SynopsisThe tensile behavior a t 20°C of polystyrene-glass-bead composites has been studied a t several glass concentrations. To gain insight into the role of interfacial adhesion, the bonding between glass and polystyrene was varied by using different silane coupling agents. In contrast to the elastic behavior, the crazing behavior of the composites was found to be considerably affected by the degree of interfacial adhesion. This is explained by means of the different mechanisms for craze formation a t adhering and nonadhering glass beads, respectively. Furthermore, it was found that both elastic and crazing behavior of the composites are influenced by the glass bead concentration.

The craze formation in polystyrene-glass bead composites subjected to a uniaxial tension has been investigated. To gain insight into the role of interfacial adhesion, the bonding between glass and polystyrene was varied by using different silane coupling agents. The distributions of several craze formation criteria around an isolated adhering glass sphere in a polystyrene matrix have been computed with the aid of the finite element analysis. It was found that the mechanism for craze formation is fundamentally different for adhering and non-adhering glass beads. In the case of excellent interfacial adhesion the crazes form near the poles of the beads in regions of maximum dilatation and of maximum principal stress. With poor interfacial adhesion the crazes form at the interface between pole and equator. It is proposed that in the latter case craze formation is preceeded by dewetting along the phase boundary.

The competition between craze formation and shear band formation at small glass beads embedded in matrices of glassy polymers has been investigated. This has been done by performing constant strain rate tensile tests over a wide range of strain rates and temperatures, and examining the deformation pattern formed at the beads with a light microscope. The glassy polymers under investigation were polystyrene, polycarbonate, and two types of styrene-acrylonitrile copolymer. It was found that besides matrix properties, strain rate and temperature, the degree of interfacial adhesion between the glass beads and the matrix also has a profound effect on the competition between craze and shear band formation: at excellently adhering beads craze formation is favoured, whereas at poorly adhering beads shear band formation is favoured. This effect is caused by the difference in local stress situation, craze formation being favoured under a triaxial stress state and shear band formation under a biaxial stress state. The kinetics of crazing and shear deformation have also been studied, using a simple model and Eyring's rate theory of plastic deformation. The results suggest that chain scission may be the ratedetermining step in crazing but not in shear deformation.

Craze and shear band formation at poorly adhering glass spheres in matrices of glassy polymers are known to be preceded by the formation of a curvilinear interfacial crack between sphere and matrix. In this study the axisymmetric finite element method has been used to analyse the stress situation near the tip of a curvilinear interfacial crack formed between a rigid spherical inclusion and a polymer matrix upon an applied uniaxial tension. Important factors that determine the stress state near the crack tip were found to be the crack length, the orientation of the crack tip with regard to the tension direction and the extent of interfacial slip between the inclusion and matrix. The results of the analyses were compared with the physical reality of craze and shear band formation at poorly adhering glass spheres. Reasonable agreement was found with respect to both the maximum interfacial crack length that can be reached until a craze or shear band forms at the crack tip and the planar orientation of craze growth perpendicular to the direction of the major principal stress.