The specimen used for fatigue crack propagation rate test was centre cracked-plate tension CCT
specimen. Figure 3 shows the geometry of the specimen according to ASTM E647-08 standard [20].
The dimension of the specimen was determined by following equation according to the test standard:
ys W
a BW
P N
2
1
max
Here, σ
N
is the nominal stress, σ
ys
is the yield stress, P
max
is the maximum load, B is the specimen thickness, W is the width of
gauge position and a is
the crack length. A screw type fixture was used in the CCT
specimen. To avoid the excessive lateral deflection or buckling of the CCT specimen during the test, the
gauge length and thickness of gage position was limited to 12 mm and 2 mm, respectively. The gage
position was then polished with 500 to 1500 grit emery papers to obtain a smooth surface.
The fatigue crack propagation rate test was conducted by using a pneumatic fatigue testing
machine 14 kN maximum capacity and to investigate the effect of heat treatment on fatigue crack
propagation behaviour. The tests were performed at frequency of 10 Hz by using sinusoidal loading form.
A stress ratio R = 0.1 was applied in the tests. The loading direction was in the extrusion direction of the
material and the testing was carried out at room temperature.
The crack propagation curve crack propagation rate dadN versus stress intensity factor range ΔK was
obtained by using K-decreasing and K-increasing test procedures. The decreasing and increasing load steps
are 5 - 7 of the previous loading value. The stress intensity factor value for the CCT specimen was
calculated using the following equation:
Fig. 3. Centre cracked-plate tension CCT specimen used in the FCP tests
F a
K
2 Here, Fα is a boundary correction factor which
depends on the ratio of the crack length a to the width of the specimen W. For the CCT test specimen used in
this study, the boundary correction factor is given as [19],
2 4
2
sec 06
. 025
. 1
F
3 where
W a
2
4
The crack length was measured using travelling microscope. The threshold stress intensity factor ΔK
th
was determined when a crack growth is not observed for 10
6
cycles. A hole with a 1 mm diameter was drilled in the centre of the specimen before introducing a 1.35
mm notch by EDM electrical-discharge machining to facilitate fatigue pre-cracking.
The procedure for introducing a pre-crack was followed the ASTM standard [20]. The specimen was
aligned so that the load distribution is symmetrical. The load ratio R during pre-cracking is the same as the load
ratio used in the fatigue crack propagation test. The pre-cracking was interrupted after a pre-crack length
equal to 0.1 of specimen thickness was attained at maintained pre-cracking propagation rates of about 10
-8
mcycle.
III. Result and Discussion
The comparison of fatigue strength of solution treated and extruded AZ61 magnesium alloy is shown
in Figure 4. The figure shows that fatigue strength of the solution treated samples increase as that compared
to the fatigue strength of the as-extruded AZ61 samples. The fatigue limit for solution treated and as-
extruded AZ61 were 180 MPa and 150 MPa, respectively. The higher fatigue strength observed for
10
3
10
4
10
5
10
6
10
7
10
8
100 150
200 250
300
Number of cycles to failure, N
f
cycles M
ax im
um s
tr es
s M
Pa
Solution treated As-extruded
Fig. 4. Fatigue strengths of solution treated and as-extruded samples
a
a 1
TABLE 2 MECHANICAL PROPERTIES OF AZ61 MAGNESIUM ALLOY
Material type
Yield Stress,
σ
y
MPa Ultimate
Tensile Strength,
σ
uts
MPa Vickers
Hardness Hv
As- extruded
244 265
270 309
329 308
67
Ave. of 10 points
Ave. 268
315
Solution treated
308 292
288 381
324 322
71
Ave. of 10 points
Ave. 296
342
the solution treated sample is believed due to higher tensile strength and also higher hardness properties
compared to that of the as-extruded sample as shown in Table 2
. After the solution treatment the increment in
hardness from Hv 67 to Hv 71 is believed due to the
solid solution strengthening. In the heat treatment process, the solution treated samples were heated into
the solid solution zone where atoms of alloying
elements dissolved into the matrix. In this condition, the samples were quenched in water, which limit the
time for precipitation to takes place.
