2 1 Wave Loading Design
Chapter 4 Analysis Work on Fatigue Test Results
4.5 Discussion on Weided Joint Ciassification for T-butt Weided Piate
4.5.1 Inclusion o f SCF Instead o f Using Weided Joint Classification
In Chapter 3, the difficulties of identifying the class for the four point bending tests for the T-butt welded plates were highlighted. The classification would be more specific to the particular problem if the geometric stress range were used instead o f the nominal stress range. This would also eliminate the need to classify the welded joints and would require fewer or even one fatigue life curve. This would be similar to tubular joints where only one line is employed for fatigue design curve. Thus the inclusion o f SCF for welded joints should be the way forward for more realistic analysis.
Chapter 4 Analysis Work on Fatigue Test Results
The practice o f using nominal stress range could stem from the difficulties in calculating the SCFs in the past or the lack of consistency o f the weld qualities of the joints. However, with the availability of new parametric equations for calculating SCF, it is no longer considered too difficult a task. There had been work carried out at UCL by Brennan et al [4.10] for calculating SCF for T-butt welded plate using parametric equation. As for the argument on the inconsistency o f the weld toe, the weld toe radius should be consistent anyway. In the past, weld toe dressing and post weld heat treatment were not always employed. This would be more to do with the quality control problem, rather than compromising the stress analysis work. If the joint quality is not up to a recommended standard, then it should not use the fatigue design line.
The different classes for the welded joints are just different fatigue life reduction factors. This could be viewed as similar to SCF. For the higher classes, i.e. F, the joint is expected to have higher SCF than joints from class B. So the different classes are in one sense, sorting different order of SCF. This could be shown using a single fatigue life curve with SCF o f the welded joint. However, this new idea will only work if the main differences between all the different joints are mainly due to the effect of SCF.
For example, the new single S/N trend line for class B could be employed instead of F and F2. Where line F and F2 were used for T-butt welded plates fatigue tested under 4 point bending work. The 50D S/N curve for class B, F and F2 are shown in Figure 4.37. The fatigue results from Chapter 3 are calculated for S/N plot with the equivalent stress range, which include SCF o f the weld toe radius. The new equivalent stress range is also thickness corrected for 16mm thick plate. The new S/N values with SCF are shown in Table 4.8 along with their nominal stress range values. For the air tests. Figure 4.38 shows the fatigue lives comparison for nominal stress range against class F and F2 50D mean line with the stress range with SCF against class B 50D mean line. Similarly, Figure 4.39 shows the new values for tests done in seawater with CP. Figure 4.40 combined all the fatigue results from air and seawater tests together Figure 4.1. Figure 4.41 shows only the stress range with SCF against class B 50D mean line.
Chapter 4 Analysis Work on Fatigue Test Results
Figure 4.38 separates the plates into two types, double sided welded T-butt and ceramic backed welded T-butt. Both types of plates were tested with the same nominal strength, but have different SCF. The new plot shows that with the inclusion o f SCF, the fatigues lives for the ceramic backed plate is more in line with the double sided welded T-butt. This would explain that the improvement of the ceramic backed welded T-butt could be due to the lower SCF.
The new S/N plot using the hot spot stress shows that all the points lie above the class B 50D line. With the nominal stress, the points lie just above the F line. It is also important to consider that the material tested was higher strength steels rather than the 50D steels, thus it was expected that the fatigue performance to be slightly better than 50D. By observing Figure 4.40, it could be assumed that the fatigue performance o f the Jack-up steels are better than 50D, in term of nominal stress and hot spot stress. However, the big question is how much is this type of Jack-up steels better by? With the hot spot stress, this seems to suggest that Jack-up steels has much better fatigue performance as for nominal stress, it is only slightly better. The most important thing that the S/N plot (Figure 4.40) shows that the differences in the relative change in fatigue lives due to SCF for T-butt plate is similar to the change o f S/N line class F to class B. This exercise does show that perhaps SCF could be used instead of having to define which class the welded joints belong to. However, experimental work is needed to back up the claim for a single design line and that SCF is responsible for the changes in fatigue lives for most o f the joints.
4.6
Summaries and Conclusions
By recalculating the equivalent stress range the scatter o f the S/N curves were reduced. Also the revised plot shifts the trend line up and the slope closer to m = -3, the same as for the HSE 50D mean line. However the data for the 50D mean line used for comparison may not be true representative of the material performance. Thus it would be desirable to complete tests under the same condition as the high strength steels for the 50D type material.
Chapter 4 Analysis Work on Fatigue Test Results
Comparison o f fatigue crack initiation stage to the overall fatigue life shows that the initiation percentage increase by reducing the stress range. Also from limited results suggested that the initiation percentage is higher for over protection. However, this does not result in as increase o f overall fatigue life.
