TOUGHNESS MEASUREMENT IN DIRECT RESIN COMPOSITES USING QUANTITATIVE FRACTOGRAPHIC ANALYSIS toughness: controlled crack techniques [ 14 ] and direct observation of“ natural” flaws or cracks [ 15 ].“ Natural” here means cracks or processing defects caused by fabrication and handling of the material before testing. It was not possible to develop controlled cracks in the material so the controlled crack technique could not be used. This result of difficulty in forming controlled cracks agrees with a similar observation in a previous study by other authors [ 16 ]. Using the“ natural” crack means that an assessment of the fracture toughness of the material as used in clinical practice can be found. Finishing operations will yield cracks of size on the order of“ natural” cracks. The advantage of this technique over others is that it provides a tool for forensic analysis. Once the toughness is determined from flaws of the size considered in this work, any strength from field failures of the same material will be able to be determined. There are limited studies in the field of dental composites using quantitative fractographic analysis. Therefore, the aim of the study was to outline a procedure to determine the fracture toughness of direct resin composites failing from“ natural” flaws. The materials used in this study are compared to those in analogous studies using different materials and fabrication techniques.
2. Methodology The material used in this study was a hybrid conventional dental composite( Tetric EvoCeram, Ivoclar Vivadent) 1. The Tetric EvoCeram composite is a light cured resin composite. The standard composition and physical properties of Tetric EvoCeram are listed in Table 1 as given by the manufacturer [ 17 ]. Tensile“ hour glass” samples with average cross-sectional dimensions of 1.76 mm by 1.51 mm and a 3 mm gauge length were made by filling a mold with the resin and curing the samples for 10 seconds each. The mold was covered with a thin Mylar strip to ensure a flat surface. The curing process was done using an LED light curing unit( Bluephase Style, Ivoclar Vivadent) which emits light with an approximate intensity of 1000 mW / cm 2. The light cure unit was calibrated prior to use by means of a dental radiometer( BluePhase meter II, Ivoclar Vivadent). The tip of the light cure unit was positioned directly on top of the Mylar strip and stabilized with the plastic tip. Once the samples were cured they were polished with very light pressure to ensure that the corners were smooth. This was done using Sof-Lex 2 extra thin polishing discs of medium grit followed by fine grit at 6000-10000 rpm. The polished samples were then broken in tension using a universal tensile testing machine 3 loaded at a crosshead speed of 1 mm / min( ≥ 10MPa / s) using an anti-torsion parallel holder, and the load at failure, P, was recorded for each sample. The load-displacement graphs were linear until there was fracture with little or no non-linear behavior before fracture. The fracture stress, σ, was calculated from the load at failure and the dimensions of each specimen using equation 1:
1
Lot Number V23426, Exp. 2020-5, Ivoclar Vivadent AG, Schaan, Liechtenstein
2
3M, 3M Center St. Paul, MN 55144
3
Instron, 825 University Ave, Norwood, MA, 02062
( 1)
where A is the cross section within the narrow region( gauge section) of the specimen( 3 mm). Any sample that did not break in the narrow cross section was discarded and not used for the data presented. Weibull parameters were calculated by maximum likelihood estimation according to ASTM C1239 – 13 [ 18 ].
Table 1. Composition and physical properties of the Tetric Evoceram Dental Composite.
Standard – Composition( in weight %) Bis-GMA, Urethane dimethacrylate, Ethoxylated Bis-EMA 16.8 Barium glass filler, Ytterbium trifluoride, Mixed oxide 48.5 Prepolymers 34.0 Aditives 0.4 Catalysts and Stabilizers 0.3 Pigments < 0.1 Physical Properties Flexural Strength( Mpa) 120 Flexural Modulus( Mpa) 10,000 Compressive Strength( Mpa) 250 Vickers Hardness HV 0.5 / 30( Mpa) 580 Density( g / cm 3) 2.10
Fracture toughness was calculated using the quantitative fractographic analysis. The method uses optical and scanning electron microscopy to locate and measure the size of the origin of the fracture for each specimen [ 13 ]. Once the flaw, or crack, at the origin starts to propagate it travels with increasing speed spreading out in all directions. As the speed increases, the surface increases in roughness. The origin of the fracture can be determined by the observation of the characteristic markings surrounding the fracture origin on the fracture surface. Generally surrounding the fracture origin there is a relatively smooth region, sometimes called the“ mirror” region, that transitions to a slightly rougher region, sometimes termed the“ mist” region. These regions and other markings, such as twist hackle, can be used to identify the location of the failure origin [ 13, 15 ]. The fracture origin is situated approximately at the center of the surrounding topography. All surface cracks were treated as elliptical cracks for calculating the fracture toughness. Images were taken using a scanning electron micrograph SEM 4. Once the crack sizes were obtained the fracture toughness was calculated using equation 2 where σ is the stress at failure, a the crack size, and Y is a geometric factor of loading, the crack shape, and location. Y was calculated using the solutions of Newman and Raju for locations at the surface of the crack or internal cracks [ 19 ]:
( 2)
The hardness, H, was determined in a conventional manner using a Vickers pyramidal diamond with an indentation load of 0.5 kg at a loading and unloading time of 30 s [ 20 ]. The Vickers diamond was used for hardness because it offers an equi-axed diamond
4
Phenom Pro SEM, Phenom World, Eindhoven, Netherlands
Original Article
Stomatology Edu Journal
19