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