Yoshitaka Natsume

Behavior of crack propagation in the test piece of circular section with "Fish eye".

The behavior of crack propagation in the complete reversed tensile and compressive fatigue test piece of circular section with Fish eye of the heat treated high-speed steel was investigated by using an acoustic emission technique. Moreover, the property of crack propagation in the 1/2CT test piece was investigated.The results obtained are summerized as follows;(1) The number of stress cycles NAE for crack initiation and propagation and the number of stress cycles Nf for fracture were accurately estimated by measuring continuously AE total ringdown counts during the fatigue process.(2) The fatigue life was good accord with the value of Nf estimated by the acoustic emission technique. The life of fatigue crack propagation (Nf-NAE)Lab. was 25-50% of the fatigue life Nf and decreased with decreasing stress amplitude.(3) The life of fatigue crack propagation (Nf-NAE)cal. calculated from the property of crack propagation in the 1/2CT test piece showed good agreement with the value of (Nf-NAE)Lab. obtained by the acoustic emission technique.(4) The behavior of crack propagation in the complete reversed tensile and compressive fatigue test piece of circular section showed good corrrelation with the behavior of crack propagation in the 1/2CT test piece, so that it is suggested that the initiative mechanism of residual stress on the fracture surface with Fish eye can be discussed by using the 1/2CT test piece.

On the Main Factors Contributing to Improvement of Fatigue Strength of Induction Hardened Steel

In the previous study the influence of induction hardened steel on its fatigue strength was investigated with respect to the hardened depth and residual stress. It is hard, however, to explain, on the bases of hardness and residual stress alone, the increase of endurance limit due to induction hardening. In the present paper it is intended to find out main factors that will contribute to improvement of fatigue strength.It is found that all the specimens tested have shown a linear relation existing between the endurance limit and the integral width of X-ray refraction line, regardless of the hardening methods, whether by progressive hardening or by static quenching. The specimen obtained from the integral width according to Halls analysis has also shown a linear relation existing between its micro strain and particle size and the endurance limit. It is this micro strain and particle size evidently rather than the residual stress that have influence on the endurance limit of induction hardened steel.