Measurements of fiber bundle interfacial properties of three-dimensionally reinforced carbon/carbon...

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Carbon 45 (2007) 459–467

Measurements of fiber bundle interfacial propertiesof three-dimensionally reinforced carbon/carbon

composites up to 2273 K

Takuya Aoki a,*, Yuhsuke Yamane c, Toshio Ogasawara a, Takeshi Ogawa c,Sunao Sugimoto a, Takashi Ishikawa b

a Advanced Composite Technology Center, Institute of Aerospace Technology, Japan Aerospace Exploration Agency,

6-13-1 Osawa, Mitaka, Tokyo 181-0015, Japanb Aviation Program Group, Japan Aerospace Exploration Agency, 7-44-1 Jindaiji-higashi, Chofu, Tokyo 181-8522, Japanc Department of Mechanical Engineering, Faculty of Science Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe,

Sagamihara, Kanagawa 229-8558, Japan

Received 14 March 2006; accepted 31 July 2006Available online 9 October 2006

Abstract

Experimental techniques for evaluating the interfacial properties between fiber bundles and the matrix of three-dimensionally rein-forced carbon/carbon composites were examined. Specially arranged fiber bundle push-out and pull-out tests were conducted up to2273 K in vacuum. In these tests, a fiber bundle in the specimens was extruded or pulled out by external compressive or tensile loads.Post-fracture observations revealed that a shear fracture was successfully induced within the carbon matrices at the loaded fiber bundleinterface. The interfacial shear strength si

max and initial sliding stress sinis of the fiber bundle monotonically increased with the test tem-

perature. The relief of residual thermal stress and increases in the frictional resistance and anchor effect at the fiber bundle interface wereconsidered to be the major mechanisms that caused the enhancements. An increase in the heat treatment temperature during the pro-cessing of the composites resulted in a significant decrease in si

max and sinis .

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1. Introduction

Carbon-fiber-reinforced carbon matrix composites (C/Cs) have been applied to structures subjected to severeheating conditions because of their high heat resistance[1–3]. For thick structures, three-dimensionally reinforced(3D) C/Cs are often preferred over laminated (2D) C/Cssince the latter have low interlaminar shear strength.Recently, Hatta et al. evaluated the mechanical propertiesof various 3D-C/Cs [4–6]. The results revealed that the3D-C/Cs exhibit several superior mechanical responsesover 2D-C/Cs, e.g., higher fracture toughness and strain

0008-6223/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2006.07.028

* Corresponding author. Tel.: +81 422 40 3381; fax: +81 422 40 3549.E-mail address: [email protected] (T. Aoki).

capability. In double-edge-notch tensile tests of the 3D-C/Cs, cracks were induced from the pre-introduced cracktips; however, they propagated along the fiber bundle inter-faces without the breakage of the load-bearing fibers. Con-sequently, an extremely high toughness was obtained [4]. Inshear tests of the 3D-C/Cs, debonding also occurred at thefiber bundle interfaces and subsequent slipping at the deb-onded interfaces enabled a large shear deformation,exceeding 10% [5]. Further, the shear and compressionstrengths of the 3D-C/Cs increased with the test tempera-ture [5,6]. At 1873 K, the strengths, as determined fromthe shear and compression tests, were 2.5 and 1.3 timesgreater than those at room temperature. Based on theresults of a fiber bundle push-out test, it was concludedthat these strength enhancements occurred due to anincrease in the interfacial strength of the fiber bundles.

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460 T. Aoki et al. / Carbon 45 (2007) 459–467

These results suggest that the characterization of the fiberbundle interfacial properties is necessary for understandingthe unique mechanical behavior of 3D-C/Cs.

