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Effect o f CP and Different Stress Level

In general, the results obtained have shown that the T-plates have a lower fatigue life when compared to ground welded plates. This is expected and is due to the weld toe geometry and hence the low initiation life o f the welded T-plates. From the S/N data presented in Figure 3.14 and Figure 3.15, it can be seen that there are differences in the performance of T-plates when compared with parent and ground welded plates. Looking at the performance in air for example, increasing the stress level had different effects on the three categories of specimens. On average, a 30% reduction in life was observed for the welded T-plates compared with the ground welded plates.

The behaviour in air and in seawater is markedly different for parent plate and flush ground weld specimens but less so for the T-butt plates. In practice, the seawater behaviour o f the three types of specimens is quite close, thus supporting the suggestion that the initiation life is the distinguishing feature among the test results.

The comparison o f fatigue lives for T-butt plates and parent plates cannot be straight forward, as the T-butt plates were conducted under variable amplitude loading, while the parent and ground welded plates were tested under constant amplitude loading. In most cases, under variable amplitude loading, the higher peak loads may lead to early crack initiation. Alternatively severe plastic deformation at the crack tip may lead to plasticity induced crack closure, which may have an influence on the crack propagation under variable amplitude loading conditions. The resulting fatigue lives therefore, for tests conducted under variable amplitude loading conditions will depend largely on the balance between crack advance and crack retardation mechanisms. Where there is this balance, one may expect lives under variable loading to be comparable to those under constant amplitude loading. This will be particularly applicable at stress levels where the clipping

Chapter 3 Fatigue Testing o f High Strength Steels

ratio is significant as is typical for offshore applications where structures are exposed to multi sea-state loading. Under this type o f loading, where high clipping ratios are involved, there is a higher probability o f larger than expected plastic deformation at the crack tip and this may lead to significant differences between constant and variable amplitude behaviour.

With respect to the effect of CP, results show that the CP level has a detrimental effect on the fatigue life for both constant and variable amplitude tests. The main factor involved in shortening the fatigue life o f specimens under higher CP potentials can be attributed to hydrogen. The mechanism o f hydrogen embrittlement is known to increase the crack propagation rate possibly through enhanced grain boundary attack. But under variable amplitude loading, where there is a combination o f crack retardation and acceleration, the effects o f hydrogen are not known fully. In addition, the rate o f charging at the crack tip with hydrogen will depend largely on the different level o f protection used. At the higher level o f protection more hydrogen will be generated from the electrochemical reactions involved and this may explain the higher crack growth rates observed for higher CP potentials when compared with lower potentials. Since most of the lives are shorter than that o f air, it can be assumed that the retardation effects were not dominant. The oxide induced retardation mechanism did not occur, as this could be put down to the length of the tests. In longer test on tubular joints, calcareous deposits were observed on the crack surfaces. This was also observed on the surface of tests as shown in Figure 3.36.

For the constant amplitude loading of the parent and flush ground welded plates, the S/N plot (Figure 3.14) shows a distinct difference for air and seawater tests under CP. This clear distinction is not seen for tests conducted on T-butt welded plates.

The percentage changes in fatigue life for different loading and CP conditions are given in Table 3.17 to Table 3.20 show the relevant S/N data matrices for parts 1, 2, and 3 respectively. Table 3.19 shows that a CP level of -1050mV leads to a reduction in life of 87% and 56% respectively for tests conducted at 350 MPa and 412 MPa for the parent plates. For ground welded plates, the effect of stress range seems to be less significant

Chapter 3 Fatigue Testing o f High Strength Steels

when compared to the effect o f CP since tests conducted at 146 MPa and 172 MPa show a reduction in life o f 83% and 88% respectively.

Table 3.19 shows that the effect o f CP seems to be less severe on the fatigue behaviour of thick T-butt welded plates. For tests conducted at an equivalent stress range of 146 MPa and a CP level of-1050mV, the observed reduction in fatigue life ranges from 48% to 57% when compared to tests conducted in air. Table 3.19 also shows that the corresponding reductions in life observed for tests conducted at the lower CP level of -800mV falls in the range of 20% to 33%. At the higher equivalent stress range of 172 MPa, the reduction in life due to CP falls between 52% and 70% for tests conducted at- 1050mV. On the other hand, tests conducted at the lower CP level of -800mV show reductions in the range of 42% to 64%. The results obtained from this study show that there is a higher percentage reduction in life for the higher CP level of-1050m V when compared with tests conducted at a CP level of-800mV. Looking at tests conducted on T- butt welded plates, increasing the CP level from -800mV to -1050mV leads to, on average, a 40% reduction in life for 146 MPa. An average reduction o f about 30% seems to be applicable for tests conducted at 172 MPa apart from test T07 that shows an increase in life o f about 40% when compared with T12 carried out under similar conditions. These results are consistent with results obtained from a previous study [3.16] on large-scale tubular welded joints, where a reduction factor o f 30% was observed for cases where the CP level was increased from -800mV to -lOOOmV for hot spot stresses ranges in the region o f 200-225 MPa.

Different Sub-block Duration Period

As for the fatigue life for different duration period, the effect o f increasing or decreasing the time was not so distinctive. From the S/N plot shown in Figure 3.16 it could be seen that the fatigue lives are not so distinct for different duration periods. The results are also similar to those obtained in part 2 of the study and the relevant reduction factors are shown in Table 3.19. However, there are differences between the two duration periods examined especially in terms o f crack propagation. The results obtained for the 30 minutes duration period show a tendency for higher crack propagation when compared with results obtained for tests conducted with a duration period of 10 minutes. The

Chapter 3 Fatigue Testing o f High Strength Steels

sequence used in most o f the tests is 20 minutes duration period. However, the results in Figure 3.21 do show that the 30 minutes period is more similar to the 20 minutes than the 10 minutes period.

