Abstract
Altering the microstructure in order to improve the tensile properties of bow shackles resulted in inconsistency in the fatigue performance. This raises the question whether the inconsistency in fatigue life can be attributed to microstructural changes along the profile of the shackle or to decarburization at the surface. Bow shackles forged from 080M40 (EN8) material were subjected to different heat treatments in order to alter the microstructure. The shackles were subjected to five different fatigue load cases, which represented typical loads experienced at termination points for an overhead power line with a span length of 400 m, with changes in conductor type, configuration, wind, and ice loading. Although the change in microstructure does improve both the tensile and fatigue performance, we found that the depth of the decarburization layer has a greater effect on the high cycle fatigue life of bow shackles than the non-homogeneous microstructure.
Highlights
In order to reduce the cost of an overhead power line, larger overhead conductor diameters and longer line span lengths are used, reducing the number of structures used for a specific line length (Calitz et al, 2005).The prime function of overhead line hardware is to connect the phase conductor to the insulators and the insulators to the structure (Eskom, 2014)
The change in microstructure does improve both the tensile and fatigue performance, we found that the depth of the decarburization layer has a greater effect on the high cycle fatigue life of bow shackles than the non-homogeneous microstructure
When bow shackle failures occurred as a result of fatigue damage on certain span lengths on the 765 kV electrical network in particular, it raised the question what impact poor control during the heat treatment of bow shackles has on their fatigue performance
Summary
In order to reduce the cost of an overhead power line, larger overhead conductor diameters and longer line span lengths (distance between two structures) are used, reducing the number of structures used for a specific line length (Calitz et al, 2005).The prime function of overhead line hardware is to connect the phase conductor to the insulators and the insulators to the structure (tower) (Eskom, 2014). The design philosophy of reducing the number of structures results in an increase in the mechanical loading on termination points at structures, especially at termination (strain) structures, as a result of the increase in conductor mass (Calitz et al, 2005). Quenching and tempering heat treatment methods are typically used to increase the mechanical strength of line hardware, which often result in a nonhomogeneous microstructure along the profile, of forged components with a complex geometry, due to the difference in cooling rates for the different cross-sectional areas (Wieser, 1980). When bow shackle failures occurred as a result of fatigue damage on certain span lengths (sections) on the 765 kV electrical network in particular, it raised the question what impact poor control during the heat treatment of bow shackles has on their fatigue performance
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