The effect of fretting marks introduced during strand winding on the fatigue performance of transmission line conductors.

Abstract

In this research, the Elasto-plastic interlayer contact of the TERN ACSR conductor used on Eskom’s 400kV transmission lines is investigated. Characterization of the elliptical contact marks using eucentric tilting in SEM stereomicroscopy for depth measurement is conducted. The size of these manufacturing defects (fret marks) resulting from the strand winding process is quantified. The already established observation that two geometrically dissimilar contacting surfaces may exhibit the same contact mechanics is extensively used to explain how the variations in stranding lay ratios affects the size of the fret marks and hence the surface quality of strands. From this supposition, an equivalent contacting sphere radius is calculated and used for both nonlinear and linear elastic finite element Analysis (FEA) in MSC Marc mentat. An inner conductor contact mechanics model for determining the normal contact force per defect is also presented and used with existing inner conductor mechanics models for tension determination. The calculated equivalent radius shows strong linear correlation with the defect size, contact force, plastic strain and stress and can therefore be used in the design of conductors. Fatigue testing was then conducted on two (02) TERN conductors. Fractographic analysis of the samples exposed to fatigue cycles was conducted using the Scanning Electron Microscopy (SEM) and Field Emission SEM (FEGSEM). Energy Dispersive Spectroscopy was also used for Surface Elemental analysis before and after the fatigue testing to analyse the changes in material composition on the fret mark surfaces.