Fig. 6: Creep-fatigue test with initial compression is compared with
that with initial tension: (a) and (b) the time evolution ofσ n and ρ for the test condition oft t=100 s, t c=20 s (i.e.t t>t c); (c)
and (d) the time evolution of σ n and ρ for
the test condition of t t=20 s,t c=100 s (i.e.t t<t c)
Fig. 6d depicts the time evolution of ρ curves for these two test
conditions. After each cycle, the ρ curves drop to 0, indicating
that all of the cavities nucleated in the tension phase are sintered
during the subsequent long compressive hold
(t c=100 s). By comparison of Fig. 6b with 6d, it
can be known that the nucleation rate in the tests witht c=100 s is almost ten orders of magnitude higher
than that in the tests with t c=20 s. This is due
to the fact that the maximum σ n is higher in the
test with t c=100 s (1043 MPa in Fig.6a) when
compared with that test with t c=20 s (761 MPa in
Fig.6c). However, this does not necessarily lead to the overall increase
of cavitation damage as the prolonged compressive hold would also cause
sintering and close all cavities nucleated in the previous tension
phase.
3.3 Combined effect of tensile and compressive
holds
Combined effect of t t andt c on the final ρ after 10 cycles is
presented in Fig. 7a by the contour plot. Note that except for thet t and t c, the default
value as given in Table 2 is used for all the other load-waveform
parameters and temperature. The contour curves of 0.1 and 0.0001 in Fig.
7a are very close to each other, suggesting that the contour curve of
0.1 can be used as the lower bound value. In other words, below this
value, the final ρ is negligible. There is a threshold value oft t for cavitation whent c=0 s, indicating that the nucleated cavities
can be sintered by the load reversal from tension to compression. In
addition, under the condition of t c=0 s, the
contour curves with values of 0.5 and beyond do not intercept with the
x-axis. This indicates that there is an upper bound value fort t; any prolonged tensile holdt t than this critical value would not further
contributing to the final ρ . This is probably attributed to the
rapid decrease of σ n during tensile hold as
informed by the exponential decay function in Eq.(2).
Now let’s focus on the changing values of t c.
When t t is less than 60 s, increasingt c helps to reduce the final ρ . But whent t is greater than 60 s, an increasedt c promotes the increased final ρ until
reaching the allowable value as highlighted by the magenta dash curve in
Fig. 7a. It is also evident that the allowable t cincreases with the increasing t t. The presence of
allowable t c is the result of two competing
effects: (i) the increased t c can promote the
cavity nucleation rate by affecting the maximumσ n in the next tension phase; (ii) the prolongedt c increases the likelihood of cavity sintering
process, given the compressive stress.
Furthermore, the combined effect of t t andt c on the final f is presented in Fig. 7b.
The distribution of contour curves looks similar by the comparison of
Fig. 7a with 7b, suggesting that the dependence of final ρ andf on the load waveform parameters are highly consistent, i.e.
both are positively correlated with the maximumσ n.