ino

, R

Geo

24e 21

ls,Hh d

susceptibility to fretting than 4340 steel, when operating in the mixed fretting regime. The use of fretting pads with dierent surface pro-les showed that contact geometry did not signicantly inuence the fretting fatigue behavior of either steel for the range of loadingconditions considered. The fretting fatigue lives are discussed in light of the low cycle fatigue and crack growth rate behavior of these

between contacting components, creates surface and sub-

helicopters. PH 13-8 Mo stainless steel has been used toreplace 4340 steel to enhance the corrosion fatigue resis-

within 200 cycles under conditions that lead to severe

and (2) a bulk cyclic stress that is sucient to grow thosecracks into the body. In this case, most of the fretting fati-gue life is dominated by crack growth.

This paper focuses on the susceptibility of both materi-als to fretting fatigue, in strength conditions typical of thesecomponents. Most tests are conducted in the mixed fretting

* Corresponding author. Fax: +1 404 894 0186.E-mail address: rick.neu@me.gatech.edu (R.W. Neu).

1 Presently with General Electric Aircraft Engines, Cincinnati, OH.

Available online at www.sciencedirect.com

International Journal of Fatigue

Internationalsurface damage from which fatigue cracks nucleate andgrow in the presence of a fatigue load. This can occur atstress levels well below the fatigue limit of a material. Ifunchecked, these cracks may eventually cause componentfailure, which could have tragic consequences.

The current investigation examines the fretting fatiguebehavior of two high strength structural steels: PH 13-8Mo stainless steel and quenched and tempered AISI 4340steel. These materials are often used in fatigue critical com-ponents, such as the rotor hubs and connecting links of

knockdown on fretting fatigue life. Observations suggestsevere cyclic and ratchet plastic strains accumulate in thesurface layers. Microstructure changes often occur in thevolume that is heavily strained. In addition, under the lim-ited normal load and displacement amplitude conditionsconsidered, there was minimal inuence of contact cong-uration on fretting fatigue life [2]. The conditions that tendto promote this minimal inuence include (1) a severe fret-ting condition that leads to crack nucleation early in thefretting process under dierent contact congurationssteels. The life trends in fretting fatigue correlate more closely to the low cycle fatigue behavior. 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Fretting fatigue is a widespread problem in engineeringapplications where two components are in contact and oneor both of them are subjected to alternating fatigue loads.Fretting, a small amplitude, oscillatory, relative motion

tance of these components. However, these componentsinvolve attachments that are susceptible to other failuremodes besides corrosion fatigue.

Earlier work focused on the evolution of fretting fatiguedamage in PH 13-8 Mo stainless steel under relatively highload conditions [1]. Fretting cracks were found to nucleateA comparative study of the frettand PH 13-8 M

J.A. Pape 1

The George W. Woodru School of Mechanical Engineering,

Received 9 July 2004; received in revised formAvailable onlin

Abstract

The fretting fatigue behavior of two high strength structural steeis investigated. Both were heat treated to a similar hardness (43-44Both materials experienced signicant reductions in fatigue strengt0142-1123/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.ijfatigue.2006.12.016g fatigue behavior of 4340 steelstainless steel

.W. Neu *

rgia Institute of Technology, Atlanta, GA 30332-0405, USA

February 2005; accepted 7 December 2006January 2007

PH 13-8 Mo stainless steel and quenched and tempered 4340 steel,RC), comparable to the condition used in structural components.ue to fretting, with PH 13-8 Mo stainless steel exhibiting a greater

www.elsevier.com/locate/ijfatigue

29 (2007) 22192229

JournalofFatigue

regime, which is characterized by gross slip in the initialcycles with rapid transition to partial slip as frictionincreases due to the generation of fretting damage [3,4].This is typically the fretting regime that leads to the largestknockdown factors. Similarities and dierences betweenboth the plain fatigue and fretting fatigue behavior of PH13-8 Mo stainless steel and 4340 steel are highlighted.

and modied ConManson equation,

ea r0f

E2N f b e0f 2N f c

are provided in Table 2. Fatigue limits for several stress ra-tios, R, based on stress amplitude, are given in Table 3.

