Hydrostatic stresses and their effect on the macroflow behavior and microfracture mechanism of two-phase alloys
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INTRODUCTION
IT is well known that the hydrostatic stress or stress triaxiality has an important influence on the macrodeformation behavior and microfracture mechanism of engineering materials. The flow stress at a given strain and the total fracture strain generally increase with superimposed hydrostatic pressure, t1-41 In addition, the fracture strain of the notched tension specimens, which develop local hydrostatic tension, is obviously lower than that in smooth ones. Bridgman pointed out that it is impossible to use parameters only related to stress or strain to describe the complicated fracture phenomena. I51 The results [6,7,8t show that both the stress and strain at failure are influenced by the hydrostatic stress or stress triaxiality. Recently, much efforts have been made to investigate the macrofracture criteria and the microfracture mechanisms. Having discussed the limitation of the J-dominance elastic-phastic fracture and the effects of the stress triaxiality, Hutchinson t9~ pointed out that two parameters, J-integral and a measure of near crack-tip stress triaxiality, might suffice to characterize the full range of near-tip fracture environments. And two parameter fracture criteria incorporating the stress triaxiality and J-integral t~~ or critical values of crack-tip opening displacement t13] have been proposed. The micromechanism of ductile fracture may be referred to initiation, growth, and coalescence of voids. The importance of the stress triaxiality in determining both the rate of void growth and shape change of the growing voids has been c o n f i r m e d , H4-19] and the fact that the rate of the void growth observed in notched specimens with higher stress triaxiality is much higher than those observed in smooth tension specimens was also demonstrated, t2~
LI ZHONGHUA, Associate Professor, Department of Engineering Mechanics, and GU HAICHENG, Professor, Research Institute of Strength of Materials, are with Xian Jiaotong University, Xian, People's Republic of China. Manuscript submitted August 21, 1990. METALLURGICAL TRANSACTIONS A
Compared with the knowledge of the influence of the stress triaxiality induced by geometry change (notch, crack, etc.) on the mechanical behavior of materials, there is still a lack of understanding of the effect of the stress triaxiality developed as a result of interaction between soft and hard phases to maintain compatibility on the flow behavior and microfracture mechanism of the twophase alloys. Recently, the authors' work t21,221 revealed that during preloading, the residual hydrostatic tension stress and the residual hydrostatic pressure stress produced in ferrite and martensite play an important role on the pronounced Bauschinger effect of the dual-phase steels. The present study focuses our attention on examining the aspects of the hydrostatic stress or stress triaxiality in two-phase alloys both on the macro- and microscale. The flow behavior of composite alloys, the hardening and softening of the constituents, and the microfracture mechanisms are dis
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