fatigue analysis of steels used in offshore structures
TRANSCRIPT
Characterization of heterogeneous arc welds through miniature tensile testing and Vickers hardness mapping
Stijn Hertelé1, Nenad Gubeljak2, Primož Štefane2, Patricia Verleysen3, Wim De Waele1
Presented at the 23th International Conference on Materials and Technology27-30 September 2015, Portorož, Slovenia
This research was possible thanks to the financial support of BOFUGent, the FWO Vlaanderen and ARRS Slovenija (grant G.0609.15N),and the kind permission of TU Delft to use their 5 kN test stage.
1 Ghent University, Dept. Energy and Systems, Soete Laboratory, Belgium2 University of Maribor, Laboratory for Machine Parts and Structures, Slovenia3 Ghent University, Dept. Materials Science and Engineering, Belgium
Basic fatigue properties
Corrosive environment decreases fatigue life
Two types of specimens are evaluated:
1) Round bar for S-N curve analysis:
Seawater is corrosive and accelerates the fatigue proccess. Anenvironmental chamber was built to test fatigue in a realisticcorrosive situation.
Corrosion is a slow process and must be accelerated to allow anincreased fatigue testing frequency.
The authors would like to acknowledge the financial support of the IWT (Agency for innovation by science and technology – grant n°131797).
Nahuel Micone*, Wim De Waele. Ghent University, Dept. Energy and systems, Soete Laboratory, Belgium
Fatigue Analysis of Steels used in Offshore Structures Subjected to Variable Amplitude Load Conditions
Two high strength low alloy steels (yield stress of 520 and 610 MPa) are tested in“realistic” conditions (variable amplitude loads and corrosive environment).
2) ESE(t) for Fracture Mechanics analysis:
Corrosion chamber Tensile testing machine
Number of cycles up to failure Crack growth rate (CGR)
Variable amplitude (VA) analysis is more accurate
Offshore industry is growing fast. Since new generation steels and designtechniques are not updated in the standards, designs can be over conservative.
Wave spectrum
Structural response
Wav
e he
ight
(m
)
-5
5
Time
For
ce (
kN)
-3000
3000
Time
For
ce (
kN)
-3000
3000
Easier to test and interpret
Difficult to test and interpret
Analysis
Time
Random
Crack growth (mm)
Cycles
Slow
Fast
a0
Cycles
Stress (MPa)
Infinite life
Finite life
Fatigue failure
Possible corrosion-fatigue interaction
Infrared &Potential Drop
Presented at FEA symposium 201509 December 2015, Gent, Belgium
Accelerated corrosion test setup Physical kinetics analysis*
Cycles
Temp (°C)
Failure (Nf)Voltage (V)
T1
V1
V0T0
Area=damage
Nfi=f(Areai)
Cycles
Stress (MPa)
Nf1 Nf2 Nfi
Si
S2
S1
Digital Image Correlation &Electrical Measurements
Crack growth (mm)
Cycles
a0
S-N Fracture mechanics
Realistic test conditions give accurate predictions.VA loading can accelerate/retard the CGR.Corrosion might be the driven damage mechanism at lowstress levels
Cycles (x106)
Stress (MPa)
Corrosion influence
Stress influence
0 2 4 6
CA+Air
VA+Seawater
Curve for CA
Curve for VA ?
Crack growth (mm)
Cycles (x105)
Underloads=acceleration
Overloads=Retardation
Unconservative
Over conservative
a0
0 1 2 3
Low-High blocks
Time
For
ce (
kN)
-3000
3000
Electrical resistance
FminFmax
Fmin
Fmax
Heater
TempOxigen
Seawater
Easier to test and interpret
High-Low blocks
Temperature (°C)
Corrosion rate (mm/y)
Log da/dN
Log DKCorrosion influence
Stress influence
Stress intensity factor:
CA+Air
VA+Seawater