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Toughening of polymethylmethacrylate (PMMA) is achieved, as usually done for many brittle polymers, through the
addition of a dispersed rubbery phase. In this work, the effect of dispersed phase content and dimensions on the
fracture toughness has been examined at varying strain rate. It has been assumed that the energy dissipation
occurring during fracture may be separated into two contributions, the first associated with generalized yielding
occurring at the crack tip and the second due to deformation processes highly localized at the crack tip, in which new
surfaces are formed. Results indicate that toughness is related to energy dissipation mechanisms taking place in the second zone.