Several attempts have been made to determine the fiber/matrix interfacial properties of C/Cs. The typical test meth-ods are push-out and pull-out tests [7–11]. In these tests, amonofilament or fiber bundle is extruded and pulled outfrom specimens by an external load. Sakai et al. [7], Dom-nanovich et al. [8], and Valette et al. [9] conducted fiberbundle pull-out tests at room temperature for unidirection-ally reinforced (UD) and 2D-C/Cs. Domnanovich reportedthat the bundle interfacial shear strength of a 2D-C/Cdecreased with an increase in the heat treatment tempera-ture (HTT) during the processing of the composite. Thisreduction was explained in terms of the graphitization ofthe matrices, i.e., as the HTT increased, weakly bondedgraphite layers of the matrices aligned themselves alongthe fiber/matrix interfaces in a more parallel manner. Hattaet al. conducted single-fiber and bundle push-out tests for2D- and 3D-C/Cs [10,11]. In the single-fiber push-out tests,they determined a critical load below which compressivefracture of the loaded carbon fiber can be avoided. Byusing thin specimens (�50 lm), they successfully evaluatedthe fiber/matrix interfacial shear strength. Through theseefforts, the techniques for evaluating the fiber/matrix inter-facial properties of C/Cs have been improved. However, tothe author’s best knowledge, except the preliminary results[5], no clear experimental data is available on the high-tem-perature fiber/matrix interfacial properties of C/Cs.

The objective of this study is the development of exper-imental techniques for determining the fiber bundle interfa-cial properties at elevated temperatures. Specially arrangedbundle push-out and pull-out tests were conducted fororthogonally reinforced 3D-C/Cs at temperatures up to2273 K in vacuum. In order to determine the specimen con-figurations, tests with different embedded lengths of fiber

Fig. 1. Microstructures of HTT-2673 K 3D-C/C

bundles were initially conducted at room temperature.The influence of HTTs on the fiber bundle interfacial prop-erties was also evaluated at elevated temperatures.

2. Experimental procedures

2.1. Materials

The investigated 3D-C/Cs were fabricated using a pitch impregnation/carbonization process. Shikibo Ltd., Japan, supplied an orthogonal 3Dpreform containing polyacrylonitrile-based carbon fibers. The fabric com-prised thin fiber bundles in the x- and y-directions and thick bundles in thez-direction. The total fiber volume fraction was �45% (�15% in eachdirection). In order to examine the influence of HTTs on the interfacialproperties of the bundle, specimens heat-treated at 2673 K, 2973 K, and3273 K were prepared. In this study, evaluations were primarily conductedfor the HTT-2673 K material.

X-ray computed tomography images of HTT-2673 K 3D-C/C areshown in Fig. 1. These images show that the 3D-C/C possesses a coarsefiber bundle architecture. The x- and y-bundles have a rectangularcross-section with dimensions of 1.65 · 0.47 mm2, while the z-bundleshave a square cross-section with dimensions of 1.65 · 1.65 mm2. As-pro-cessed flaws existing in the composite were classified into the followingthree types: debonding, axial split cracking of the fiber bundles, andmatrix voids. The matrices in C/Cs are known to significantly shrink atthe carbonization stage during the processing of C/Cs; this is due to theconversion of resin into carbon. They also shrink during the cooling stagesfrom the carbonization temperature and HTT due to the large coefficientsof thermal expansion (CTE). Further, fiber bundles contract significantlyin the transverse direction due to the anisotropy of the carbon fibers.Hence, as-processed debonding and splitting were considered to occurdue to the shrinkage stress during carbonization and the tensile thermalstresses during the cooling stages from the carbonization temperatureand HTT [4,12].

2.2. Bundle push-out and pull-out tests at room temperature

In order to determine the specimen configurations, tests with differentembedded lengths of the fiber bundle were first conducted at room temper-ature. For ease in operation, evaluations were performed on thick z-bundles.

obtained by X-ray computed tomography.

T. Aoki et al. / Carbon 45 (2007) 459–467 461

Fig. 2(a) shows the presently proposed specimen geometries for thebundle push-out tests. The specimen has a circumferential notch machinedperpendicular to the z-bundles. A 1-mm-thick notch was introduced usinga wheel saw by cutting into the center z-bundle up to a depth of 0.05–0.15 mm. The specimen was placed on a stainless plate with a 3-mm-diam-eter hole with the center z-bundle set above the hole. A compressive loadwas directly applied to the specimen ends; the center z-bundle wasextruded into the hole. The embedded length L of the loaded z-bundlewas varied from 1 mm to 15 mm.

Fig. 2(b) shows the specimen and test setup of the bundle pull-out tests.This test method is similar to that used by Domnanovich et al. [8]. The spec-imens were placed in a stainless fixture, and a tensile load was applied usinga mechanical grip. The L was varied from 5 mm to 15 mm.

