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It was deemed important to investigate the role of fibers added to composite compared to other commonly used bulk fill composites, and to examine the extent to which fiber reinforcement would enhance the mechanical properties of the materials. Multiple laboratory investigations have been used to evaluate dental composite resins; standardized tests present the advantage of being easily reproducible in laboratories, and allowing values obtained by different institutes to be compared. Moreover, they provide preliminary information about the material suitability in the oral environment and the extent to which they conform to the indications prescribed by the manufacturer.19 Heintze et al. found that flexural strength and flexural modulus tests can be used as a good indicator for the material durability under stress, and correlate well with the clinical longevity.19 Fracture toughness test was considered by Ilie et al. as another important method that investigates the material's ability to endure stress without fracture and monitor the crack propagation inside the material before failure.11 On the other hand, Vickers hardness assay, one of the most used mechanical experiments examines the material surface hardness, and scanning electron microscope observations reveal important information about the samples used and the mode of failure of the material.9 Standard ISO flexural strength and modulus tests consider only 2 mm thickness samples. However bulk fill composites are indicated to be used clinically in 4 mm thick increments, and accordingly investigating the material at this thickness seems more appropriate.
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Results obtained acknowledge the role of fibers in increasing the material's resistance to fracture, and coincide with those of previous studies.18,22 The single edge notched beam method used in this study is one of the most commonly used fracture toughness test methods, which are used to predict resistance to fracture. The method is widely used in dental material research and is usually conducted by means of a 3 point bending apparatus, and the sharp crack created could be easily measured. This method is also very sensitive to the notch width and depth, thus making comparison difficult between different studies.11,18 In the present work, no correlation was found between the fracture toughness value and the filler volume percentage or the filler particle size.
The enhancement of the material properties was explained to be due to the stress transfer from the matrix to the fibers and also due to the action of the fibers in stopping crack propagation through the material.23 It was found that the mere insertion of fibers is not enough to enhance the composite properties, that is, the fibers length and diameter play a critical role in this mechanism. Peterson found that fibers incorporated into a material, greatly enhances its mechanical properties, on the condition that the fibers have a length that exceeds a certain minimum length. This is known as the critical fiber length, which could be calculated using the following formula:24
One interesting observation found from the fracture toughness and the flexural strength test samples alike was that all fiber reinforced composite EXP samples remained attached, even after failure of the sample and formation of crack line, unlike the samples from other bulk fill brands which separated in two pieces the time the failure load was reached, as can be seen in Figure 5. Scanning electron microscope observations performed on fractured samples show the fibers traversing the crack line and between the fractured parts, as can be seen in Figure 3. Further investigation of this property is important clinically, since not only it shows the material resistance to fracture, but also its resistance to displacement at the more vulnerable interface, thus preventing cavitation and food impaction. Moreover, this property would render the material with better potential for repair.
First of all, it should be mentioned that despite of essential difference in the content of β-stabilizing elements in the alloys #1, #2, and #3, there were no noticeable differences in their fracture surfaces. Due to this we present and analyze here only fracture surfaces of tensile tested specimens of alloys #3 and #4 as typical examples (Fig. 10). In as-rolled state all alloys, regardless the content of alloying elements, had similar fracture surfaces: at the macro-level a lamination along microstructural elements was observed (compare Figs. 3a, 10a, c), while at the micro-level fracture surfaces had a dimpled relief indicating the ductile nature of fracture (Fig. 10b, d). The same situation was observed in the specimens tested after annealing (Fig. 10e), and, of course, the fracture was also ductile (Fig. 10f). The annealing did not cause recrystallization of the β-phase grains which retained the shape elongated in the rolling direction. The surface dimples indicate that fracture was still ductile (compare Fig. 10f with b, d). The lamination cannot be explained by the crystallographic texture of β-phase (see Fig. 6b, d). Obviously, the lamination is a result of secondary cracking (perpendicular to the fracture surface which was formed by main crack growth) along high-angle boundaries between neighboring non-recrystallized β-grains elongated in the rolling direction.
The microstructures close to the fracture surfaces of the same (from Fig. 10) specimens are presented in Fig. 11. As can be seen, elongated in tension (and plastic flow) direction particles of α-phase and β-phase interlayers are observed near the fracture surface in as-rolled (Fig. 11a, c) and annealed (Fig. 11b, d) materials. At the same time, it is clearly seen that the β-phase appears to have significantly higher ductility, as its tips formed the ridges of the dimples (indicated by the letter A in Fig. 11a, b). Absolutely the same situation was observed in the alloys after annealing (Fig. 11c, d). It should also be noted that in both as-rolled and annealed states, neither pores nor cracks were observed in the fractured specimens either near the fracture surface or at some distance from it.