Abstract
ABSTRACTMarciniak–Kuczynski and Nakajima tests of the dual‐phase steel Docol 600DL (www.ssab.com/) have been carried out for a range of stress‐states spanning from uniaxial tension to equi‐biaxial tension. The deformation histories of the specimens have been recorded by digital images, and the displacement and strain fields have been determined by post‐processing the images with digital image correlation software. The fracture characteristics of the material are presented by means of the stress triaxiality, the Lode parameter and the equivalent strain. These parameters are evaluated on the surface of the specimens based on the optical field measurements and assumptions regarding the mechanical behaviour of the material. Additionally the minor versus major principal strains up to fracture are presented. It is found that the material displays a significantly lower ductility in plane‐strain tension than in uniaxial tension and equi‐biaxial tension, and that it, in the tests exposed to local necking, undergoes large strains between the onset of necking and fracture. Fractographs of selected specimens reveal that fracture is due to growth and coalescence of voids that occur in localised areas governed by shear‐band instability.
Highlights
Ductile fracture is controlled by nucleation, growth and coalescence of microvoids as explained by McClintock [1] and Rice and Tracey [2]
Three of the M-K set-ups are close to plane-strain tension (MK-155, MK-160 and MK-165), while the fourth is close to equi-biaxial tension (MK-205)
The results in form of major vs. minor principal logarithmic strains and equivalent strain as function of stress triaxiality are shown in Fig. 6(A) and (B), respectively
Summary
Ductile fracture is controlled by nucleation, growth and coalescence of microvoids as explained by McClintock [1] and Rice and Tracey [2]. Increased hydrostatic pressure tends to decrease the rate of void growth, and so increase the ductility. The ductility can be expressed by the equivalent strain at fracture, ε f = ε (t f ) , where t f is the time at fracture and ε is the equivalent strain, defined as ε = ∫0t 2 / 3D : Ddt where D is the rate-of-deformation tensor. A commonly used parameter to describe the hydrostatic stress state is the stress triaxiality, σ ∗ , defined as σ∗ = σh (1). [3,4,5] indicate that the deviatoric stress state influences the ductility in the range of low stress triaxiality σ ∗. The deviatoric stress state can be expressed by the Lode parameter, μ , defined as [6]
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