Room-temperature tests were performed using a screw-driven testingmachine (Model 4502, Instron, USA) at a displacement rate of 0.1 mm/min. In these tests, the displacements were obtained from crosshead move-ments. Under the assumption that the shear stress is uniformly distributedalong the interface, the average shear stress at the loaded z-bundle inter-face si was calculated as follows:

si ¼ P4lzL

ð1Þ

where P, lz, and L denote the applied load, width of the z-bundle, andembedded length of the loaded z-bundle, respectively.

3L

L

Center z-bundle

Notch

9.5

6.5

3-through

Stainless plate (t=1)

70

1

X

Z

Y

3L

L

Center z-bundle

Notch

9.5

6.5

3-through

Stainless plate (t=1)

70

1

X

Z

Y

X

Z

Y

Fig. 2. Shapes and dimensions of fiber bundle push-out (a) an

7

12

12

18

3

1.2

Center z–bundle

Notch

Z

X

Z

Y

1

7

12

12

18

3

1.2

Center z–bundle

Notch

Z

X

Z

Y

X

Z

Y

1

Fig. 3. Fiber bundle push-out (a) and pull-out (

2.3. Bundle push-out and pull-out tests at elevated temperatures

In order to evaluate the fiber bundle interfacial properties at elevatedtemperatures, we modified the specimens that were used in the room-tem-perature tests. For the push-out tests, a flat-bottom hole with a diameter of3 mm was engraved on the specimen bottom, as shown in Fig. 3(a); the cen-ter z-bundle was extruded into the hole. The value of L was fixed at 5.8 mm.This specimen did not require a stainless plate, as used in the room-temper-ature push-out tests. Fig. 3(b) shows the specimen used in the high-temper-ature pull-out tests. A circumferential notch was also introducedperpendicular to the z-bundles. In order to induce a pull-out fracture, athrough-thickness hole with a diameter of 3 mm was machined at a dis-tance of 8 mm from the notch. Thus, the value of L was 6.5 mm. This spec-imen geometry was determined based on that proposed by Valette et al. [9].

High-temperature tests were conducted up to 2273 K in vacuum(<2 · 10�2 Pa) at a displacement rate of 0.2 mm/min. A screw-driven test-ing machine (Model 8862, Instron, USA) equipped with a tungsten heaterwas used to apply the loads. In these tests, the displacements were obtainedfrom crosshead movements. Prior to loading, the temperature of the spec-imens was maintained at the selected test temperature for 15 min. The spec-imen temperature was measured using a tungsten/rhenium thermocoupleset near the specimen up to a temperature of 1873 K and using a two-colorinfrared thermometer at temperatures exceeding 1873 K. The validation ofthe test temperature during testing was ±2 K.

10.5

3.23.2

L

15

25

Center z-bundle

Stainless fixture

X

Z

Y

Z

10.5

3.23.2

L

15

25

Center z-bundle

Stainless fixture

X

Z

Y

Z

d pull-out (b) specimens used in room-temperature tests.

25

9.5

30

1273-through

8Notch

Centerz–bundle

X Y

Z

6.525

9.5

30°

1273-through

8Notch

Centerz–bundle

X Y

Z

6.5

b) specimens used in high-temperature tests.


3. Results and discussion

3.1. Bundle push-out and pull-out behaviors at room

temperature

Fig. 4(a) and (b) shows the typical interfacial shearstress–displacement (si–d) curves for the push-out andpull-out tests at room temperature. Initially, si rapidlyincreased up to the maximum stress si

max. It should be notedthat a slight nonlinearity was observed before si

max wasattained in both the test methods. Above si

max, a gradualstress drop generally occurred in the push-out tests. In con-trast, in the pull-out tests, an instantaneous stress dropoccurred beyond si

max. Then, si continued to decrease witha further increase in the displacements. In some room-tem-perature tests, si increased again after the first stress dropand maintained a nearly constant si

max value, as indicatedby the dotted lines. In 25% and 20% of the push-out andpull-out tests, respectively, si increased after the first stressdrop. This behavior appeared independent of L within theexamined range. The reason for the constant si

max value willbe discussed in Section 3.2.

