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DENTAL MATERIALS
TOUGHNESS MEASUREMENT IN DIRECT RESIN COMPOSITES USING QUANTITATIVE
Original Article
FRACTOGRAPHIC ANALYSIS
Brendan M. Angus1a*, John J. Mecholsky Jr.1b, Nader Abdulhameed2c
1
Materials Science and Engineering Department, College of Engineering, University of Florida, Gainesville FL, USA
2
College of Dentistry, University of Florida, Gainesville FL, USA
a
BS, Research Assistant
b
PhD, Professor and Associate Chair of the Materials Science and Engineering Department
c
BDS, MS, PhD Student, Clinical Assistant Professor of Restorative Dental Science
ABSTRACT DOI: 10.25241/stomaeduj.2018.5(1).art.1
Objective: To outline a procedure to determine the fracture toughness of direct resin OPEN ACCESS This is an Open Access
composites failing from “natural” flaws. article under the CC BY-NC 4.0 license.
Methodology: Tensile (hour glass) tests (n = 30) of a conventional hybrid dental Peer-Reviewed Article
composite (Tetric EvoCeram, Ivoclar Vivadent) were fabricated and fractured in uniaxial Citation: Angus BM, Mecholsky Jr JJ,
tension loaded at a crosshead speed of 1 mm/min (≥ 10 MPa/s). The fracture toughness Abdulhameed N. Toughness measurement in
direct resin composites using quantitative fracto-
of the material was then calculated using the stress at failure and measurement of the graphic analysis. Stoma Edu J. 2018;5(1):18-23.
crack size from fractographic analysis using SEM. Hardness (H) measurements were Academic Editor: Nicoleta Ilie, Dipl. Eng,
taken using a Vickers pyramidal diamond indenter. Elastic modulus (E) was calculated PhD, Professor, University Hospital, Ludwig-
Maximilians-Universität München, Munich, Germany
from the E/H ratio using a Knoop indenter.
Results: The values for fracture toughness found were similar to other Bis-GMA based Received: January 09 ,2018
Revised: February 04, 2018
dental composites 0.5 ±0.2 MPa . The Vickers Hardness was 509 ± 27 MPa and the Acccepted: February 22, 2018
Knoop Hardness was 495 ± 14 MPa using 0.5 kg/30 s, while the elastic modulus was Published: February 26, 2018
9.5 ±1.4 GPa. *Corresponding author: Brendan M. Angus,
Conclusion: The differences found in fracture toughness between this study and BS, Research Assistant, Materials Science and
Engineering Department, College of Engineering,
previous published studies are most likely due to variation in technique and material. University of Florida, 1724 Gale Lemerand Dr,
Gainesville, FL 32603, USA, Tel: (561) 927-8584,
Quantitative fractographic analysis offers a different method to evaluate the toughness e-mail: BrendanAngus@ufl.edu
of direct resin composites.
Copyright: © 2018 the Editorial Council for the
Keywords: fracture toughness, resin composites, fractography, dental materials. Stomatology Edu Journal.
1. Introduction be time dependent. Under a rapid stressing rate, the
Dental composites are a mixture of polymers and stress-strain response will be primarily linearly elastic.
glass particles used in dental restorations to mimic the Under a slow stressing rate, the stress-strain response
appearance and performance of teeth and are often will be viscoelastic [8]. The presence of filler also has
used to repair damaged teeth [1]. Their mechanical a high impact on the mechanical properties [9]. The
properties have improved over the last years and greater the amount of fillers, the greater the modulus
consequently a lot of research has been performed to of elasticity. The combination of the viscous matrix
assess these properties and how they are affected by and greater modulus reinforcement leads to a tougher
variations in particle size, polymerization depth, and composite [10]. The main concern with the increase
viscosity [2-4]. in fracture toughness of the restorative material is
In one longitudinal study it was found that out of 926 maintaining or improving the lifetime service in the oral
restorations investigated, 8% failed by fracture. This environment while maintaining the esthetic value. The
number increases to 18% when considering the failure mechanical properties of dental composites have been
of only resin composites restorations [5]. In a review highly improved in the last few years [11]. Even though
of prospective studies, it was found that fracture of there are many tests done to correlate in vitro data with
dental restorations is the most common cause of the clinical behavior of dental composites, there still is a
restoration failures in the first 5 years. [6] Therefore need to improve these methods to be more realistic and
fracture toughness is a property that has received a lot comparable to material behavior while in service [12].
