To study fatigue strength design method for suspension coil springs for automobile, especially as the fundamental study of fatigue strength for spring steel, torsional fatigue and rotating bending fatigue tests were conducted for shot-peened spring steel of SUP7 having various Vickers hardness through tempering temperature variation. The effect of shot peening on the torsional fatigue strength and rotating bending fatigue strength were studied. Furthermore, the difference between the morphology of torsional fatigue failure and rotating bending fatigue failures were also studied. As a result, following findings came to light. (1) The torsional fatigue limit of peened specimen exceeded that of non-peened one in any hardness condition. On the other hand, as for rotating bending fatigue limit, there was no difference between non-peened and peened specimen, with the exception of specimen with maximum hardness. (2) The relationship between torsional fatigue limit and rotating bending fatigue limit for non-peened specimen was well explained by the shear strain energy theory. On the other hand, as for peened specimen, the relationship was explained by the combination of the maximum principal stress and the shear strain energy theories. (3) The fatigue failure morphology for non-peened specimen due to torsional fatigue indicated that shear fatigue failure initiated parallel to the axial line and then the failure propagated toward principal stress direction. As for peened specimen, the fatigue failure initiated along the 45 degree direction to the axial line (principal stress direction), and then propagated. On the other hand, as for all specimen due to rotating bending fatigue tests, the fatigue failure initiated toward principal stress direction, and then propagated.
