Stress Distribution in Diametral Compression Tests

Abstract

In an attempt to find an explanation for an experimentally found strong connection between the extension of the loaded area and the load required for fracture, an analysis of the diametral compression test was carried out by means of the finite element method. The distribution of the stresses in a quadrant of a circular specimen section was studied for varying contact surface but at a given load distribution within this surface. Below a limit value for the width of the contact surface tensile stresses appeared peripherically outside the contact area, which had a magnitude exceeding those stresses which are centrally located and should normally initiate fracture. For that reason, peripheral fracture initiation must be regarded as a possibility. Only marginal importance could be attached to changes in the extension and magnitude of the central tensile stresses, as compared with the potential impact of the peripheral tensile stresses.

Reevaluation of the diametral compression test for tablets using the flattened disc geometry

“…The highest tensile strains are located slightly under the contact point. For the tensile stresses, there is a clear 3D effect as already mentioned in other publications (Ehrnford, 1981;Li and Wong, 2013). The maximum tensile stress is located on the surface of the compact and is not centered.…”

“…It thus supposes that there is no 3D effects. Nevertheless, 3D effects were demonstrated by both analytical developments (Wijk, 1978) and numerical simulation using the Finite Element Method (FEM) (Ehrnford, 1981;Li and Wong, 2013). All these studies show that, if the contact surface between the cylinder and the platen is too small, high tensile stresses develop on the outer surface of the compact away from the central axis.…”

“…The value and position of these stresses depend on the contact area, on the thickness of the compact and also on Poisson's ratio. The quantitative determination of these stresses is thus complicated, but as stated by Ehrnford a long time ago, due to these stresses, "peripherical fracture initiation must be regarded as a possibility" (Ehrnford, 1981). Surprisingly, the presence of the maximum tensile stress away from the center is generally not considered and studies about 3D effects in the diametral compression test mainly focus on the variation of the tensile stress at the center along with the thickness of the compact (Podczeck et al, 2013;Yu et al, 2006).…”

Theoretical investigations into the influence of the position of a breaking line on the tensile failure of flat, round, bevel-edged tablets using finite element methodology (FEM) and its practical relevance for industrial tablet strength testing

“…This would still mean that there should be a difference of 15 N in the breaking load of the experimentally tested tablets, which is not the case (see Table 1). Large discrepancies between FEM-predictions and experimental data have previously been noted for simple flatfaced tablets (Ehrnford, 1980(Ehrnford, , 1981, and Darvell (1990) concluded that "finite element analysis can only be as good as the theory used to make the calculations, and the theory is at present insufficiently worked out". While the theory behind FEM calculations has advanced considerably since Darvell's statement, for bevel-edged tablets with a breaking line there is no analytical solution available, making it difficult to identify the reasons for the discrepancies observed.…”

Green strength testing of pressed compacts: An analysis of the different methods

“…The diametral compression test, also called the indirect tensile test or Brazilian test, which was introduced around 1950 to measure the tensile strength of concrete [7][8][9] , has gained popularity because of simplified piece preparation, simple geometry, and quickness of testing. Thus, it was very early implemented in many technological fields, such as rocks 10,11 , ceramics [12][13][14][15] , metal composites 16 , materials used in dentistry [17][18][19] , and materials used in the processing of pharmaceutical dosage forms. [20][21][22][23][24] A significant distinction is to be noted, however, between the tensile fracture stress obtained from the bending tests and that obtained from the diametral compression test: the former pertains to a strictly uniaxial stress state, while the latter is associated with a transverse compressive stress considerably greater than the tensile stress.…”