of attention in dental composites. Fracture toughness is There are several ways to measure fracture toughness
a property that represents the ability of some materials such as the single edge notch test, compact tension
to resist crack propagation [7]. The load continuously test, or double torsion test [13]. However, the tests for
applied in different directions with temperature and toughness, in almost all cases for resin composites,
humidity variation leads to the progressive degradation involve large crack techniques. Resin composites mostly
and failure of the restoration, mainly due to the crack fail from small cracks so it is important to develop small
propagation. The introduction of imperfections in the crack techniques to determine fracture toughness.
material during processing, finishing, and/or in service The quantitative fractography method offers the
has a high impact on the restoration failure probability. advantage of using flaw sizes of those encountered
The organic matrix of dental composites has viscoelastic in service, i.e., small cracks. There are two approaches
properties, which means that the response to stress will when using fractographic procedures to determine
18 Stoma Edu J. 2018;5(1): 18-23. http://www.stomaeduj.com
TOUGHNESS MEASUREMENT IN DIRECT RESIN COMPOSITES USING QUANTITATIVE
FRACTOGRAPHIC ANALYSIS
toughness: controlled crack techniques [14] and direct
Original Article
observation of “natural” flaws or cracks [15]. “Natural” (1)
here means cracks or processing defects caused by where A is the cross section within the narrow region
fabrication and handling of the material before testing. (gauge section) of the specimen (3 mm). Any sample
It was not possible to develop controlled cracks in the that did not break in the narrow cross section was
material so the controlled crack technique could not discarded and not used for the data presented. Weibull
be used. This result of difficulty in forming controlled parameters were calculated by maximum likelihood
cracks agrees with a similar observation in a previous estimation according to ASTM C1239 – 13[18].
study by other authors [16]. Using the “natural” crack
means that an assessment of the fracture toughness Table 1. Composition and physical properties of the Tetric Evoceram
of the material as used in clinical practice can be Dental Composite.
found. Finishing operations will yield cracks of size Standard – Composition (in weight %)
on the order of “natural” cracks. The advantage of this
Bis-GMA, Urethane dimethacrylate, Ethoxylated Bis-EMA 16.8
technique over others is that it provides a tool for
forensic analysis. Once the toughness is determined Barium glass filler, Ytterbium trifluoride, Mixed oxide 48.5
from flaws of the size considered in this work, any Prepolymers 34.0
strength from field failures of the same material will be Aditives 0.4
able to be determined.
