Microscale characterization of granular deformation near a crack tip

Abstract

This paper presents a study of microscale plastic deformation at the crack
tip and the effect of microstructure feature on
the local deformation of aluminum specimen during fracture
test. Three-point bending test of aluminum specimen was conducted
inside a scanning electron microscopy (SEM) imaging system.
The crack tip deformation was measured in situ utilizing SEM imaging
capabilities and the digital image correlation (DIC)
full-field deformation measurement technique. The microstructure feature
at the crack tip was examined to understand its effect on
the local deformation fields. Microscale pattern that was suitable
for the DIC technique was generated on the specimen surface
using sputter coating through a copper mesh before the fracture
test. A series of SEM images of the specimen surface were
acquired using in situ backscattered electronic imaging (BEI) mode
during the test. The DIC technique was then applied to these
SEM images to calculate the full-field deformation around the
crack tip. The grain orientation map at the same location
was obtained from electron backscattered diffraction (EBSD), which
was superimposed on a DIC strain map to study the
relationship between the microstructure feature and the evolution of
plastic
deformation at the crack tip. This approach enables to track
the initiation and evolution of plastic deformation in grains
adjacent to the crack tip. Furthermore, bifurcation of the
crack due to intragranular and intergranular crack growth was observed.
There was also localization of strain along a grain boundary
ahead of and parallel to the crack after the maximum load was
reached, which was a characteristic of Dugdale–Barenblatt
strip-yield zone. Thus, it appears that there is a mixture of effects
in the fracture process zone at the crack tip where the
weaker aspects of the grain boundary controls the growth of the crack
and the more ductile aspects of the grains themselves
dissipate the energy and the corresponding strain level available for
these processes through plastic work.

Journal of Materials Science;

Volume 46, Number 20, 6596-6602

http://www.springerlink.com/content/k516648156343122/