FATIGUE CRACK NUCLEATION and PROPAGATION in LOCKED COIL WIRE ROPES under FORCED BENDING

ABSTRACT

ln order to examine the fatigue behaviour of locked coil ropes bent over a roller chain, full scale fatigue tests were run using two different ropes. For better understanding of how the fatigue damage develops, microscopical investigations of single wires were carried out. The fatigue cracks of all tested locked coil wire ropes started in their second layers. The morphology of the damage clearly revealed multiple mechanisms of crack initiation and propagation. By using SEM and metallographical investigation a differentiation of the predominant damage mechanism involved could be established. ln the ropes with sufficient lubrication the fatigue crack initiation is a result of friction wear and fretting under high pressure stress. ln ropes with insufficient lubrication the corrosion pits and cracks can nucleate the fatigue cracks. Typical features of each damage mechanism are described. lt could be shown that the local fretting and abrasive wear, which lead to crack nucleation, are mainly the result of misfit in the construction of the ropes. Based on the fractographical investigations of single wires the instrumental causes of the crack propagation could be assessed.

Author(s): Woodtli, J., Kopanakis, G.