Composite Structures 74 (2006) 145–152

www.elsevier.com/locate/compstruct

Strength of functionally gradient composite hemispherical bearings

Seung Min Lee, Dong Chang Park, Byung Chul Kim, Dai Gil Lee *

Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Mechanical Design Laboratory

with Advanced Material, ME3221, 373-1, Guseong-dong, Yuseong-gu, Daejeon-shi 305-701, Republic of Korea

Available online 13 June 2005

Abstract

Carbon fibre phenolic woven composites are used as journal bearing materials because they have self-lubricating properties and

high specific strength at high temperature without the danger of seizure problem with the metallic journals.

In order to reduce the frictional coefficient of carbon phenoilc woven composite, in this work, polyetheretherketone (PEEK)

powders were mixed near the surface plies to make a functionally gradient material, because the PEEK powder lowers not only

the friction coefficient but also the strength of the composite. The effects of PEEK powders and the bottom vent hole in the com-

posite hemispherical bearing (CHB) on the strength were experimentally investigated. Based on the investigation, a new CHB was

fabricated by molding rather than machining the hemispherical surface to eliminate crack and delamination and tested in an exper-

imental set-up.

The experimental results showed that the new CHB outperformed the existing carbon PEEK CHB both in the endurance life and

compressive strength.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: PEEK; PAN; Phenolic; Composite; Micro crack; Drilling; Hemispherical; Bearing; Air channel

1. Introduction

Carbon–phenolic composite materials have self-lubri-

cating characteristics and high-specific strength charac-

teristics at high temperature, and allow inexpensivemolding and fast curing characteristics comparable to

polyester resin used for SMC (sheet molding com-

pound). Therefore, recently, the carbon–phenolic com-

posite materials are widely employed in areas such as

lubricating parts in pistons and bearings, which require

low friction and non-seizure characteristics [1].

One of the successful applications of the phenolic

composites is the composite hemispherical bearing(CHB) of an in-arm suspension unit (ISU) for military

tracked vehicles. The CHB, where the connecting rod

journal rotates like a ball and socket joint, transfers

0263-8223/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compstruct.2005.04.005

* Corresponding author. Tel.: +82 42 869 3221; fax: +82 42 869 5221.

E-mail address: dglee@kaist.ac.kr (D.G. Lee).

the compressive load from the connecting rod to the pis-

ton as shown in Fig. 1(a) [2].

Since the CHB whose hemispherical surface was

machined by a CNC milling machine was failed fre-

quently earlier than the intended life due to the cracksand delaminations generated by the machining opera-

tion [3], it is necessary to develop a new molding method

for the CHB without machining. Also the bottom hole

drilled at the center of the conventional CHB for the

ventilation of air in the piston during assembly of

CHB, generates the stress concentration and delamina-

tion of the composite layers, which usually reduce the

composite performance [4–7].Since it has been known that the addition of fillers

such as carbon and graphite into polymers not only re-

duces the coefficient of friction and wear rate, but also

reduces the thermal expansion coefficient [8], in this

work, the effect of filler in PAN–phenolic composite

on compressive strength was investigated. Also, a new

Fig. 1. Schematic diagram of CHB assembly: (a) in-arm suspension

unit and (b) assembly of connecting rod, CHB, and piston.

Fig. 2. Schematic cross sectional diagram of the conventional CHB.

146 S.M. Lee et al. / Composite Structures 74 (2006) 145–152

manufacturing method for the CHB with less stress con-

centration and easy assembly was developed.

2. Effect of the bottom hole in the CHB on the

compressive strength

The ISU (In-arm Suspension Unit) mounted ontracked vehicles in Fig. 1(a) reduces the transmission

of the irregular load from a roadwheel and dissipates

it. The CHB (composite hemispherical bearing) is the

interface between the connecting rod and the piston

[9]. When an irregular load makes the roadwheel move

up and down, the compressive load is transmitted to

the CHB by the connecting rod in the ISU. The average

and peak bearing pressures on the CHB during opera-tion are about 57 MPa and 270 MPa, respectively.

The hemispherical shape of the CHB has been created

by CNC machining a thick carbon–PEEK woven com-

posite block, and the vent hole at the center of CHB

used for the assembly of the CHB into the piston was

drilled as shown in Fig. 2. These machining operations

not only cause cracks and delaminations but also in-

crease the cost and process time. During the endurance

testing of carbon–PEEK CHB, the crack initiatedaround the bottom hole as the bottom thickness of the

CHB became thinner due to the wear after test cycle

of 1.0 · 105 as shown in Fig. 3.

