  In this work we present the design of a new test geometry inspired by the
Tapered Double Cantilever Beam (TDCB) specimen that is shown to provide an
improved characterization of the fracture properties of brittle solids. First,
we show that our new design results in an exponential increase of the specimen
compliance with crack length, leading to an extremely stable crack growth
during the test. We determine an analytical description of this behavior, which
provides a simple procedure to extract the fracture energy without depending on
finite element calculations. Validation tests are done on
polymethylmethacrylate (PMMA) specimens. We use both finite element simulations
and our analytical model to interpret the data. We find a very good agreement
between the toughness determined by both methods. The stable nature of crack
growth in our improved TDCB specimens results in a precise control of the crack
speed. This feature is employed to go one step further and characterize the
variations of toughness with crack speed. We propose an original optimization
procedure for the determination of the material parameters characterizing the
kinetic law describing the toughness rate dependency. Overall, the approach
proposed together with the newly designed test geometry offer unprecedented
possibilities for the full and accurate characterization of the fracture
behavior of brittle materials such as rocks, sandstone, mortar etc.