0

2

4

6

8

10

12

0 0.2 0.4 0.6 0.8 1Displacement (mm)

0

2

4

6

8

10

12

0 0.1 0.2 0.3 0.4 0.5 0.6Displacement (mm)

HTT-2673K

HTT-2673K

Bundle push-out tests

Bundle pull-out tests

τd, τinis

τd

τinis

L=10 mm

L=10 mm

L=2.3 mmL=13 mm

Inte

rfac

ial s

hear

str

ess

(MP

a)

0

2

4

6

8

10

12

0 0.2 0.4 0.6 0.8 1Displacement (mm)

0

2

4

6

8

10

12

0 0.1 0.2 0.3 0.4 0.5 0.6Displacement (mm)

HTT-2673K

HTT-2673K



τd, τinis

τd

τinis

L=10 mm

L=10 mm

L=2.3 mmL=13 mm

Inte

rfac

ial s

hear

str

ess

(MP

a)In

terf

acia

l she

ar s

tres

s (M

Pa)

Fig. 4. Typical si–d curves of HTT-2673 K 3D-C/C in the bundle push-out (a) and pull-out tests (b) at room temperature.

The fracture process observed in the push-out and pull-out tests is summarized in Fig. 5(a)–(c). Before the tests,as-processed debonding was partly observed within thematrices at the fiber bundle interfaces, as shown inFig. 5(a). Above si

max, debonding propagated along theloaded z-bundle interface and complete debondingoccurred (Fig. 5(b)). Upon further loading, sliding of thefiber bundle continued, as shown in Fig. 5(c). As observedin this photograph, the matrix block fragments are embed-ded to the surfaces of the loaded z-bundle. Hence, the slid-ing surfaces are periodically undulated at a regular intervalof �1 mm (twice the thickness of the x- and y-bundles).SEM images of the tested push-out and pull-out specimensare shown in Fig. 6(a) and (b), respectively. It is evidentthat a shear fracture was successfully induced at the loadedz-bundle interface.

3.2. Debonding and sliding at room temperature

In the bundle push-out and pull-out tests conducted inthis study, debonding was considered to have initiatedbefore si

max since a slight nonlinearity was observed in thesi–d curves. In this nonlinear region, the as-processed deb-onding at the loaded z-bundle interface gradually propa-gated as the displacements increased. Large friction at thepartially debonded interface was considered as the reasonfor the stable debonding process, wherein the applied loadincreased as debonding progressed. However, the propaga-tion of the as-processed debonding in the nonlinear regionwas not clearly identified by cross-sectional observationssince many debondings had already occurred after the pro-cessing of the composite.

In the bundle push-out and pull-out tests of C/Cs, alarge instantaneous stress drop was frequently observedafter si

max in the si–d curves [5,8,10,11]. It is known thatan instantaneous drop occurs when the fiber and the matrixare bonded before the tests and the friction after the deb-onding is small [13]. However, the 3D-C/Cs used in thisstudy exhibited gradual stress drops, particularly in thepush-out tests, thereby indicating high friction at the deb-onded interface. As shown in Fig. 5(a) and (b), the as-pro-cessed debonding and shear fracture surfaces are stronglyrough. In addition, the sliding surfaces are periodicallyundulated due to the matrix block fragments (Fig. 5(c)).These interfacial roughnesses cause a high friction due tothe anchor effect. Consequently, si

max obtained from thepush-out tests is regarded as the apparent interfacialstrength and the initial sliding stress of a fiber bundle. Inthis study, the initial sliding stress represents the frictionalresistance immediately after complete debonding. In con-trast, in the pull-out tests, si

max represents the apparentinterfacial strength; however, it differs from the initialsliding stress due to the instantaneous stress drops. In thisstudy, the value of si

max denotes an average strength. Thetrue shear strength of the intact portion at the interfaceshould be considerably higher due to the extensive as-pro-cessed debonding.

Fig. 5. Typical fracture process observed in the fiber bundle push-out and pull-out tests. (a) Before tests. (b) Just after smax. (c) Fiber bundle sliding.

Fig. 6. SEM images showing the interfacial fractures in the bundle push-out (a) and pull-out (b) tests.