The sub-block duration period is the fixed time the sea-state remains before moving to the next sea-state or remaining in the same state. The longer the transition period, the more likely it is that once a more damaging sea-state with a lower probability o f occurrence is reached, it will persist in the sequence for longer leading to more fatigue damage. Thus this could be the case with the 30 minutes sub-block duration sequence, where the crack propagation was at its most damaging. This could have been caused when the most damaging sea-states was selected as the starting sea-state. A further investigation into the effect of different sub-block duration was needed and this could be done analysing the Markov chain process and sea-states selection process. Some further analysis is reported in Chapter 5.

Different Backing Material

Two different backing materials were considered, ceramic and metallic. It was noted from the start that the quality of welds, were the same on one side, but very different on the other side. The differences are shown in Figure 3.42 for ceramic backed T-butt and Figure 3.43 for metallic backed T-butt. For the ceramic backing plate, the finish on the backside was smooth with less steep weld angle (where the ceramic plate was attached and later knocked out). The metallic backing plate was completely different, with the backing plate still attached to the plate itself as shown in Figure 3.43. This provided difficulties, in terms o f inspection and monitoring of the weld toe and perhaps raising the geometric stress around the weld toe region.

The plates were tested in air at two different stress levels, 146 MPa and 104 MPa. The results perhaps were not surprising, as the ceramic backing plates performed better than the metallic backing plate in air. As for the double sided welded T-butt, at 146 MPa in air, the ceramic backing plates also performed better, while the metallic backing were about the same. For the single sided welded T-butt, all the cracks appeared on the non-backed

Chapter 3 Fatigue Testing o f High Strength Steels

side for ceramic backed plate and on the backed side for the metallic backed plate (as shown in Figure 3.44 and Figure 3.45).

The improved performance o f ceramic backed plate may be due to the improved weld profile with smaller stress concentration factor. Stress concentration factors (SCF) were calculated for both side of the ceramic backed plate using parametric equations [3.17]. The weld toe radiuses for both side o f the ceramic backed plate were relatively large but similar at around 4mm. The real difference was in the weld angles where it was 64 degrees for the non-backed side and 45 degrees for the ceramic backed side. Thus the SCF is slightly higher at the non-backed side at 2.13, while it was 2.05 for the ceramic backed side. This also could explain the improved performance when compared to the double sided welded T-butt as the SCF here is higher at 2.4.

However, all this benefit is lost for metallic backed plate due to the attached metallic plate. Figure 3.43 has a drawn weld profile in the metallic backed plate to illustrate how far the weld has penetrated. Figure 3.45 shows that the crack grows underneath the weld profile o f the metallic backing material. The fatigues lives fi-om this experiment has shown that the metallic backed plate is comparable to double sided welded T-butt but less than ceramic backed plate. However, the metallic backed plate should have a penalty in its fatigue design life as it cannot be inspected and that the crack initiated under the metallic backing material.

Two tests were carried out in seawater for the ceramic backing plates, under two CP levels o f -SOOmV and -1050mV at 146 MPa. With a limited number o f tests, the results should be looked at with caution. At -SOOmV, the fatigue life was very good, with only a small reduction of life. This fits in well with the results obtained earlier for the double sided welded T-butt joints. The result for -1050mV is also similar to the double sided welded T-butt joints, showing a bigger reduction in fatigue life. Looking at these results for the CP, one can conclude that CP is detrimental to the fatigue life, especially at higher level of protection (overprotection) while at lower level; the effect is not so severe.

Chapter 3 Fatigue Testing o f High Strength Steels

The results from the LLT showed that for the thinner plates 690MPa yield steel; the effect of CP at long life can be quite severe. The low-alloy steel tested in the current programme had an average fatigue life o f around 500,000 cycles at a stress range o f 108 MPa with a CP o f -1050mV. In comparison, the tests at -SOOmV at the same stress had a run-out at 3,800,000 cycles, a factor of seven. However, it should be remembered that this was a different material tested with a different dimension.

In contrast, the tests at higher stresses in Part 2 on the thicker steel showed only a factor of only 1.42. This difference is either a feature o f lifetime and/or composition and it would be valuable to conduct further tests in the long life region with the thicker plate steel with the same minimum yield stress of 690MPa.

Conclusion

Figure 3.15 shows all the data for T-butt welded specimens from both Part 2, 3 and 5. Of concern is the short fatigue life of the long life tests, which are close to and below the P curve (F and F2 Class) CP line. The figure also shows that the behaviour at a CP of -800mV is clearly superior.

Overall, all the results show that the steels perform satisfactorily as can be seen in the S/N curve where the performance o f T-butt plates in air, is compared with results for 50D tests (Figure 3.17). All o f the tests in air for the thick SE702 T-plates lie above the class F and F2 50D mean curve (Figure 3.15). Also the seawater results for thick T-butt tested under CP all lies above the class F and F2 50D mean line for CP. Thus there is a slight improvement in fatigue performance o f these specimens compared to the 50D steels. The results are limited though and more tests are needed to confirm the trends.

Chapter 3 Fatigue Testing o f High Strength Steels