318  Prakt. Met. Sonderband 46 (2014) 
[2], the significant drop of fatigue strength takes precedence over the notch effect. Both 2. Mi 
the strength reduction due to fretting and the coefficient of friction increase with increasing a 
life [1], [2]. There are no materials completely insensitive to this failure mechanism. 2.1 CI 
However, the susceptibility to fretting fatigue varies with the material and alloy. The 
contact situation is very important for the resulting fretting damage. Surface roughness, During 
residual stress and surface hardness all influence the propensity of a tribological system initiate 
towards fatigue. Remedial measures and preventative action aim for influencing these outwa 
parameters by modification of the surface through treatments such as shot peening or liberat 
anti-fretting coating. Such palliatives aim for the reduction of friction and hence surface ‘visual 
stresses (anti-fretting coatings), while others try to improve fatigue properties of the 2.9 Ci 
materials in the tribological system (shot peening). However, caution is needed with all om 
such palliative measures when excessive fretting wear is involved that could partly or, ‘Lockir 
completely remove those topmost surface zones [2]. There is an interrelation between after 
fretting wear and fretting fatigue [2], i.e. fretting wear does not necessarily result in fretting hardel 
fatigue cracking. Fretting processes have an incubation period [2]. It might vary only a 
considerably, from only a few percent of the total lifetime to much longer, depending on some. 
the materials and system parameters. "When two metal surfaces are in contact desigr 
undergoing fretting, the sequence of events may be summarized as follows: (1) disruption 
of air-formed oxide or other surface films, (2) intimate intermetallic contact with the 2.3 FF 
possible formation of cold welds, local plastic deformation, surface roughening and the PAE 
initiation of fatigue cracks, (3) development of wear debris, usually the relevant oxide..." While 
[2]. The latter can be conceived as a delamination process, leaving some fretting oxides ‘moder 
intact on the surface, while having removed them elsewhere so that eventually there are oT he st 
patches on the fretted surface with and others without oxide scales. The sometimes very a fretty 
fine-grained debris is probably produced by the comminution of plate-like metallic wear of a he 
particles [2]. The response of a tribological system to fretting is therefore determined by 2 4 FF 
its susceptibility to all those individual sub-processes [2], which makes the behavior of oo 
tribo-systems so difficult to predict under fretting conditions. It is interesting to note that ‘In this 
corrosion-resistant alloys that rely on the formation of protective oxide layers for their ‘statior 
good corrosion properties are all the more sensitive to fretting and particularly vulnerable, ‘unrela 
because those oxides are continuously disrupted [2]. Materials that fall into that category induce 
are, apart from titanium and aluminum alloys, the stainless steels [2]. According to the shape 
literature, the high strength-reduction factors reported for surfaces damaged by fretting ‘mating 
are attributable, besides the material damage by contact fatigue, to the notch effect ‘mating 
caused by the sharp surface damages ensuing from fretting damage. Of course, this only mobile 
worsens once the first crack is induced that will then act as a stress raiser due to the oo 
sharp crack tip. A drop of the ‘fretting fatigue endurance limit’, if such a category existed, 3. RE 
to 20 - 50 % of the unfretted fatigue strength appear to be not unusual, depending on the 
alloys and tribo-systems involved [2]. Even though "fretting-initiated fatigue cracking can 1M 
be quite easily overlooked if the failure analyst is not alert”, "there are some features of 
fretting-fatigue cracking that are unique.” "The fretted areas are roughened and, in steel, 
are usually decorated with reddish-brown deposits." "Cracking normally starts at an angle [2] 
of less than 90° to the surface. The geometric stress concentrations present on the com- 
ponent may be overridden because of fretting, and cracks may initiate on previously 
smooth surfaces." [7]. There, the authors state that "the significant cracks were not I 
associated with the most severe surface damage but were near one boundary of the area [3] 
of surface damage...", i.e. "close to the boundary between the damaged and undamaged 
areas". Fretting fatigue cracks might even "ignore" stress concentrations at corners and, I 
radii and initiate elsewhere, where fretting damage dominates, i.e. "fretting fatigue can 14) 
override the geometric stress concentration that is present." [7]. Further reading on the 
subject of fretting fatigue is provided in the references ~~