Failure analysis of overhead power line yoke connector Miroslav Dzˇupon a , Ladislav Falat a,⇑ , Ján Slota b , Pavol Hvizdoš a a Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 040 01 Košice, Slovak Republic b Technical University of Košice, Faculty of Mechanical Engineering, Department of Technologies and Materials, Mäsiarska 74, 040 01 Košice, Slovak Republic article info Article history: Received 28 February 2013 Received in revised form 4 April 2013 Accepted 26 April 2013 Available online 7 May 2013 Keywords: Hot-dip galvanized steel Yoke connectors of overhead power lines Liquid metal assisted cracking Brittle failure abstract This paper deals with investigation of failure reasons of yoke connectors of overhead power lines. These components were produced by cold bending of steel plates (grade S355J2+AR) and their subsequent hot-dip galvanizing. Approximately 6 months after production, sud- den failures of the connectors occurred by manual manipulation during assembling. The failures were localized in bent parts of the connectors. Performed investigations included metallographic analyses, semi-quantitative chemical microanalyses, microhardness mea- surements and numerical simulations. It was found out that the failures were caused by several degradation processes related to liquid metal assisted cracking which occurred dur- ing the galvanizing from the initial small surface cracks induced during bending. Remedial action to the production process should involve a systematic control of bending tool func- tional surface. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Overhead power line yoke connectors (see scheme in Fig. 1) are the components in construction of hanging fitting of over- head power lines. Any failure of these components may lead to serious problems in electric power distribution system. The investigated yoke connectors were produced by cold bending of plates of structural carbon steel and subsequent hot-dip gal- vanizing. The steel plates were bent using the bending press schematically shown in Fig. 2. During the bending process, metallic materials are known to be deformed by inhomogeneous plastic deformation at the locations of maximum bending moment with simultaneous effects of external bending moments from longitudinal and transversal forces. The first analytical solutions of bending process were focused mainly on plain-stress state, pure bending and bending with a great ratio between the bending radius and plate thickness [1–3]. Later, the solutions have been intro- duced for spring-back effect during the bending of steel sheets/plates with different ratios between the bending radius and sheet/plate thickness, e.g. [1,2,4]. The intensity and distribution of stresses and deformations are affected by the materials properties, materials processing route and technological treatment, hardening effects, variation in elastic properties, elastic and plastic anisotropy and the Bauschinger effect. A sophisticated design of forming processes frequently involves finite ele- ment method (FEM) simulations with experimentally adjusted data of process parameters [3,5–10]. Anti-corrosive protection of the yoke connectors is usually achieved by hot-dip galvanizing which is the most common process being exploited to protect structural steels against corrosion [11]. The protection is reached by a barrier effect of complex surface coating (Zn–Fe based intermetallic phases, pure zinc and its oxides) against oxidizing agents. If the integrity of galvanized steel surface is disrupted, the zinc acts like an anode. If the zinc-coating contains small defects, zinc corrosion products (ZnO) may tamp corrosion cells formed of steel substrate, zinc layer, air and humidity [12]. During the last decade, 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.04.024 ⇑ Corresponding author. Tel.: +421 55 7922447; fax: +421 55 7922408. E-mail address: lfalat@imr.saske.sk (L. Falat). Engineering Failure Analysis 33 (2013) 66–74 Contents lists available at SciVerse ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal