Improve Fatigue Limit On Coil Spring

Author:  N. Hu, X. Liu, J. Yao and K. Jin
Source:  Conf Proc: ICSP-3, (p.541-546)
Doc ID:  1987043
Year of Publication:  1987
Abstract:  
To increase the fatigue strength of coil spring by shot peening has been discussed in many papers, the major topic was to correlate the peening intensity with fatigue strength in terms of redistribution of residual stress and change of surface topography. However, failure analysis was not so emphasised in the previous works and the leading factor which is responsible to the increase of fatigue strength was not clarified. Since fatigue test for coil spring is tedious and time consuming, it would be beneficial to the practical application to find the vital factor for the quality control. Paper (1) summerized the effect of shot peening for coil spring with a broad spectrum of testing parameters and found that with the increase of peening intensity, the surface roughness increases; the peak value of compressive residual stress holds in constant while the surface value decreases; the depth of the peak value shows little change but the total depth increases. The fatigue strength depends on the size of coil wire, with the increase of peening intensity, for the diameter smaller than 2mm, a maximum fatigue strength corresponds to a certain peening intensity; for 3mm, fatigue limit shows little effect with a large extent of peening intensities; for those diameter larger than 3mm, fatigue strength gradually increases with the increase of peening intensity(1,2). Nevertheless, it seems to be not appreciable so far to set up a correlation between residual stress distribution and surface roughness with the fatigue limit for coil spring. Leaf spring, on the other hand, exhibits reasonable good relation with peening intensity. Paper (3) presented an equation correlating the integral value of surface residual stress and the fatigue limit. Paper(4) found surface roughness is the leading factor for the fatigue strength of leaf spring. The difference of testing results between coil and leaf spring could be resulted from the following facts: First, the surface layer of leaf spring is usually decarburized and the fatigue crack initiates there, it should be beneficial to improve the surface strength by reducing the roughness or increasing the compressive residual stress. Second, The fatigue fracture of leaf spring is caused by normal stress under bending, biaxial compressive residual stresses are in the principle loading stress plane, so the Goodman or Dang Van relation can be employed for fatigue limit evaluation. Coil spring is suffered from torsion and shear stresses, since the mean stress does not change shear fatigue behavior(5), the compressive residual stress would show little contribution on the shear fatigue strength too. Decarburization is negligible for coil spring, so its strengthening behavior is different. It would be helpful to find out the fracture mechanism prior to the prediction of peening parameters for coil spring production. In this experiment, different processing ways were carefully selected to obtain different depths and magnitudes of residual stress and different surface roughnesses as well. Fracture characteristics were examined by fractography, so as to find out the effects of above factors on the fatigue behavior for coil spring.