Teoría del Significado y de la Creencia: la Interpretación Radical

All the fatigue tests carried out on T-butt welded plates in Chapter 3 were completed under variable amplitude sequence. There were three variable amplitude sequences employed. All o f the sequences were generated using the Jack-up standard offshore sequence generator, JOSH. The 20 minutes sub-block duration sequence was employed in most o f the tests. However, it was noticeable that the 20 minutes sequence was not stationary. This point will be discussed in more detail in Chapter 5. The consequence for the sequence not being stationary is that the calculated equivalent stress ranges is not stable for different length of the sequence. This implies that the equivalent stress range for 10,000 cycles test will be different from 20,000 cycles. Thus, the actual equivalent stress ranges had to be recalculated again for each tests. The recalculated stress ranges were plotted again on the S/N curves to give a fairer comparison o f the fatigue performance. The revised S/N plots are categorised into three different groups, air tests and the seawater tests with two different CP level, -800mV and -lOSOmV.

Chapter 4 Analysis Work on Fatigue Test Results

Usually the ways in which the equivalent stress is calculated are as follow. The sequence is generated using JOSH with specific sub-block duration time. Normally 20 minutes is chosen as the sub-block duration length. In the case here, the generated sequence is made up o f the twelve sea-states employed by JOSH. The final sequence is displayed as turning points or peaks and troughs against time. The amplitudes o f these point ranges from +100 to -100 units. Cycle counting is performed on these points; in this case rainflow counting technique was employed. The equivalent amplitude range for the sequence can be found firom the stress ranges calculated by cycle counting. This can then be used to find the factor for amplifying the sequence into stress ranges. Thus the equivalent stress range calculated using this process is for one complete sequence. However, if the sequence was not a stationary process, the equivalent stress range will be different if the tests ended before the complete sequence or in between the start and the end o f the sequence. For the revised S/N plots, the rainflow counting is done for the actual length o f fatigue life o f each tests, thus the equivalent stress range calculated will be the actual stress ranges experienced.

4.1.1 Discussion on Revised S/N Plots

Most o f the T-butt welded plates were tested using 20 minutes sub-block JOSH generated sequence. The revised S/N plot for air tests is shown in Figure 4.1. For the seawater tests, the revised plots for CP at -SOOmV and CP at -1050mV are presented in Figure 4.2 and Figure 4.3 respectively. All the plots are displayed in comparison to the original equivalent stress range.

For the fatigue air results, the recalculated equivalent stress range affected the S/N curve for the higher stress range, resulting in shorter fatigue life than expected. In this region the loading sequence has not reached the stationary point and the equivalent stress range is likely to fluctuate. Figure 4.1 shows the old equivalent stress range against the new stress range for the air tests. The new equivalent stress range decreased the scatter for the air curve and it also shifted the trend line closer to the expected slope o f m = -3 .

Chapter 4 Analysis Work on Fatigue Test Results

Overall the air fatigue result for the variable amplitude loading is slightly better than BS4360 50D [4.1] material tested under constant amplitude loading. It was expected that the higher strength steels should perform marginally better than the BS436 50D material. The reason for the similar performance could be due to the data obtained for the BS436 50D S/N curve employed here. The fatigue result for 50D material under a similar set up and condition would give a more realistic comparison study. However, the fatigue result is satisfactory.

By recalculating the equivalent stress ranges, similar effects was expected for the corrosion test as for the air tests. The recalculated S/N curve is shown in Figure 4.2. This did not reduce the scatter as much but it did shift the slope o f the line closer to m = -3. As discussed in the earlier chapter, the effect o f CP level at -SOOmV on the fatigue life of SE702 material was not as detrimental.

For the overprotection CP level of -lOSOmV, the recalculating o f the equivalent stress ranges was expected to affect the S/N curve the most since most o f the fatigue lives under this condition were short. The recalculated S/N curve is shown in Figure 4.3. All the fatigue results were shifted up, thus showing an improved performance. The scatter o f the data was also reduced

Most o f the fatigue tests on T-butt welded joints were double sided welds. Small number o f tests was carried out on novel welded Joints. These plates are welded from from one side, where backing plates made from ceramics or steels were placed on the opposite side to hold the tee in place. The representative diagrams of the plates are demonstrated in Figure 4.4. On one side the weld profiles are similar to that of the double-sided weld. However on the other side, the weld profile is not. This could result in different stress concentration factor and different cooling rate post heating. The recalculation o f the equivalent stress range did not affect the overall fatigue lives o f such plates since they were tested at lower level o f stress ranges. The limited number o f fatigue results shows the performance of single sided welded plates to be comparable to the double-sided weld plates. All the failure for the single sided through weld occurred on the side with the backing material. This was expected due to the weld profile creating higher stress concentration factor.

Chapter 4 Analysis Work on Fatigue Test Results