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Contact Fatigue Copyright © 1996 ASM International® ASM Handbook, Volume 19: Fatigue and Fracture All rights reserved. ASM Handbook Committee, p 331-336 www.asminternational.org Contact Fatigue W.A. Glaeser and S.J. Shaffer, Battelle Laboratories CONTACT FATIGUE is a surface-pitting-type the direction of ball travel. Not all spalls in ball- automobile engine tests (Ref 3). Lifters were failure commonly found in ball or roller bearings. bearing races are of the shape shown in Fig. 2. nodular iron, and cams were flake graphite cast This type of failure can also be found in gears, Figure 3 shows a fatigue spall near the race iron. Fatigue cracks were associated with cracked cams, valves, rails, and gear couplings. Contact shoulder of a deep-groove ball bearing. The spall carbides, graphite flakes, and hard inclusions. fatigue has been identified in metal alloys (both appears to have been formed by the joining of Contact fatigue occurs in gears along the pitch ferrous and nonferrous) and in ceramics and cer- several pits. The fact that the spall occurred close line. The geometry of tooth mesh is such that mets. to the race shoulder may have distorted the con- rolling occurs at the pitch line while sliding oc- Contact fatigue differs from classic structural tact state of stress, causing a multiple origin. curs at the addendum as the gears come out of fatigue (bending or torsional) in that it results Fatigue in roller bearings may differ from ball- mesh. An example of pitch line contact fatigue is from a contact or Hertzian stress state. This local- beating contact fatigue. Quite often the pitting shown in Fig. 5 (Ref4). The pits seen on the teeth ized stress state results when curved surfaces are occurs in the inner race at the contact zone of the will grow in size and depth, ultimately resulting in contact under a normal load. Generally, one roller ends. In some cases, contact stress peaks at in tooth fracture. the roller ends and pitting originates in these Another form of contact fatigue, known as mi- surface moves over the other in a rolling motion locations. Roller-end pitting can be a sign of cropitting, occurs in bearings. An example is as in a ball rolling over a race in a ball bearing. misalignment. shown in Fig. 6. This feature can show up over The contact geometry and the motion of the roll- Cams and Gears. Valve lifter cams and rollers the entire raceway surface. It is often the result of ing elements produces an alternating subsurface are subject to contact fatigue. An example is too thin a lubricant film or excessive surface shear stress. Subsurface plastic strain builds up shown in Fig. 4 (Ref 3). The character of the roughness and sometimes heavy loading. with increasing cycles until a crack is generated. damage is very similar to that found in rolling In gears, micropitting is termed frosting and in The crack then propagates until a pit is formed. contact bearings. The example shown in Fig. 4 the present ANSIdAGNA standard it is consid- Once surface pitting has initiated, the bearing was found in both cam nose and lifters during ered a form of contact fatigue. For bearings, becomes noisy and rough running. If allowed to continue, fracture of the rolling element and cata- strophic failure occurs. Fractured races can result from fatigue spalling and high hoop stresses. Rolling contact components have a fatigue life (number of cycles to develop a noticeable fatigue spall). However, unlike structural fatigue, contact fatigue has no endurance limit. If one compares the fatigue lives of cyclic torsion with rolling contact, the latter are seven orders of magnitude greater (Ref 1). Rolling contact life involves ten to hundreds of millions of cycles. Examples of Contact Fatigue Contact fatigue produces a surface damage that is unique and well recognized. Familiar examples are found in fatigue of ball and roller bearings. A typical failure in a roller bearing is shown in Fig. 1. Although this spall is small, it would grow in size until roller fracture would occur, as bearing operation continues. One classic shape of a fatigue spall in a ball bearing is a delta shape, as shown in Fig. 2 with a diagram of the pit (Ref 2). The apex of the pit is the initiation point, usually the location of a sur- face defect like a dent. The pit grows in a fan Fig. 1 Scanning electron micrograph of a fatigue spall on a roller from a roller bearing after 630,000 cycles. Roller is AISI shape, becoming wider and deeper as it grows in ] 060 steel, hardened to 600 HV. Spall is 400 ltm wide by 700 p.m long. 332 / Fatigue Strength Prediction and Analysis X ; ¸¸¸¸¸¸¸¸¸%¸~ /: ..... ~ ::~ ~:~ i J c ~ i i i Spall depth in o i //• AB C D E F G 0.001 in. .... i I I I i I I 0 20 40 60 80 100 120 = .... Spall length in 0.001 in. ', F = la) i i Fig, 3 Multiple spall near a race shoulder i i i × maximum shear stress depth is about the same as the dimension of B. However, with increased (hi traction or tangential force, the maximum shear Fig. 2 Anatomy of a race spall in a ball bearing. (a) lypical delta shape with the apex at the origin. (b) Profiles of the spalL stress moves closer to the surface. In hardened Source: Ref 2 martensitic steels used in ball and roller beatings, the subsurface shear stresses produce plastic de- formation in the martensitic structure. The resid- ual strain increases with increase in rolling cy- gears, and any contact, micropitting can be re- Analysis of the subsurface stress state indicates cles. This has been shown by x-ray measurements duced by improved surface finish, reduced tem- that a maximum shear stress exists at a given during rolling contact experiments (Ref 7). perature or loads, and providing sufficient elasto- depth below the surface. The stress distribution is Many researchers have studied the microstruc- hydrodynamic film. shown in Fig. 8. The maximum shear stress is tural changes that occur as a result of the buildup Rails. Spalling and "shelly"-type failures occur shown increasing with depth below the surface as of subsurface strain (Ref 8-10). In AISI 52100 discussed by Kloos and Schmidt (Ref 6). on track rails from wheel-track rolling contacts. steel, a common rolling-contact-bearingmaterial, The curves shown are based on two different An example of shelly failure is shown in Fig. 7 the accumulation of strain initially is associated from Kilburn (Ref 5). The name comes from the mathematical approaches to the estimation of contact state of stress. Both approaches produce with the formation of a dark etching Zone below morphology of the fracture surface in the bottom the surface. Further swain causes the formation of a shear stress distribution quite close to each of the spall. Shelly failures are serious because light etching bands caused by the formation of a other. The z axis scale is Z/B, where B is the minor they lead to rail fracture and derailments. Rail new ferrite phase. Then carbides in the high stress axis length of the contact ellipse and the usual spalling has been reduced in recent years by the region begin to show decay and break up. Other direction of rolling motion. Note in Fig. 8 that the use of higher carbon steels for rails. microstructural features include "butterflies" or Mechanisms of Contact Fatigue The state of stress produced by rolling contact is concentrated in a small volume of material and produces intense plastic strain. The strain accu- mulates as the same volume is stressed with each rolling cycle until a crack is initiated and forms a spall. In the real world of contact fatigue, the mechanisms involved can be quite complex. Most models assume a condition of ideal geomet- ric surfaces and little input by heat generation, environmental conditions, and inhomogeneities of materials. Hertz stress analysis assumes a cir- cular, elliptical, or line contact surface area be- tween curved surfaces (depending on the geome- try of the contacts) and a parabolic pressure distribution with the maximum pressure at the center of the contact. The subsurface state of stress involves a hydro- static component that inhibits tensile fracture. Fig. 4 Contact fatigue spalling of cam lifter surface, Source: Ref 3 Contact Fatigue / 333 Fig. 5 Pitch line spalling of medium-hardenedgears. Source: Ref4 (a) (b) Fig, 6 Micr•pitting•fr•••erbearingouterrace•Scanninge•ectr•nmicr•graph•(a)57xand(b)athighermagnificati•n Large shell evidenced by black spot Shell with piece broken out Gage Fig. 7 Shelly rail spall from wheel-rail contact fatigue. Source: Ref 5 334 / Fatigue Strength Prediction and Analysis x B z 3D contact pressure distribution for curved contacts Oo Surface 0.2X. TM 0.6 0.8 1.0 1.0 - 2~Max shear stress Fig. 9 Deve oping span. (a) Top view of developing spall at race surfacedent. (b) Sectionthrough developing spall show- 2.0- ing subsurfacecracking Source:Ref 4 contact fatigue. With shear stresses higher and 95 closer to the surface, surface defects (dents, Fig. 8 Stressdistributions at a contact subsurface c scratches, etc.) all contribute to higher incidence = .oL / of surface-originating fatigue. Figure 2 shows a "g" 60 ...................... race spall that started at a dent in the race. This white etching wings radiating out from large hard produced a delta-shaped spall as the cracking 401" /o inclusions. Some features depend on the level of progressed from the origin. Sections through the contact stress. For example, butterflies are often spall show it to be shallow at the origin and 20 ,7, associated with high contact stress (2000 to 4000 deeper at the other end.
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