Mechanics and Physics of Brittle to Ductile Transitions in Fracture

[+] Author and Article Information
A. S. Argon

Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139

J. Eng. Mater. Technol 123(1), 1-11 (Jun 19, 2000) (11 pages) doi:10.1115/1.1325408 History: Received June 19, 2000
Copyright © 2001 by ASME
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Dependence of TBD on loading rate K̇1 for a cleavage crack on a (111) plane parallel to a [112] direction. Data from 18. Line (1) result from Eqs. (11) and (14) using direct data from 21. Line (2) with adjustment for α/σo3, more appropriate for actual experimental conditions (after reference 18, courtesy of Pergamon Press).
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Crack-tip stress relaxation zone of an arrested crack tip viewed on an etched median plane of a DCB sample of Si
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Etch pit density distribution along the spine at θ=θc=π/2 of the plastic arrest zone
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Linear dislocation etch pit density, comparing measured quantities with densities assessed from the Riedel-Rice crack tip stress relaxation model
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Sketch showing the geometrical arrangement of the set of two “vertical” slip planes α and β on which dislocation emission has been observed in Si DCB specimens
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Depiction of a relatively idealized jerky crack advance scenario in which the step-wise advancing crack is systematically put into new environments at progressively increasing temperatures until it is eventually arrested
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Dependence of the TBD on the average crack velocity vo as defined by Eq. (15). The measured dependence reflects an activation energy of 1.82 eV in Si DCB specimen.
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Etched fracture surface of a crack tip region in a sample with an arrested crack front. The etch pits outline the intersection of the dislocations emitted from the crack front with the crack plane. The trails enamating from the sources (s) indicate that the dislocations are placed alternatively on the α and β “vertical” slip planes in the Si DCB specimen.
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Configurations of crack tip processes leading from brittle to ductile behavior: (a) formation of a dislocation embryo which can expand freely to result subsequently in unimpeded intense dislocation multiplication as is approximately the case in BCC transition metals; (b) emission of a train of dislocations, sluggishly moving away from the crack tip in materials with high lattice resistance to kink motion along dislocations such as in Si and compounds
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Three modes of dislocation emission from the tip of a cleavage crack: (a) on an inclined plane going through the crack front; (b) on an oblique plane intersecting the crack front; (c) on a small cleavage surface ledge at the crack front
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A generic semi-infinite crack with an inclined plane going through the crack front defining the parameters of the variational boundary integral method of analysis
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Two activation (saddle-point) configurations of nucleated embryos at a cleavage ledge along a crack front: (a) at a high energy release rate level GIII/GIIIcd=0.75; (b) at a lower energy release rate level of GIII/GIIIcd=0.5 (after reference 7, courtesy of Taylor and Francis Ltd.)
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Dependence of activation energies on levels of crack tip driving “forces” for dislocation embryo formation for the three modes of emission given in Fig. 2, for alpha iron (after reference 7, courtesy of Taylor and Francis Ltd.)
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Nomarski interference contrast micrograph of the (110) cleavage fracture surface of a typical specimen showing both straight striations parallel to the crack front and cleavage ledges roughly parallel to the crack growth direction
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Two complementary Berg-Barrett X-ray imaging topographs produced from (333) Bragg reflections from a 1 μm thick zone parallel to the cleavage fracture plane: (a) region behind the crack front; (b) region ahead of the crack front. These images indicate that the entire set of plastic zone dislocations are a result of multiplications of screw dislocation embryos emitted from crack tip cleavage ledges.
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Estimates of the B-D transition temperature for alpha iron for the three modes of dislocation emission from the crack tip in a scenario of arrest of a brittle cleavage crack travelling up a temperature gradient (after reference 7 courtesy of Taylor and Francis Ltd.)




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