Dislocation structures ahead of advancing cracks
- PDF / 1,721,182 Bytes
- 7 Pages / 594 x 774 pts Page_size
- 3 Downloads / 243 Views
I.
INTRODUCTION
P R E D I C T I V E modeling of a material's response to load requires information on the microstructural response of the material. In particular, information on the microstructural development which accompanies propagating cracks and the crack propagation mode is required. One way of obtaining detailed information on the crack tip structure is the in situ transmission electron microscope (TEM) deformation technique, where the crack response to an applied load can be directly observed at high magnifications. Several research groups are now employing this technique to investigate various microscopic aspects of fracture. The experiments and theory pioneered by the late Dr. S.M. Ohr and co-workers [~-9] were instrumental in the development of the now popular dislocation-free zone (DFZ) theory of fracturefl 'gj This theory is primarily based on microscopic observations in ductile materials that have shown the plastic zone ahead of a crack to be comprised of a linear array of dislocations separated from the crack tip by a region free of dislocations. The length of this DFZ is material-dependent but has been reported to be typically on the order of micrometers. [1-6,8] The dislocation structures that we have observed during in situ TEM deformation studies in a number of metal tl~ and ceramic systems have not been supportive of the DFZ theory. Dislocations have been observed to be emitted from the crack tip and to form stable dislocation arrangements vicinal to the crack. In Section II of this paper, we briefly outline the general DF-Z theory (a detailed review was presented by Ohr[7]), then consider the effect of introducing an obstacle (such as a sessile dislocation) in the region of the plastic zone on the length of the DFZ, and discuss artifacts inherent to TEM specimens as they relate to the theory. The geometry of the straining stage and samples used in the present in-
D.K. DEWALD, Research Assistant, T.C. LEE, Visiting Assistant Professor, I.M. ROBERTSON, Assistant Professor, and H.K. BIRNBAUM, Professor, are with the Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801. This paper is based on a presentation made in the symposium "Interface Science and Engineering" presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26-29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee. METALLURGICAL TRANSACTIONS A
vestigation are considered in Section III along with the experimental results. In Section IV, we discuss the implications of our results on the theory before summarizing our conclusions in Section V.
II.
THEORY
A. General Outline of the Dislocation-Free Zone Theory The DFZ model for the dislocation structure ahead of a crack is based on a modification of the Bilby, Cottrell, and Swinden tl~] (BCS) model of crack tip plasticity. The model balances the stresses exerted on dislocations which form a coplanar pileup ahead of a Mode III crack in
Data Loading...
