Investigation on early fatigue fracture for automobile suspension springs

55CrSi steel with different tensile strength levels by different treatment were used to manufacture automobile suspension springs. The springs were processed on same shot peening machine with identical procedure supplied by different manufacturerers. The experimental results exhibit significant difference in the fatigue (LSF) life, namely the one is N=4 × 105 cycles the other is N=2.6 × 105 cycles. It is considered that the lower tensile strength of the spring should be responsible for its early longitudinal shear fatigue (LSF) fracture. However, the analysis based upon a great number of test results reveals that the instability of shot peening operation, the un-proper shot peening intensity as well as the poor quality control of the shot peening, all of which produce the lower effects of microstructure strengthening, are the inevitable consequence resulting in the early longitudinal fatigue fracture. Only if the manufacturerer correctly recognizes the microstructure strengthening mechanism in shot peening process, it could be avoided effectively to produce the suspension springs with early fatigue fracture for both longitudinal and transverse shear fracture (LSF/TSF) modes, and manufacture the suspension springs with high fatigue fracture resistance by normal tensile fracture mode.