The long crack growth behavior of PH 13-8 Mo stainlesssteel and 4340 steel for R = 0.1 and 0.5 are shown in Fig. 3[8,9]. The long crack growth rate for PH 13-8 Mo and 4340

i C Cu Mn P

.1 Max 0.05 Max 0.03 Max 0.1 Max 0.01 Max

.23 0.43 0.16 0.7 0.008i N O V Nb(Cb)

1000

1200

1400

1600

(M

Pa)

4340 steel

4340 steel

PH 13-8Mo

Fig. 1. Monotonic and cyclic stressstrain curves of 4340 steel and PH 13-8 Mo stainless steel.

Fig. 2. Strainlife curves for 4340 steel and PH 13-8 Mo stainless steel.

2220 J.A. Pape, R.W. Neu / International Journal of Fatigue 29 (2007) 221922292. Experimental methods

2.1. Materials

PH 13-8 Mo stainless steel (UNS13800) is a martensitic,precipitation hardenable stainless steel that combines cor-rosion resistance with material properties similar to thoseof traditional quenched and tempered alloy steels [5,6],while 4340 steel is a medium carbon, low alloy steel thatcan be quenched and tempered to achieve a signicantincrease in strength. The elemental composition of bothmaterials is shown in Table 1. The PH 13-8 Mo stainlesssteel was aged at 566 C for 4 h and air cooled (H1050),which resulted in a hardness of 44 HRC. The resultingmonotonic yield and ultimate tensile strengths were1286 MPa and 1325 MPa, respectively. The 4340 steelwas hardened at 829 C for 1.5 h, oil quenched, temperedat 427 C for 2 h, and then air cooled. The resulting hard-ness of the 4340 steel was 43-44 HRC, and the correspond-ing monotonic yield and ultimate strengths were 1456 MPaand 1548 MPa. The elastic moduli for PH13-8 Mo stainlesssteel and 4340 steel are 192 GPa and 208 GPa, respectively.

Strain-controlled fatigue experiments were performedon both materials to determine their respective low cyclefatigue properties. The monotonic and cyclic stressstraincurves are shown in Fig. 1. PH 13-8 Mo stainless steel isnearly cyclically stable, with only very slight cyclic soften-ing occurring, while the 4340 steel softens signicantly.The 4340 steel has higher monotonic yield and ultimatestrengths than the PH 13-8 Mo while PH 13-8 Mo has ahigher strength under cyclic loading than the 4340 steel.The strainlife curves are compared in Fig. 2. 4340 steelexhibits a higher fatigue strength for lives less than 103

cycles compared to PH 13-8 Mo, while the PH 13-8 Mohas a greater fatigue strength for lives greater than 103

cycles. The additional parameters for the cyclic stressstrain curve

ea raE raK 0

1n0

Table 1Elemental composition of 4340 steel and PH 13-8 Mo stainless steel

Alloy Cr Ni Mo S

PH 13-8 Mo 12.2513.25 7.508.58 2.02.50 04340 0.81 1.75 0.25 0

S Al Co T

0.008 Max 0.901.35 0.01 0.010.003 0

200

400

600

800

0 0.005 0.01 0.015 0.02

Stre

ss

Strain

monotoniccyclic0.01 Max 0.0025 Max 0.6 0.003

tes

n

00

JourTable 2Strainlife fatigue constants of 4340 steel and PH 13-8 Mo stainless steel

Alloy Incremental step test Individual fatigue

K 0 (MPa) n0 K 0 (MPa)

PH 13-8 Mo 1550 0.0352 17434340 N/A N/A 1770

Table 3107 cycle fatigue endurance limits of 4340 steel and PH 13-8 Mo stainlesssteel [7]

Alloy 107 cycle life

R = 1.0 R = 0.1 R = 0.5 R = 0.75PH 13-8 Mo 577 377 N/A N/A4340 486 332 261 196

10-6

J.A. Pape, R.W. Neu / Internationalare quite similar at R = 0.5, while cracks grow signicantlyfaster in 4340 steel at R = 0.1.

2.2. Experimental procedure

The apparatus used to carry out the fretting fatigueexperiments is shown in Fig. 4. Two bridge-type frettingpads were clamped to a specimen of the same material bya calibrated proving ring to create a contact load of343 N at each pad foot. Fretting pads had either at feet,or cylindrical feet with a radius of curvature of 1.5, 15,or 150 mm. Most experiments were conducted with frettingpads having dierent surface proles on either edge of thespecimen. Strain gages were bonded to the underside of thefretting pads to measure the frictional forces between thepads and the specimen. After the apparatus was assembled,the specimen was fatigued at a frequency of 10 Hz in a uni-axial, servohydraulic test system in laboratory air.

