The load necessary to fracture lactose monohydrate tablets under diametral compression has been determined using an Instron physical testing instrument, so that true tensile failure was obtained in all cases, leading to improved reproducibility. Four ranges of tablet thickness were examined at 12.7 mm diameter. All tablets gave a linear increase of breaking load with compaction pressure up to 310 MN/m2. Expressing the tablet strength as the breaking load gave a separate regression line for each range of tablet thickness, whereas the use of tensile strength provided a common regression line, within given statistical limits, for all but the lowest range of tablet thickness. The fact that such a correlation is possible shows that the tensile strength is a property of the "as compacted" material and provides a new and useful parameter to maintain constancy of properties when tablet size is changed.Pharmaceutical tablets, when compressed diametrically, as in any of the tablet crushing tests normally applied, may fracture in any of the five ways shown in the upper part of Fig. 1. Failure by any of the first four mechanisms (a-d) will lead to greater variability in the crushing strength measurements than will failure by mechanism 1 (e). This is purely tensile fracture, giving a straight crack dividing the tablet into two semi-circular parts. It occurs only when the force applied to break the tablet is carefully controlled: in such circumstances the stress distribution within the tablet is calculable (Frocht, 1948) and is as shown in the lower part of Fig. 1. The vertical stress component my varies along the vertical diameter, as does the maximum shear stress T. The horizontal stress component mx, however, is virtually constant along the vertical diameter and tends to split the tablet into two equal halves. The value of this uniform horizontal stress is, at failure, the ultimate tensile strength of the "as compacted" material forming the tablet, and is given by 2P nDtwhere P is the load necessary to cause fracture, D is the tablet diameter and t is its thickness. The various versions of the crushing strength tests have been compared by . To ensure correct load application, it is sometimes necessary to have packing pieces between the tablet and the loading platens, and experiment appears to be the only method of assessing the nature and quantity of padding required. Recognition of tensile failure is, however, readily made by inspection of the tablets after fracture.
