Diametral compression test

The diametral compression test or Brazilian test was used to characterise the ultimate strength of tablets made of starch powder only. This test, based on Hertz [11] and Michell [12], [13] works, provided the value of the tensile stress σt along the loaded diameter D of a cylindrical sample of thickness t as described in Eq. (3):

P being the applied load (Fig. 5). This equation is valid with the assumption that the material exhibits a brittle elastic behaviour. In addition, it has been shown by Fell and Newton [14] and Fahad [15] that the error in Eq. (3) is under 4% when the thickness t is not higher than 0.2×D. In this study, the diameter of the tablet was around 12 mm and its thickness was approximately 2 mm.

Principle of the diametral compression test, typical curve definitions of the extracted properties.

Because of their mechanical behaviour and structure, starch tablets did not respect strictly the assumptions of the Brazilian test. However, results of this test enabled a qualitative comparison of the mechanical behaviour of each sample. A typical curve obtained by testing a starch tablet is shown in Fig. 5. One can also see how the different properties were measured. The mechanical tests were conducted using an Instron 5965 testing machine with a 50 N force sensor and a displacement velocity of 2 mm min⁻¹. The samples were stored in the testing room at 23 °C and 50% RH. Each test was repeated five times.

It must be noted that the diametral compression test was the only suitable test, considering the shape of the samples made of starch only. Indeed, it was difficult to design a homogeneous oblong sample made of starch only. The size of defects that were induced in the samples with this processing route was close to the transversal dimensions of the samples themselves. For this reason, the sonotrode with a circular tip was used to obtain samples made of starch only and the mechanical properties could be determined.