Fatigue Performance of Copper Power conductors
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- Institutt for marin teknikk 
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.