dissertation global and local fracture properties of metal matrix ...

4.1.1.3. Fracture mechanics tests
Section 4
Compact tension (CT) specimens with a thickness of B = 12.5 mm, a width of W = 40 mm,
and an initial crack length of a0 ≈ 20 mm are machined for the fracture mechanics tests of the
cast MMCs (Fig. 4.4). The specimens have a longitudinal-transverse (LT) crack plane
orientation.
To insert the pre-crack in the CT specimens, they are subjected to cyclic compression loading
with ∆K = 10 MPa m and further to cyclic tensile loading with ∆K = 5 MPa m . Fracture
mechanics tests are conducted using the “ZWICK” testing machine which is equipped by a
special computer program for fracture mechanics tests. The cross-head velocity is of 0.02
mm/min for the reinforced materials, and 0.08 mm/min for unreinforced materials. A potential
drop method is employed to determine the crack extension during the fracture mechanics
tests. In this method, a constant electric current is sent through the specimen. An increase of
the crack length changes the electric resistance of the system and the measured potential.
From the change of the potential, the crack extension can be calculated by the Johnson
equation (4.1) [69],
⎡
⎤
⎢
⎥
⎢
⎥
⎢
π
cosh
⎥
2W
⎢
⎥
a = ⋅ arccos
2W
⎢
⎥ , (4.1)
π
⎢
⎡
πy
⎤
⎥
⎢
⎢ cosh
U
⎥
cosh
⎥
⎢
⎢ ⋅ arccos
2W
⎥
⎥
⎢
⎢U
πa
o
o
cos ⎥
⎥
⎣ ⎢⎣
2W
⎥⎦
⎦
where 2y is the distance between the points of the potential measurement, U is the current
potential value, and Uo is the initial potential value.
A schematic view of the potential drop method is presented in Figure 4.5. The electrical
current intensity, I, is 10 A. The potential, U, is determined by a nanovoltmeter with an
accuracy of ± 100 nV, which corresponds to an accuracy of the crack length of ± 0.01 mm. A
clip-gage is used to determine the load line displacement of the specimen during the fracture
tests of the cast MMCs. The data on the load, load line displacement, and potential are saved
in a file by the computer program every 10 s. The J-integral resistance curves were
determined according to [73]. The J0.2/Bl -, Ji-values determined from these curves are given in
Table 4.2, as well.
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