Fretting wear is a complex damage process: Dobromirski  proposed that there are up to 50 variables that affect fretting behaviour. Fretting is a surface damage mechanism that occurs in the contact region between two materials under combined normal load with non-uniform, typically small-amplitude, cyclic relative tangential motion. The present work is motivated by the potential for fretting damage in the pressure armour layers of flexible marine risers. Flexible risers are a key component in the delivery of offshore hydrocarbons from the seabed to sea level, typically to a floating structure, such as a platform or vessel and are comprised of a large number of layers with different functions. The primary function of the helically wound interlocked metallic pressure armour layer is to contain the internal pressure due to conducted hydrocarbons, primarily by hoop stress resistance. However, the riser itself will also be subjected to significant large bending and torsional deformations, as well as axial tensile forces, due to the combined effects of vessel motion, buoyancy and hydrodynamic loading, for example. These loading conditions result in high contact pressure and micro-scale relative displacement in the nub-groove contact of the pressure armour layer.
In this paper, the design, development, control and validation of a new fretting test rig to represent fretting in the pressure armour layer will be presented. A fretting rig must be able to: (i) apply a known (typically constant) normal load to achieve a clamping force between two or more test specimens; (ii) apply a small-amplitude (typically micro-scale) reciprocating tangential displacement between the test specimens; and (iii) test at high frequencies.
In addition to the mechanical requirements for the design of fretting test rigs, the test rig must be capable of measuring and recording the cyclical variation of displacement at the contact (or as close to the area of contact as possible) and the avociated (induced) tangential force. The main components of the new fretting test rig include a linear drive line with a piezoelectric actuator to produce reciprocating micro-scale tangential displacement, a collet to fix test specimen in place and dead weight configuration for application of constant normal load, as shown in Fig. 1.
Fig.1 Fretting rig with dead weight normal load applied.
Test parameters (applied displacement, number of fretting cycles and test frequency) are controlled via a LabView GUI and data acquisition (DAQ) unit. Displacement and force sensors connecting to a PC via the DAQ unit allow for real-time visualization of fretting test conditions. The material tested is pre-service pressure armour wire.
Fretting loop and coefficient of friction evolutions with number of cycles for tests will be presented. These results show that both partial slip and gross slip regimes are achieved using this test rig. Wear scars have been measured using a profilometer; from this the wear rate is calculated. Frictional and wear results from this test rig have been compared to previous tests conducted on a similar material , to characterize the fretting performance of the flexible riser material, under peak contact pressure and contact width conditions representative of in-service operation. The results and trends are directly relevant to design of flexible marine risers against fretting fatigue and wear.