Spheroidal graphite cast iron can be made with shape freedom, and can be used often in critical parts of automobiles and industrial machinery that are designed using the allowable stress design method. However, spheroidal graphite cast iron has been regarded as having low ductility deriving from fracture. It has therefore never been used for structural members for construction purposes. Previous studies have not clarified the plastic deformation capacity and ultimate strength of the spheroidal graphite cast iron members, even though the material properties were estimated using many experimentally obtained results. As described in this paper, compression tests were performed to investigate the plastic behavior of the spherical graphite cast iron members, and to assess the relation between the collapse mechanism and the width-to-thickness ratio. Tensile strength of samples corresponded to 700 N/mm~2, 450 N/mm~2, and 400 N/mm~2. In addition, steel pipe material corresponding to 490 N/mm~2 was selected. Consequently, six specimens were used for the compression test. One was a steel pipe. The specimen diameters were 165.2 mm; the lengths were 450 mm, corresponding to approximately three times the diameter. The specimen thickness was 4 mm or 6 mm, with a width-to-thickness ratio of 41.3 or 27.5. The ultimate strength of spheroidal graphite cast iron pipe members is about twice the yield strength, even though the ultimate strength of the steel piles is equal to the yield strength. A crack was found on the pipe surface at the ultimate strength because the axial stress on the surface changes compression to tension by growing plate bending deformation of local buckling. Spheroidal graphite cast iron pipe members under compression loading exhibit brittle behavior after post local buckling.
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