Determination of the tensile strength of elongated tablets

The tensile strength of a tablet is an important attribute as the tablet needs to be mechanically strong enough to withstand further handling such as film-coating, packaging, transport and end-use by the patient, but to be weak enough to break apart in the human body and so release its contents. Mathematical solutions to calculate tensile strength are known for flat-faced and convex-faced circular tablets. The work described here aims to extend this knowledge to capsule-shaped and oval-shaped tablets by means of 2-dimensional (2D) and 3-dimensional (3D) Finite Element Analysis (FEA). The stress analysis showed that as the tablet was elongated from a standard circular tablet to become that of an extended tablet shape, the peak principal tensile stress reached a limiting value. This limiting value was reached as the ratio of the length to width dimensions exceeded 1.7:1, which encompasses most modern pharmaceutical tablets. In addition the stress analysis shows that this limiting value approximated to 2/3 that calculated for circular tablets. Hence for a convex-faced elongated tablet the calculation for tensile strength generated by Pitt et al. [1] would become: sigma(t) = 2/3 (10P/pi D-2(2.84t/D - 0.126t/W + 3.15W/D + 0.01)) where sigma(t) is the tensile strength, P is the fracture load, D is the length of the short axis, t is the overall thickness and W is the wall height of the tablet, as shown in Fig. 1c. The solution was then checked by applying it to commercial tablets of differing shapes to demonstrate its utility. (C) 2011 Elsevier B.V. All rights reserved.