Aeroengine Safety

Since there are many different operating influences and parameters that determine fretting (chapter 6.1), the number of different remedies is correspondingly high. These remedies are specifically designed for the problem they are intended to solve, but there is no universal remedy. Fretting remedies can be divided into several main groups:

• Constructive design and optimized layout with regard to mechanical stresses and elastic deformations.

• Optimizing the material combination and lubrication of the tribo-system.

• Avoiding changes in the tribo-system due to damaging operating influences.

• Workable regulations, specifications (e.g. overhaul intervals, damage limits for re-use and/or repair), and tested and proven repair methods.

Design layout

The following sentence must be taken to heart: where there are no micro-movements, there is no fretting. Since fretting fundamentally reduces dynamic strength, fretting must be avoided in highly stressed zones. An example is disk damage that can be traced back to a crack initating in the region where a metal sheet layed against the hub (Ref. 6.3-2). The designer should always question whether a detachable connnection is necessary, or if a permanent connection (e.g. welding, soldering, one-piece assebly) is feasible. This requires comprehensive reconsideration, since compromises must usually be made. One need only think of the difference between a welded and a threaded compressor rotor or the difference between a blisk and a conventional disk/blade system. Anti-twist devices with bolts on rotor flange that are braced across the shaft, rather than screwed, have been shown to be susceptible to fretting (Fig. "Fretting damage at turbine rotor"). Therefore, threaded connections are preferable to the former type of bolt connection. If a detachable connection cannot be avoided, the design of the attached components in the contact zone should be optimized. Because micro-movements occur even in tight positive fits (when the parts in the connection area elastically deform in different ways under operating loads), emphasis must be on creating tuned, equal elastic behavior. Of course, this is not necessay, if the operating loads are so low that no noteable elastic deformation occurs (Ref. 6.2-2). This task is made easier by computer-based deformation calculations, which should by this time be standard in design layouts. In connections whose function entails glide movments (e.g. plug tooth systems in shaft couplings), sufficient lubricant supply must be ensured.
Reducing the oscillation amplitude can be accomplished by the aforementioned optimization of stiffness in the contact parts, and also by weakening the vibration stress. Before a targeted reduction of vibration stress is possilbe, a theoretical and/or practical analysis (e.g. modal analysis) of the vibration types and their causes must be completed. Only then can targeted changes to the elasticity, mass, and damping be undertaken with any hope of success.
Cylinder supports for pipes must be equipped with appropriate intermediate layers. It is important that warping of the pipe during manufacture and operation is safely avoided.
The design of contact surfaces and their transitions can make a decisive contribution to operating safety. Important factors include surface pressure, position of the surfaces with regard to the transmitted forces (e.g. dove tail angles), as well as strain-relieving fillets in the transition to the contact surface (Fig. "Reducing wear and stress in contact zone", top diagram).
Putting a pattern into the contact surfaces (Fig. "Blade foot wear symptoms") can also improve their operating behavior, but it must be designed for and be proven effective in the specific application.

Overhaul and maintenance

Determining and adhering to proper overhaul intervals is important. A determining criterion is the change in the contact zone due to operating conditions. If changes are expected to increase stress levels to the point that they are not covered by the design, then the part must be reworked or regenerated. Dangerous changes include local metal removal accompanied by an increase in or local relocation of the stress in the contact surfaces. Glide coating failure that increases the coefficient of friction or a drop in the pressure residual stress due to creep (e.g. in shot-peened surfaces) requires repair.
The technology required for regeneration must be developed, proven, and made avaliable in time.
Overhaul guidelines are necessary in order for overhauls to be suitable, workable, and effective. These should be given in engine-specific overhaul manuals. Problems can arise from foreign-language technical terms or translations of these terms in the guidelines. If implementation results in implausible results (e.g. unusually frequent damage), one should consult a specialist and/or if in doubt, consult the original foreign-language document. It is reccomended that clear damage descriptions accompanied with pictures and diagrams are included, as well as clear specifications of the allowable tolerances for re-installation, repair, and replacement; i.e. parts that are no longer usable. Meaningful pattern tables for the damage types are useful for classifying damages. However, it must be remembered that assessing damage (i.e. loss in dynamic strength) on basis of the external damage symptoms is always problematic and requires sufficient experience. Additionally, repair measures (specifications and working plans) must be provided. Repairs are usually orientated according to the production process of new parts.
Decisions regarding type and extent of damages depend on a sufficiently reliable damage report. This requires suitable non-destructive trials and/or destructive trials of random samples. It is vital to be aware of the limitations there are to assertions made based on evidence from these trials. For example, it is very difficult to find oxide-filled micro-cracks in lay-on surfaces.
When combining broken-in parts with new ones or combining parts with different wear rates in their contact surfaces (Fig. "Fretting wear induced fir tree cracks"), it is important that no unallowable surface pressures or tensions are created
by the changes. A suitable reworking may be necessary in some cases.
If it becomes known that unusual operating conditions (e.g. windmilling) lead to increased fretting stress in certain engines, then these engines must undergo special inspections.
It is important to ensure that fretting damage is not promoted by engine maintenance. Constantly lubricated shaft connections (e.g. multiple splining), for example, must have a sufficient grease reservoir. Problems can occur due to oxidation products bleeding out or increased play, for example. Re-lubrication must be done only with approved greases. Cleaning fluids should not be used in such a way that necessary glide materials or coatings are dissolved, changed, or washed away. This includes cleaning materials used for compressor washing that can enter the gap at the blade roots if the engine is stopped or running at low RPM.

