Fatigue Strength and Crack Propagation Behavior of Press-Fitted Axle of Carbon Steel

An Experimental study has been carried out to determine fatigue strength and crack propagation behavior of press-fitted axles of a S45C carbon steel. Six kinds of specimens were prepared by means of heat treatments. That is, material A and B were normalized and annealed after heating at 850°C for 30min, C and D were normalized and annealed after heating at 1000°C for 4h, H was quenched and tempered after heating at 850°C for 30min, and R is as-received condition. For rotating bending fatigue tests, the press-fitted specimens, inserted two axles of 8mm in diameter into a joint with an interference of 10μm, were used to evaluate the fatigue strength, and the taper-fitted specimens, jointed the same axle with a tapered boss, were used to observe intermittently the crack propagation behavior. Although the fatigue strength of smooth specimens for all material conditions varied from 220 to 435MPa, that of the press-fitted specimen was in the narrow range from 140 to 190MPa. The fatigue strength of press-fitted axles was independent of the static strength or the plain fatigue strength of the material. From the observations of the crack propagation behavior at stress amplitude of 200MPa with the taper-fitted specimens, it was found that crack initiation was earlier in high strength materials, such as H and R, than in low strength materials, such as B and D. And, the crack propagation rate in the material R was higher than that in the material B or D, and that in the material H was lowest. Consequently, the fatigue strength of press-fitted axles was found to be lowest for the material R and highest for the material H.