article by Mattson and Roberts on thepre-straining of leaf springs. Go to the ShotPeener Library, select Search function and enter as Key Word "Strain Peening". The article is the third one that appeares and is downloadaable in pdf format.
