SecMet (Pty) Ltd was requested to investigate the failure of a 7 strand wire rope. The investigation concentrated on analysing the fracture surfaces in order to elucidate the failure mechanism. The fracture surfaces of wires were examined using stereo microscopy and Scanning Electron Microscope (SEM).
The overall condition of the cable bore evidence of poor maintenance as dust, sand and regions with a lack of lubrication were noticed. Areas with substantial external corrosion were also found suggesting that for a prolonged time insufficient lubrication prevailed or that a corrosive medium was in contact with the cable. Although most of the cable length showed little evidence of mechanical damage, the region in the vicinity of the failure showed significant localised damage.
The fracture surface of most wires appeared to be contaminated with foreign matter. The residue was analysed by means of Energy Dispersive Spectroscopy (EDS) and the results of one of the contaminated regions is shown in Table 1. The source of the contaminant warrant further investigation as the chlorine and sulphur could cause significant corrosion and premature failure of any replacement wire rope.
Table 1: EDS analysis of the contaminated fracture surface
After dissolving the surface contaminants in an ultrasonic bath with hot water and subsequently rinsing with acetone, the fracture surfaces were scrutinised.
While brittle, intergranular failure was observed for some of the outer strand wires, ductile failure and significant necking of most wires was confirmed using both electron and stereo microscopy. This finding would suggest that the majority of wires failed in overload. Such overload could be due to loads exceeding the rated capacity or due to normal load exceeding the remaining strength after intergranular failure of the wires/strand. Some individual wires from various strands were corroded, worn and mechanically damaged insofar that the cable’s load bearing capacity had been reduced.