Comparison of different failure tests for pharmaceutical tablets: Applicability of the Drucker–Prager failure criterion

Abstract

Several tests can be used to study the strength of pharmaceutical tablets. Equations exist in the literature to transform the failure force measured into a failure stress which can be considered as a characteristic of the strength of the material. For each failure test, the stress state at failure is different, and as a consequence, the failure stresses obtained are also different. It would thus be interesting to find a failure criterion to unify the different results. In this study four different tests were performed on pharmaceutical compacts of various densities: diametral compression, three-point flexure, biaxial flexure and uniaxial compressive tests. The Drucker-Prager criterion was tested as a possible fracture envelope. The results showed that this criterion is well suited to explain the failures obtained by diametral compression, three-point flexure and biaxial flexure. Nevertheless, for the uniaxial compressive test, the use of this criterion led to a significative underestimation of the experimental value of the failure stress. As a consequence, the Drucker-Prager criterion must be used with caution and is not able to explain all the failures that can occur in a pharmaceutical compact.