Abstract
The fatigue phenomena due to pulsating repeated bending in a large plate specimen of spring steel which had been surface-strengthened by shot-peening were investigated experimentally. The change in the surface residual stress during fatigue process and the roughness in the fractured surface after rupture were measured by X-ray and surface roughness tester and the fractured surface conditions were observed under the optical and electron microscopes. The results obtained are as follows.
(1) Up to the time just before a rupture in the fatigue process, the surface residual stress on the tension side changes only slightly, but just before the rupture it drops sharply. On the compression side, regardless of the measured position, the stress diminishes remarkably in the initial stage of the fatigue process, followed by a slight decrease continuing up to the occurrence of a rupture and then by a sharp drop just before the rupture.
(2) A large number of fatigue cracks initiate near the boundary between the surface-strengthened layer on the tension side and the non-strengthened inside. These cracks propagate at a velocity of about 0.5∼4.0×10-4mm/cycle and, coupled with an acceleration of failure, lead to a rupture. Thus, the fractured surface presents a complicated look with coexistence of a fatigue fracture and a fracture by accelerated failure.
(3) The electron microscope observation showed coexistence of complicated striation patterns and dimple patterns at the fatigue fracture surface.
