Effect of Crack Blunting on Subsequent Crack Propagation
- PDF / 371,026 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 51 Downloads / 231 Views
*Department of Physics, Washington University, St. Louis, MO 63130-4899 "**NationalInstitute of Standards and Technology, Gaithersburg, MD 20899
ABSTRACT Theories of toughness of materials depend on an understanding of the characteristic instabilities of the crack tip, and their possible interactions. In this paper we examine the effect of dislocation emission on subsequent cleavage of a crack and on further dislocation emission. The work is an extension of the previously published Lattice Greens Function methodology[l, 2, 3]. We have developed a Cavity Greens Function describing a blunt crack and used it to study the effect of crack blunting under a range of different force laws. As the crack is blunted, we find a small but noticeable increase in the crack loading needed to propagate the crack. This effect may be of importance in materials where a dislocation source near the crack tip in a brittle material causes the crack to absorb anti-shielding dislocations, and thus cause a blunting of the crack. It is obviously also relevant to cracks in more ductile materials where the crack itself may emit dislocations.
INTRODUCTION When a sharp crack in a material is loaded until it deforms plastically at the crack tip, two fundamentally different modes of deformation can occur. The crack may propagate (possibly leading to cleavage of the specimen), or it may emit a dislocation. In the first case the material is said to be intrinsically brittle, in the second it is intrinsically ductile. Even for single crystals, many other aspects of the microstructure of the material influence the behavior of the material, such as dislocation activity in the material surrounding the crack and shielding of the stress fields by the dislocations present in the vicinity of the crack. In this paper we will however concentrate on the intrinsic behavior of the crack. The intrinsic behavior of the crack can be predicted by comparing the critical stress intensity factors necessary for cleavage and for dislocation emission. The cleavage is well described by the Griffith criterion[4], but several criteria have been proposed for the dislocation emission. Rice[5] has proposed an emission criterion given by a welldefined solid-state parameter, the unstable stacking fault energy (Ts), characterizing the barrier to displacement along the slip plane. In this model the emission criterion becomes a "balance" between two energies: The surface energy (%-) and the unstable 237 Mat. Res. Soc. Symp. Proc. Vol. 408 01996 Materials Research Society
stacking energy. Zhou et al.[2, 6] extended this model to include the energy of the small ledge at the end of the crack, and showed that for most physically realistic force laws this term dominates the energetics, leading to an emission criterion containing both -Ysand -y, and a ductility criterion that is independent of Y,. These ductility criteria all assume that the crack is sharp at the atomic level. However, emission of a dislocation will change the local geometry of the crack, possibly changing both the emiss
Data Loading...