Catalysts and Stabilizers 0.3
There are limited studies in the field of dental
composites using quantitative fractographic analysis. Pigments <0.1
Therefore, the aim of the study was to outline a Physical Properties
procedure to determine the fracture toughness of Flexural Strength (Mpa) 120
direct resin composites failing from “natural” flaws. The
Flexural Modulus (Mpa) 10,000
materials used in this study are compared to those
in analogous studies using different materials and Compressive Strength (Mpa) 250
fabrication techniques. Vickers Hardness HV 0.5/30 (Mpa) 580
Density (g/cm3) 2.10
2. Methodology Fracture toughness was calculated using the
The material used in this study was a hybrid quantitative fractographic analysis. The method uses
conventional dental composite (Tetric EvoCeram, optical and scanning electron microscopy to locate and
Ivoclar Vivadent)1. The Tetric EvoCeram composite is a measure the size of the origin of the fracture for each
light cured resin composite. The standard composition specimen [13]. Once the flaw, or crack, at the origin
and physical properties of Tetric EvoCeram are listed starts to propagate it travels with increasing speed
in Table 1 as given by the manufacturer [17]. Tensile spreading out in all directions. As the speed increases,
“hour glass” samples with average cross-sectional the surface increases in roughness. The origin of the
dimensions of 1.76 mm by 1.51 mm and a 3 mm gauge fracture can be determined by the observation of
length were made by filling a mold with the resin and the characteristic markings surrounding the fracture
curing the samples for 10 seconds each. The mold was origin on the fracture surface. Generally surrounding
covered with a thin Mylar strip to ensure a flat surface. the fracture origin there is a relatively smooth region,
The curing process was done using an LED light curing sometimes called the “mirror” region, that transitions
unit (Bluephase Style, Ivoclar Vivadent) which emits to a slightly rougher region, sometimes termed the
light with an approximate intensity of 1000 mW/cm2. “mist” region. These regions and other markings, such
The light cure unit was calibrated prior to use by means as twist hackle, can be used to identify the location of
of a dental radiometer (BluePhase meter II, Ivoclar the failure origin [13, 15]. The fracture origin is situated
Vivadent). The tip of the light cure unit was positioned approximately at the center of the surrounding
directly on top of the Mylar strip and stabilized with topography. All surface cracks were treated as elliptical
the plastic tip. Once the samples were cured they were cracks for calculating the fracture toughness. Images
polished with very light pressure to ensure that the were taken using a scanning electron micrograph
corners were smooth. This was done using Sof-Lex2 SEM4. Once the crack sizes were obtained the fracture
extra thin polishing discs of medium grit followed by toughness was calculated using equation 2 where σ is the
fine grit at 6000-10000 rpm. stress at failure, a the crack size, and Y is a geometric factor
The polished samples were then broken in tension of loading, the crack shape, and location. Y was calculated
using a universal tensile testing machine3 loaded at a using the solutions of Newman and Raju for locations at
crosshead speed of 1 mm/min (≥ 10MPa/s) using an the surface of the crack or internal cracks [19]:
anti-torsion parallel holder, and the load at failure, P,
was recorded for each sample. The load-displacement (2)
graphs were linear until there was fracture with little The hardness, H, was determined in a conventional
or no non-linear behavior before fracture. The fracture manner using a Vickers pyramidal diamond with an
stress, σ, was calculated from the load at failure and the indentation load of 0.5 kg at a loading and unloading
dimensions of each specimen using equation 1: time of 30 s [20]. The Vickers diamond was used for
1
Lot Number V23426, Exp. 2020-5, Ivoclar Vivadent AG, Schaan, Liechtenstein hardness because it offers an equi-axed diamond
2
3M, 3M Center St. Paul, MN 55144
3
Instron, 825 University Ave, Norwood, MA, 02062 4
Phenom Pro SEM, Phenom World, Eindhoven, Netherlands
Stomatology Edu Journal 19
TOUGHNESS MEASUREMENT IN DIRECT RESIN COMPOSITES USING QUANTITATIVE
FRACTOGRAPHIC ANALYSIS
Original Article
Figure 1. Optical Micrograph of Knoop Indentation Demonstrating the
Measurement for Elastic Modulus.
Figure 3. Example of a corner crack. The lines indicate the dimensions of
the crack at the fracture origin.
Figure 2. Example of an internal crack. The arrow points to fracture origin.
and thus increases the number of measurements and
increases precision. For completeness, the Knoop
hardness value was also determined at 0.5 kg at a
loading and unloading time of 30 s.
The elastic modulus was determined using a technique
developed by Marshall [21] and Conway [22]. The
method used the geometry of the Knoop indenter,
i.e., due to the asymmetrical shape of the indenter,
one direction of the impression is elongated and
the transverse direction is considerably shortened.
