Table of Contents
6. Fretting
- 6.1 Fundamentals of Fretting Damage
- 6.2 Damage Caused by Fretting
- 6.3 Preventing Fretting Damage
The term fretting is used to describe phenomena including fretting corrosion, wear oxidation, contact erosion, false brinelling, coke formation.
In the following text, fretting is taken to mean friction wear that occurs between oscillating contact surfaces. Fretting can cause mechanical wear (geometric/volume wear) as well as chemical reactions with the surrounding materials (corrosion, oxidation) and material damages. These damages can locally lower the strength of the affected area considerably, especially its resistance to dynamic loads. The term “wear oxidation” is used if at least one of the surfaces involved is an iron-based alloy. However, fretting occurs with any and all material combinations, especially the metallic materials used in engine construction. Titanium alloys tend to show an especially pronounced drop in their resistance to dynamic loads.
The mechanics of fretting are usually considerably different from those of unidirectional sliding wear (Fig. "Load-specific wear types"). Conclusions drawn from the symptoms of one of these types of wear are therefore not always applicable to the other type. Corrosion and oxidation, especially, have a pronounced influence on fretting.
Especially in aircraft engines many components (Fig. "Fretting zones in an aeroengine") are affected by fretting. This is primarily due to the design singularities that result from the need for high transmission capacity/power at the lowest possible weight. This results in high elastic deformations that result in relative movements of removable spliced contact surfaces with different elasticities. The high dynamic loads on the often filigreed elastic components and/or temporal temperature changes with large gradients promote dynamic relative movements. This is compounded by the respective operating loads of different, optimally-matched component material properties such as the modulus of elasticity and behaviour during thermal expansion.
Figure "Fretting zones in an aeroengine": Engines have many components that are subjected to considerable fretting stress, including:
Rotor blade roots and disk grooves (Fig. "Fretting threatened engine parts"):
Strain due to centrifugal force, temperature changes and oscillations lead to fretting damage and blade failures in blade roots subjected to high static stress in the seating (centrifugal force, bending force from the gas flow).
Socket splining (multiple splining, Fig. "Failure due to fretting wear at spline coupling" ):
These torque-transmitting shaft connections are prone to damaging relative movements due to misalignment, shaft vibrations, and manufacturing errors. In extreme cases, these can cause the shaft connection to belt-in.
Ducts, socket joints with large surface areas (Fig. "Fretting at combustion chambers"):
In the combustion chamber area, mounting and guiding surfaces of thin-walled gas lines as well as the chamber itself are subjected to considerable dynamic loads by the combustion process. They are also stressed by large thermal strain. These factors combined with oxidation can lead to serious, especially erosive, wear damage.
Fastening connections with bolts or holding catches as well as guides (e.g. of injection nozzles) are also at risk for fretting damage.
Axial fastening of rotor blades:
Rotor blades are subjected to considerable axial stresses by the gas forces that can lead to oscillating blades moving even under centrifugal force. The blades then lie on the axial fastenings (e.g. spacers and labyrinth racks) and can create wear notches, from which fatigue fractures can spread into the ring.
Dampers and lay-on surfaces:
In turbine rotor blades, especially, “cowbell” elements are used that dampen vibrating blades through friction resulting from their lying against the root platform. It must be safely guaranteed that this functionally necessitated fretting stress is controllable throughout the life of the cowbell element.
The same applies to supporting lay-on surfaces such as clapper catches on larger compressor rotor blades (usually in the fan) or shrouds on turbine rotor blades, which are often braced against one another.
Guide vanes and housings (Fig. "Fretting wear at compressor guide vanes"):
Guide vanes in compressors and turbines are usually fastened to the housing in a removable manner. Fastening devices such as holding catches or fastening elements (e.g. pins or rivets) are used to prevent them from rotating under the gas loads. All contact surfaces are subject to considerable fretting due to housing oscillations and heat strain. This can cause intolerable weakening of cross-sections, which in turn can lead to blades breaking out or “wandering”.
Screw connections, mating surfaces, centering collars, lay-on surfaces, bolt holes:
The expansion cycles caused by RPM and temperature changes (especially during the start-up/shut-down process), along with oscillations (e.g. flexure of the rotor) can cause a drop in dynamic strength and reduction in the life span of rotor parts in highly stressed regions.
Example "Fretting damage trials for a training aircraft" (Ref. 6.1-1):
The document excerpt describes trials of an engine for a training aircraft. The trial program with accelerated mission testing (AMT) was designed to simulate longer overhaul intervals. Fretting damage was found in these trials:
“…wear at the bearing race of the lever, actuating compressor variable stator vanes” (see also Figs. "Play increasing by fretting" and "Wear in protective sleeves").
“…wear and peeled coating of the combustion liner lock pin” (see also Fig. "Fretting at combustion chambers").