Other fatigue result for tests completed at lower stress range for the T-butt welded plates were compared. The S/N plots shows that the overall performance of the SE702 type material is comparable to the conventional fixed platform material, 50D. This ties in with the similar conclusion for earlier work for SE702 tubular joints.
The idea of using one design line for welded joint could be feasible with the inclusion of SCF. This would also be more specific to the individual welded joints and eliminating the difficult task o f selecting the class for the welded joint. However, this idea will have to be backed up by experimental results.
Chapter 4 Analysis Work on Fatigue Test Results
4.7
References
[4.1] BS 4360:1990, Weldable Structural Steels, British Standard Institution, London 1990
[4.2] Dover W D, Collins R, ‘Recent Advances in the detection and sizing o f cracks using alternating current field measurements (ACFM)’, British Int Jnl of NDT, Nov 1980, pp 291-295
[4.3] Technical Software Consultants Ltd., ACFM Crack Microgauge - Model UlO User Manual, April 1991, Milton Keynes
[4.4] Austin J A, The role o f corrosion fatigue crack growth mechanisms in predicting the fatigue life of offshore tubular joints, PhD thesis. Department o f mechanical engineering. University College London, October 1994
[4.5] Etube L S, Variable Amplitude corrosion fatigue and fracture mechanics o f weldable high strength Jack-up steels, PhD thesis. Department of mechanical engineering. University College London, 1998
[4.6] Myers P T, Corrosion fatigue and fracture mechanics o f high strength jack up steels, PhD Thesis, Department of mechanical engineering. University College London, February 1998
[4.7] Vinas-Pich J, Influence of environment loading and steel composition on fatigue of tubular connections, PhD Thesis, Department o f mechanical engineering. University College London, January 1994
[4.8] Tantbirojn N, Bowen R J, Etube L S and Dover W D (UCL NDE Centre) and Kilgallon P J, Roberts T and Spurrier J (SIMS, Cranfield University), Variable Amplitude Corrosion Fatigue of Jack-up Steels, Phase 2 VACF-T: Thick Plate Specimens, Phase 3 LLCF: Long Life Tests. To be published by HSE in 2002
[4.9] Lindley C and Rudd W J, Influence o f the level of cathodic protection on the corrosion fatigue properties of high-strength welded joints. Marine Structures 14, pp 397-416, 2001
Chapter 4 Analysis Work on Fatigue Test Results
[4.10] Brennan P, Dover W D, Karé R F, Hellier A K, Parametric equations for T-butt weld toe stress intensity factors. International Journal o f Fatigue 21 (1999) 1051-
1062
[4.11] ASTM E 647-88, Standard method for measurement o f fatigue crack growth rate. 1988
[4.12] Health and Safety Executive, Offshore Installation, Guidance on design, construction & certification. Department of Energy. 4^^ Edition 1990
[4.13] Gurney T R, Fatigue of Welded Structures. Cambridge university press, first published 1968. ISBN 0 521 07115 1
Chapter 4 Analysis Work on Fatigue Test Results
4.8 Tables
Table 4.1: Data of the initiation life of the fatigue tests
Test No. Sh (MPa) VA Sequence Test Condition Ni Np Nt initiation %
101 350 JOSH 20 minutes air 70,000 257,000 327,000 21.4
103 350 JOSH 20 minutes air 53,000 218,000 271,000 19.6
T02 412 JOSH 20 minutes air 20,000 243,000 263,000 7.6
104 412 JOSH 20 minutes air 9,500 155,500 165,000 5.8
105 350 JOSH 20 minutes CP -SOOmV 42,000 176,000 218,000 19.3
109 350 JOSH 20 minutes CP -SOOmV 29,000 270,361 299,361 9.7
106 350 JOSH 20 minutes CP-1050mV 20,000 120,257 140,257 14.3
n o 350 JOSH 20 minutes CP-1050mV 6,000 149,477 155,477 3.9
107 412 JOSH 20 minutes CP -SOOmV 15,000 80,825 95,825 15.7