Two types of specimen were used during this investiga-tion, a at dogbone specimen and a at rectangular speci-

10-9

10-8

10-7

1 10 100

PH13-8, R = 0.5PH13-8, R = 0.14340, R = 0.54340, R = 0.1

Crack

gro

wth

rate

(m/cy

cle)

K (MPa-m1/2)Fig. 3. Long crack growth rate curves for 4340 steel and PH 13-8 Mostainless steel at stress ratios of 0.1 and 0.5.men. The dogbone specimen design was used for all of the

ts

0 r0f (MPa) b e0f c

.059 1997 0.0739 0.525 0.74

.0911 2059 0.0859 1.7624 0.848

Fig. 4. Experimental fretting fatigue apparatus.

nal of Fatigue 29 (2007) 22192229 2221PH 13-8 Mo experiments, and the rectangular specimendesign was used for all of the experiments on 4340 steel.The gage section of all specimens had a width of19.0 mm and a thickness of 3.81 mm. All of the specimenswere fretted on the thickness dimension. The contactregions of both the fretting pads and specimen were pol-ished to a 600 grit nish, with the polishing marks orientedin the direction of fretting. The dierence in specimendesigns led to a dierence in the nal polished surface, withthe fretting surface of the dogbone specimens having aslight curvature while the rectangular specimen edgesremained at. To facilitate a comparison between frettingfatigue experiments on PH 13-8 Mo and 4340 steel despitethis dierence, some specimens of 4340 steel were re-pol-ished, which resulted in a curvature along their edge similarto that present on the PH 13-8 Mo specimens.

Surface roughness measurements of 4340 steel speci-mens were obtained by using a Zygo NewView 200 3-Dsurface proler (Zygo Corporation, Middleeld, CT). Theaverage surface roughness (Ra) of polished specimens wasdetermined to be between 0.10 and 0.16 lm for traces withor across the fretting direction. Average roughness valuesfor polished specimens of PH 13-8 Mo stainless steel weremeasured with a laser prolometer using two dierent cut-o lter lengths [2]. The resulting surface roughness valueswere 0.04 lm and 0.2 lm across the direction of fretting forcuto lter lengths of 100 lm and 1000 lm, respectively,and 0.05 lm and 0.3 lm with the fretting direction for cut-o lter lengths of 100 lm and 1000 lm.

3. Results

3.1. Fretting fatigue lives

Fretting fatigue experiments on PH 13-8 Mo stainlesssteel and Q&T 4340 steel showed that fretting caused a sub-stantial decrease in fatigue endurance for both materials.Both materials had similar fretting fatigue lives, but thereduction in fatigue strength was greater for the PH 13-8Mo than for the 4340 steel. This can be seen in Fig. 5 forR = 0.1. Arrows indicate specimens for which failure didnot occur at either of the fretting pads. For the experimentsdiscussed in this paper, cycles to fretting fatigue failurerefers to the presence of a through crack of approximately3 mm in length. This crack is detected by a rapid decreasein frictional force amplitude, resulting from the change inlocal compliance due to the presence of the crack. Hence,the failure life incorporates both the nucleation of a fretting

0

50

100

150

200

250

300

104 105 106 107 108

S=16.5, Flat/15S=12.0, 15/15

Stre

ss a

mpl

itude

(MPa

)

Cycles to failure

R = 0.1

0

100

200

300

400

500

600

104 105 106 107 108

Fretting fatigue data R = 0.5Fatigue endurance limit R = 0.5Fretting fatigue data R = 0.75Fatigue endurance limit R = 0.75

Stre

ss a

mpl

itude

(MPa

)

Cycles to failure

Fig. 6. Fretting fatigue SN curves for 4340 steel at stress ratios of 0.5 and0.75.

0

50

100

150

200

250

300

104 105 106 107 108

S=12.0, 15/150S=12.0, 15/150, spS=16.5, Flat/15S=16.5, Flat/15, spS=12.0, 1.5/15

Stre

ss a

mpl

itude

(MPa

)

Cycles to failure

R = 0.1

Fig. 7. Fretting fatigue SN curves for 4340 steel at a stress ratio of 0.1and various contact geometries.

