Bayer R.G. Mechanical Wear Fundamentals and Testing, Revised and Expanded

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3.4. REPEATED-CYCLE DEFO RMATION MECHANISMS

Repeated-cycle deformation mechanisms are wear mechanisms that require repeated

cycles of deformation. There are a number of these mechanisms. Some of these mechan-

isms involve progressive deform ation processes, like creep, compression set, and subsur-

face ﬂow. However, these are usually limited to particular types of materials in speciﬁc

wear situations. The more general ones, surfa ce fatigue, delamination, and ratcheting

involve fatigue-like or fatigue processes. Such processes involve the accumulation of plas-

tic strain, which ultimately leads to the nucleation and propagation of cracks or fracture,

which is similar to conventional fatigue. Micrographs of wear scars associated with these

common forms are shown in Figs. 3.24, 3.25, 3.27 and 3.30–3.32. Examples of creep and

subsurface ﬂow are shown in Fig. 3.26. In general, the severity of these mechanisms is pro-

portional to some power, often high, of the ratio of an operating stress to a strength prop-

erty of the material, such as contact pressure to compressive yield stress. The exact form

depends on material and wear mechanism. As a class, repeated-cycle deformation mechan-

isms are not limited to a particular type of motion. They can occur as a result of sliding,

rolling, or impact. They are also not limited to contact between two bodies but can occur

as a result of contact between surface and abrasive particles. However, they only are

important in the latter case when the surface is harder than the particle.

Surface fatigue is a generic term used for repeated-cycle deformation wear mechan-

isms that result from fatigue processes, which occur on and below the surface of contact.

These processes result in the formation of cracks and crack networks on and below the

surface and in deformed material. Such processes can also result in the formation of pits.

Examples of these features are shown in Figs. 3.24, 3.25, 3.27 and 3.30–3.32. Delamination

is a particular form of surface fatigue, which is related to the accumulation of

Figure 3.22 Transition in wear behavior when the wearing material becomes harder than the abra-

sive. (From Ref. 177.)

dislocations in a narrow band below the surface. This type of wear is illustrated in Fig. 3.27.

Ratcheting is another particular form of repeated-cycle deformation wear that is based

on incremental plastic ﬂow, the accumulation of plastic strain, and mechanical

Figure 3.23 Changes in wear scar appearance as a function of the amount of sliding. Data are for

lubricated sliding between a steel sphere and a single crystal copper ﬂat. (From Ref. 20.)

shakedown. This is illustrated in Fig. 3.28. Again fracture ultimately resul ts from crack

formation and propagation, that is, fatigue.

The common concept associated with the typical forms of repeated-cycle deforma-

tion wear is fatigue or, more appropriately, fatigue wear. The basic concept of fatigue wear

is that with repeated sliding, rolling, or impacting, material in the vicinity of the surface

experiences cyclic stress. As a result of this, stress cycling, plastic strain accumulates

and cracks are ultimately formed. With further cycling, the cracks propagate, eventual ly

intersecting with the surface and themselves. These intersections then produce free parti-

cles, which are easily removed from the surface by a subsequent motion. This worn surface

also experiences stress cycling and the process continues, resul ting in progressive loss of

material from the surface. This concep t is illustrated in Fig. 3.29.

This type of wear mechanism is most evident in rolling and impact wear situations,

where it is generally recognized as the principal mechanism (71–75). Figs. 3.24 and 3.30

show examples of fatigue wear under such conditions. Fatigue wear is also possible with

sliding (21,23,25,76–78). Examples are shown in Figs. 3.25, 3.27, 3.31 and 3.32. In the case

of rolling and to a lesser degree with impact, the topological features of the wear scar are

often quite suggest ive of crack initiation and propagation. Under sliding conditions, the

topological features are generally not as suggestive. There are several reasons for this.

Features associated with adhesive and abrasive mechanisms frequen tly confound the

appearance in sliding situations. Smearing on the surface also tends to hide surface cracks

Figure 3.24 Examples of surface fatigue wear in metals under conditions of normal impact (A–D)

and rolling (E–H). (‘‘A’’–‘‘D’’ from Ref. 75; ‘‘E’’–‘‘G’’ from Ref. 73; ‘‘H’’ from Ref. 71; (‘‘A’’–‘‘D’’

reprinted with permission from Elsevier Science Publishers; ‘‘E’’, original source The Torrington

Co., and ‘‘F’’ and ‘‘G’’ reprinted with permission from Texaco’s magazine Lubrication: ‘‘H’’ rep-

rinted with permission from ASME.)

with sliding. In addition, the crack network under rolling and impact tends to be more

macroscopic or coarser than those often encountered under sliding conditions and fre-

quently result in larger particles or pits being formed. This tends to make fatigue features

Figure 3.24 (continued )

Figure 3.25 Examples of surface fatigue wear in metals (‘‘A’’, ‘‘B’’, and ‘‘C’’) and plastics (‘‘D’’) as

a result of sliding. (‘‘A’’ from Ref. 9; ‘‘B’’ from Ref. 24; ‘‘C’’ from Ref. 20; ‘‘D’’ from Ref. 21; A’’ and

‘‘D’’ reprinted with permission from ASME; ‘‘B’’ and ‘‘C’’ reprinted with permission from Elsevier

Sequoia S.A.)

