Development/Preliminary Validation of a Modified Notchless Triangular Prism Protocol

Objective: Dental specimens are incompatible with standard fracture toughness testing methods due to size constraints. The purpose of this study was to develop and preliminarily validate a modified notchless triangular prism (NTP) technique that more closely simulates the fracture mechanics of standard chevron notch short rod testing (CNSR). Methods: CNSR and NTP specimens (n=7) were machined from polymethyl methacrylate (PMMA). The dimensions of the NTP specimens were 4x4x4x8mm. The dimensions of the CNSR specimens were based on the ASTM E1304-97 standard (diameter=25.4mm, W/B=1.45). NTP and CNSR specimens were loaded in tension (0.1mm/min and 1.0mm/min, respectively) according to the original NTP protocol (Ruse et al.) and ASTM standard until failure. Peak load was recorded and used together with specimen dimensions to calculate plane-strain fracture toughness according to the standard equation and with the proposed alteration to the NTP equation using specimen width in place of specimen-holder width. Results: There was a significant difference (p<.05) in the modified NTP fracture toughness values (0.89±0.30 MPa٠m1/2) as compared to the CNSR values (1.4±0.06 MPa٠m1/2). Despite the significant difference between the test values, both CNSR and NTP results were within range of the standard values for PMMA, 0.7-1.6MPa٠m1/2. Conclusions: The preliminary evidence suggests the modified NTP protocol might be utilized to accurately measure fracture toughness of dental biomaterials. However, more testing is required using additional brittle polymer-based materials to increase confidence in the methodology. Supported by K23 DE01623 and T32 DE07294.