The Influence of Ageing and Hydrothermal Fatigue (Thermocycling) on Degradation and Fracture Toughness of Light-Cured and Hybrid Resin-Based Nanocomposites (RBCs)

The aim of this study was to evaluate the influence of artificial saliva ageing and cyclic hydrothermal loading on the mechanical properties of dental composite materials. Two commercial composites (Filtek Z550 and Filtek Ultimate Flow) and two experimental materials representing flow-type and hybrid composites were investigated. SENB specimens were prepared in accordance with ASTM E399, together with flat specimens intended for impact strength testing using the Dynstat method. All samples were aged in artificial saliva for approximately one month at 37 ± 1 °C, and subsequently, half of the specimens were subjected to thermocycling in the temperature range of 10–65 °C for 10,000 cycles. Static mechanical tests, including three-point bending (TFS), biaxial flexural strength (BFS), and compression strength (CS), were performed before and after thermocycling. In addition, impact strength and fracture toughness expressed by the stress intensity factor KIC were determined. The results were analyzed in terms of the residual work of fracture (WOF), while the durability of the materials was evaluated using Weibull distribution parameters. The experimental analysis was complemented by SEM observations of the microstructure. The obtained results demonstrated a pronounced deterioration of mechanical properties after hydrothermal loading. The average impact strength after artificial saliva ageing reached 11.69 J/mm2 for Filtek Z550, 11.57 J/mm2 for Ex-hyb(P), 16.39 J/mm2 for Filtek Ultimate Flow, and 10.27 J/mm2 for Ex-flow(P), whereas after thermocycling, these values decreased to 5.38 J/mm2, 8.86 J/mm2, 4.55 J/mm2, and 4.39 J/mm2, respectively. A similar trend was observed for the fracture toughness parameter KIC, which decreased considerably after thermocycling for all investigated materials. The analysis of the residual work of fracture revealed the influence of thermocycling on the energy-related parameters of the composites. In the case of TFS, the average WOF decreased, among others, from 13.65·10−3 J to 1.90·10−3 J for Filtek Ultimate Flow and from 4.76·10−3 J to 2.37·10−3 J for Filtek Z550. For BFS, a noticeable decrease in WOF was also observed, particularly for Ex-flow(P) and Filtek Ultimate Flow. In the compression tests (CS), the changes were less unambiguous, and some materials exhibited an increase in WOF after thermocycling. Furthermore, changes in the scale and shape parameters of the Weibull distribution were identified, indicating degradation of composite durability under hydrothermal loading. The results confirmed that cyclic hydrothermal loading exerts a greater influence on impact strength and fracture toughness than on static flexural strength. While all investigated materials exhibited degradation, the extent of changes was material-dependent, and compression behaviour showed non-uniform responses. Weibull analysis confirmed reduced reliability and increased heterogeneity of the composites after ageing, indicating that hydrothermal fatigue is a dominant factor governing long-term mechanical deterioration of dental resin-based composites.