Biomedical Engineering Reference
In-Depth Information
Fig. 5
Graphs of tensile stress,
σ
, versus tensile strain,
ε
,representing(
a
) Five initial loading-un-
loading cycles and final loading until failure of non-degraded sample (
T
=
0 days) and samples
degraded for 10 and 30 days, respectively (
T
10, 30 days) and (
b
) Close-up of five initial load
cycles of non-degraded sample (
T
=
0 days) and samples degraded for 30 days (
T
=
30 days),
showing decrease of stress at upper limit of cyclic strain of 20 % with increasing number of cycles.
The graph for the sample with
T
=
=
10 days degradation was omitted to improve clarity of the
illustration
value of strain of
ε
0, 4, 8, 12, 16 and 18 %. Stress-strain graphs are presented
in Fig.
5
(a) for the entire strain range of the tests for samples at degradation time
points
T
=
0, 10, and 30 days and in Fig.
5
(b) for limited to the strain range of the
load cycles with upper limit of
ε
=
=
20 % for degradation time points of
T
=
0 and
30 days.
The maximum stress
σ
max
and associated strain
ε
max
versus degradation time
T
are illustrated in Fig.
6
(a). The stress did not exhibit a change after the first five days
of degradation and decreased steadily thereafter. The maximum stress ranged from
σ
max
=
0
.
52
±
0
.
12 MPa at
T
=
0 days to
σ
max
=
0
.
033
±
0
.
028 MPa after a degra-
dation period of
T
=
34 days. When compared to
T
=
0 days, the decrease in
σ
max
was statistically non-significant up to
T
=
14 days but became statistical significant
thereafter. The strain
ε
max
decreased statistically significantly between
T
=
0 days
(
ε
max
=
176
.
8
±
21
.
9 %) and
T
=
14 days (
ε
max
=
46
.
72
±
2
.
35 %). After
T
=
14
days, the decrease of
ε
max
to the minimum of 24
.
6
34 days occurred
at a reduced rate and was non-significant. Figure
6
(b) illustrates the stress
σ
20 %
,i
for each repetitive loading event (
i
=
±
3
.
0%at
T
=
1 to 6) at each degradation time point. Gener-
ally, the stress
σ
20 %
,i
decreased with repeated loading. The reduction in stress due
to repeated loading (cycling) was less pronounced, and not statistically significant,
for the degradation periods up to
T
=
18 days. During the sixth loading, the stress
σ
20 %
,
6
reached between 92
.
4
18
days) of the initial value at the first loading
σ
20 %
,
1
, see Fig.
6
(c). At degradation of
T
±
2
.
1%(at
T
=
0 days) and 90
.
6
±
1
.
9%(at
T
=
22 days and longer, the reduction of
σ
20 %
,i
due to repeated loading increased
with degradation time and the ratios
σ
20 %
,
6
/σ
20 %
,
1
became statistically significant
at
T
=
=
30 and 34 days (
p
=
0
.
00016 and 0.00014, respectively, when compared
to
T
=
18 days). At
T
=
34 days, the ratio
σ
20 %
,
6
/σ
20 %
,
1
was at a minimum of
28
.
5
±
16
.
4%.
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