In a fracture experiment of polymers, the total fracture work usually consists of an essential part associated with the plastic deformation at the crack tip and a nonessential part related to the viscous relaxation in the background. In this article, the essential fracture work in nylon 6-silicate nanocomposites is measured through a series of double-notched-sample (DNS) tests. The samples are of different silicate contents and crystallization conditions. As the cooling rate rises, the essential work is nearly constant while the total fracture work increases considerably, which should be attributed to the change in effective viscosity. (C) 2004 Wiley Periodicals, Inc.