The mechanical properties of compressed beam specimens of microcrystalline cellulose (Avicel pH 101) have been assessed in terms of the tensile strength (sigma t), Young's modulus (E) and the following fracture mechanics parameters: the critical stress intensity factor (KIC), the critical strain energy release rate (GIC) and the fracture toughness (R). Increase in the compaction pressure used to form the beams resulted in compacts with higher values of tensile strength, Young's modulus, KIC, GIC and R, indicating that the compacts became less brittle as their porosity decreased. Extrapolation of the values of sigma t, E, KIC, GIC and R to provide values at zero porosity indicated that the material had values of 30 Nm m-2, 0.0103 GPa, 1.21 MN m-3/2, 1.98 x 10(2) Nm-1 and 2.19 x 10(3) Nm-1, respectively. These provide a range of values whereby a fuller characterization of the mechanical properties of pharmaceutical materials can be made.

The critical stress intensity factor KIC has been determined for compacts prepared at different pressures from both Avicel PH101 and sorbitol "Instant', by means of a double torsion method. This method avoids the need for displacement measurement and the introduction of notch into the specimens, both of which present problems with pharmaceutical materials. Measurements of fracture loads for such specimens were reproducible. The values of KIC derived for both materials were found to increase linearly with increase in compaction pressure and linearly with decrease in specimen porosity. Extrapolation of the latter relationship to zero porosity gave values of 1.81 and 0.69 MN m-3/2 for the KIC of Avicel PH101 and sorbitol "Instant', respectively.

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