抄録
Much work has so far been made on the problem of the strength and rupture of materials under non-uniformal stress distribution in such cases as bending, torsion and tension of notched bars. It is well known that the beams of gray cast iron can support 15-25% larger bending moment than the theoretical rupture moment calculated upon the maximum tensile stress hypothesis. This is attributed to the existence of non-uniformal stress distribution by bending, though the relation between the rupture strength and stress distribution is not clear.
The author carried out some experiments about the influence of non-uniformal stress distribution on the rupture strength of cast iron by the bending test. Using rectangular specimens of different depths each specimen was measured regarding its rupture moment and strain on the surface.
The results obtained are summarized as follows;
(1) In static bending of gray cast iron, the ratio of the measured rupture moment Mb and the theoretical moment Mth, of which the latter was calculated upon the maximum tensile stress hypothesis, changes with increase of its depth h. Mb/Mth increases with decrease in h, and the rupture moment decreases to the theoretical value by increasing h.
(2) The author defined the strain gradient. xε by xε=1/ε1(dε/dx)1, and found that there was relation between xε and Mb/Mth, where Mb/Mth=1+√δxε. The type of the relation as mentioned above was similar to that which was proposed by Siebel and Petersen on the fatigue strength of notched bars.
(3) In the relation, δ is regarded as material constant. For the gray cast iron used in the present experiment, δ was about 0.25mm. This value was almost the same as that obtained by Stieler on the fatigue strength of notched cast iron.
(4) The rupture moment of cast iron beams can not be calculated by the bending strength which is determined as material constant. The bending strength varies according to the stress or strain gradient in the beam.