Optical micrographs revealed that there is no precipitation of second phase observed in the solution
treated sample. Further, the aging processes performed
Fatigue crack propagation
Fatigue crack initiation A
s- re
ce iv
ed
L ow
h ar
dn es
s m
at ri
x
S ol
ut io
n tr
ea te
d
H ig
h ha
rd ne
ss m
at ri
x
Cyclic load
Matrix Inclusion
Cyclic load
Inclusion
Matrix Crack
Crack Crack
Crack Matrix
Matrix Su
rf ac
e
Su rf
ac e
P.S.B
High stress Concentration site
Δa
i
Δa
i+1
Δa
i
Δa
i+1
Δa
i+1
Δa
i+1
Δa
i
Δa
i
Fig. 5. Mechanisms of crack initiation and propagation for as-extruded and solution treated AZ61 magnesium alloy
at different aging times and temperatures were unable to achieve higher hardness compared to that of solution
treated sample due to limitation of second phase precipitation. This result was in aligned with the results
obtained by Uematsu et al. who reported that precipitation of Mg
17
Al
12
in AZ61 magnesium alloy is very limited due to low percentage of Al content as
compared to other magnesium alloy with higher Al content such as AZ80 [22].
The increased in hardness of solution treated sample resulted in difficulty for the extrusion and
intrusion of persistence slip bands to occur and consequently the fatigue crack tends to initiate at
higher stress concentration sites such as inclusions or foreign particles at sub-surface. For the as-extruded
samples, the extrusion and intrusion
play a prominent role in initiating fatigue crack. Therefore, the increased
in hardness and yield strength in solution treated samples delay the fatigue crack initiation and result in a
higher fatigue life. The initiation and propagation
mechanisms of fatigue crack in both samples are illustrated in Fig. 5.
Detailed fracture surface observations showed that foreign particle was found at the fatigue fracture origin
of the solution treated samples especially for the samples which exhibited higher fatigue life more than
10
5
cycles as shown in Fig. 6a. The foreign particle size observed was about 20 to 30µm. Pile-up of slips
deformation at near the foreign particle during fatigue cycles contributed to high stress concentration at
around the foreign particle and resulted in fatigue crack initiation. In contrast, SEM observation results on
fracture surface of as-extruded samples showed that there was no evidence of foreign particle at the fatigue
fracture origin. The fatigue crack initiation site was relatively flat as shown in Fig. 6b.
The FCP curve of solution treated AZ61 magnesium alloy as a function of stress
intensity factor
range ΔK at room temperature is shown in Fig. 7. The FCP curve for as-extruded AZ61 magnesium alloy is
also plotted in the same figure for comparison [8]. From the figure, it can be noted that there is not much
difference in the FCP resistance for both curves at a low ΔK
region. However the difference of fatigue crack propagation resistance can be seen at a higher ΔK
region above 2.0 MPa√m. Fatigue crack propagation resistance for solution treated samples is found lower
as compared to the as-extruded samples. It can be considered that the crack in as-extruded AZ61 has
more frequent chances of encounter with grain boundaries due to the smaller grain size, resulting in a
slower propagation rate. This argument is similar to the FCP behaviour of AZ31B-L as mentioned by Uematsu
et al. [22].
The arrows in the figure indicate that the threshold value of stress intensity factor range at ΔK
th
. From the figure, the threshold value for solution treated AZ61
magnesium alloy is at 0.91 MPa√m. This value is almost at par to that of the extruded magnesium alloy
where the threshold value is at 0.92 MPa√m. From the result, it can be concluded that heat treatment does not
affect the threshold value of AZ61 magnesium alloy. However, a slight difference in the fatigue crack
propagation resistance is been demonstrated at higher ΔK
region. Similar FCP behaviour of the AZ91D magnesium alloy was also reported by Kobayashi et al.
[17]. The dadN-ΔK curves obtained at the Paris regime
can be expressed as:
m
K C
dN da
where C is a constant and m is the slope of the curve on the log-log plot. Values of constants C and m were
calculated using the least square method and the results are shown in Table 3.
Fig. 6. Observations were done using SEM equipped EDX. a Foreign particle b Flat surface
0.1 0.5
1 5
10 10
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
Stress intensity factor range, Δ K MPa.m
12
Fa ti
gu e
cr ac
k pr
op ag
at io
n ra
te , d
a dN
m c
yc le
Solution treated As-extruded
Sajuri et al. [21]
Fig. 7. Fatigue crack propagation behavior of solution treated and as- extruded AZ61 magnesium alloy
a
b
5
TABLE 3 CRACK PROPAGATION PARAMETER AT THE PARIS
REGIME FOR AZ61 MAGNESIUM ALLOY
a Overview
b Micrograph of fracture surface at ‘a’ in a at low ΔK region
c Micrograph of fracture surface at ‘b’
in a at high ΔK region Fig. 8. Fracture surface observations of FCP test for solution treated
specimen.
IV. Observation of Fracture Surface
