## 4 Discussion
The stresses generated during the simulation of the composite material under vertical loading (simulation of functional occlusal loads) were greater than 400 MPa and those under oblique loading (simulation of lateral chewing forces) were approximately 8 MPa [28]. The flexural strength of modern dental materials, which additionally meet the higher requirements of ISO 4049 standards, is approximately 10 times greater [29] than the average chewing force (8-12 MPa), considering the stresses generated in the restoration. Therefore, if the fatigue flexural limit is on average approximately 40% of the flexural strength post-aging, the flexural strength limit for composite materials after aging should be within the range of 32-48 MPa. Among the most common causes of failure in dental treatment are fractures and secondary caries. Flaws in dental materials may occur during restoration preparation or can develop during use [30]. Therefore, describing the composite resistance to the propagation of flaws under stress is very important [22]. In our study, there was no relationship between FT and FS or FFL (Table A1). Other studies reported a significant correlation between FTand FS [20] but this study was conducted for non-aged materials only. To the best of the authors' knowledge, no study has evaluated the relationship between FT and FS after aging, and this requires further verification. Additionally, it is important to note that FT values depend on the chosen research method and pre-cracking conditions, which may affect the observed correlations [13; 31; 32]. The samples for FS and FFL had the same dimensions, which may further explain the strong correlation between these results. However, the samples for FT had a different geometry, which may also have contributed to the lack of observed correlation. Unfortunately, despite emphasizing the importance of conducting fracture toughness studies and comparing the most commonly used testing methods, a standardized approach for this test in dental composites is still lacking [13; 33; 34; 35; 36]. It is necessary to specify certain limits of FT that materials should meet, especially following accelerated aging. Studies have indicated that the fracture toughness of human enamel ranges from 0.7 to 1.3 MPa\(\surd\)m while that of dentine rangers from 1.7 to 3 MPa\(\surd\)m [37]. The literature shows that the fracture toughness of dental composites typically ranges from approximately 1 to approximately 2 MPa\(\surd\)m [31] while for unfilled materials, it is approximately 0.8 MPa\(\surd\)m [38], which is consistent with the obtained results. After aging, the FT values of the tested materials decreased by an average of approximately 30%. Other studies also show that fracture toughness decreases significantly after different techniques for material aging [39; 40; 41; 38]. The FT values after aging can indicate the stability and longevity of materials in the oral cavity, suggesting that fracture toughness should be included in standard tests for dental composites. There is also a need to standardize the testing procedure due to varying existing methodologies in the literature. According to our results, accelerated aging significantly reduced the strength properties (FS, FT, DTS, FFL) compared to those of the control condition (with decreases from 13 to 70%, on average, by approximately 31%, Figure 4). Our study revealed a decrease in hardness after aging (Figure 4). This was consistent with other findings wherein fluctuations in hardness changes were observed over a duration or used medium [42; 43; 44]. In general, numerous studies have agreed that artificial aging results in a notable decline in the mechanical properties of dental composite materials [45; 46; 47]. The underlying cause of these alterations stems from inherent degradation processes within the materials.