Comparing failure tests on pharmaceutical tablets: Interpretation using experimental results and a numerical approach with cohesive zone models

The mechanical strength is an important quality attribute of pharmaceutical tablets. It can be determined using different failure tests like the Brazilian test or the three-point bending test. Nevertheless, literature shows that different failure tests often give conflicting values of tensile strengths (TS), which are generally calculated using the maximum stress criterion as a failure criterion. This work started from the hypothesis that these discrepancies are in fact due to the application of this criterion which is not suited to study pharmaceutical tablets, first due to heterogeneity of the stress distributions during the tests and second due to the quasi-brittle nature of pharma-ceutical tablets. As an alternative, a numerical fracture criterion which is known to be well-suited for quasi -brittle solids (cohesive zone model, CZM) was used and calibrated using experiments. Using this approach, the breaking forces obtained numerically were shown to be in fair agreement with the experimental ones. Above all, the numerical results made it possible to catch the trends when comparing the different failure tests one to another. Especially, the model made it possible to retrieve the factor 2 between the TS obtained by three-point bending and by diametral compression found in the literature.

Automated summary

The mechanical strength of pharmaceutical tablets is crucial and can be tested using different failure tests. However, these tests often yield conflicting values of tensile strengths (TS) due to the inappropriate application of the maximum stress criterion. This study proposes an alternative approach using a numerical fracture criterion (CZM) that is suitable for quasi-brittle solids. The numerical results show good agreement with experimental data and enable comparisons between different failure tests. Notably, the model explains the factor of 2 difference in TS obtained by three-point bending and diametral compression tests observed in literature.