Effect of Indenter Size on Failure Behaviour of Amalgam

Objective: To investigate the effect of indenter size on load bearing capacity and failure behaviour of amalgam under Hertzian indentation. Methods: Discs of dental silver amalgam (Logic+, SDI, Australia) 2 mm and 5 mm thickness, 10 mm diameter were fabricated, 120 for each thickness. After 7 d storage at 37 ºC in artificial saliva, specimens of each thickness were randomly divided into 8 groups and tested resting freely on a reinforced-polyamide substrate with spherical indenters of diameter 2, 3, 5, 8, 10, 20, 30 and 40 mm. Load at the first failure was detected acoustically; mode was determined visually. At least 1/3 of specimens in each case were examined under scanning electronic microscope for corroboration. Results: For 2 mm specimens, the failure load was constant for all indenter diameters; for 5 mm specimens, the failure load first increased with indenter size with a quadratic relationship up to about 10 mm and then tended to remain constant. Large diameter indenters tended to produce radial cracking while small diameter indenters promoted the occurrence of cone cracking and plastic deformation. Conclusion: Indenter size has a significant effect on both failure load and failure mode; this effect is closely related to specimen thickness.