Introduction
During an operation to install a pipeline, a winch wire failed (Figure 1). It was observed to have tangled approximately 3m above the connection to the bridle. The load then dropped approximately 30cm as though the wire had untangled, and failure occurred at this point. The wire rope is specified as EN 2160 galvanised
Investigation
The fracture surfaces of ten individual wires were examined at high magnification and the surfaces of the wires were examined for any effects that may have contributed to the failure. A materials characterisation involving hardness testing, microstructural analysis and scanning electron microscopy was conducted on one of the wires
Results
The rope had an area of bird caging approximately 2m from the final area of failure. This is a form of structural damage which occurs when a difference in length between the wire core and the outer strands results in the outer strands being pushed up. This can occur due to shock loading.
All of the ten wires examined show signs of severe plastic deformation, indicating ductile overload failure i.e. either tensile overload or shear failure (Table 1). In some instances, there is evidence of fatigue cracking.
The SEM analysis shows that the galvanised layer is cracked and these cracks extend into the metal substrate (Figure 2). Results of the compositional analysis by means of EDS indicate that the coating is an intermetallic layer, with no evidence of a pure galvanised layer. This is likely due to sacrificial corrosion in order to protect the underlying steel, as is the intended purpose of this layer. The intermetallic layer that remains is brittle and acts as initiation sites for fatigue cracking. Thus under normal operating conditions, the wires became susceptible to fatigue cracking.
The microstructure and hardness of the wire indicate that is in the quench and tempered condition, as specified.
Conclusions
The shock load compromised the structural integrity of the wire rope by uncoiling the strands. This resulted in a considerable decrease and thus the loading forces became an overloading scenario. and the individual wires were not able to withstand these forces.
