It is important to determine the distribution of plastic strain at the tip of a crack, in order to clarify the mechanisms of fracture. In this paper, a method is presented for measuring local plastic strain by the X-ray microbeam diffraction technique.
The total misorientation β, which is calculated from the tangential breadth of the spotty diffraction image in the stationary photography, is closely correlated to the equivalent plastic strain εp
. This correlation is independent of the deformation modes; tension or torsion.
The total misorientation can not be measured for a heavily deformed material because the diffraction image becomes a continuous ring. The radial breadth of a diffraction ring is adopted as a measure of the change in diffraction image. Measurement of this quantity is acculately performed by using a specially designed X-ray micro-camera with the film- and specimen-swinging devices. The half-value breadth B
of a diffraction intensity profile, representing the radial breadth of a diffraction ring, is also uniquely correlated to the equivalent plastic strain εp
independently of the deformation modes.
The correlation between εp
and β or B
obtained above offers us a method for plastic strain measurement in a localized region by the X-ray microbeam diffraction technique. The technique based on the measurement of the half-value breadth may particularly suit for the investigation of the fracture process at the tip of a notch or a crack, because of its ability for measuring the large plastic strain up to about 3.6.
The utility of this technique is confirmed by applying it to the measurement of the plastic strain distribution at the neck in a stretched specimen.