Study On Fatigue Tests And Fatigue Life Estimation Methods Of Stranded Copper Conductors Of Dynamic Submarine Cables

With the exploitation of offshore wind power gradually shifting from offshore to deep sea,the development and application of floating wind power has been paid more and more attention.As an important part of the floating wind power system,the dynamic submarine cable plays a very important role in the development of deep sea wind power.However,the complex marine environment brings great challenges to the application of dynamic submarine cables,especially the higher requirements for the fatigue performance of the components in dynamic submarine cables.As a key component,the stranded conductors may cause a series of engineering accidents and huge losses once fatigue failure occurs.Therefore,it is necessary to study the fatigue performance of dynamic submarine cable conductor.In this paper,the tensile fatigue property of the stranded copper conductor of dynamic submarine cable was studied.The main research contents include:(1)Tensile fatigue test of stranded copper conductor with different compaction degreesThe surface irrugularities data and mechanical properties of three-layer stranded copper wires with different compaction degrees were obtained by the size measurement and tensile test.The tensile fatigue test of three-layer stranded copper conductor with different compaction degrees was carried out.It is found that the tensile fatigue life of stranded copper conductor with different compaction degrees is not distributed in the same S-N data standard scatter band,indicating that the tensile fatigue life of stranded copper conductor is affected by the compaction degrees,the influence cannot be ignored.(2)Numerical simulation of compaction process of stranded copper conductor and normalization of experimental tensile fatigue life based on stress concentration coefficientA compaction model was established to simulate the compaction process of stranded copper conductor,the correctness of the model results was verified by comparing with the parameters of actual stranded copper conductor.Based on the results of the compaction model,the calculation model of the stress concentration coefficient at the copper wire irrugularities in the outer layer of stranded copper conductor was established,the variation law of the stress concentration coefficient with the nominal stress at the end of the model was analyzed.It was found that the experimental tensile fatigue life of stranded copper conductor with different compaction degrees could be “normalized” into the same S-N data standard scatter band after considering the stress concentration coefficient.The expression of normalized S-N data scatter band was given.(3)Prediction method of tensile fatigue life of stranded copper conductor based on normalized S-N data scatter bandThe influence of compaction coefficient and outer copper wire pitch diameter ratio on outer copper wire stress concentration coefficient of stranded copper conductor with different nominal sections was analyzed and its variation rule was summarized.The prediction formula of stress concentration coefficient of stranded copper conductor was fitted,which could consider different compaction coefficient,different outer copper wire pitch diameter ratio and different nominal end stress,the change mechanism of stress concentration coefficient of outer copper wire was analyzed.A prediction method of tensile fatigue life of stranded copper conductor based on normalized S-N data scatter band(abbreviated as “normalized S-N data scatter band method”)was proposed,a prediction table of tensile fatigue life of stranded copper conductor based on this method was compiled.(4)Tensile fatigue life prediction of stranded copper conductor based on the macroscopic and mesoscopic elastic-plastic constitutive model and fatigue damage evolution modelIn order to consider the influence of plastic damage caused by compaction,based on the theory of hybrid strengthening theory and double scale theory,the macroscopic and mesoscopic elastic-plastic constitutive model of TU2 oxygen-free copper material was constructed,the macroscopic and mesoscopic UMAT subroutines based on ABAQUS software were written.By calling the macroscopic and mesoscopic UMAT subroutines,the elastic-plastic stress-strain of the circular copper wire model under cyclic uniaxial tensile load was calculated.The results show that the macroscopic and mesoscopic elastic-plastic constitutive model established in this paper can fully describe the mixed strengthening characteristics of TU2 oxygen-free copper material.The fatigue life prediction method based on the macroscopic and mesoscopic elastic-plastic constitutive model and damage evolution model(abbreviated as “fatigue damage model method”)was used to predict the tensile fatigue life of stranded copper conductor,the results were compared with the experimental tensile fatigue life,the correctness and applicability of the fatigue damage model method was proved.(5)Comparison and verification of two fatigue life prediction methodsThe normalized S-N data scatter band method and fatigue damage model method were used to predict the tensile fatigue life of stranded copper conductor,the results were compared with the experimental tensile fatigue life.It was found that the prediction result of the fatigue damage model method was closer to the experimental result,while the prediction result of the normalized S-N data scatter band method was safer.The errors of the two methods were both small and safe.Therefore,both methods can be used to predict the tensile fatigue life of stranded copper conductor.The tensile fatigue life of stranded copper conductor with different parameters predicted by the two methods was compared,the reliability of the normalized S-N data scatter band method proposed in this paper was verified.