P. Stanley

Tensile fracture of doubly-convex cylindrical discs under diametral loading

Doubly-convex cylindrical gypsum discs have been fractured under the action of two diametrically opposed in-plane forces. The disc diameter was constant throughout the test series. The ratio of cylinder length to diameter ranged from 0.06 to 0.30; the ratio of cylinder diameter to radius of curvature of the disk faces was varied from 0 to 1.43. The fracture loads obtained have been correlated with stress data obtained from the photoelastic analysis of Pitt et al. An empirical equation, valid for any brittle material, relating the tensile strength of the material to the fracture load and dimensions of a doubly-convex disc has also been developed.

Theoretical considerations of the influence of polymer film coatings on the mechanical strength of tablets

A theoretical analysis of the influence of polymer film coatings on the mechanical strength of tablets has been undertaken. Making some basic assumptions, the theory predicts that neither the thickness of the substrate nor that of the coating has any influence on which fractures first, this being solely determined by the ratio of the tensile fracture strength to the Youngs modulus for the two materials. Such a finding suggests that in practice for film‐coated tablets the substrate will usually fracture before the coating. Simple measurements of maximum failure loads are of little value in assessing the influence of film coating on the mechanical strength of tablets.

THE TENSILE FRACTURE STRESS OF CAPSULE - SHAPED TABLETS

A recent paper by Gold et al (1980) has suggested that the determination of the mechanical strength of capsuleshaped tablets can be undertaken by what would be described, in the field of material science, as a flexure test. This type of test has been proposed for conventional tablets by David & Augsburger (1974). A feature of such a test is that under the correct conditions of loading, the specimen will be subjected to a pure longitudinal tensile stress along a line on the surface opposite to that on which the load is applied; see Fig. 1. The loading and support point details are important in this form of test; correct conditions are readily recognizable by the occurrence of a clean break across or near to the loading line of the specimen. The purpose of the present communication is to show how it is possible to determine the tensile strength of capsuleshaped tablets from this type of flexure test. In general, for a beam specimen subjected to bending, the tensile fracture stress ur can be calculated from the following expression

A modified Weibull treatment for the analysis of strength-test data from non-identical brittle specimens

Powder compacts (e.g., pharmaceutical tablets) manufactured on commerically available machines are not strictly identical but show inevitable variability in their weights, thicknesses and compaction pressures. Consequently, the variability in fracture-stress data obtained from such brittle specimens is greater than that due to the inherent strength variability of the material itself. A modified Weibull analysis has been developed so that a more accurate estimate of the inherent variability of the mechanical strength of the material can be derived from test data obtained from commercially produced compacts; its application is illustrated.

EFFECTS OF COMPACTION VARIABLES ON POROSITY AND MATERIAL TENSILE STRENGTH OF CONVEX-FACED ASPIRIN TABLETS

Abstract— The porosity and tensile strength of convex‐faced aspirin tablets formed under a compaction pressure in the range 40–320 MPa and at punch velocities in the range 0.008 to 500 mm s−1 have been determined. The material tensile strength, σf, was calculated from the observed fracture load, Ps, using the equation of Pitt et al (1988):

The graphical interpretation of fracture load data for doubly-convex cylindrical discs

A set of graphs is developed for the determination of the material tensile strength of a brittle, doublyconvex cylindrical disc from the disc dimensions and the fracture load in in-plane diametral compression. Graphs are also presented relating the normalized geometric variables of the disc and giving the normalized volume of the disc in terms of the geometric parameters.

An assessment of systematic errors in beam tests on brittle materials

In deriving the unit volume strength of brittle materials from beam bend test data, the shear and compressive stresses in the beam specimens are usually ignored. Depending on the span-to-depth ratio of the beam, these omissions may give rise to significant errors in the unit volume strength value, and in component failure probabilities derived from this strength value. In this paper, the effects of these systematic errors are considered for the 3-point beam as a function of span-to-depth ratio and for different values of the Weibull modulus. The relative errors are found to be small and conservative provided a sufficiently large span-to-depth ratio is used and the compressive/tensile strength ratio of the material is relatively high.

Assignment errors in histograms with equal class intervals of arbitrary width

It is shown that significant distortions can occur in histograms constructed with a class interval of arbitrary width, due to the inclusion (or omission) of readings in a class interval which do not (do) properly belong to that interval. Unless the class interval width is at least ten times greater than the reading interval the proportion of unrepresentative readings may exceed 10%. In any event caution is required in the interpretation of a histogram in which this effect is present.

A MODIFIED WEIBULL ANALYSIS FOR FRACTURE TEST DATA OF TABLETS WHICH VARY IN VOLUME AND DENSITY

The inevitable flaws in brittle materials give rise to a wide variation in the mechanical strength of nominally identical test specimens, This variability can be characterised by statistical techniques; the analysis m s t widely adopted for this purpose is that proposed by Weibull (1939). There is evidence (STANLEY and NEWTON, 1978) that the Weibull-based Statistical model Satisfactorily represents the strength variability of nominally identical right circular Cylindrical powder compacts, as measured by the diametral Compression test. However, powder compacts manufactured on conventional production machines exhibit inconsistences in their weights, thicknesses and forming pressures in addition to the strength variability inherent in the material. These additional forms of variability will give rise to an increase in the observed strength variability of such compacts over and above that attributable to the material itself.