Breaking pharmaceutical tablets with a hole: Reevaluation of the stress concentration factor and influence of the hole size

Abstract

Mechanical strength is an important property for pharmaceutical tablets. Its study using the theory of linear elastic fracture mechanics has been introduced in the pharmaceutical field through the Brittle Fracture Index (BFI). This index is based on the stress concentration factor (SCF) and contradictory results have been published in the pharmaceutical literature about the value of the SCF during the diametral compression of a disc with a hole. In this work, thanks to the use of numerical simulations (FEM) and analytical results, the value of the SCF was proved to be equal to 6. The result was also applicable for the case of the flattened disc geometry that was introduced in a previous work. The value of the SCF is found to be nearly independent of the hole size if the ratio between the hole and the tablet diameterswas lower than 0.1. Nevertheless, experimental results presented in this paper show that the load needed to break a compact varieswith the hole size. This influence is due to the change in the stress distribution around the hole when the hole size is changing. Criteria such as the average stress criterion, which takes into account the stress distribution, made it possible to explain the influence of the hole on the breaking load.