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All the T-butt welded plates had Alternating Current Potential Drop (ACPD) [4.2, 4.3] probes spot welded to the weld toes. So when the plates were fatigue tested, the area around was under constant surveillance for any signs o f a surface crack. The ACPD technique can also be used to monitor the fatigue crack growth as it works by measuring the surface crack depth. Thus this technique can be used to detect the fatigue crack initiation life. The technique should be able to detect crack increments o f 0.05mm relatively accurately. The initiation depth of 0.1mm was employed by the fatigue studies on tubular joints at UCL [4.4, 4.5, 4.6 and 4.7]. For this particular experimental work on T-butt welded plates, the output potential was low and was prone to scatter, 0.2 mm was used instead as the crack initiation depth.

4.2.1 Crack Initiation Results

The fatigue initiation data from the tests are displayed as a ratio of the initiation life (Ni) to the total fatigue life (Ni). The fatigue initiation lives are displayed against the fatigue lives as bar chart and S/N curves. The resultant bar chart is shown in Figure 4.5 as the proportion o f initiation life to the joint fatigue life. The initiation lives are recalculated as the percentage o f the fatigue initiation life against the fatigue life presented in Figure 4.6. The stress ranges in these charts are the hot spot stress ranges. The resultant fatigue initiation data are displayed in Table 4.1. Figure 4.7 represents the fatigue life initiation and the total fatigue life as an S/N curve.

Chapter 4 Analysis Work on Fatigue Test Results

4.2.2

The notable effect of increasing the stress range is the ratio o f initiation life to the total fatigue life, , decreases. This was more noticeable for the air test, but the effect was less for the seawater tests with CP. For the air test, the average initiation percentage at 350 MPa is 20.5% and for 412 MPa is 6.7%. For the single side welded plates with ceramic backing, the initiation percentage for the test at 250 MPa is 55.4% and at 350 MPa is 28.4%. The air S/N curve (Figure 4.7) shows the trend line o f the initiation life to be linear with the slope being less steep than the fatigue life trend line. For the seawater tests under CP, the S/N curve for the initiation point is shown in Figure 4.8. The seawater fatigue lives had more scatter than the air data.

For the same stress range at 350 MPa, the changing o f the CP level from -800mV to -1050mV changes the initiation life percentage from 14.5% to 9.1% respectively. For the stress level at 412 MPa, the initiation life percentage is 20.2% for -800mV and 13.4% for -1050mV. Most o f the initiation life percentages are small so that the difference in the percentage changes is not significant. Also the fatigue lives are short due to overprotection thus the initiation lives are expected to be low. It can be concluded that by increasing the CP level, the initiation life percentages as well as the fatigue lives decreases. When the initiation percentages for the CP tests are compared to the air tests for the 350 MPa stress range, the overall initiation life percentage is slightly higher for air tests than seawater tests. However, for the higher stress range, 412 MPa, the overall air initiation percentage is slightly lower than the seawater tests. Thus the effect of CP at higher stress ranges increases the initiation life percentage. At higher stress range, the tests in seawater are carried out under extreme condition, which resulted in shorter fatigue lives. Most of the initiation lives are also relatively short compared to the overall fatigue life. Also the scatter in the data was relatively high making it very hard to establish a trend and more tests results are needed to confirm the effect of CP on the initiation life.

Chapter 4 Analysis Work on Fatigue Test Results

Other high strength steels results from past studies at UCL for the initiation life were available for tubular welded joints. In Table 4.2, the initiation lives for tubular joints are represented as well as the thick parent plate tested in Chapter 3. Austin [4.4] and Vinas- Pich [4.7] studied corrosion fatigue on tubular joints fabricated from BS 4360 50D. The tests were carried out under pseudo variable amplitude sequence generated by WASH. Etube [4.5] tested SE702 tubular joints in corrosive environment under pseudo variable amplitude loading sequence generated by JOSH., while Myers [4.6] tested SE702 tubular joints in corrosive environment under constant amplitude. The initiation life and overall fatigue life from Myers and Etube studies are presented as S/N plot in Figure 4.9 (air tests) and Figure 4.10 (seawater under CP tests). The air data as shown in Figure 4.9 is limited and nothing conclusive can be concluded. Figure 4.10 shows the results for CP tests. The data are for different type of tubular joints and one set was tested under variable amplitude loading, while the other was tested under constant amplitude. The limited data shows that the initiation life percentage was higher for the -lOOOmV than the -SOOmV. This shows that the crack initiate at later stage for higher CP level. This could be due to several factors, such as formation of calcareous deposit on the joint surface, which could inhibit the growth o f small cracks. Also the hydrogen produced by CP, could affect the steels mechanical properties by making it more brittle, thus harder for smaller cracks to initiate. However, the overall fatigue life for the -lOOOmV is lower than that of -SOOmV, thus the benefit gained by longer initiation life is cancelled.

For the parent plates. Alternating Current Frequency Measurement (ACFM) [4.2] array probes were used for crack monitoring. ACFM probes do not require electrical contact with the steel surface to operate. This is useful as the spot welding o f the probes used by ACPD technique can create points for cracks to initiate from. The ACFM array probes has eight probes aligned in a series. The method could not calculate the crack depth as the crack initiated from the side, which does not fit the technique requirement. However, the initiation point can be identified or rather the first changes in the recorded data from the array probes. The fatigue life and the initiation life of the parent plates tested in air are presented in Table 4.2 and in Figure 4.9 as an S/N plot. The initiation life for 350 MPa is 93% and for 412 is 59%. This seems to fit in with the trend for most fatigue air tests that by increasing the stress range, this reduces the initiation ratio to the overall fatigue life.

Chapter 4 Analysis Work on Fatigue Test Results