Fig. 4 Fatigue growth behavior of a short surface crack whose results
are presented in Fig. 3 should its length be measured at intervals of
(A) 10000 cycles, (B) 20000 cycles and (C), and 30000 cycles
The above example considers one crack out of fifteen cracks at a surface
site. Should other individual cracks be in focus, difference in results
is expected. Further, cracks existing at other surface sites are
expected to have different activities. All the above aspects give rise
to the experimentally observed anomalous fatigue growth behavior of
short cracks.
Figure 5 shows examples of the computed \(A_{C}\) plotted against the
number of cycles \(N\) as a result of running four virtual tests
relevant to the experimental parameters listed in Table 1(a). The four
runs started with the same specimen configuration. The two lines
O-A-D\ and
OBC\ present the results relatively out
of the two constant stress amplitudes (CSA) 390 MPa and 432.5 MPa. In
both two-step loading (TSL) tests, the application of\(\sigma_{o_{1}}\)for \(N_{1}\) cycles results in a behavior similar to
that given due to the application of the CSA of 432.5 MPa and 390 MPa ,
i.e. the line O-A in Fig. 5 for the L-H sequence and the line O-B for
the H-L sequence. The remaining life\(\text{\ N}_{2}\ \)at\(\text{\ σ}_{o_{2}}\) results
in\(\ \sum{\ \frac{N}{N_{f}\ }}\) equal to 1.25 for the virtual
L-H test and 0.79 for the H-L virtual test. Here, \(N_{f}\) refers to
the number of cycles to failure at a CSA test. The corresponding
experimental \(\ \sum{\ \frac{N}{N_{f}\ }}\) are 1.35 and 0.823. The lines B-C and A-D give the duration\(\text{\ N}_{2}\ \)for the sequences L-H and H-L respectively. Another
specimen differently configured was utilized to repeat the above four
virtual runs with the same\(\ N_{1}\). The resulting\(\ \sum{\ \frac{N}{N_{f}\ }}\) is and 1.18 for the virtual L-H
test and 0.83 for the H-L virtual test.