Figure 4. Example of a surface crack. Yellow lines indicate the width and
Upon loading, the geometry of the impression will be
depth of the crack at the fracture origin. Black arrows indicate direction of
determined by the shape of the diamond (cf. Figure crack propagation away from the crack origin. White arrows indicate twist
1). Upon unloading, the shorter direction (2b*) will hackle marks on the fracture surface.
contract (2bR) due to the elasticity of the material. The
elongated end (2a*) will not be measurably changed Table 2. Average values of obtained data.
because of the length. Thus, the difference between the Physical Property Standard Deviation
original measurement of the diagonals (from the shape
of the diamond) and the impression on the material Fracture toughness (MPa ) 0.5 0.2
will provide a measure of the elasticity of the material.
Vickers Hardness (MPa) 509 27
The E/H ratio can be calculated from the measurement
of the diagonals as shown in equation 3 [22]: Y (geometric constant) 1.28 0.04
Elastic Modulus (GPa) 10 1.7
(3) 90% Confidence Interval
Unbiased Weibull Modulus 4.4 3.4 - 5.6
where H is the Knoop hardness, ν is Poisson’s ratio (0.3),
γ = 75 (the average half angle of the Knoop indenter), Characteristic Strength (MPa) 26 24 - 28
b* is half the minor diagonal at maximum load, and bR
is half the residual minor diagonal that is measured. The 7.11a*, where a* is half the major diagonal, assumed to
value of b* can be calculated for the Knoop indenter be the same before and after indentation.
because it is related to the major diameter, i.e., b* =
20 Stoma Edu J. 2018;5(1): 18-23. http://www.stomaeduj.com
TOUGHNESS MEASUREMENT IN DIRECT RESIN COMPOSITES USING QUANTITATIVE
FRACTOGRAPHIC ANALYSIS
Original Article
3. Results
The average values of toughness, hardness, Y, elastic
modulus, Weibull modulus, and characteristic strength
can be found in Table 2. The detailed strength and
toughness data obtained are presented in Table A-1
in the Appendix. The average fracture toughness for
this material was found to be 0.5 ± 0.2 MPa . Of the
30 samples investigated, most cracks originated at
the surface of the material, often at a corner that was
polished. Only 3 of the 30 samples had internal crack
origins. An example of an internal origin is shown in
Figure 2. Figures 3 and 4 show the more common edge
and corner cracks at the surface of the samples. They
are also representative of the measurement technique.
Figure 3 also illustrates the presence of voids in various
samples often found near crack origins. The Vickers
Hardness was 509 ± 27 MPa using 0.5 kg/30 s, while the
elastic modulus was 10 ±1.7 GPa.
A Weibull graph for the data is presented in Figure 5.
The unbiased Weibull modulus and characteristic
strength were calculated using MATLAB and found to
be 4.4 (90% confidence intervals as per [16]: 3.4 - 5.6)
and 26 MPa (90% confidence intervals as per [16]: 24
MPa - 28 MPa) respectively. The locations of the fracture
origins are also depicted on the Weibull graph. All
origins appear to be uniformly distributed. Figure 5. Weibull graph of the composite strengths.
26] and determination of toughness values for resin
4. Discussion composites [27, 28], to our knowledge there is no record
While the values for fracture toughness could not be of toughness values for direct resin composites measured
found from the manufacturer, the value obtained agrees using the quantitative fractographic technique used
with other Bis-GMA based dental resin composites [23]. here. Thus, we provide useful information for use in in
Our value is less than the value of 1.11 MPa found by vitro analysis because the size of the cracks are those
Cho et al. for the same material [24] and less than the expected in clinical failures. The results here and from
value of 1.1 MPa found by Quinn et al. for materials Quinn et al. suggest that the fractographic technique
that are resin based, but manufactured in a different way may be used to determine differences in manufacturing
[16]. Note that a different technique was used by Cho et techniques as well as differences in particle loading.