2222 J.A. Pape, R.W. Neu / International Journal of Fatigue 29 (2007) 22192229fatigue crack and some growth. This criterion was used pri-marily because of its reliability and ease of systematicimplementation by monitoring the evolution of the fric-tional force response.

Quantitatively, the 107 cycle fretting fatigue endurancelimits for PH 13-8 Mo stainless steel and Q&T 4340 steelwere 26% and 36% of the smooth specimen endurancelimits, respectively, for R = 0.1. This corresponds to areduction in the endurance limit from 377 MPa withoutfretting to 100 MPa under fretting fatigue conditions forPH 13-8 Mo, and from 332 MPa to 120 MPa for 4340steel. For R = 0.5 and 0.75, fretting fatigue endurancelimits for 4340 steel were 46% and 38% of the smoothspecimen endurance limits, respectively. This is shown inFig. 6.

Fretting fatigue stresslife curves are shown for eachmaterial individually in Figs. 7 and 8. These plots breakdown the fretting fatigue experiments according to the test-ing conguration used. For example, the top entry in the

0

80

160

240

320

400

480

560

640

104 105 106 107 108

PH13-8 fatigue4340 fatiguePH13-8 fretting4340 fretting

Stre

ss a

mpl

itude

(MPa

)

Cycles to failure

R = 0.1Fig. 8. Fretting fatigue SN curves for PH 13-8 Mo stainless steel at astress ratio of 0.1 and various contact geometries.

Fig. 5. Fretting fatigue SN curves for 4340 steel and PH 13-8 Mostainless steel at a stress ratio of 0.1.

legend in Fig. 7 indicates that fretting pads having a padspan (S) of 12.0 mm were used, and pads having radii ofcurvature of 15 mm and 150 mm were used on either edgeof the specimen. The pad span is dened as the distancebetween the centers of the fretting pad feet. The additionof sp to the second legend entry indicates that the speci-mens used for these experiments were re-polished andhad a slight curvature along the specimen edge. As in Figs.5 and 6, an arrow indicates those specimens for which fail-ure did not occur at either of the fretting pads.

From these data it appears that contact geometry is nota major factor contributing to the reduction of fatigue lifefor either material under the experimental conditionsemployed. In general, experimental data for 4340 steel(Fig. 7) all follow the same trend, regardless of contactpad geometry, specimen edge curvature, or pad span used.In addition, none of the pad geometries appeared to bepreferential for nucleating catastrophic cracks, as crackswere nucleated from all four geometries within similar life-times. Similar results were observed from experiments onPH 13-8 Mo, further strengthening the conclusion thatcontact geometry does not signicantly inuence fretting

signicantly change from fretting wear since most testswere operating in either the partial slip or mixed fretting

50

100

150

200

250

300

104 105 106 107 108

R = 0.1R = 0.5R = 0.75

Stre

ss am

plitu

de (M

Pa)

Cycles to failure

Fig. 9. Inuence of stress ratio on the fretting fatigue life of 4340 steel.

J.A. Pape, R.W. Neu / International Journal of Fatigue 29 (2007) 22192229 2223fatigue life of PH 13-8 Mo under these particular experi-mental conditions [1,2].

It should be noted that the size of the fretting fatigueprocess volume is a more signicant factor in inuencingfretting fatigue life [1015]. The size of the volume dependson both the pressure (i.e., normal force and prole of thebodies) and size of contact [10,11]. The fretting fatigue pro-cess volume is comparable for the contact geometries con-sidered in this study because the normal force was notvaried. Hence, it is not surprising that there is little eectof contact geometry. Also, the prole of the pads did notFig. 10. Fretting fatigue scars from: (a) cylindrical contact and (b) at contact fThe direction of fretting is from top to bottom for each image.regime. Fig. 8The eect of mean stress on the fretting fatigue life of

4340 steel is shown in Fig. 9. Experiments were performedat R = 0.1, 0.5, and 0.75. Little dierence was observedbetween experiments performed at R = 0.1 and 0.5. At astress amplitude of 120 MPa, no failure occurred for eitherstress ratio. For R = 0.75, however, failure occurred after211,000 cycles at a stress amplitude of 120 MPa and after301,000 cycles at a stress amplitude of 100 MPa, indicatingthat the fretting fatigue limit in terms of stress amplitude atR = 0.75 is signicantly lower than that at R = 0.1 andR = 0.5.rom an experiment on PH 13-8 Mo stainless steel showing scar orientation.

