抄録
The X-ray diffraction techniques have remarkably been developed and have been recognized as a valid experimental means in the fields of both fundamental and applicational researches concerning the strength of engineering metallic materials. The techniques, however, have been mainly applied to the studies on the strength of steels and light metals, and hardly been applied to the studies on inhomogeneous metallic materials in spite of so much expectation being made in this line, for example to establish the method of X-ray stress measurement.
Cast iron, an example of inhomogeneous metallic materials, has graphite as a part of its structure, so that the deformation behavior is complicated and its mechanical properties are difficult to be defined, as compared with ordinary carbon steel. Many investigations have been made on the effect of graphite on the strength of cast iron. It has been considered that the strength of cast iron is mainly subjected to stress concentration effect around graphite and to decrease in effective cross sectional area due to graphite. However, the above idea has not yet been verified experimentally.
Cast iron is composed of three phases in the structure in general, which are ferrite, cementite and graphite. Therefore, it is advantageous to know the deformation behavior of each phase. The X-ray diffraction technique may be available for this purpose, since the diffraction line can be obtained from only ferritic phase.
As a series of the studies on the deformation mechanism of inhomogeneous metallic materials by the X-ray diffraction techniques, the authors propose to present in this paper a report of the investigations conducted on spheroidal graphite cast iron with pearlitic matrix. The changes in lattice strain during and after stress cycles on the tensile or on the compressive surface of the plate bending specimens were mainly measured.
Many interesting features were obtained from the experimental results, and the conclusions are as follows:
(1) The deformation behavior of test material is very complicated, and local plastic deformation appears in the matrix due to stress concentration effect and decrease in effective cross sectional area caused by graphite under even low tensile stress of about 10kg/mm2. This plastically deformed area spreads with stress cycles until it reaches the limit.
(2) On the other hand, the deformation of graphite itself, the formation of voids around graphite, and the formation and opening of cracks during loading are also considered to make up the tensile deformation of cast iron in addition to the elastic and the plastic deformation of the matrix.
(3) In the case of compression, such characteristic features as in the case of tension do not occur.
(4) The X-ray elastic modulus of cast iron can be measured after 3 to 10 times of stress cycles with the same stress amplitude for an annealed specimen. However, it is necessary to investigate further on the possibility of using this modulus for different stress levels, especially for practical X-ray stress measurement on cast iron.
