EXPERIMENTAL-MEASUREMENT OF THE NEAR-TIP STRAIN FIELD IN AN IRON SILICON SINGLE-CRYSTAL

EXPERIMENTAL RESULTS are presented for the plastic deformation field n ear a crack (200 mum wide notch) tip in an iron-3% silicon single crys tal. The specimen was loaded in four point bending and the measurement s were made at zero load after extensive plastic deformation had occur red. Results are given for a crack on the (011) plane with its tip alo ng the [011BAR] direction. The surface deformation field was measured using moire microscopy and a grating on the specimen surface. The in-p lane Almansi strain components have been obtained by digitally process ing the moire fringes. A well-structured asymptotic field has been fou nd at a distance of 350-500 mum from the notch tip, where the maximum plastic strain is about 9%. The asymptotic field is observed to be com posed of many distinct angular sectors. Three (six symmetric) of these sectors are found to have approximately constant strains. In a fourth (two symmetric) sector, the surface strains are approximately 1/r sin gular. Between these sectors there are interconnecting transition sect ors. The location of the stress state on the yield surface and the act ive slip systems in each sector are identified by assuming that the pl astic strain rates are normal to a Schmid law yield surface. The slip systems identified in this manner show excellent agreement with direct observations of the slip texture on the surface and dislocation etch pits in the interior of the specimen. The experimental strain measurem ents also show that the constant strain sectors are regions in which u nloading occurs. Because of this unloading, the crack tip stress and d eformation state is substantially different from an HRR type field whi ch assumes proportional loading. This strong nonproportional loading i s thought to be caused by the presence of material anisotropy. The non proportional loading also provides a large amount of crack tip shieldi ng that is evidence of a toughening mechanism that results from the pr esence of material anisotropy.