3.2. Surface analysis of fretting scars

Fretting scars produced on the PH 13-8 Mo stainlesssteel specimens were, in general, elliptical in shape and werelocated near the center of the specimen, whereas most ofthe scars produced on the 4340 steel were rectangularand extended to the edges of the specimen. The ellipticalscar on the stainless steel specimens is due to curvatureof the specimen edges, leading to a crossed cylinder contactas opposed to a true cylinder on at contact. Similar con-tact conditions were produced on 4340 specimens by pol-ishing the specimen edges such that a slight curvatureresulted.

As seen in Fig. 10, there was a fundamental dierence inthe orientation between the at and cylindrical frettingscars, as the fretting direction is from top to bottom foreach gure. Fretting scars created by at pads exhibitedsignicant variations in shape, whereas the overall charac-teristics of scars produced by cylindrical fretting pads weremuch less variable. Fig. 11 shows fretting scars on 4340steel specimens for fretting pads with radii of curvatureof 1.5, 15, and 150 mm. As the fretting pad radius of cur-vature increases, the fretting scar width increases as well.

assumes that only elastic deformation occurs within thecontact, and that there is no relative sliding between con-tacting components. As can be observed from Fig. 12,

Fig. 12. Measured fretting scar widths as a function of fretting pad radiusof curvature for various ranges of relative slip.

2224 J.A. Pape, R.W. Neu / International Journal of Fatigue 29 (2007) 22192229However, in general, scars created by pads with 15 and150 mm radii of curvature appear to be quite similar.

Fretting scars from numerous experiments were mea-sured, and the results are shown in Fig. 12 as a plot of fret-ting scar width (in the fretting direction) as a function ofthe fretting pad radius of curvature for relative slip rangesfrom 4.3 to 19.8 lm. Also shown is the contact width pre-dicted by Hertzian contact theory for cylindrical pads hav-ing radii of curvature of 1.5, 15, and 150 mm. Hertz theoryFig. 11. Fretting fatigue scars from experiments on 4340 steel made by frettingdirection of fretting is from top to bottom for each image.the general trend of increasing contact width with increas-ing fretting pad radius of curvature holds well for radii ofcurvature of 1.5 and 15 mm. All of the data points fallabove the Hertz predictions for pads with these radii ofcurvature, as would be expected due to the relative slip,plastic deformation, and wear that occurs within the mea-sured fretting scars. Also, the measured scar widthsapproach those predicted by Hertz theory as the range ofrelative slip decreases, as one might predict. On the otherpads with: (a) 1.5 mm; (b) 15 mm and (c) 150 mm radii of curvature. The

hand, the scar widths for pads having a radius of curvatureof 150 mm are nearly the same as those for pads with aradius of curvature of 15 mm, as was observed from photosof the fretting scars. This is likely due to inaccuracies inmachining or polishing of the 150 mm pads, leading to adecrease in the actual radius of curvature from thatexpected.

Trends in average surface roughness (Ra) with increas-ing relative slip range and fretting pad radius of curvature

are shown in Fig. 13 for fretting scars on 4340 steel. Thesemeasurements were taken within the fretting scar using theZygo 3-D surface proler. Fig. 13a indicates a slightincrease in surface roughness as the relative slip increases,while Fig. 13b shows that the average roughness does notseem to be dependent on the radius of curvature of the fret-ting pads. A similar trend as in Fig. 13a is observed for sur-face roughness as a function of fatigue stress amplitude.This is no surprise, as the relative slip is proportional to

0

1

2

3

4

5

0 5 10 15 20

Ra

rou

ghne

ss (

m)

Ra

rou

ghne

ss (

m)

Relative slip range (m)

0

1

2

3

4

5

1 10 100Fretting pad radius of curvature(m)

Fig. 13. Average surface roughness (Ra) as a function of: (a) relative slip range and (b) fretting pad radius of curvature for experiments on 4340 steel.

J.A. Pape, R.W. Neu / International Journal of Fatigue 29 (2007) 22192229 2225Fig. 14. Fretting scars from PH 13-8 Mo stainless steel at fatigue stress ampltrailing edge of contact is at the bottom of each image.itudes of: (a) 217 MPa; (b) 166 MPa; (c) 120 MPa and (d) 100 MPa. The

the fatigue stress amplitude for the experimental apparatusused during this study.

