|
4-4
.
Figure 4-4: Figures of a double-torsion setup and the loading scheme of the specimen
.
Experimental
61
Fracture toughness was determined in DT testing simply by loading a specimen rapidly and
recording the maximum load at failure (
F
IC
). The used fracture toughness calculation formula
[193] :
𝐾
𝛪
= 𝐹
𝐼𝐶
∙ 𝑊
𝑚
∙ [
3(1 + 𝜈)
𝑊 ∙ 𝑡
4
∙ 𝜉
]
1
2
⁄
(4-1)
𝜉 = 1 − 1.26(𝑡 𝑊
⁄ ) + 2.4 ∙ 𝑒𝑥𝑝 [−𝜋𝑊 2𝑡
⁄ ]
(4-2)
where
W
,
L
and
t
are the width, length and thickness of the specimen, respectively,
W
m
is the
moment arm,
ν
the Poisson’s ratio,
𝜉
a thickness correction factor.
The list of double torsion tested specimens and corresponding experimental conditions are given
in
Table
4-5
.
Experimental
62
Table 4-5: Details of double torsion tests.
Material
Type
Number of
specimens
Test conditions
Environmental
conditions
Analyzed
effect
NiO-3YSZ
Standard
3
Pre-crack +
1000 µm/min
RT
Pre-crack
20
1000 µm/min
RT
Standard
1
2000 µm/min
RT
SCG
4
100 µm/min
RT
5
10 µm/min
RT
3
1 µm/min
RT
2
1000 µm/min
800 °C
HT
Ni-3YSZ
Standard
3
Pre-crack +
1000 µm/min
RT
Standard
3
Pre-crack +
1000 µm/min
800°C
HT
NiO-8YSZ
4
2
1000 µm/min
RT
No pre-
crack
4
5
Pre-crack +
1000 µm/min
RT
Standard
4
2
Pre-crack +
100 µm/min
RT
SCG
4
2
Pre-crack +
10 µm/min
RT
3
4
Pre-crack +
1000 µm/min
RT
Type
2
3
Pre-crack +
1000 µm/min
RT
4
2
Pre-crack +
1000 µm/min
800 °C
HT
Ni-8YSZ
4
2
Pre-crack +
2000 µm/min
RT
Type
3
1
Pre-crack +
2000 µm/min
RT
2
1
Pre-crack +
2000 µm/min
RT
4
1
Pre-crack +
5000 µm/min
RT
SCG
4
1
Pre-crack +
1000 µm/min
RT
4
1
Pre-crack +
100 µm/min
RT
4
2
Pre-crack +
2000 µm/min
800°C
HT
4 re-
oxidized
2
Pre-crack +
2000 µm/min
800 °C
Re-
oxidation
Experimental
63
4.3.2.
Creep behavior
4.3.2.1.
Compressive test
Compressive creep behavior was analyzed in tests in which bar-shaped (cuboid) specimens were
uniaxially loaded between two supports. The tests were performed using an Instron 1362 testing
machine. A linear variable differential transducer (Sangamo, LVDT, range ±1 mm, precision
1.25 μm) was used for measuring the vertical displacement. The transformer was assembled with
the half-sphere in the clamping device by an alumina rod (
Figure
4-5
). The load was measured
by load cells with 10 KN measuring range (1210 BLR, Interface Company). The temperature
was monitored close to the outer specimen surface with a thermocouple type K.
Figure 4-5: Compressive creep test set-up.
The surfaces of the specimen were previously ground and polished to ensure flatness and
parallelism. The creep strain was determined by following equation:
𝜀 =
∆ℎ
ℎ
0
(4-3)
where
∆h
is the deformation measured during the test and
hₒ
is the initial height of the specimen.
A Norton law (Equation 3-10) was used in current study to obtain creep parameters of materials.
By plotting the natural logarithm ln of the steady state creep rate
𝜀̇
against
ln (σ)
, at a constant
temperature
T
,
n
is determined as the slope in the plot. The activation energy
Q
was calculated
by plotting the
ln
(
𝜀̇
) against the reciprocal of the absolute temperature (
1000/T
) at a constant
stress.
Experimental
64
Tests were carried out on reduced anode bar-shape specimens from 800°C to 900°C under 4 %
H
2
/Ar to protect the material from oxidation occurring at elevated temperatures. The specimens
were heated with a heating rate 8 K/min. The thermal equilibrium was considered to be reached
after 1 h dwell time. A variable stresses was applied during the experiments as illustrated in the
test scheme in
Table
4-6
.
Table 4-6: Compressive creep tests.
Material
Applied stress (MPa)
Temperature (°C)
Number
of test
Environmental
conditions
Ni-8YSZ
(~3 x 3 x 8 mm
3
)
30
800
1
H
2
/Ar
850
1
900
1
63
800
1
850
1
900
1
100
800
1
850
1
900
1
4.3.2.2.
Bending test
Several types of bending tests were performed using an electromechanical testing machine
(Instron 1362). The central displacement was measured with a sensor attached to the tensile
loaded sample surface. A ceramic extension rod connected to a linear variable differential
transducer (Sangamo, LVDT, range ± 1 mm, precision 1.25 μm) provided the actual
displacement. The load was measured with a 1.5 KN load cell (Interface 110 BLR). The set-up
permitted measurement from room temperature up to 1000°C. The temperature was monitored
close to the outer specimen surface with a thermocouple type K. Two different bending
techniques have been used, ring-on-ring test and four-point bending test, for plate- and bar-
shaped specimens, respectively. The distinct atmospheres (air or 4% H
2
/Ar) were varied
according to the measurement requirements.
Experimental
65
(1) Ring-on-ring bending test
The technique is based on the bending of a thin circular (or rectangular) plate-shaped specimen.
A loading ring bends the specimen vertically constrained by a supporting ring (
Figure
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