111 412 JOSH 20 minutes CP -SOOmV 40,000 121,400 161,400 24.8
108 412 JOSH 20 minutes CP-1050mV 10,600 68,555 79,155 13.4
113 350 JOSH 10 minutes CP-1050mV 132,000 133,731 265,731 49.7
114 412 JOSH 10 minutes CP-1050mV 59,000 83,914 142,914 41.3
115 350 JOSH 30 minutes CP-1050mV 47,000 104,774 151,774 31.0
Tie 412 JOSH 30 minutes CP-1050mV 61,000 80,042 141,042 43.2
TCOl 350 JOSH 20 minutes air 125,000 314,528 439,528 28.4
TC02 250 JOSH 20 minutes air 815,000 655,669 1,470,669 55.4
TMOl 350 JOSH 20 minutes air 52,000 252,750 304,750 17.1
Chapter 4 Analysis Work on Fatigue Test Results
Myers
Table 4.2: Data of the initiation life of the other fatigue tests
SE702
Type Test No. S(MPa) CP level (m V ) Nt Ni Np initiation % Comment
I Joint 11 400 air 74,000 13,000 61,000 18 CA
I Joint 12 300 air 180,000 24,000 156,000 13 CA
I Joint 13 225 -lOOOmV 194,000 115,000 79,000 59 CA I Joint 14 225 -SOOmV 548,000 105,000 443,000 19 CA
I Joint 15 180 -lOOOmV 2,300,000 - - - CA: Run out
I Joint 16 300 -SOOmV 138,000 10,000 128,000 7 CA
Etut)e SE702
Type Test No. S(MPa) CP level (m V ) Nt Ni Np initiation % Comment
Y Joint Y1 ISO air 2,130,000 619,830 1,510,170 29 JOSH
Y Joint Y2 250 -SOOmV 380,000 9,880 370,120 3 JOSH
Y Joint Y3 200 -SOOmV 1,545,000 49,440 1,495,560 3 JOSH Y Joint Y4 200 -lOOOmV 1,140,000 180,120 959,880 16 JOSH
Parent Plate Dillimax
Type Test No. S (MPa) CP leveK mV) Nt Ni Np initiation % Comment
Parent Plate PI 412 air 348,000 180,000 168,000 52 CA Parent Plate P3 350 air 1,532,000 1,428,000 104,000 93 CA
Vinas-Pich Lower yield material
Type Test No. S(MPa) CP ievel( mV) Nt Ni Np initiation % Comment
Y Joint YOPBA1 200 air - - 546,000 - WASH
Y Joint Y0PBC1 180 -850mV 464,000 163,000 301,000 35 WASH Y Joint YOPBC2 180 -lOOOmV 435,000 152,000 283,000 35 WASH Y Joint Y0PBC3 180 -llOOmV 252,000 20,000 232,000 8 WASH
Austin Lower yield material
Type Test No. S(MPa) CP levei ( mV ) Nt Ni Np initiation % Comment
T Joint Cl 162 -850mV 615,000 100,000 515,000 17 WASHWA T Joint C2 143 -850mV 670,000 120,000 550,000 16 WASHWA T Joint C3 181 -850mV 385,000 65,000 320,000 18 WASHWA T Joint C4 142 -850mV 515,880 70,000 445,880 14 WASHWB I Joint D1 125 -lOOOmV 1,076,000 220,000 856,000 20 WASHWC I Joint D2 90 -lOOOmV 2,940,000 900,000 2,040,000 31 WASHWC
Chapter 4 Analysis Work on Fatigue Test Results
Table 4.3: Fatigue life result for LLCF
Note: *+’ signifies a run-out
Plate Number
Test type Frequenc
y
Hz CP level mV Stress Range MPa Cycles to failure HSSl Air 2 NA 83.33 1,550,000 (A) HSS2 Seawater 0.2 -800 104.17 2,600,000+ HSS3 Seawater 0.2 -800 83.33 1,900,000 + HSS4 Seawater 0.2 -1050 104.17 690,000 (B) HSS5 Seawater 0.2 -1050 104.17 340,000 (B) HSS6 Air 2 NA 83.33 3,800,000 + HSS7 Air 2 NA 104.17 1,220,000 (B)Table 4.4: Spectrographic metallurgical analysis of DILLIMAX690E-Z15 steel (16mm thick)
Element Fe Mn Si C Or Mo AI V P Ni 8 Ti Cu Pb
% composition 97.85 1.12 0.285 0.177 0.096 0.095 0.083 0.04 0.021 0.021 0.008 0.006 0.003 0.0002
Table 4.5: Chemical composition of DILLIMAX690E % (85mm plate thickness)
C Si Mn P S A1 Mo Ni Cr 0.155 0.328 1.43 0.01 0.000 8 0.079 0.42 0.81 0.86 B FO- 02 FO- 31 N Cu Ti V Nb 0.001 7 0.71 0.34 0.005 0.032 0.005 0.03 0.001 - 172-
Chapter 4 Analysis Work on Fatigue Test Results
Table 4.6: Mechanical properties for LLCF Material Steel grade Material Specification Measured Yield Measured UTS
DILLIMAX690E-Z15 DILLING-E06/97 790 MPa 840 MPa
Table 4.7: Mechanical properties for 85mm plates Material Steel grade Material Specification Minimum Specified YS (MPa) UTS (MPa) Tests Used DILLIMAX690E- Z15
DILLING-E06/97 690 814 Parent and Ground Welded
Plates Requirement to U""S from yield/tensile<0.95
Table 4.8: Data of the fatigue tests
Air 1