Fracture of a galvanized steel U-bolt stirrup of an overhead electrical transport line

A.C. Ferro, L. Calado
2016 Procedia Structural Integrity  
During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation
more » ... mmercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data. Abstract Overhead electric transport lines, cables and support systems, experiment complex stress and environment requirements due to ice formation and release, wind exposure, temperature fluctuations and corrosion. The burst of any element in the chain brings the cables to the floor, cutting the transport of the current with important liabilities. This work reports the rupture of a galvanized U-bolt steel stirrup of a 60 kV overhead electric transport line of a Eolic Park in the North of Portugal. This component is subject to a complex load system and variable attack angles. Thus, the component is subjected to fatigue, wear, static crush and corrosion. The fractured stirrup was manufactured from a hot rolled C1-S235JRC steel rod with Ø 14.5 mm. The final component was hot dip galvanised and centrifuged according standard ISO1461. Fracture surface and multiple cross section microscopy and chemical analysis of the fractured stirrup as well as of new unused stirrups was carried out to identify the rupture mechanisms responsible for the collapse of the structure. Residual resistance of the broken stirrup was evaluated via dedicated designed tensile testing. The original steel has a wide number of big inclusions, same over 400 m long, developed parallel to the axis of the rod. These inclusions intercept the surface of the rod, inducing surface indentations and cracking of the galvanised coating. Also a considerable number of pronounced notches are observed at the steel surface, at the steel/zinc coating interface. The fracture surface displays two symmetrical glossy burnish areas, characteristic of the fatigue processes, separated by a central ductile central zone. In the fractured component several cracks due to inclusions are observed intercepting the fracture surface. Zinc and Oxygen were identified of all over the fatigue grown crack surface indicating that hydrolysis of the coating took place with continuous wet transport to the cracked surface with precipitation and oxidation. The analysis carried out allows concluding that the failure of the component took place by fatigue. Fracture was initiated at the 1 st /2 nd thread of the component, close to the screw joint. The variable load acted perpendicularly to the plane of symmetry of the component. Corrosion took place simultaneously with fatigue crack propagation speeding the failure process. The inclusions
doi:10.1016/j.prostr.2016.02.034 fatcat:vjbedi22d5cbtmeft2wsbfcccy