Avoiding Unfavorable Operating Conditions
Some operating conditions promote fretting wear. In many cases, these conditions cannot be avoided, but in individual cases they sometimes can be avoided. For example, windmilling (free rotation of the rotor of a shut-down engine caused by the airstream) is known to cause heavy fretting stress in the fan blade roots. This is caused by the weak centrifugal force due to low RPM. This can result in the blade roots undergoing large relative movements with contact surfaces seperating and hammering wear occuring. This is an unusual damage mechanism for these parts.

Testing Fretting Systems
A sufficiently certain statement as to the suitability of a fretting system during operation can only be made based on operating experience or, as the case may be, testing under conditions that simulate those during operation. As a basic principle, testing in the engine should be preferred. This requires that the trial runs sufficiently correspond to conditions in serial operation. For example, a corrosive or eroding influence that occurs during normal operation with marked downtimes (e.g. military use) is almost impossible to sufficiently realistically simulate in a testing rig. Shortened trials with increased loads or single-stage trial instead of load collectives similar to those present during operation should be viewed very sceptically.

Selection of suitable material combinations
There are various methods of ensuring that material combinations in contact zones act to minimize fretting damage:

• Suitable base material combination without after-treatment

• Applying metallic surface coatings to one or both surfaces

• Use of loose inserts (metal sheets)

• Applying firmly adherent glide coatings to one or both surfaces

• Use of paste-like or fluid lubricants

Base material combinations in compressor disks and compressor rotor blades are determined by operating requirements such as heat resistance, dynamic strength, and thermal strain. This makes after-treatment of the contact zones possible.
Metallic surface coatings can be applied in various ways: with the aid of a galvanic process, streamless treatment in a coating bath, thermal spraying, vapor deposition techniques (PVD), or chemical gas reactions (CVD) and diffusion treatments. Many factors must be considered when selecting coatings. These include optimal coating thickness, sufficient heat proofing, plastic deformability (evens out imprecise application; e.g. Ref. 6.3-1, CuNi or CuNiIn coatings on titanium alloys), a low coefficient of friction with the opposite surface, wear resistance, low sensitivity to damage caused by micro-movements, sufficient oxidation resistance, and no formation of damaging oxides (which increase the coefficient of friction, initiate cracks, and ccelerate wear).
Thermally sprayed WC-Co coatings, the tungsten carbide particles of which are embedded in a cobalt matrix, have proven themselves as wear surfaces under high surface pressure and hammering stress. These coatings are typically used in areas 1, 2, 4, and 9 of Fig. "Fretting threatened engine parts".
Welded armors (e.g. Co alloys, stellite) have shown to be effective for turbine rotor blade shrouds. Hard cast-in or soldered-on wear plates or plasma spray coatings (chrome-carbide/nickel-chrome base; Ref. 6.3-1) are also in use.
The wear-protection layer must be inspected for possible negative propeties. In hard layers, especially, the dynamic strength in the contact region of the surface may be at risk for dropping suddenly. Problematic coatings include galvanic Cr and Ni coatings. The coating process itself must not damage the base material. With thermal spraying and PVD coatings, for example, a high substrate temperature decreases protective pressure-internal stress in peened coatings to a dangerous level. Overly high operating temperatures cause the coating elements to diffuse into the substrate, resulting in possible property changes (loss of dynamic strength, embrittlement) in both the coating and substrate. This effect limits, for example, the use of silver coatings above about 250°C, even though these coatings seem to perform very well on compressor blade roots, especially in russian engines. Silver coatings are used successfully on bearing carriers (silvered rings) and screw shafts (and, as the case may be, mating surfaces).
The after-treatment of coatings by hardening (e.g. ball peening) acts to increase dynamic strength by decreasing tension-internal stress and/or creating an advantageous surface structure.
As shown in Fig. "Minimizing fretting by low friction coefficient", the coefficient of friction, and therefore the stress in the contact surfaces, can be lowered with a glide coating (see Figs. "Fretting processes in contact zone" and "Fretting load parameters in dovetail").
Loose metallic intermediate layers such as thin Cu plates (Fig. "Fretting processes in contact zone") between Ti surfaces are especially effective. However, foil inserts for blade roots have are not known to be effective in serial implementation. This is probably due to the fact that shifting or sliding-out of the foil inserts cannot be prevented with a sufficient degree of certainty. This would promote the failure of the tribo-system along with the risk of blade root failure and foreign object damage from foil fragments. On the other hand, the insertion of bushings in bores has found effective serial use (Ref. 6.3-2).
The most commonly used glide coatings are firmly adherent ones. These are graphite-based lacquers. MoS₂-filled coatings can disintigrate and release sulfur at temperatures above about 300°C, which can damage the tribo-system. Application of a glide coating should be preceded by shot peening, in order to take advantage of the reservoir effect of the calotte structure. The operating life of lacquer glide coatings is shortened by wear, oxidation, or disintigration. Accordingly, the coefficient of friction increases with operating time, while the wear protection decreases. The stressing of the contact surfaces and risk of fatigue fractures increase with operating time. Overhaul intervals for engines with this type of coating must be set with these characteristics in mind.
The life span of contact surfaces that are not continually supplied with fluid or paste-like lubricants is greatly shortened. Multiple splining must be continually lubricated from a reservoir or oil circuit. Fluid or paste-like lubricants cannot be used in blading due to the operating temperatures, centrifugal force, and high surface pressure, among other factors.
Coatings must be simply and workably reparable. A coating that, when damaged, cannot be removed during engine overhaul in a serially applicable procedure (e.g. not damaging to the base material) is not suitable. The frequency of recoatings and/or repairs during the predicted life span of the part must be determined. For example, if every recoating requires metal removal, the allowable dimensional tolerances determine the possible number of recoatings. A further criterium is, if the customer wants to conduct the repairs himself and if the necessary requirements are available to him.

Mechanical Surface Treatment
The most important and most frequently used method (Fig. "Shot-peening as strengthening of surfaces") for improving operating behavior, especially for ensuring sufficient dynamic strength during fretting, is shot peening. This procedure does not noticeably alter the geometry and can also be done during repair. It plastically deforms metal surface, hardens the same, and induces pressure-residual stresses. Additionally, it creates a callote structure, corresponding to the geometry of the steel shot. This has a beneficial effect on the contact behavior of the two surfaces. Other hardening methods, such as rolling (in symmetrically rotating parts, such as shaft sockets), stamping ,and flaring (bores), are rarely used in fretting-sensitive areas during engine construction. A special profiling of the contact surfaces (Fig. "Blade foot wear symptoms") would be expected to be beneficial, but no serial implementation is known. Surface hardening through use of laser peening is being developed. This process utilizes shock waves in a coating on the surface to be hardened in order to achieve mechanical hardening.

• As early as the design/development stages of parts affected by fretting, factors that could influence the configuration must be precisely analyzed and evaluated with the help of experts. If possible, the design should be based on prior experience, i.e. proven configurations, materials, and auxilliary materials should be used.
  

• If damage occurs, targeted remedies are only promising if the damage-causing influences (damage mechanism) have been determined as exactly and completely as possible. For example, it is important to determine whether it was hammering wear or gliding wear that caused the damage.
  

• In contact surfaces that experience combined shearing loads and tension loads, the coefficient of friction (not only when the part is new, but especially after long periods of operation) is very important for the damaging stress.
  

• Fretting causes especially large decreases in the dynamic strength of titanium alloys.
  

• Technical trials for determining the life span and hardness of parts subject to fretting stress (e.g. rotor blade roots and disk slots) be conducted under conditions that are sufficiently close to those during operation (this includes the surface of the part and the operating atmosphere). For example, hot parts must be preoxidized (to simulate real operating conditions) before being tested in a vacuum testing rig.
  

• Protective coatings against fretting wear must be tested for suitability under conditions sufficiently close to those during operation. It is vital that the coatings retain the necessary properties for the duration of the part`s life span.
  

• Shot peening has shown itself to lengthen the life span of engine parts in many cases, especially if it is used in combination with lubricants or coatings. Hard armorings, such as WC in a Co-matrix or stellite, have proven effective against hammering loads.
  

6.3-1 J.D. Schell, K.P. Taylor, General Electric Aircraft Engines, “Wear of Jet Engine Components”, ASM Handbook, Volume 18 “Friction, Lubrication and Wear Technology”, pages 588-592.

6.3-2 J.S. Alford, General Electric Co., “Design Criteria and Configuration for Long-Life Aircraft Gas Turbines”, SAE-Paper No. 670344, Proceedings of the National Aeronautic Meeting, New York, April 24-27, 1967, pages 1-16.

6.3-3 H.-J. Böhmer, “Wälzverschleiß und -ermüdung von Bauteilen und Maßnahmen zu ihrer Einschränkung”, periodical “Materialwissenschaft und Werkstofftechnik” 29, pages 697-713 (1998).

6.3-4 P. Adam, “Fertigungsverfahren von Turboflugtriebwerken”, Birkhäuser Verlag 1998,, ISBN 3-7643-5971-4, page 110.

6.3-5 Published by the “Metal Improvement Company”, pages 1-47.