Interactive effect of indenter size and specimen thickness in Hertzian indentation test

To investigate the effect of indenter size on the failure mode of amalgam and its interaction with thickness.

Dental silver amalgam (Lojic+, SDI) discs, 10 mm diameter, thickness h=0.4-8.0 mm, were stored in air at 37 degrees C for 7 days before testing in 'Hertzian contact', resting freely on a 30% glass fibre-reinforced nylon 6,6 substrate (E: 10GPa) at 23 degrees C in air at a crosshead speed of 0.2 mm/min. The load at the first crack, detected acoustically, was recorded. Two series of tests were conducted. In the first, a 3 or 20 mm diameter hard steel ball was used to apply the load to discs with h=0.4-8.0 mm. The second was performed on discs with h=2.0 and 5.0 mm with indenter diameters D=2-40 mm.

The failure loads for these two groups were substantially different, with most values for D=3 mm being lower than those for D=20 mm. The transitions of the main failure mode with increasing thickness were similar, i.e., from RC-dominant for h small to CC-dominant for h large, except that this occurred over a greater range for the larger indenter. For h=2 mm, failure load was constant for D=2-40 mm; all specimens consistently failed from bottom RC. For h=5 mm, failure load increased with D with a quadratic relationship up to about 10 mm (r(2)=0.85), after that, values were near constant; the failure mode gradually changed with D from PD- or CC-prevailing for D small (< or =5 mm) to RC-prevailing for D very large (30 and 40 mm).

The failure mode of amalgam is dependent on the indenter size; indenter size and coating thickness have reciprocal effects.