The implications of the determination of the mechanical strength of powder compacts containing a pre-formed hole

Abstract

Powder compacts of two pharmaceutical model materials, microcrystalline cellulose (MCC) and α-lactose monohydrate (LM), have been prepared, at approximately the same porosity, in the form of discs with and without a central hole. The ratio of the inner to outer diameters for the ring compacts was 0.088. To achieve the equivalent porosities, higher pressures were required for the discs in which a central hole was formed. The compacts were subjected to diametral compression, and the force to cause failure was recorded. The values of the tensile stress to cause failure in the discs with holes were derived from the approach of Durelli and Lin [J. Appl. Mech. 53 (1986) 213]. For MCC, the values of tensile strength for compacts without a hole were higher than for those with a hole, but the opposite was true for LM. The approach also allowed the calculation of values for the compressive and tensile stress at the inner and outer boundaries of the ring. Values for both the ratio of the inner tensile stress and the inner compressive stress to that of the tensile strength of a compact without a hole clearly differed from those suggested by Hiestand et al. The value for the stress concentration factor is an essential feature of the calculation of a factor used by these authors in the prediction of the compaction properties of powders, the “brittle fracture propensity” (BFP). These differences, coupled with the incorrect method of calculating the value of the tensile stress of a compact with a central hole from the breaking force, indicate that the derivation of the BFP has no theoretical basis.