Since the through-thickness compressive strength

(TTCS) of the CHB made of carbon–PEEK woven com-

posite was about 600 MPa, which was much lower than

the compressive strength of carbon–phenolic woven

composite [10], in this work, a new CHB made of car-bon–phenolic composite was developed by molding

method to eliminate the cracks generated during the

machining of hemispherical shape. Also, the effect of

the vent hole of 3 mm diameter in the bottom of the

CHB on the compressive strength of CHB was investi-

gated by the finite element (FE) analysis. In the FE anal-

ysis, the quadratic quadrilateral elements (type CAX8)

in ABAQUS 6.4 (Hibbitt, Karlsson & Sorensen, Inc.,USA) with axisymmetry were used. The connecting

rod and the piston were modeled as rigid bodies as

shown in Fig. 4. The number of elements was 7502.

The mechanical properties of the carbon–phenolic com-

posite used in the analysis are listed in Table 1.

The maximum compressive stress (rzz) in the CHB

with and without the bottom hole under the load of

500 kN are 415 MPa and 340 MPa, respectively (Fig.5), which are lower than the compressive strength of

the composite as shown in Table 1. The maximum shear

stresses (sxz) in the CHB with and without the bottom

hole under the same condition are 53 MPa and

5.0 MPa, respectively. Since the shear stress in the

CHB with the bottom hole is larger than the shear

strength of carbon–phenolic composite (39.2 MPa), the

failure may occur at the edge of hole as shown in Fig. 6.In order to obtain the effect of a hole on the compres-

sive strength, the static and dynamic tests were

performed using the flat specimens made of carbon–

phenolic prepreg (KPI Co., Ltd., Chang Won, Korea).

The specimens were fabricated with a hot-press under

the curing cycle which had two degassing processes to

increase compaction as shown in Fig. 7. The cured

plates were cut to the specimen size by a diamond wheelcutter. The length, width, and thickness of the specimen

Fig. 3. Photographs of damage and crack around the vent: (a) damage around the vent hole during machining and (b) crack creation and

propagation around the vent hole after endurance life test of 1.0 · 105

Fig. 4. FE modeling of CHB between the connecting rod and the

piston.

Table 1

Mechanical properties of PAN-based carbon–phenolic woven

composite

Young�s modulus, Exx (GPa) 61.5

Young�s modulus, Eyy (GPa) 61.5

Young�s modulus, Gzz (GPa) 14.3

Poisson�s ratio, mxy 0.039

Poisson�s ratio, mxz 0.46

Poisson�s ratio, myz 0.46

Shear modulus, Gxy (GPa) 5.8

Shear modulus, Gxz (GPa) 2.8

Shear modulus, Gyz (GPa) 2.6

Compressive strength (MPa) 890

Shear strength (MPa) 39.2

S.M. Lee et al. / Composite Structures 74 (2006) 145–152 147

were 10 mm, 10 mm, and 6 mm, respectively as shown in

Fig. 8(b). The depth of cut during cutting operation was

maintained less than 0.25 mm with sufficient cooling

water to minimize the damage on the machined surface.

During the drilling of carbon–phenolic woven compos-

ite, the ball nose type core drills and the helical-feed

method were used for high quality and delamination-

free machining [4].Static and impact test of the test specimens were per-

formed using an INSTRON 5208 with a 150 kN load

cell and drop weight impact tester. In the static test,

the cross head speed was maintained at 0.5 mm/min

according to the load history of the ISU [9]. For the

Fig. 5. Calculated compressive stresses (rzz) in the CHB with and

without the bottom hole: (a) with the hole and (b) without the hole.

Fig. 6. Calculated shear stresses (sxz) in the CHB with and without the

bottom hole: (a) with the hole and (b) without the hole.

0

20

40

60

80

100

120

140

160

180

0 20 40 60 80 100 120 1 160 180

0

2

4

6

8

10

12

14

16

18

20

140

TemperaturePressure

2nd degassing

1st degassing

Time (min)

Tem

pera

ture

(o C

)

Pressure (M

Pa)

Fig. 7. Curing cycle of carbon–phenolic composite using hot-press.

148 S.M. Lee et al. / Composite Structures 74 (2006) 145–152

impact test of the specimens, the impactor of 25 kg wasdropped at the height of 900 mm, and the initial impact

speed and force history data were measured by the pho-

toelectric sensor (E32-T11L and E3X-F21, Omron,

Japan) and the force transducer (PCB234B, PCB,

USA), respectively. Fig. 8(a) shows the schematic dia-

gram of the weight drop impact tester. As shown in Figs.

