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I.

INTRODUCTION

FERRITIC spheroidal graphite (SG) cast iron is an important engineering material which is high in ductility. However, some brittle phenomena may exist and affect its reliability.[~-s] Possible factors of the brittle effect include impurity segregation, dynamic strain aging, and others, t~j As far as graphite nodules are concerned, they are extremely low in strength, and hence their presence can induce triaxial stress in the deformed ferritic matrix.t2'6'7] For a given stress state, its deviatoric component causes plastic deformation, whereas the hydrostatic component affects ductility. The overall deformation and fracture behavior can be described by a triaxiality ratio, which is defined as the ratio of hydrostatic stress over von Mises equivalent stress, t8j The higher the triaxiality ratio, the lower the ductility. A model to estimate the triaxiality ratio of ferritic SG iron under uniaxial tension has been proposed by Yanagisawa and Lui t2j in which they assumed graphite nodules as voids. The model, which will be briefly summarized in Section IV-A, reveals that the triaxiality ratio at the center region of the ferrite matrix is a function of graphite nodule interparticle spacing and nodule size. By changing nodule spacing and nodule shape simultaneously, Chao e t al. |31 took an additional step toward the investigation of the shape effect on this triaxiality ratio. Although the aforementioned two studies have formed a theoretical basis for the analysis, systematic experimental data of varying separately the individual graphite nodule variables need to be provided. Moreover, the triaxiality ratio at the center region of ferrite matrix was considered in these studies. The effect of stress concentration near the graphite nodules was neglected without firm clarification. To check whether this consideration of local triaxiality ratio is proper or not, the ductility and fracture morphologies will be examined in the following

Y.F. LIN, Graduate Student, and T.S. LUI and L.H. CHEN, Professors, are with the Department of Materials Science and Engineering, National Cheng-Kung University, Tainan, Taiwan 70101, Republic of China. Manuscript submitted January 11, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A

investigation by choosing graphite nodule spacing as the main material variable. In order to obtain a larger variation in the triaxial stress state, the hydrostatic component will also be varied through the application of fluid pressure during tension.

II.

EXPERIMENTAL PROCEDURE

Four ferritic S.G. cast irons were used for the experiment. As revealed in Table I, the carbon concentration of these SG irons was varied from 1.96 to 3.89 wt pct. The remaining elements were maintained about constant, with an average silicon concentration of 2.6 wt pct. To vary graphite nodule interparticle spacing without significantly changing other microstructural variables, these SG irons were carefully prepared by melting electrolytic iron, ferrosilicon, and electrode-graphite powder in a high-frequency induction f