Prakt. Met. Sonderband 46 (2014) 317
ly the cyclic Erniedrigung der urspriinglichen Schwingfestigkeit des Werkstoffes, die je nach Quelle
ices will be bis zu 30 % und mehr betragen kann. Hauptursache hierfir diirfte die zyklische plastische
roduces the Verformung der Werkstoffoberfliche sein. Außerdem aktiviert die beschriebene
hat is more, Relativbewegung der Reibpartner die gepaarten Oberflaichen immer wieder neu und
ic volume of bildet so das typische Reiboxid, das seinerseits abrasiv wirkt und den Rissfortschritt über
das größere spezifische Volumen in bis zur Rissspitze oxidierten Rissen fördert.
>omachinery An Beispielen aus dem GroBgasturbinenbau werden im Beitrag die vielfältigen
sting failure Erscheinungsformen dieses interessanten Schadensmechanismus' gezeigt.
1. ON FRETTING FATIGUE
The mechanisms of fretting fatigue are by no means common knowledge, therefore some
t, ist ein consideration should be given to this interesting phenomenon. Fretting fatigue is a
nur im common failure mechanism in turbomachinery, particularly aircraft engines, aircraft
lurchgesetzt, structures, industrial gas turbines, steam turbines, generators, chemical plants, and
automotive parts, in short, it might affect all components that are somewhat excited
dynamically and have a tribological partner (fraying surfaces). It might occur in dry
ng, der le.8. conditions as well as with electrolytes or even lubricants. Sources of fretting in a blade-
net, die eine disk system are manifold and schematically depicted in Fig. 1. According to early works,
AUS. WIE steel specimens subjected to fretting might show a reduction in fatigue strength of some
eitung und - 15 % [1], [2]. More recent studies put that figure much higher, as high as 30 % [1], [2]. In
this respect fretting fatigue resembles corrosion fatigue, in so far as there is a synergistic
nd kleinste effect, or, in other words, a "conjoint action" [1]. It is interesting to note that post-war
werden 0,1 - studies of fretting fatigue in aircraft structures found no relation between the amount of
issen, damit fretting wear, i.e. of fretting debris, and the actual diminution of fatigue strength [1]. In one
1g mehr auf. of the older literature sources [1], the authors found a remarkable statement as to the
plexer Natur importance of fretting for fatigue strength in general. It is stated there that, "The possibility
igen deshalb of fretting occurring in structures and multi-component systems which are subjected to
cle Fatigue vibration is so likely that the fatigue behaviour of such assemblages can be said to be
ar HCF-Riss entirely determined by the response of the contacting surfaces to fretting." "Fretting
damage occurs on the contacting surfaces of components which are clamped together
and which, although nominally at rest relative to each other, are subject to vibration." [1].
1s in allen That same source states elsewhere that lower clamping forces rather promote fretting
ationen o0.dgl. fatigue failure as opposed to higher ones. Fretting "is caused by repeated alternating
eugteile und slipping of the surfaces over part of their mutual contact area. Visible damage is found
i Bauteile im when the amplitude of slip is as little as 10% mm [1], [2], i.e. some tenths of a micrometer.
nkbar, wo es Other sources and more recent discussions in the scientific community put that figure a
“allem durch little higher in the multi-micrometer range. The question arises why the production of
uckstrahlver- fretting damage on the surfaces of specimens and components subjected to alternating
n. stresses can reduce fatigue strength so drastically [1]. The coefficient of friction steadily
increases under fretting fatigue conditions with service time, from about 0.2 to maybe
tstehens von about 0.8 in an aluminum alloy [1]. For one, this increases the alternating elastic stress
lensystemen, exerted on the contact areas and the ensuing submicroscopic plastic deformation which
alten dieser will eventually lead to the known dislocation cell arrangements and to fatiguing the
materials subjected to fretting. In addition, the fretting mechanism will permanently re-
{omponenten activate the metal surfaces of the mating tribological partners, continuously removing
empfindliche surface oxides, thereby producing abrasive particles, enabling cold weld and transfer of
material, and creating pits with the resulting notch effects and stress raisers. According to