9 and 10, both the static and dynamic compressive

strengths of the composite specimens with the hole wereabout 25% lower than those without the hole, which is

the similar to results of the CHB.

Since the bottom hole is necessary to vent the gas in

the piston during the assembly of the CHB, the position

of the hole was moved to the wall of CHB as shown in

Fig. 11.

3. Effect of filler on the compressive strength of

composite specimens

Since the lubricating fillers in the composite may de-

crease the friction coefficient between the CHB and the

connecting rod, which is important for the bearing life

[8], polyetheretherketone (PEEK) powders were addedto the carbon–phenolic woven composite. Then, the sta-

tic and dynamic tests were performed to investigate the

effect of filler on the compressive strength of the carbon–

phenolic woven composite. Specimens were fabricated

by a hot-press using the same curing cycle as shown in

Fig. 7 and the dimensions of the specimen were same

as those of Fig. 8(b). The weight amounts of PEEK

powder added to carbon–phenolic woven composite

10.0

6.0

10.0

Hole (Ø3mm)

10.0

6.0

10.0

a

b

Photoelectric sensor

Shock absorber

Force transducer

Lift and separator

LM shaft

Rubber pad

Mass Photo detection bar

Fig. 8. Schematic diagram of the weight drop impact tester: (a)

instrument and (b) specimen dimensions (mm) with and without hole.

Fig. 9. Measured static compressive strengths of the composite

specimens with and without the hole: (a) load versus displacement

curve and (b) static compressive strength.

S.M. Lee et al. / Composite Structures 74 (2006) 145–152 149

were 2%, 5%, 8%, and 10%, respectively. From these test

results, it was found that the compressive strength of the

composite decreased as the amount of PEEK filler was

increased, as shown in Fig. 12.

However, when 10% PEEK powders by weight were

added, the friction coefficient decreased and wear resis-

tance increased [11]. Therefore, PEEK powders were

mixed in the two top plies of specimen as shown inFig. 13, which realizes a functionally gradient material.

Then, the static and impact tests were performed again

and compared with the previous test results. From the

test result, it was found that when PEEK powders were

mixed in the top two plies, the compressive strength

decreased less than 5% compared with those specimens

without PEEK powder as shown in Fig. 14. Also the

compressive strength of carbon fiber PEEK compositewas much lower than of carbon–phenolic woven com-

posite as shown in Table 2.

Fig. 10. Measured impact strengths of the composite specimens with

and without hole: (a) dynamic loads versus displacement curves and

(b) dynamic compressive strengths.

4. Manufacturing of the new CHB

The new CHBs whose top two plies were mixed with

10% PEEK powder and whose two air channels were

Fig. 11. Sectional view of the hole and air channels in the CHB.

Fig. 12. Static and impact strengths of the carbon–phenolic woven

composite with respect to the addition amount of PEEK powder.

Mixing of PEEK powders in the top two layers

Fig. 13. Fabrication method for the functionally gradient specimen.

Fig. 14. Comparison of the strengths of 10% of PEEK powder added

with that of every ply with to two top layers.

Table 2

Static and impact strengths with respect to PEEK powder

Strength (static) Strength

(impact)

Carbon–PEEK

composite

673 MPa 862 MPa

Carbon–phenolic

composite

0% PEEK powder 881 MPa 934 MPa

2% PEEK powder

(all plies)

766 MPa 865 MPa

5% PEEK powder

(all plies)

762 MPa 854 MPa

8% PEEK powder

(all plies)

725 MPa 842 MPa

10% PEEK powder

(all plies)

696 MPa 741 MPa

10% PEEK powder

(top two plies)

862 MPa 880 MPa

Fig. 16. Photograph of the CHB endurance life test set-up.

Fig. 15. Photograph of the new CHB with air channels.

150 S.M. Lee et al. / Composite Structures 74 (2006) 145–152

placed at the outside wall were molded with a hot-press

rather than CNC machining. The mold was designed to

fabricate the CHB in a near net-shape at 155 �C inside a

hot-press. Fig. 15 shows the refined CHB with air

channels.

The dimensions of CHB after molding were measured

by CMM (coordinate measuring machine) and found to

satisfy the allowable tolerance of ±50 lm.

Fig. 17. Photograph showing wear debris in the PEEK CHB with the

bottom hole after endurance test of 7.0 · 104 cycles.

