Otros motivos

The results obtained from the fatigue tests conducted in this study are presented in two forms, the fatigue lives (S/N data) of the specimens and the fatigue crack growth data. Both S/N data and fatigue crack growth results are discussed in the following sections.

3.4.1 S/N Data

The fatigue lives obtained from the tests are presented as S/N curves. These data are for the thickness effects for 16mm thick plates. The thickness effect was discussed earlier in detail in Chapter 1. In order to compare all fatigue lives o f different thickness specimens, the results have to be modified. Thus the stress ranges were corrected according to the equation below, from the HSE offshore fatigue guidance [3.11], for 16mm thick plates.

Sg = S

\ J

Where: Sb is the stress range at the reference plate thickness S is the stress range at the plate thickness t

ts is 16mm y is 0.3

For comparison, the fatigue results for the high strength weldable steels were plotted as S/N points against the conventional offshore structural steel design curves, BS4360 GrSOD [3.12]. The specimens tested have to be divided into different categories or classes so that the correct S/N curve was employed. The parent and flush ground welded plates

Chapter 3 Fatigue Testing o f High Strength Steels

were plotted with the class C BS4360 50D mean line. The T-butt welded plates were plotted against class F and F2 for the BS4360 50D mean line. It was not clear from the Joint Classification guidance [3.13] description as to which class the T-butt welded plates tested belonged to. The T-butt welds belong to section 5.1. Thus both lines, F and F2 were employed. All the mean lines and design lines for the BS4360 50D steels were obtained from the HSE offshore fatigue guidance [3.11]. The equations for the mean curves BS 4360 5CD type steels are given in the equations (3.5) to (3.7). They are for the three different welding classes, C F and F2.

Class C: L og,,(N ) = 14.0342 - ^.Log,,(S,)

Class F. Log’jo(A) = 12.2370 — 3 . L o g ^ Q { S (3.6)

Class F2: Log^^^N) = 12.2370 - ^.LogyQ^Sg) ^ 7^

The S/N data obtained for the thick parent plate and flush ground welded plates under constant amplitude loading conditions are given in Table 3.12 while results from variable amplitude loading o f the T-butt plates are shown in Table 3.13. For the LLT, the fatigue results are shown in Table 3.14. For fatigue tests conducted under variable amplitude loading conditions, the stress range shown is the equivalent stress range for the variable amplitude sequence used while the life represents the number o f fatigue cycles to failure. In each case, the specimen was considered to have failed once the fatigue crack reached half the plate thickness. The resulting S/N curves are shown Figure 3.14 in to Figure 3.17.

Figure 3.14 shows the results o f tests conducted on parent and flush ground welded parent plates under constant amplitude loading conditions in Part 1 o f the study. One of the results obtained for the parent plate P I, was tested at two stress range levels. The first level was at 206 MPa, which lasted for 1,000,000 cycles and the second level at 412MPa for 268,000 cycles. This could explain the shortened fatigue life. The data was corrected using Miner’s rule. The new value calculated is shown in Table 3.15.

Chapter 3 Fatigue Testing o f High Strength Steels

The results obtained under variable amplitude loading for Part 2 are shown in Figure 3.15, which shows the comparison between the results. Figure 3.16 shows results obtained from Part 3 for different transition periods. Results from this study have also been compared with those obtained from HSE [3.11] 50D BS4360 steels. This comparison is shown in Figure 3.17.

For the S/N plots, mean lines were drawn through the data points. These were done in Figure 3.18 to Figure 3.24. However, bearing in mind the limited number of tests conducted, these lines were included to show the general trends rather than the actual mean line. The slopes of the lines were all -3 . In Figure 3.22, Figure 3.23 and Figure 3.24, the lines below two times the standard deviation from the mean result, were plotted for the T-butt plates in Air and CP. The standard deviation values were obtained from the results o f the tests conducted here. The same analysis was not carried out for the parent and ground welded plates, as the number of data points was too small.

In Table 3.16, the results o f T-butt plates are compared, with reference to different transition period and CP level. The results for T-plates TOI to T12 were tested under 20 minutes sea-state duration.

The fatigue lives are also compared as percentages in Table 3.17 and shows the changes in performance under different testing conditions, relative to other tests in the programme, and show the same fatigue life reduction factors, relevant to Parts 1, 2 and 3 respectively. This relative difference in fatigue life obtained for different test conditions is also shown as a bar chart in Figure 3.25.

3.4.2 Fatigue Crack Growth data

All the crack growth data is presented in two forms. These include crack shape evolution curves and through thickness crack propagation curves obtained as the number of fatigue cycles is increased for each test. These are explained below.

Chapter 3 Fatigue Testing o f High Strength Steels

Crack shape evolution data

Crack shape evolution data show crack front profile and how this front advances as the number o f fatigue cycles is increased. This is obtained from measurements taken at suitable intervals along the entire length of the weld toe at appropriately defined ACPD inspection intervals. Sample results obtained from tests are shown in Figure 3.26 and Figure 3.29 for T09 and TIO. Each plot represents only the failed side o f the weld but in most cases, there were cracks on both sides of the attachment plate.

Through thickness crack growth data

Through thickness crack propagation data is obtained by monitoring the crack advance at a single location of interest along the weld toe. This information was obtained from 19 sites on each side o f the weld for every test. Figure 3.28 and Figure 3.29 show typical crack growth curves obtained corresponding to T09 and TIO respectively.

Figure 3.30 to Figure 3.33 show the comparison in through thickness crack growth for different combinations o f environmental and loading conditions. Figure 3.30 compares tests of similar stress range. In this figure, the fatigue crack growth curves for air and two different CP levels are compared at an applied equivalent stress range of 146 MPa. A similar comparison is shown in Figure 3.31 for the higher equivalent stress range of 172 MPa. In both o f these figures, data from air tests are compared with those obtained from tests conducted under CP levels of-SOOmV and-1050mV. Figure 3.32 shows the effect of transition period and compares results obtained for 10 and 30 minutes transitions periods. All the results are further compared in Figure 3.33. The two thick vertical lines seen in each figure represent the repetition of the loading sequence, where the starting cycles tend to have higher overall amplitude. Figure 3.35 shows the crack growth at a lower stress range for the ceramic backing plates.

Chapter 3 Fatigue Testing o f High Strength Steels