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.