Strain peening has to involve stressing below the yield point. The yield point for a given steel can vary according to its thermomechanical history. 51CrMoV4 steel, for example, is quoted as having a tensile strength in the range 1400-1700 MPa with a minimum 0.2% proof strength of 1200 MPa. The proof strength must be less than the tensile strength. It follows that stressing of this steel MUST be less than 1700MPa even in its strogest condition. Customers should decide their stated values on the basis of knowing the yield strength of their particular material. Have you asked your customers for details of the yield strength of their supplied spring material?
A practical test is to carry out incremental loading of a test spring. A plot of deflection against load should be linear up to its yield point. You can then calculate the stress that is being applied when the spring yields.
