Intersections of coarse shear bands in polystyrene

Abstract

Three types of intersection between coarse shear bands are described. The first is the most numerous and happens in a single compression. The two bands shear each other. The reason why a new band can be sheared by an existing old band seems to be that it is easier for the new band to propagate along the striation direction of the old band rather than along its own direction. The merging of striations, the stretching of molecules, and the displacement of bands are described in some detail. The second type of intersection can be created by two mutually perpendicular compressions. As in the first type, the material inside the intersection is sheared twice, first by the old band and then by the new band. However, unlike the first, the two shears are in opposite directions in the second type of intersection. Both intersections cause sufficient disturbances so that microcracks and new shear bands (both coarse and fine bands) are generated to relieve some stresses. A third type of intersection which takes place also during two mutually perpendicular compressions has a small angle of intersection. The two shears of the material in the intersection are also in nearly opposite directions. However, the disturbance is so small that no microcracks or new shear bands are generated. As a result the bands offer little resistance to each other at the intersection of the third type. For all three types of intersection, the displacement of one band corresponds approximately to the shear strain of the other intersecting band. These observations suggest that molecular ordering or directional defects exist in the coarse shear bands in polystyrene.