Figure 3.26 ‘‘A’’ shows the wear of a lead coated c-ring as a result of small amplitude oscillations,

which results from creep. An example of wear resulting from progressive subsurface ﬂow is shown in

‘‘B’’. These micrographs show the wear of an electrical tab on a circuit board as a result of small

amplitude oscillations. The left-hand micrograph shows the deformation of the substrate. ‘‘C’’ shows

the worn surface of an elastomer slab subjected to repeated impact. Two modes are shown. In the

left-hand micrograph, there is no material loss but the material is permanently deformed, which

results from a compression set type of behavior. In the right-hand, there is material loss resulting

from fatigue. (‘‘A’’ is from Ref. 178, ‘‘B’’ is from Ref. 179, ‘‘C’’ is from Ref. 180.)

more easily detected in the case of rolling and impact. Because of these aspects, often the

only way to determine the existence of cracks under sliding conditions is by means of

microscopic examination of cross-sections through the worn surface, such as those shown

in Figs. 3.25b, 3.27, 3.30, 3.31b and 3.32d. Magniﬁcations of several hundred times or

more are generally required for this.

While fatigue wear and fatigue, that is structural fatigue, share a common basic con-

cept, namely the formation and propagation of cracks, they have different characteristics.

While both have an incubation period, the periods are not the same. With fatigue, the

incubation period is the period of crack formation. With fatigue wear the incubation per-

iod extends beyond this. For fatigue wear, the incubation period involv es the propagation

of the cracks to the surface and generally the formation of loose particles. Some topolo-

gical changes might be evident during this initial period of fatigue wear, including some

evidence of plastic deformati on. Howeve r, there is no loss of material from the surface

or formation of free particles. There are also further distinctions between fatigue wear

and fatigue. With fatigue, the process simply involves the formation and propagation

of cracks. With fatigue wear the process is a continuous cycle of crack formation,

Figure 3.27 Crack structure in delamination wear. (From Ref. 24, reprinted with permission from

Elsevier Sequoia S.A.)

Figure 3.28 Conceptual illustration of the ratcheting wear mechanism. The diagram shows the

accumulation of strain as a result of repeated stress cycling, which leads to fracture.

propagation, and removal. For fatigue, most materials exhibit an endurance limit, that is,

a stress level below which fractu re will not occur. In the case of fatigue wear, there does

not appear to be such a limit at least in terms of macroscopic loads and stresses. For prac-

tical load conditions, no matter how small the load or stress, sufﬁcient rolling, sliding or

impact results in the generation of fatigue wear. A further difference is that with fatigue a

distinction is often made between low cycle fatigue and high cycle fatigue. A similar dis-

tinction is not made with fatigue wear.

For rolling situations, there is a generally accepted empirical relationship between

load and the number of revolution deﬁning the incubation period (77,79–81). The general

form of the relationship for both point and line contact situations is

¼ N

ð3:29Þ

where N

is the number of revolutions req uired for a load of P

and N

the number of

revolutions required for a load of P

. For point contact situations, such as in a ball

bearing, n is 3; for line contact, such as in a roller bearing, n is 10=3. Frequently this

relationship is referred to as Palmgren’s equation (81,82). A more fundamental form of

this equation relates stress to number of revolutions. Since according to elastic contact

theory (83) , the maximum stress in a point contact situation, S

, is proportional to P

1=3

the stress form of Eq. (3.29) becomes

¼ N

¼ S

ð3:30Þ

Similar relationships exist for sliding and impact, as described later in this section (21,84).

The progression of wear scar morphology for fatigue wear under sliding conditions

was studied in Cu. (21) The sliding system consisted of a hardened steel sphere sliding

back and forth across the ﬂat surface of Cu single crystals. Boundary lubrication was used

and stress levels were maintained well under the yield point of the Cu. Three stages were

Figure 3.29 General model for surface fatigue wear. Stage I, stress cycling of surface; Stage II,

nucleation of cracks in near-surface regions; Stage III, crack growth; Stage IV, crack coalescence;

Stage V, crack intersection with surface; Stage VI, formation of loose particles.

identiﬁed and are shown in Fig. 3.32. In the ﬁrst stage, grooves and striations in the direc-

tion of sliding were the predominate feature. There was no material loss and the topo-

graphy would suggest single-cycle deformation. During this stage, as sliding increased, the

Figure 3.30 Examples of cracks formed under impact conditions. (From Ref. 181, reprinted with

permission from ASME.)

density or number of these grooves increased. In the second stage, damage features per-

pendicular to the sliding direction appeared. Again, there was no loss of material. This fea-

ture, termed crosshatching, implied something other than a single-cycle deformation mode

was occurring. As sliding continues in this stage, the crosshatching became more pro-

nounced until ultimately spalling and ﬂaking occurred. This is the start of the third and

ﬁnal stage. In this stage, material loss occurs and, with continued sliding, a wear groove

of increasing depth is formed. The start of the third stage was considered to be the end

of the incubation period.

The striations of the ﬁrst stage are the result of local stress systems associated with

individual asperity contact. However, the crosshatching features occur over many stria-

tions and are therefore probably associated with the overall stress syst em associated with

the macro-geometry of the contact. This feature is also considered to be associated with

the initiation and growth of subsurface cracks. Micrographs of cross-sections through

the wear scar conﬁrmed the existence of sub-surface cracks in this situation, as shown

in Fig. 3.32d.

In the same study, it was found that the number of cycles required to initiate the

third stage could be correlated to the maximum shear stress associated with the macro-

geometry. In fact, a relationship identical to Eq. (3.30) was found. This c orrelation is

shown in Fig. 3.33. It is signiﬁcant to note that the same type of correlation wi th stress

is found in impac t wear situations when the macro-stresses are within the elastic limit

of the materials (84). As stated earlier, a similar correlation is found with rolling.

Figure 3.31 Crack structure in extrusion wear. In ‘‘A’’, the surface morphology of the wear scar is

shown. In ‘‘B’’, a view of the cross-section through the wear scar. (From Ref. 87, reprinted with

permission from ASME.)