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.
4. Discussion
While the values for fracture toughness could not be
found from the manufacturer, the value obtained agrees
with other Bis-GMA based dental resin composites [23].
Our value is less than the value of 1.11 MPa found by
Cho et al. for the same material [24] and less than the
value of 1.1 MPa
found by Quinn et al. for materials
that are resin based, but manufactured in a different way
[16]. Note that a different technique was used by Cho et
al. to measure the fracture toughness. The notched bend
test is noted for producing increased values of fracture
toughness unl ess the notch is artificially sharpened [13].
In the present study, as well as the one in Quinn et al., we
were not able to produce a sharp crack artificially due to
the viscoelastic nature of the material [16]. The condition
at the crack tip can explain the difference in the
numerical values between the notched bend test and
the “natural” flaws. The material used in Quinn et al. [16]
is an indirect resin composite block (Paradigm, 3M ESPE,
MN) used for indirect restorations. The composition of
the indirect material used in their research contains a
high fraction of filler particles (85 wt% ultrafine zirconia-
silica ceramic to reinforce a highly crosslinked polymeric
matrix). Thus, as the authors state, this material is closer
to ceramic behavior. The materials used in this research
is a direct dental composite material which contains 40-
48 wt% Baria-aluminosilicate glass filler as well as 34.0%
pre-polymer fillers. In addition, the sample preparation
was different in the two studies. The present study used
a prefabricated mold followed by a light cure and then
shaped for tensile specimens, while in the Quinn et al.
article, a hard block was used and it was sectioned to
get the desired shape for flexural tests. Thus, we should
not expect the values to be comparable. While there
are many fractographic studies of resin composites [25,
Stomatology Edu Journal
Figure 5. Weibull graph of the composite strengths.
26] and determination of toughness values for resin
composites [27, 28], to our knowledge there is no record
of toughness values for direct resin composites measured
using the quantitative fractographic technique used
here. Thus, we provide useful information for use in in
vitro analysis because the size of the cracks are those
expected in clinical failures. The results here and from
Quinn et al. suggest that the fractographic technique
may be used to determine differences in manufacturing
techniques as well as differences in particle loading.
Further research in this area should be pursued.
The unbiased Weibull modulus was 4 for this specific
material, which is less than the value of 8 found by
Quinn et al. for their material [16]. Of course, the Weibull
modulus is just an indication of the distribution of the
values of strength obtained. This distribution is related
to the uniformity of the flaws in the material which,
in turn, is related to manufacturing procedures and
handling. Thus, both values found in the two studies
are relatively low, indicating a wide spread of flaw sizes
and locations. As observed in the Weibull graph, there
does not appear to be an effect of the location of flaws
as to the strength of the material. The characteristic
strength was 26 ± 2 MPa. Since the fracture initiating
flaws were “natural”, they were not controlled except by
the fabrication and finishing procedure. The sizes should
be comparable to those observed in clinical procedures.
Better control of the fabrication procedures could result
in greater toughness values, but most likely not greater
than ~ 1 MPa , and thus, in greater strengths for the
same size flaws.
The method used to determine the elastic modulus in
this work is relatively straightforward and unique for
resin composites. Since the value agrees with the value
provided by the manufacturer for flexural modulus,
we think this is encouraging in that this presents
TOUGHNESS MEASUREMENT IN DIRECT RESIN COMPOSITES USING QUANTITATIVE
FRACTOGRAPHIC ANALYSIS
21