The increasing behavior of si after the first stress drop(dotted lines in Fig. 4) was also caused by the high frictionat the debonded interface. The first stress drops occurred ata fairly low stress. This indicates that the interface of theloaded z-bundle was intensively debonded before the tests,thereby leading to complete debonding at a small load.However, a further load increase was required to overcomethe high friction at the debonded interface. The transitionto a constant si

max without an appreciable stress drop sup-ports this proposed fracture process. In these cases, thesi

max equals the initial sliding stress. In this study, the testresults that showed an increase in si after the first stressdrop were also used to evaluate si

max and sinis .

Fig. 7(a) and (b) shows the values of simax and sini

s

obtained at room temperature as a function of L. No clear

dependence is evident for either the test method or L. Thisindicates that si

max and sinis were not affected by the stress

concentrations at the loaded bundle ends [14]. Hence, theaverage shear stress determined from Eq. (1) wasconcluded to be approximately adequate. Based on this,the value of L in the high-temperature tests was set as�6 mm.

In principle, simax and sini

s in the push-out tests areexpected to exceed their corresponding values in the pull-out test results because of Poisson’s expansion and contrac-tion of the loaded bundles [13]. The instantaneous stressdrops in the pull-out tests probably reflect the influenceof Poisson’s contraction. However, Poisson’s effects onthe absolute values of si

max and sinis were not evident due

to considerable scattering.

0

5

10

15

20

0 5 10 15



Embedded z-bundle length (mm)

HTT-2673 K

0

5

10

15

20

0 5 10 15Embedded z-bundle length (mm)

HTT-2673 K

Bundle push-out testsBundle pull-out tests

Inte

rfac

ial s

hear

str

engt

h(M

Pa)

Initi

al s

lidin

g st

ress

(MP

a)

0

5

10

15

20

0 5 10 15



Embedded z-bundle length (mm)

HTT-2673 K

0

5

10

15

20

0 5 10 15Embedded z-bundle length (mm)

HTT-2673 K

Bundle push-out testsBundle pull-out tests

Inte

rfac

ial s

hear

str

engt

h(M

Pa)

Initi

al s

lidin

g st

ress

(MP

a)

Fig. 7. Values of simax and sini

s of HTT-2673 K 3D-C/C obtained from thebundle push-out (a) and pull-out (b) tests at room temperature.

Fig. 8. Typical si–d curves of HTT-2673 K 3D-C/C in the bundle push-out (a) and pull-out (b) tests at elevated temperatures.

0

5

10

15

20

25

30

35

0 500 1000 1500 2000 2500

Push-out (HTT-2673 K)

Pull-out (HTT-2673 K)

Push-out after de-gassing(HTT-2673 K)


Temperature (K)

0

5

10

15

20

25

30

35

0 500 1000 1500 2000 2500


Pull-out (HTT-2673 K)

Push-out after de-gassing(HTT-2673 K)


Temperature (K)

Inte

rfac

ial s

hear

str

engt

h(M

Pa)

Fig. 9. Temperature dependence of simax obtained for the HTT-2673 K

and HTT-3273 K 3D-C/Cs.


3.3. Bundle push-out and pull-out behaviors at elevated

temperatures

Fig. 8(a) and (b) shows the typical si–d curves in thepush-out and pull-out tests at various temperatures up to2273 K. It is evident that si

max significantly increases withthe test temperature T. In addition, gradual stress dropsoccurred after si

max in both the test methods, particularlyat high temperatures. Since sliding of the fiber bundleoccurs immediately after si

max with a gradual stress drop,the values of si

max in the high-temperature push-out andpull-out tests are regarded as the apparent interfacialstrength and the initial sliding stress. Fig. 9 shows the tem-perature dependence of si

max. The push-out and pull-outtests yielded nearly identical averaged values of si

max,although significant scattering was observed in each result.

In order to understand simax enhancement at elevated

temperatures, the candidate factors—strength enhance-ment of the carbon matrix, degassing of absorbed water,relief of residual thermal stress, and increase in frictionalresistance—are discussed below.