Fig. 14 shows the damage produced by fretting at fourdierent stress amplitudes on PH 13-8 Mo stainless steelat R = 0.1. The trailing edge of the scar, i.e. the edge fur-thest from the line of symmetry of the pad, is located atthe bottom of each gure. Experiments at stress ampli-tudes of 217 MPa and 166 MPa (Figs. 14a and b, respec-tively) were generated from cylindrical fretting pads witha radius of curvature of 15 mm and a pad span of16.5 mm. Major fatigue cracks are observed near the trail-ing edge of these scars. Signicant damage can beobserved throughout the entire scar at a stress amplitudeof 217 MPa (Fig. 14a), but when the stress amplitude isdecreased to 166 MPa, the scar exhibited a central regionthat diered signicantly from the outer edges of the scar(Fig. 14b) even though the fatigue life did not change sig-nicantly. These scars are typical of tests operating in themixed fretting regime. Scars from experiments at stressamplitudes of 120 MPa and 100 MPa (Fig. 14c and d)were generated from cylindrical fretting pads with aradius of curvature of 15 mm and a pad span of 12.0mm. Fretting scars for both of these stress amplitudesexhibited a central stick region surrounded by a slip annu-

lus, indicating that partial slip conditions prevail through-out the test. The shorter pad span is expected to generateless relative slip than a longer pad span for a given stressamplitude, but it is expected that the scars in Figs. 14aand b would not vary signicantly if a shorter pad spanhad been used [9].

3.3. Frictional force measurements

The frictional force between the fretting pads and spec-imen was measured via strain gages mounted on the fret-ting pads. Fig. 15 shows the evolution of the frictionalforce vs. applied fatigue load hysteresis from a cylindricalfretting contact during an experiment on PH 13-8 Mostainless steel performed at a stress amplitude of217 MPa and R = 0.1 [2]. The initial hysteresis behaviorindicates the existence of a gross slip condition (Fig. 15a),which was typical during the rst one to 200 cycles forall of the experiments in this study. In most cases, this grosssliding behavior gradually transitioned towards partial slip,which is characterized by the hysteresis loops shown in Fig.15b and c. A partial slip condition was usually attainedwithin the rst few 1000 cycles of fretting, and remainedthroughout most of the experiment. Hence, most tests were

200

400

ce (N

)

200

400

ce (N

)

at a

2226 J.A. Pape, R.W. Neu / International Journal of Fatigue 29 (2007) 22192229-400

-200

0

50 10 15 20 25 30 35 40

Fric

tiona

l For

Load(kN)Cycle 60

-400

-200

0

200

400

0 10 15 20 25 30 35 40

Fric

tiona

lFor

ce (N

)

Load(kN) Cycle 60,000

5

Fig. 15. Evolution of the frictional force vs. applied fatigue load hysteresis

steel with a stress amplitude of 217 MPa, a stress ratio of 0.1, and a pad span122,000 [2].-400

-200

0

0 10 15 20 25 30 35 40

Fric

tiona

l For

Load (kN) Cycle 5,300

-400

-200

0

200

400

0 10 15 20 25 30 35 40

Fric

tiona

lFor

ce (N

)

Load (kN) Cycle 122,000

5

5

cylindrical fretting contact during an experiment on PH 13-8 Mo stainless

of 16.5 mm at: (a) cycle 60; (b) cycle 5300; (c) cycle 60,000 and (d) cycle

operating in the mixed fretting regime that promotes fret-ting crack formation [3,4].

During the early stages of an experiment, the coecientof friction (COF) changed signicantly. The COF is calcu-lated by dividing the frictional force at the onset of grosssliding by the contact load. Typical values for the coe-cient of friction during the initial loading cycle for PH13-8 Mo stainless steel tested at a stress amplitude of217 MPa, R = 0.1, and a pad span of 16.5 mm range from0.06 to 0.20. Initial COF values for 4340 steel tested undersimilar conditions range from 0.15 to 0.25. Little dierenceis observed in the COF values from fretting pads havingdierent geometries.

