FRACTURE OF POLYMERS - CRAZING, SHEAR BAN DS AND TOUGHENING MECHANISMS

Glassy polymers are essentially rather brittle materials. Their ruptur e is often due to craze propagation. A craze is a type of crack tip, p eculiar to polymers. A craze does not consume much energy, and therefo re, if polymers are used for structural applications, they are usually modified to behave in a more ductile manner and to develop large dama ged zones before breakage. The techniques used to toughen polymers are various and depend on the nature of the material. Including more or l ess small rubber particles in a glassy polymer matrix is one of the ve ry common way to achieve high toughness. The particles may be made fro m pure rubber, or may have an inner structure like core-shell, '' oign ons '' or '' salami ''. The toughening mechanisms of this type of poly mer blends have given rise to a large number of experimental research, and recently it has been experimentally shown that in some cases, rub ber particles cavitate and that this cavitation is the precursor of th e other damage mechanisms in the material. A first theoretical analysi s of the cavitation in pure rubber has been proposed, and showed that particles may cavitate even under low mean tensile strain. On the othe r hand, in many industrial polymers, core-shell particles are used rat her than pure rubber particles. Some experimental work on this type of material has been done, but the mechanical theoretical analysis of th is type of material has still to be done. In this paper, the mechanica l concept of damage (the decrease of the modulus) will be used to char acterize the changes in the inner structure of rubber toughened polyme thylmethacrylate (RT-PMMA) during the deformation process. During the same experiment, the unrecoverable volume changes, the Poisson's ratio , the lost energy and the light transmission and diffusion were record ed. This paper reviews the current knowledge on crazes, and points out some recent research on the damage mechanisms in rubber toughened pol ymethylmethacrylate.