Home Forums General Discussion Ask Dr. Peener Compression Stress In Stress Peening

Dear Dr. Peener;

My name is Taylan from Turkey. We produces spring leaves.I have some questions about stress peening process. As u know in stress peening process you applied some stresses on the leaves.How much stress is enough and how i calculate this? Yield point is enough or we need more? Thank you for your interest.

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.

Thank for your interest Socrates.Still i have questions. You think two parabolic leaf springs, one of is 100x48 51CrMoV4 and the other one is 60x21 51CrMov4. Dimensions are diffent so how i can calculate the most suitable values. In stress peening machines you can apply different forces as you know. And i calculate this forces by changing the displacement of spring. But i want to learn the correct values and calculation way.

It seems as if you need to apply classic beam bending equations. For a leaf spring in three-point loading the induced surface stress is given by 3*P*L/(2*w*t*t) where P is applied load, L is the distance between the supports, w is the width and t is the thickness. Deflection in three-point loading is given by P*L*l*L/(4*E*w*t*t*t) where E is the elastic modulus.

Thanks for formulas i have information about them. Some customers says; stress peening should be under 1650 Mpa, for another spring should be 1400Mpa. How they decide this values? Sometimes i think they give bigger values. So i want to learn the relation between yield point and this values.

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.