Fatigue Crack Propagation of Spring Steels

Abstract

Nowadays, a car suspension is used under higher stress conditions with more weight saving needs of cars. As a higher hardness suspension is used, the notch sensitivity, being beforehand taken notice of, is a more important problem to be solved.
This paper describes the investigation on influences of hardness of SUP 6, SUP 7, and SAE 9254 steels on their property of fatigue crack propagation on the basis of fracture mechanics to quantify the notch sensitivity associated with increasing hardness.
The investigated results are as follows:
(1) Properties of fatigue crack propagation hardly depend on the difference between those three steel types and greatly depend upon hardness of material within this investigation range.
(2) The fatigue crack propagation velocity in an intermediate growth rate regime is able to be formulated by Paris' law as a function of hardness; so that a higher hardness gives a greater propagation velocity.
(3) The threshold stress intensity factor range decreases against increase of hardness and the notch sensitivity grows with increasing hardness.
(4) The investigation in a high growth rate regime is not conducted yet, but the fatigue crack propagation velocity in the high growth rate regime is supposedly increased with increasing material hardness because a fracture toughness decreases with hardness.
(5) Fatigue fractographs vary with great dependence on the stress intensity factor range and material hardness.