The COF increases as the sliding conditions at the con-tact transition from gross sliding to partial slip, but thehysteresis loops no longer saturate at a constant value offrictional force. Instead, the frictional force continues toincrease until the maximum fatigue load is reached, asshown in Figs. 15b and c. At this point gross sliding isno longer occurring, and the coecient of sliding friction

is greater than the value obtained by using the maximumfrictional force reached during the cycle. Thus, only a lowerbound for the COF can be obtained directly from theresponse. Lower bounds for the COF during the periodof peak frictional force from experiments on 4340 steelare 1.05 for the at fretting pad and 0.78 for the cylindricalfretting pad. In comparison, similar values from experi-ments on PH 13-8 Mo stainless steel are 1.20 for the atpad and 0.70 for the cylindrical pad [2].

Comparisons between the frictional force range evolu-tion for PH 13-8 Mo stainless steel and 4340 steel areshown in Fig. 16. Comparisons are shown for stress ampli-tudes ranging from 100 MPa to 217 MPa. In general, thefrictional force range increases quicker in experiments on4340 steel than it does during experiments on PH 13-8Mo. At higher stress amplitudes, PH 13-8 Mo stainlesssteel experiences a higher steady state frictional force rangethan 4340 (Fig. 16a). For stress amplitudes below140 MPa, the higher frictional force range is observed inexperiments on 4340 steel.

200

300

400

500

600

700

800

900PH 13-8 Mo, S3, 15 mmPH 13-8 Mo, S3, Flat4340, S21, 15 mm4340, S21, Flat

Fric

tiona

l for

ce ra

nge

(N)

200

300

400

500

600PH 13-8 Mo, S10, 15 mmPH 13-8 Mo, S10, 15 mm4340, S24, 15 mm4340, S24, 150 mm

Fric

tiona

l for

ce ra

nge

(N)

103

umb

J.A. Pape, R.W. Neu / International Journal of Fatigue 29 (2007) 22192229 22271001 101 102 103 104 105 106

Number of cycles

100

150

200

250

300

350

400

1 101 102

Fric

tiona

l for

ce ra

nge

(N)

NFig. 16. Comparison of frictional force range evolution between PH 13-8 Mo(b) 120 MPa and (c) 100 MPa.1001 101 102 103 104 105 106 107

Number of cycles

104 105 106 107 108

PH 13-8 Mo, S13, 15 mmPH 13-8 Mo, S13, 15 mm4340, S39, 1.5 mm4340, S39, 15 mm

er of cyclesstainless steel and 4340 steel for fatigue stress amplitudes of: (a) 217 MPa;

our4. Discussion

Both materials experienced a signicant reduction infatigue strength due to fretting. However, since PH 13-8Mo stainless steel has a greater resistance to fatigue inthe life regime of interest, it sustains a greater reductionin life due to fretting than 4340 steel does. This behaviorcan be rationalized by considering the cyclic behavior ofthe two steels in the absence of fretting. Fig. 2 shows that4340 steel has a greater resistance to low cycle fatigue(

5. Conclusions

Fretting fatigue experiments on PH 13-8 Mo stainlesssteel and quenched and tempered 4340 steel showed signif-icant reductions in the 107 cycle fatigue endurance limit dueto fretting. Both materials had similar fretting fatigue lives,but the reduction in fatigue strength was greater for PH 13-8 Mo stainless steel than for 4340 steel. Thus, PH 13-8 Mostainless steel is more susceptible to fretting fatigue than

smooth specimen life. Comparing these trends to the low

References

[1] Pape JA, Neu RW. Fretting fatigue damage accumulation in PH 13-8Mo stainless steel. Int J Fatigue 2001;23(Suppl. 1):S43744.

[2] Pape JA, Neu RW. Inuence of contact conguration in frettingfatigue testing. Wear 1999;225229:120514.

[3] Zhou ZR, Vincent L. Mixed fretting regime. Wear 1995;181183:5316.

[4] Fouvry S, Kapsa P, Vincent L, Dang Van K. Theoretical analysis offatigue cracking under dry friction for fretting loading conditions.

J.A. Pape, R.W. Neu / International Journal of Fatigue 29 (2007) 22192229 2229cycle fatigue and crack growth behavior of these two steelssuggests that the superior performance of 4340 steel in fret-ting fatigue closely correlates to its superior low cycle fati-gue performance. The fretting fatigue life showed somedependence on mean stress, though the dierence in lifeis primarily attributed to the eect of mean stress on thecrack propagation behavior.

