SOME OBSERVATIONS OF COARSE SHEAR BANDS IN POLYSTYRENE

On the surface parallel to the shear direction, coarse bands appear as ridges, usually of isosceles cross-section with a height-to-base ratio of about 1/10. They split and join with thickness conservation but a corresponding change of cross-section. The shear strain along the same band is almost a constant with a most probable value of 1.8 but a spread between 1 and 3 among different bands. The bands terminate by reducing the shear strain to zero or by converting themselves to a bundle of fine bands at obstacles. Upon annealing the height of the band ridge is reduced until it disappears. When a banded surface is polished, the band pattern reappears upon either annealing or recompression. However, in the former case, the bands are valleys with reverse shear strain but in the latter they are ridges with almost no strain. Similar behaviour occurs on the polished cut surfaces in the middle of the specimen after annealing. On the side surface, the bands are wavy and they are steps rather than ridges. They also split and join with thickness conservation and they terminate freely. Based on these characteristics, the propagation of coarse bands can be viewed as the motion of macroscopic Volterra dislocations. © 1979 Chapman and Hall Ltd.