Mechanistic aspects of fracture and fatigue in resin based dental restorative composites Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1v53k237p

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  • For resin based dental restorative composites, one of the major challenges is to optimize the balance between mechanical and optical properties. Although fracture is the second leading cause of dental restorative failures, very limited mechanistic understanding exists on a microscopic level. In the present study, the fracture properties and mechanisms of two commercial dental resin composites with different microstructures are examined using double notched four point beam bending and pre-cracked compact-tension, C(T), specimens. Four point bend flexural strength was also measured using un-notched beam samples. The first material is a microhybrid composite that combines a range of nano and micro scale filler particles to give an average particle size of 0.6 μm, while the second is a nanofill composite reinforced entirely with nano particles and their agglomerates. The influences of 60 days water hydration and a post-cure heat treatment were also examined. Fracture resistance curve (R-curve) experiments have demonstrated the microhybrid composite to be more fracture resistant than the nanofill composite in both as-processed and hydrated conditions. Rising fracture resistance with crack extension was observed in all specimens, independent of the environmental conditions. Compared to the as-processed condition, a significant reduction in the peak toughness was observed for the nanofill composite after 60 days of water aging. Hydration lowered flexural strength of both composites which was attributed to hydrolytic matrix degradation with additional interfacial debonding causing larger strength decrease in the nanofill. Optical and SEM observations revealed an interparticle matrix crack path promoting crack deflection as a toughening mechanism in all cases except the hydrated nanofill which showed particle-matrix debonding. Crack bridging was another observed extrinsic toughening mechanism that was believed to be responsible for the rising fracture resistance curve (R-curve) behavior. Post-curing heat treatment changed the R-curve shape which was attributed to matrix toughening. Fatigue crack growth resistance was also measured after water aging for 60 days and testing in wet conditions. The da/dN-ΔK curve showed sigmoidal behavior with three different fatigue crack growth regions. The nanofill composite had a lower fatigue threshold, ΔK[subscript th], by ~ 0.13 MPa√m compared to the microhybrid composite, with an observed plateau in the fatigue crack growth curve suggestive of environmental attack. Toughening mechanisms of crack deflection and crack bridging were identified with evidence of cluster-matrix debonding in the nanofill composite. In general, fatigue crack growth ranged from ~10⁻⁹ to 10⁻⁵ m/cycle over ΔK of 0.54 to 0.63 MPa√m for the microhybrid composite and ΔK of 0.41 to 0.67 MPa√m for the nanofill composite.
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