In this study, a transparent acrylonitrile-butadiene-styrene polymer alloy was tested in a range of biaxial stress states and a yield locus was generated. The onset of microscopic damage was detected by in situ light transmission measurements. The macroscopic yield locus followed a linear behavior on an octahedral shear stress vs, mean stress plot and the onset of microscopic damage was dependent on both shear and mean stresses. Our findings indicate that macroscopic ductility arises from damage in the polymer at two distinct length scales. On the scale of the rubber particle (0.1-0.2 um), damage occurs via particle debonding from the matrix. At a length scale intermediate between the particle size and macro scale, damage occurs through the formation of micro crack arrays with a well-defined pattern.