Experimental analysis of near-crack-tip plastic flow and deformation characteristics (I): Polycrystalline aluminum

The in-plane components of the Almansi strain tensor were measured incrementally, at the microstructural length, near a stationary crack-tip in a four-point-bend specimen of polycrystalline aluminum. The strains were measured with a finite-deformation laser-moire interferometer. Measurements were made on the spatial (Eulerian) configuration to follow the strain history approximately of material points around the crack tip. The observed deformation characteristics are found to be well represented by the HRR-type field (Hutchinson, J. W., 1968. Singular behavior at the end of a tensile crack in a hardening material. J. Mech. Phys. Solids 16, 13-31; Rice, J. R., Rosengren, G. F., 1968. Plane strain deformation near a crack tip in a power-law hardening material, J. Mech. Phys. Solids 16, 1-12) of plastic orthotropy with a single parameter: J-value (Rice, J. R., 1968a. A path independent integral and the approximate analysis of strain concentration by notches and cracks. J. Appl. Mech. 35, 379-386), related to the applied loading. Despite strain fluctuations from one grain to another, the average radial dependence of strain closely matches the HRR-type singularity field within the observation window, and for the full range of thr applied load. The incremental strain measurement also reveals the flow characteristics near the crack-tip. Using the Mohr strain plane representation, it is found that the incremental slip directions approximately follow the direction of the normal to the assumed yield surface. It is also found that there is a zone of appreciable size near the crack tip. in which the slip-line field is close to that of Prandtl (the tested material has a very low hardening rate). The whole field grew in an essentially self-similar way. Since the tested aluminum alloy showed a degree of plastic orthotropy, the incremental slip magnitudes were quite different from the isotropic plasticity case. Experimental results were compared to a two-dimensional (2-D) finite element analysis, employing Hill's quadratic yield function. Although this study relied on surface measurements, the general features of the measured in-plane deviatoric strain field were close to those of the plane-strain simulation. Details of the discrepancies are also discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.