It is difficult to directly evaluate the matrix strength of3D-C/Cs. Hence, we refer to the test results of polycrystal-

line graphites to discuss the temperature dependence of thematrix strength. It is reported that the compressionstrength of IG-110 graphite (Toyo Tanso Co., Ltd., Japan)increased by 20% at 2273 K as compared to that at room

Fig. 10. Opening widths of the as-processed debonding at the fiber bundleinterface observed at 293 K (a) and 1673 K (b). Note that the HTT was2673 K.


temperature.1 In this test, the fracture surface was orien-tated at 45� to the loading direction. This indicates thatcompressive fracture occurred due to shear. A similarextent of strength improvement has been reported at2273 K for other polycrystalline graphites and graphitefoam [14,15]. Therefore, the increase in the matrix strengthis one of the mechanisms for si

max enhancement at elevatedtemperatures. However, its influence is too small to com-pletely explain the large enhancement of si

max; this valueincreased by four times for HTT-2673 K 3D-C/C.

Koyama et al. reported that the degassing of absorbedwater increases the interlaminar shear strength of a 2D-C/C [16]. To examine the effect of absorbed water, push-out tests of HTT-2673 K 3D-C/C were conducted at roomtemperature after heat treatment at 1273 K maintaining avacuum environment. The values of si

max obtained fromthese tests are also plotted in Fig. 9 as open triangles. Itis evident that the contribution of the absorbed water tothe si

max enhancement is low.Thus, the major mechanisms for the si

max enhancementare probably the relief of residual thermal stress andincrease in frictional resistance. During the cooling stagesfrom the carbonization temperature and HTT, a CTE mis-match between the constituents results in a large tensilethermal stress in the carbon matrices at the fiber bundleinterfaces. Most of the stress can be released by the deb-onding of the fiber bundles. However, thermal stress is con-sidered to persist locally in the matrix blocks bonded to thefiber bundles. This residual thermal stress should reducesi

max.The CTE mismatch also affects the frictional resistance

at the debonded interface. High-temperature optical-microscope observations revealed that the opening widthof the as-processed debonding at the fiber bundle interfacedecreases with T, as shown in Fig. 10. The closure of thecracks is approximately consistent with the thermal expan-sion of the matrix block and fiber bundle along the trans-verse direction [17]. Fig. 10 indicates that the narrowregions of the as-processed debonding are probably closedat high temperatures. This closure increases the frictionalresistance due to an increase in the contacting area. Fur-ther, the anchor effect due to the rough debonding surfacesand fragmentized matrices is enhanced by the closure,causing a further increase in si

max. In fact, an increase inthe frictional resistance can be observed in Fig. 8; thedecrease in si after si

max was considerably rapid at high Tdue to increased friction and wear at the sliding surfaces.Hence, the identical averaged values of si

max that wereobtained from the push-out and pull-out tests were proba-bly due to the greater influence of friction than Poisson’seffects at the debonded interface.

1 Sato Y., Unpublished work, Institute of Aerospace Technology, JapanAerospace Exploration Agency, 6-13-1 Osawa, Mitaka, Tokyo 181-0015,Japan.

3.4. Influence of heat treatment temperatures

The simax values of HTT-3273 K 3D-C/C obtained from

the push-out tests are also plotted in Fig. 9. It can beobserved that si

max significantly decreases as the HTTincreases from 2673 K to 3273 K. To clarify the factorsresponsible for this si

max reduction, we discuss the candidatefactors—changes in the mechanical properties of carbonmatrices, extension of the as-processed debonding, andrelief of residual thermal stress.

In order to discuss the influence of HTTs on the matrixproperties, tensile tests of unidirectionally reinforced (UD)C/Cs were conducted at room temperature perpendicularto the fiber axis (90� tensile tests) with HTTs of 2673 Kand 3273 K. The UD-C/Cs were reinforced with identicalfibers and fabricated using a processing method similar tothat of the 3D-C/Cs. In these tests, a final fracture occurredwithin the carbon matrices and fiber/matrix interfaces.Fig. 11 shows a comparison of the typical results of the90� tensile tests. Variations in the initial elastic modulusand ultimate strength are listed in Table 1. Although thenumber of tests is limited, these results suggest that theincrease in HTT from 2673 K to 3273 K did not signifi-cantly modify the strength of the carbon matrices in the3D-C/Cs.

