Fig. 9 Examples of the assessment of the present simulation for the
average crack length plotted against cycle ratio due to the application
of CAL having different stress amplitudes utilizing specimens with
different surface configurations.
In the TSL test with the H-L sequence shown in Figs. 5 and 6, the
specimen at point B due to the application of the CSA 432.5 MPa
experiences less Z, and cracks relatively longer than those at point A
due to the application of the CSA 390 MPa, Thus, this virtual test
starts the application of the lower stress with relatively long cracks
generated by the coalescence of smaller cracks at the higher stress. To
illustrate, one of those long cracks is assumed generated by the
coalescence of a number of smaller cracks. That crack can be easily
proved to have a growth rate higher than the summation of the growth
rates of the smaller cracks together. Thus, point D in Fig. 5 is on the
left of point D\.
In the TSL test with the L-H sequence, the virtual test starts the
application of the higher stress with relatively short cracks developed
at the low stress compared with those cracks resulting in if the higher
stress were applied from the start of the test. Following the above
discussion of the behavior due to the H-L sequence, point C is expected
on the right of point C\. However,
point C in Fig. 5 is slightly on the left of point
C\. The results of the present runs
show that the rate of crack coalescence sharply increases for a period
just after the start of the application of the higher load. That action
adjusts to have the CDC of the specimen similar to that of the CSL at
the higher load. The shift C-C\ is due
to the fast transition at the higher load from short cracks to
relatively longer cracks.
The above two configured specimens were used to construct Fig. 10. This
figure presents the results of virtual TSL tests utilizing the two
stresses listed in Table 1(a) but at different ratios of\(\ \frac{N_{1}}{N_{f_{1}}}\) with the L-H and H-L sequences together
with the corresponding two experimental data. As experimentally observed
in relevant literature, \(\sum\frac{N}{N_{f}}\ \)is generally
greater than 1 for the tests of L-H load sequence and\(\sum\frac{N}{N_{f}}\) is less than 1 in the case of H-L load
sequence.