| dc.description.abstract | The present work is concerned with the fatigue strength of copper power conductors applied
in umbilicals and power cables. Two cross-sections of conductors were investigated, namely,
95 mm2 and 300 mm2. The work included fatigue testing of both individual wires and full
cross-section conductors in tension-tension and tension-bending modes. Moreover, fatigue
strength assessments by employing 3D finite element and analytical methods were performed
to attempt bridging the gap between individual wire fatigue test data and full cross-section
conductor fatigue performance. The thesis is composed as a collection of research papers and
consists of two parts. The first part presents an introduction to the topic and summarizes the
findings of the PhD study. The second part gives the research papers (journal articles).
The experimental investigations were divided into three major parts, including:
1. Individual wire fatigue tests in tension-tension mode and in load control applying a
constant amplitude with a stress ratio R = 0.1. The loading was sinusoidal with load
frequency f = 2 Hz. The tests were performed for wires taken from centre, inner and
outermost layer of the 95 mm2 cross-section. A similar procedure was employed for
testing of the outermost layer of the 300 mm2 conductor.
2. Full cross-section fatigue tests in tension-tension mode were performed for the 95mm2
copper conductor under constant amplitude loading with a stress ratio R = 0.1, in load
control, and at load frequency 2 Hz.
3. Displacement controlled bending fatigue tests of 95 and 300 mm2 full cross-section
conductors were carried out. The specimens were prestressed to 100 MPa mean stress
and exposed to reversed bending motion with a constant curvature. The loading was
sinusoidal with load frequency f = 0.4 Hz. The intention of the test was to simulate the
loading of a conductor hanging from a floater through a bellmouth (bending stiffener)
and subjected to the motions of a floating structure. In this test, all motions were
applied in one plane.
Moreover, in order to assess the initiation and growth mechanism of fatigue cracks (fatigue
failures due to tension-tension and tension-bending loads) fractography was investigated using
SEM (Scanning Electron Microscopy) for both 95 mm2 and 300 mm2 conductors.
Finite element (FE) analysis of individual wires was carried out to investigate the combined
effects of surface irregularities resulting from the manufacturing processes and material plasticity
on the fatigue performance. The obtained results from FE analysis were used to transform
S-N fatigue data based on the nominal stress range to S-N fatigue data based on the
actual stress ranges.
A series of friction tests of copper wire against copper wire was conducted. The purpose
of these tests was to investigate the coefficient of friction. Several constant normal force conditions
were applied in the tests. The tangential and normal loads were monitored throughout
the tests and the associated coefficient of friction was calculated.
A calibration test was used to tune the FE model of the 95 mm2 and 300 mm2 copper power
conductors. This was performed by investigating the measured axial stiffness vs. FE and
analytical results. A copper power conductor with free span length 1000 mm was mounted
in a servo-hydraulic test machine. On each individual wire of the outer layer, strain gages
were attached for measuring the axial strain. In addition, the specimen was equipped with an
extensometer for monitoring the average axial strain of the conductor.
FE analysis of both cross-sections was carried out applying the measured coefficient of friction.
A simplified description of the contact behaviour was adopted, considering the large
contact area resulting from plastic deformations of the wires during manufacturing, and calibrated
by the axial tension testing. Numerical simulations and analytical calculations were
performed in order to bridge the gap between the S-N data obtained from full cross-section
fatigue tests and S-N data obtained from individual wires fatigue tests. | nb_NO |
