Strongly nonlinear surface acoustic waves (SAWs) with shock fronts were used to study impulsive fracture in anisotropic silicon crystals and isotropic fused quartz. With this method, spatially localized dynamic fracture was studied without an artificial pre-cracking. SAWs allow the investigation of mode I tensile stress and mode II shear stress fracture. For silicon, the difference between the measured critical fracture stress of 1-2 GPa and the theoretical tensile strength of 22 GPa is discussed in terms of Griffith's approach. However, due to the biaxial stress field applied with SAWs and the low ideal shear stress of 6.8 GPa, the nucleation process may not be uniaxial and purely tensile in silicon. In fused quartz, nucleation occurred via tensile crack opening at the surface and propagation into the bulk proceeded at an angle of 55 degrees-60 degrees to the surface normal in the direction of SAW propagation. This behavior could be described theoretically by calculating the energy release rate as a function of direction and assuming that stable tip propagation is obtained under pure mode I conditions. (c) 2005 Elsevier B.V. All rights reserved.