al. to measure the fracture toughness. The notched bend Further research in this area should be pursued.
test is noted for producing increased values of fracture The unbiased Weibull modulus was 4 for this specific
toughness unless the notch is artificially sharpened [13]. material, which is less than the value of 8 found by
In the present study, as well as the one in Quinn et al., we Quinn et al. for their material [16]. Of course, the Weibull
were not able to produce a sharp crack artificially due to modulus is just an indication of the distribution of the
the viscoelastic nature of the material [16]. The condition values of strength obtained. This distribution is related
at the crack tip can explain the difference in the to the uniformity of the flaws in the material which,
numerical values between the notched bend test and in turn, is related to manufacturing procedures and
the “natural” flaws. The material used in Quinn et al. [16] handling. Thus, both values found in the two studies
is an indirect resin composite block (Paradigm, 3M ESPE, are relatively low, indicating a wide spread of flaw sizes
MN) used for indirect restorations. The composition of and locations. As observed in the Weibull graph, there
the indirect material used in their research contains a does not appear to be an effect of the location of flaws
high fraction of filler particles (85 wt% ultrafine zirconia- as to the strength of the material. The characteristic
silica ceramic to reinforce a highly crosslinked polymeric strength was 26 ± 2 MPa. Since the fracture initiating
matrix). Thus, as the authors state, this material is closer flaws were “natural”, they were not controlled except by
to ceramic behavior. The materials used in this research the fabrication and finishing procedure. The sizes should
is a direct dental composite material which contains 40- be comparable to those observed in clinical procedures.
48 wt% Baria-aluminosilicate glass filler as well as 34.0% Better control of the fabrication procedures could result
pre-polymer fillers. In addition, the sample preparation in greater toughness values, but most likely not greater
was different in the two studies. The present study used than ~ 1 MPa , and thus, in greater strengths for the
a prefabricated mold followed by a light cure and then same size flaws.
shaped for tensile specimens, while in the Quinn et al. The method used to determine the elastic modulus in
article, a hard block was used and it was sectioned to this work is relatively straightforward and unique for
get the desired shape for flexural tests. Thus, we should resin composites. Since the value agrees with the value
not expect the values to be comparable. While there provided by the manufacturer for flexural modulus,
are many fractographic studies of resin composites [25, we think this is encouraging in that this presents
Stomatology Edu Journal 21
TOUGHNESS MEASUREMENT IN DIRECT RESIN COMPOSITES USING QUANTITATIVE
FRACTOGRAPHIC ANALYSIS
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TOUGHNESS MEASUREMENT IN DIRECT RESIN COMPOSITES USING QUANTITATIVE
FRACTOGRAPHIC ANALYSIS
Original Article
Brendan M. ANGUS
BS, Research Assistant
Materials Science and Engineering Department
College of Engineering, University of Florida
Gainesville, FL, USA
CV
Brendan Angus is a Masters Student at the University of Florida pursuing a degree in Materials Science and Engineering. He
earned his Bachelor’s degree in Materials Science and Engineering at the University of Florida in May of 2017. His research
focuses on the fractography of dental composites.
Questions
1. The fracture toughness of the resin composites was measured using:
qa. Single edge notch test;
qb. Compact tension test;
qc. Quantitative fractography;
qd. Double torsion test.
2. The elastic modulus was calculated using:
qa. The slope of the stress-strain curve;
qb. The sonic modulus technique;
qc. Knoop indentation;
qd. Vickers indentation.
3. The value for the fracture toughness found was:
qa. In agreement with comparable studies;
qb. Lower than values found in comparable studies;
qc. Greater than values found in comparable studies;
qd. Not compared to values found in other studies.
4. Quantitative fractography uses what measurements to calculate fracture
toughness?
qa. Crack size and stress at failure;
qb. Crack size and elastic modulus;
qc. Elastic modulus and stress at failure;
qd. Stress at failure and Vickers hardness.
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Stomatology Edu Journal 23