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

Abstract

were studied. FEM results obtained for fully elastic and elasto-plastic tablets were 22 fairly similar, but they highlighted large differences in stress distributions depending 23 on the position of the breaking line. The stress values at failure were predicted to be 24 similar for tablets tested at an angle of 45° or above, whereas at lower test angles the 25 predicted breaking loads were up to 3 times larger. The stress distributions 26 suggested that not all breaking line angles would result in clean tensile failure. 27Practical results, however, did not confirm the differences in the predicted breaking 28 loads, but they confirmed differences in the way tablets broke. The results suggest 29 that it is not advisable to convert breaking loads obtained on scored tablets into tablet 30tensile strength values, and comparisons between different tablets or batches should 31 carefully consider the orientation of the breaking line with respect to the loading 32 plane, as the failure mechanisms appear to vary. Flat, round tablets usually have bevel-edges to reduce chipping of the tablet edges 38 during packaging, transport and handling, and very often they carry a breaking 39 ("score") line. The provision of a breaking line is an attempt to reduce the number of 40 tablet dosing strengths required to cover a range of dosing options for a drug. At the 41 same time, breaking lines might help patients who have swallowing difficulties and 42 provide some flexibility in the amount of drug taken in a single dose (van Santen et 43 al., 2002). However, as noted in the USP monograph on testing tablet breaking 44 forces (Method 1217, USP38/NF33, 2014) the presence of a breaking line might 45 influence the breaking forces recorded, and they hence advise that during the 46 standard diametral compression test the orientation of the breaking line should be 47 kept constant, either horizontally or vertically. However, in line with fracture 48 mechanics knowledge Newton et al. (1977) recommended that the breaking line 49 should be positioned perpendicular to the platen surfaces i.e. be parallel to the 50 direction of loading to increase the chance of tensile failure to occur along the 51 breaking line. As an alternative to the diametral compression test, Sovány et al. 52(2010) used a three-point bending test, whereby the breaking line was positioned 53 below the upper, slightly blunted loading edge, presumably facing downward (this is 54 not clearly specified in their paper). The advantage of a three-point bending test over 55 the diametral compression test is that the bending moment increases linearly from 56 zero at either support of the tablet to a maximum value at the mid-span location, and 57 assuming linear elastic behaviour, shear effects will not affect the maximum tensile 58 stress developing at the lower tablet surface (Stanley, 2001). Mazel et al. (2014) also 59 suggested that for pharmaceutical compacts the three-point bending test should be 60 preferred over the diametral compression test, because it reflects the ten...

The bending strength of tablets with a breaking line—Comparison of the results of an elastic and a “brittle cracking” finite element model with experimental findings.

“…Similar effects, but more detailed due to the use of different breaking line positions in terms of angles relative to the loading plane, were found using FEM (Podczeck et al, 2014), and their theoretic work also predicted that not all breaking line angles would result in clean tensile failure. The practical results, however, only confirmed some deviations from a clean tensile failure due to the position of the breaking line, whereas the breaking forces as such were only marginally affected by the position of the breaking line relative to the loading plane.…”

“…The load was kept constant for all models. Similar to previous work (Podczeck et al, 2013(Podczeck et al, , 2014 only one linear elastic model with the properties of Araldite CT200, hardened with 30% w/w Hardener 901, for which Young's modulus of elasticity (2.58 GPa) and Poisson's ratio (0.35) were taken from the literature (Burger, 1969), was studied. The theory of elasticity (Timoshenko and Goodier, 1987) predicts that relative stress distributions are independent of Young's modulus and Poisson's ratio, and that this holds in FEM studies has previously been confirmed Heasley, 2013, Podczeck et al, 2013).…”

“…Under industrial manufacturing conditions, the breaking force of round tablets should be tested using the standard diametral compression test (Method 1217, USP38/NF33, 2014. Recently, Podczeck et al (2014) investigated the influence of the position of the breaking line in terms of its angle relative to the loading plane during a diametral compression test of commercially available beveledged flat-round tablets. They compared their practical findings with theoretical investigations using finite element method (FEM).…”

“…Mazel et al (2014) suggested that pharmaceutical compacts of round, cylindrical shape should be tested using a three-point bending test, because it reflects the tensile failure stress more accurately than the diametral compression test. The advantages and disadvantages of this test in terms of its practical applicability under routine settings in the pharmaceutical industry can be found in previous reports (Podczeck, 2012;Podczeck et al, 2014). However, this test has not yet been studied in terms of its applicability and accuracy when breaking round-flat, bevel-edged tablets with a breaking line.…”

“…Similarly to the previous paper (Podczeck et al, 2014) two different brands of commercially available, flat-round, bevel-edged tablets with a single central breaking line were studied. Breaking line positions tested, relative to the upper loading roll, were 0°, 22.5°, 45°, 67.5° and 90°.…”