Fracture toughness testing: a discriminatory mechanical testing performance indicator for glass-ionomer restoratives?

OBJECTIVES: To investigate the single-edge notched (SEN) bend fracture toughness (KIC) testing methodology as a reproducible and discriminatory mechanical testing protocol for encapsulated and hand-mixed glass-ionomers (GI).

METHODS: SEN bend test-pieces (35.0±0.1mm length, 6.0±0.1mm width, 3.0±0.1mm thickness with a sharp notch formed at mid-length by embedding a scalpel blade) were prepared for KIC testing using three encapsulated GI products (Chemfil Rock, Fuji IXGP Fast Capsule and Ionofil Molar AC). In addition, test-pieces were prepared from a hand-mixed GI product (Ionofil Molar) which contained between 100% and 20% of the manufacturer's recommended powder content (in 10% decrements) for a constant weight of liquid. Groups of 20 test-pieces were prepared for each encapsulated GI product (n=3) and hand-mixed GI powder:liquid mixing ratio (n=9). Data were statistically analyzed and the coefficients of variation (CoV) determined for each encapsulated GI product and hand-mixed GI powder:liquid mixing ratio.

RESULTS: The KIC testing methodology failed to discriminate between the encapsulated GI products that were investigated (p=0.225). For the hand-mixed GI, the KIC testing methodology also failed to discriminate between the powder:liquid mixing ratios investigated (R(2)=0.576). The pooled CoV (10%) for the encapsulated GI products and for the powder:liquid mixing ratio groups (12%) identified the reproducibility of the test for this experiment. For the hand-mixed GI mixing ratio groups with between 100% to 50% of the recommended powder content, no trend could be discerned.

SIGNIFICANCE: The KIC testing methodology failed to discriminate between different encapsulated GI products and hand-mixed GI powder:liquid mixing ratio groups investigated, despite KIC being an intrinsic material property and the coefficient of variation being acceptable.