The strength of bilayered tablets

The tensile strength of model materials (dicalcium phosphate dihydrate, microcrystalline cellulose and pregelatinised starch) compacted to form tablets in the form of beams consisting of two layers of equal thickness has been determined by three-point loading. The values of the tensile strength of the materials were sometimes higher and sometimes lower than the tensile strength of beams of the same thickness composed of a single material. Correction of the values for the tensile strength of the layered beams for the differences in the elasticity of the materials in the layered tablets failed to correct for these differences, as did considering the layered beams as beams of half thickness. For a layered tablet with pregelatinised starch at the bottom and microcrystalline cellulose at the top, the value of the tensile strength recorded appeared to be that of the microcrystalline cellulose as the fracture propagated across the boundary between the layers and into microcrystalline cellulose. What appeared to be the important factor was the way the failure of the beam crossed the interface between the two layers.