TOUGHNESS MEASUREMENT IN DIRECT RESIN COMPOSITES USING QUANTITATIVE
FRACTOGRAPHIC ANALYSIS
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.
Upon loading, the geometry of the impression will be
determined by the shape of the diamond (cf. Figure
1). Upon unloading, the shorter direction (2b*) will
contract (2b R ) due to the elasticity of the material. The
elongated end (2a*) will not be measurably changed
because of the length. Thus, the difference between the
original measurement of the diagonals (from the shape
of the diamond) and the impression on the material
will provide a measure of the elasticity of the material.
The E/H ratio can be calculated from the measurement
of the diagonals as shown in equation 3 [22]:
(3)
where H is the Knoop hardness, ν is Poisson’s ratio (0.3),
γ = 75 (the average half angle of the Knoop indenter),
b* is half the minor diagonal at maximum load, and b R
is half the residual minor diagonal that is measured. The
value of b* can be calculated for the Knoop indenter
because it is related to the major diameter, i.e., b* =
20
Figure 4. Example of a surface crack. Yellow lines indicate the width and
depth of the crack at the fracture origin. Black arrows indicate direction of
crack propagation away from the crack origin. White arrows indicate twist
hackle marks on the fracture surface.
Table 2. Average values of obtained data.
Physical Property
Fracture toughness (MPa
Standard Deviation
)
0.5
0.2
Vickers Hardness (MPa) 509 27
Y (geometric constant) 1.28 0.04
10 1.7
Elastic Modulus (GPa)
90% Confidence Interval
Unbiased Weibull Modulus 4.4 3.4 - 5.6
Characteristic Strength (MPa) 26 24 - 28
7.11a*, where a* is half the major diagonal, assumed to
be the same before and after indentation.
Stoma Edu J. 2018;5(1): 18-23.
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