S.M. Lee et al. / Composite Structures 74 (2006) 145–152 151

5. Endurance life characteristics of the CHBs

To investigate the wear characteristics of CHB, the

endurance life test was performed using the laboratory

experimental set-up as shown in Fig. 16 [11]. The com-

pressive load was applied to the CHB by a hydraulicjack, which applied vertical load of 1.5 · 105 N on the

CHB, which was the normal operating load of a real

ISU [11].

The endurance life tests for the CHBs with respect to

PEEK powder and bottom hole were performed for

1.0 · 105 cycles, which is the required cycle for the

tracked vehicle. Fig. 17 shows the test result of the

PEEK CHB with the bottom hole, where large amountof wear occurred in the loading direction, and a lot of

wear debris were observed in the CHB, although test

was performed under the oil-lubricated condition. The

wear debris expelled from the CHB during operation

were piled up around the mouth of the CHB, and

blocked the oil passage, which caused dry wear condi-

tion and accelerated wear rate of the CHB. While, the

new CHB made of carbon–phenolic woven compositewith 10% of PEEK powder in the top two plies showed

good wear characteristics as shown in Fig. 18. The wear

depth of the PEEK CHB and the new CHB were mea-

Fig. 18. Photographs of the CHB with

sured by a coordinate measuring machine (CMM).

Fig. 19 shows the CMM result of CHBs after the endur-

ance test of 1.0 · 105 cycles, where the 1-direction repre-

sents the parallel direction to the sliding direction, while

the 2-direction represents the perpendicular direction tothe sliding direction. From the test results, it was found

that the new carbon–phenolic woven CHB with the side

air channels showed much better wear resistance than

the PEEK CHB with the bottom hole.

air channels after endurance tests.

Fig. 19. Wear test results of the CHBs after endurance test.

152 S.M. Lee et al. / Composite Structures 74 (2006) 145–152

6. Conclusion

In this work, a new composite hemispherical bearing

(CHB) for tracked vehicles was developed using carbon–phenolic woven composite rather than expensive car-

bon–PEEK composite using molding method. Since

the bottom vent hole in the CHB used for the vent hole

during assembly affected much the strength and endur-

ance life of the CHB, the vent hole of the new CHB

was replaced by two side air channels. Since the addition

of PEEK powder not only decreases the friction coeffi-

cient but also the strength of composite, only two sur-face plies of CHB were mixed with PEEK powder to

make a functionally gradient structure.

From the test results of the CHBs using a test set-up,

it was found that that the new CHB outperformed the

existing carbon PEEK CHB both in the endurance life

and compressive strength.

Acknowledgement

This work has been supported by NRL Project of

Korean Government.

References

[1] Kim SS, Park DC, Lee DG. Characteristics of carbon fiber

phenolic composites for journal bearing materials. Compos Struct

2004;6.

[2] Root LW, Bowman H. Design and fabrication of In-arm

hydropneumatic suspension unit. TACOM-TR-12165. AD-

B047449. 1976.

[3] Lee DG, Suh NP. Axiomatic design and fabrication of composite

structures, 2005.

[4] Park KY, Choi JH, Lee DG. Delamination-free and high

efficiency drilling of carbon fiber reinforced plastics. J Compos

Mater 1995;29(15):1998–2002.

[5] Andrew SD, Ochoa OO, Ownes SD. The effect of fastener hole

defects. J Compos Mater 1993;27(1):3–20.

[6] Tagliaferri V, Caprino G, Diterlizzi A. Effect of drilling param-

eters on the finish and mechanical properties of GFRP compos-

ites. J Mach Tools Manufact 1990;30(1):77–84.

[7] El-Sonbaty I, Khashba UA, Machaly T. Factors affecting the

machinability of GFR/epoxy composites. Compos Struct

2004;63:329–38.

[8] Bhushan Bhart. Principles and Applications of Tribology.

New York: Wiley-Interscience; 1999. p. 873–96.

[9] Park DC, Lee SH, Kim JS. Stress analysis of an in-arm suspension

unit housing. Technical Report GSDC-519-010322. Agency for

Defense Development, 2001.

[10] Park DC, Lee DG. Through-thickness compressive strength of

carbon–phenolic woven composites. Compos Struct, in press, doi:

10.1016/j.compstruct.2004.09.001.

[11] Park DC, Lee SM, Kim BC, Kim HS, Lee DG. Development of

heavy-duty hybrid carbon–phenolic hemispherical bearings. Com-

pos Struct, in press, doi: 10.1016/j.compstruct.2005.01.029.