Tensile fracture and fractographic analysis of 1045 spheroidized steel under hydrostatic pressure
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H.A. Kuhn University of Pittsburgh, Pittsburgh, Pennsylvania 15261
O. Richmond Alcoa Technical Center, Alcoa Center, Pennsylvania 15069
W. A. Spitzig Ames Laboratory, Iowa State University, Ames, Iowa 50011 (Received 2 March 1989; accepted 30 August 1989) Void formation in tensile test under hydrostatic pressure is characterized through quantitative metallography, and the fracture mechanism under pressure is analyzed by fractography. Transition of the fracture surface from the cup-and-cone under atmospheric pressure to a slant structure under high pressure is explained on the basis of the void development leading to fracture and the concomitant change in fracture mechanism. The concept of "shear blocks" is introduced to illustrate the features observed on the fracture surface of specimens tested under high pressure. It is postulated that shear blocks evolve to connect the central crack regions with the shear crack initiated on neck surface due to the severe necking deformation under applied pressure.
I. INTRODUCTION
Tensile deformation of steels under superimposed hydrostatic pressure has been investigated in a number of studies.1"3 The general trend shows that increasing pressure leads to a significant increase in ductility and slight increase in flow stress, which are explained by the suppression of microvoid development and dislocation mechanism under applied pressure. The correlation between the microdamage process (void nucleation, growth, and coalescence) and the concomitant change in macrofracture mechanism under pressure has not been reported. The functional relationship between global stress state and the pressure-induced transition in tensile fracture has also not been established. Previous studies on the correlation between hydrostatic stress and tensile fracture focused exclusively on the tensile hydrostatic stress induced by a notch geometry,4"6 which promotes the microdamage process but results in an effect opposite to the superimposed hydrostatic pressure. The influence of hydrostatic stress through directly applied pressure is, therefore, investigated in the present study. Using a spheroidized 1045 steel, void formation processes are quantitatively characterized and correlated with the deformation history and the fractographic features developed under applied pressure. The transition of fracture mechanism due to the suppression of dimple generation under pressure and the presence of "shear blocks" at neck center are explained by the competition of microdamage processes: the tearing crack from neck center and the shear decohesion from neck surface. FiJ. Mater. Res., Vol. 5, No. 1, Jan 1990
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nally, a clear picture of the hydrostatic pressure effect on the mechanism of tensile fracture is demonstrated. II. MATERIAL AND EXPERIMENTAL
The test specimens were prepared from a laboratory vacuum-melted heat of 1045 steel with the following composition (wt. %):
c
Mn
Si
P
s
Al
Ni
Fe
0.45
0.65
0.21
0.01
0.004
0.001
0.007
Balance
The co
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