X-Ray Investigation on the Deformation and Fatigue Fracture of Spring Steel in Plane Bending

Abstract

The existence of decarburized surface layer affects the fatigue resistance of spring steel under cyclic load very much.In our previous experiment, fatigue resistance and the plastic deformation under cyclic load of spring steel were studied with non-decarburized and decarburized specimens by means of repeated plane bending.The plate specimens used were cut from the leaf spring stock of Si-Mn steel, approximately 6mm thick by 60mm wide, then quenched and tempered. In the half of the specimens, the surface remained as rolled and decarburized, and in another half the decarburized layers were removed by grinding.The test results obtained in several stress ratio showed that the endurance limit markedly decreased and the plastic deformation in the process of repeated bending increased in different manners, due to the existence of decarburized skin.In the present experiment, the behaviours of surface layer of decarburized and non-decarburized specimens under static or repeated bending were studied by X-ray diffraction method.First, the surface stresses of the both specimens in static uniform bending were measured by X-rays, at several loaded and unloaded steps, in tension and compression side separately. Then, the relations between the nominal bending stress, the measured stress by X-rays and the total surface strain were examined.In non-decarburized specimens, the nominal bending stress and the measured stress by X-rays kept linear relation till the nominal stress nearly reached the tensile yield point. In the decarburized specimens, however, the relation ceased to be linear at the nominal stress much lower than the apparent tensile yield point. The nominal stress, where the relation between the nominal stress and the measured stress by X-rays ceased to be linear, was defined as“the proportional limit of surface layer”. The fatigue ratio of the decarburized specimens, expressed in termes of reversed bending fatigue limit divided by“the proportional limit of surface layer”, well agreed to that of non-decarburized ones.Then, the half-value breadth of X-ray diffraction line was measured on the tension side surface of the specimens that failed in the above mentioned repeated bending test. The X-ray diffraction was made at the point adjacent to the fracture and 10mm distant from the fracture in the longitudinal direction.In case of non-decarburized specimens, the half-value breadth ratio (half-value breadth of failed specimen divided by the initial value of the specimen) obtained at the point 10mm distant from the fracture (b2/b0) was observed to be 0.9-1.0 in every stress ratio. The half-value breadth ratio in the fracture adjacent point (b1/b0) appeared to be smaller than b2/b0 under the reversed stress, and larger than b2/b0 under pulsating stress.In decarburized specimens, b2/b0 was observed to remain near unity when the maximum stress did not exceed 70kg/mm2, but became larger according as the maximum stress increased. And b1/b0 appeared to be larger than b2/b0 in any stress ratio.