In this paper, fracture criterion of gray cast iron under non-uniform stress was discussed on the basis of bending and eccentric tension tests performed on the rectangular specimens at room and liquid nitrogen temperatures. The experimental results were analyzed with a concept of“over stressed depth δ”, which was proposed by the author as a fracture criterion of notched cast iron in the previous paper.
The values of δ, obtained from the calculated stress distribution at fracture and the tensile strength of the material, were almost constant and 2-5mm for both bending and eccentric tension at room temperature, coinciding with those obtained in notched plates and bars. At liquid nitrogen temperature, δ decreased to less than 1mm, also coinciding with that in notched tension. Thus the applicability of constant δ as the fracture criterion under the stress gradient conditions was confirmed.
The value of δ at liquid nitrogen temperature was almost equivalent to the size of a graphite eutectic cell which is considered to be a microstructural unit of cast iron, and at room temperature, increased by 3-5 times. From the fact that δ depends not only on material but on specimen size or stress gradient and especially on temperature, δ was infered to be a parameter which represents the propagating condition of a crack, initiated at highly stressed surface or notch root before final fracture. By the concept of over stressed depth, mechanical characteristics of gray iron such as high bending strength, low notch sensitivity and their dependences on temperature and specimen size can be explained rationally with a single parameter δ.