X-Ray Investigation on the Strength of Inhomogeneous Metallic Materials

Abstract

The X-ray method of stress measurement is taken as a unique method of non-destructive measurement of local surface stress. In the past few years, the experimental technique and theory of the stress measurement by X-rays have been remarkably improved, and the recent studies in this field have achieved the sufficiently accurate measurement of applied or residual stress for homogeneous metallic materials such as iron, steel, light metal, alloy and so on. However, the method of X-ray stress measurement is not established for inhomogeneous metallic materials such as cast iron, and there remain yet a few problems upon which there is much controversy.One of the most important problems is to find out the suitable elastic constant for the practical X-ray stress measurement of cast iron.In the previous papers, the authors discussed the effects of graphite on the deformation behavior of spheroidal and flake graphite cast iron from the lattice strain measured by X-rays. It was also suggested that the X-ray elastic constant obtained at after 3 to 10 stress cycles should be used for X-ray stress measurement of cast iron, and that its constant obtained by X-rays is different from the value obtained by the mechanical method. Besides, there is a wide difference in these values obtained by X-rays in cast iron from those obtained in wrought iron or steel.In the present investigation, further experiments were made of the application of X-ray stress measurement to cast iron. The elastic constant of spheroidal and flake graphite cast iron was measured by the mechanical method and by X-rays in detail. The X-ray measurement of applied stress and residual stress induced by grinding was carried out using the obtained X-ray elastic constant.The following conclusions were mainly obtained for these materials;(1) The macroscopic elastic modulus (EM) decreased with the increase of stress. For flake graphite cast iron it decreased having a straight line relation with stress. On the other hand, the elastic modulus pasted through two-stages for spheroidal graphite cast iron. It showed gradual decrease in a linear relation to stress to the stress of 25kg/mm2 and, after that, the rate of its decrease became larger until fracture occurred.(2) The macroscopic elastic modulus (EM) of flake graphite cast iron was markedly different from that of spheroidal graphite cast iron. It is believed that this wide difference is due to the effect of difference in the shape of the graphite on each deformation behavior.(3) On the other hand, the X-ray measurement yielded almost the same elastic constant for the two materials. It is considered that the value of the X-ray elastic constant of cast iron is controlled not so much by the shape of graphite as by the volume ratio of graphite to matrix. Therefore, it may be said in conclusion that the elastic constant measured by X-rays should be used in the X-ray stress measurement of cast iron.(4) If the X-ray elastic constant was used for stress calculation, the applied stress could be measured by the X-rays having a good correlation with mechanically applied stress.(5) A good correlation between the value measured by the X-ray method and by the mechanical method was found for the residual stress induced by grinding.