Yielding:

PhotoStress was used to study the post-yield strain behavior on "notched" tensile test specimens. Bottom photo shows a broad plastic strain field developing over most of the surface of one test sample, while the upper photo shows initial yielding occurring in the form of Lueder's lines (slip planes) in the other.

Relations between a notch and a crack in an aluminum part under tension (using the MNC-1 PhotoStress Monochromator)