An Investigation Into Fretting Behaviour in Pressure Armor Wires of Unbonded Flexible Pipes

The possibility of fretting damage in pressure armor wires of flexible pipes has been investigated. A novel experimental facility which is capable of simulating nub/valley contact conditions with dynamic slip, representative of actual pipe loading, has been developed. The test setup is equipped with a state of the art data acquisition system and a controller with transducers to measure and control the normal load, slip amplitude and friction force at the contact in addition to the hoop stress in the wire. Tests were performed with selected loading and the fretted regions were examined using an optical microscope. Results show that the magnitude of contact loading and the slip amplitude have a distinct effect on surface damage. Surface cracks originated from fretting scar were observed at very high contact loads in mixed slip sliding while surface damage predominantly due to wear was observed under gross slip. The position of surface cracks and the wear profile have been related to the contact pressure distribution. The evolution of friction force and surface damage under different slip and normal pressure conditions are presented. The effect of a general grease lubricant on friction behavior is also discussed.