Varying the geometry of the contact did not have a signif-icant inuence on the fretting fatigue life under the experi-mental conditions considered in this investigation.Experiments were performed with at/cylindrical, 1.5 mm/15 mm, and 15 mm/150 mm pad congurations. Neitherpad from each of these congurations was exclusive innucleating catastrophic fretting fatigue cracks. In addition,stresslife data points followed the same trend regardless ofwhich pad nucleated the crack that caused failure.

A clear link between the frictional force range evolutionand the sliding conditions at the fretting interface has beenmade. Unless both scars for a fretting pad show evidence ofa central stick region with a surrounding region of slip sug-gestive of partial slip, the frictional force will peak withinthe rst few 100 cycles of fretting fatigue and will thendecrease until a steady-state response is attained. Crackswere nearly always observed to form in this mixed frettingregime in both steels.

Acknowledgement

This research was funded by the Oce of NavalResearch through research Grant N00014-95-1-0539.4340 steel for the particular heat treatments and frettingconditions used. The greatest reduction was for tests con-ducted at a stress ratio of 0.1. The fretting fatigue life ofPH 13-8 Mo was only 26% of the smooth specimen life,while the fretting fatigue life of 4340 steel was 36% of theWear 1996;195:2134.[5] Scully JR, Van Den Avyle JA, Cieslak MJ, Romig Jr AD, Hills CR.

The inuence of palladium on the hydrogen-assisted crackingresistance of PH 13-8 Mo stainless steel. Metall Trans A1991;22A:242944.

[6] Hochanadel PW, Robino CV, Edwards GR, Cieslak MJ. Heattreatment of investment cast PH 13-8 Mo Stainless Steel: Part I.Mechanical properties and microstructure. Metall Mater Trans A1994;25A:78998.

[7] MIL-HDBK-5G, November; 1994. p. 2-479.[8] Patel AM, Neu RW, Pape JA. Growth of small fatigue cracks in PH

13-8 Mo stainless steel. Metall Mater Trans A 1999;30A:1289300.[9] Pape JA. Fretting fatigue damage accumulation and crack nucleation

in high strength steels. Ph.D. Thesis. Georgia Institute of Technology,Atlanta, GA; 2002.

[10] Nowell D, Hills DA. Crack initiation criteria in fretting fatigue. Wear1990;136:32943.

[11] Araujo JA, Nowell D. Analysis of pad size eects in frettingfatigue using short crack arrest methodologies. Int J Fatigue1999;21:94756.

[12] Araujo JA, Nowell D. The eect of rapidly varying contact stresselds on fretting fatigue. Int J Fatigue 2002;24:76375.

[13] Fouvry S, Kapsa P, Vincent L. A multiaxial fatigue analysis offretting contact taking into account the size eect. In: Hoeppner DW,Chandrasekaran V, Elliott CB, editors. Fretting fatigue: currenttechnology and practices. ASTM STP, Vol. 1367. American Societyfor Testing and Materials; 2000. p. 16782.

[14] Fouvry S, Kapsa P, Sidoro F, Vincent L. Identication of thecharacteristic length scale for fatigue cracking in fretting contacts. JPhys IV France 1998;8:Pr8-15966.

[15] Swalla D, Neu RW. Characterization of fretting fatigue processvolume using nite element analysis. In: Mutoh Y, Kinyon SE,Hoeppner DW, editors. Fretting fatigue: advances in basic under-standing and applications. ASTM STP, Vol. 1425. ASTM Interna-tional; 2003. p. 89107.

[16] Waterhouse RB. Physics and metallurgy of fretting. AGARD In:Conference proceedings no. 161, specialists meeting on fretting inaircraft systems; 1975. p. 8-18.

[17] Waterhouse RB. Avoidance of fretting fatigue failures. In: Water-house RB, editor. Fretting fatigue. London: Applied Science Pub-lishers Ltd.; 1981. p. 22140.

[18] Waterhouse RB. Fretting fatigue. Int Mater Rev 1992;37(2):7797.[19] Wallace JM, Neu RW. Fretting fatigue crack nucleation in Ti-6Al-

4V. Fatigue Fract Eng Mater Struct 2003;26:199214.

A comparative study of the fretting fatigue behavior of 4340 steel and PH 13-8 Mo stainless steelIntroductionExperimental methodsMaterialsExperimental procedure

ResultsFretting fatigue livesSurface analysis of fretting scarsFrictional force measurements

DiscussionConclusionsAcknowledgementReferences