Fig. 12 shows the open porosity of the 3D-C/Cs as afunction of HTT. This measurement was conductedaccording to a Japanese standard (JIS R1634). As shownin this figure, open porosity increased linearly with HTT.Cross-sectional observations revealed that the as-processeddebonding at the fiber bundle interface elongated andthickened as the HTT increased. At HTT exceeding�1773 K, carbon material is known to graphitize [18,19].Since graphitization accompanies the relaxation of internalstress, a higher HTT increases the tensile thermal stress atthe fiber bundle interface during subsequent cooling.Hence, the longer and wider as-processed debondingobserved in the high-HTT 3D-C/C was attributed to the

Table 1Results of 90� tensile tests conducted for HTT-2673 K and HTT-3273 KUD-C/Cs

HTT(K)

Testno.

Initial elastic modulusa

(GPa)Ultimate strength(MPa)

2673 1 8.17 7.322673 2 8.36 7.572673 3 8.25 8.763273 1 7.36 7.743273 2 7.12 7.26

a Initial elastic modulus was calculated within the strain range of0–0.02%.

0

2

4

6

8

10

0 0.05 0.1 0.15

Ten

sile

str

ess

(MP

a)

Tensile strain (%)

HTT-3273 K

HTT-2673 K

At 293 K

Unidirectionally reinforced C/C

0

2

4

6

8

10

0 0.05 0.1 0.15

Ten

sile

str

ess

(MP

a)

Tensile strain (%)

HTT-3273 K

HTT-2673 K

At 293 K

Unidirectionally reinforced C/C

Fig. 11. Tensile stress–strain curves perpendicular to the fiber axisobtained for HTT-2673 K and HTT-3273 K UD-C/Cs.

Bul

k de

nsity

(g/

cm3 )

0

2

4

6

8

10

1.9

1.95

2

2.05

2.1

2500 3000 3500

HTT (K)

Open porosity

Bulk density

Bul

k de

nsity

(g/

cm3 )

0

2

4

6

8

10

1.9

1.95

2

2.05

2.1

2500 3000 3500

HTT (K)

Open porosity

Ope

n po

rosi

ty (

%)

Bulk density

Fig. 12. Dependence of the open porosity and bulk density on the HTT.


increased thermal stress. When a larger as-processed deb-onding occurs, the value of si

max decreases due to a decreasein the bonding area. The greater opening width of theas-processed debonding contributes to a further reductionin si

max due to a smaller frictional resistance.

The residual thermal stress in the carbon matrices is con-sidered to decrease with increasing T and may be assumedto be zero at each HTT. Hence, in high-HTT 3D-C/Cs, alarger fraction of the thermal stress remains when com-pared with that in low-HTT 3D-C/Cs, even when theyare heated up to the same temperature. This greater resid-ual fraction of the thermal stress in high-HTT 3D-C/Csalso causes a reduction in si

max. Consequently, the lowsi

max in high-HTT 3D-C/C is assumed to be caused due tothe following factors: the small bonding area and fric-tional resistance and the large residual fraction of thermalstress.

4. Conclusions

Experimental techniques for evaluating the fiber bundleinterfacial properties of orthogonally reinforced 3D-C/Csat high temperatures were proposed and examined. Thefollowing conclusions are obtained:

1. Fiber bundle interfacial properties of 3D-C/Cs were suc-cessfully obtained up to 2273 K by the present push-outand pull-out test methods.

2. The interfacial shear strength and initial sliding stress ofthe fiber bundles increased monotonically with the testtemperature. The relief of thermal stress and increasesin the frictional resistance and anchor effect at the fiberbundle interface were considered to be the dominant fac-tors for these enhancements.

3. In this study, the value of simax is an average strength at

the fiber bundle interfaces. The true shear strength of theintact portion at the interface should be considerablygreater due to the extensive as-processed debonding.

4. Frictional resistance at the fiber bundle interface wasconsidered to play an important role in the bundlepush-out and pull-out behaviors of the present 3D-C/Cs.

5. High HTT resulted in a significant decrease in the inter-facial shear strength of the fiber bundles. The smallerbonding area and frictional resistance and the largerresidual fraction of the thermal stress at the fiber bundleinterface were considered to be the major mechanismsthat caused this reduction.

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Measurements of fiber bundle interfacial properties of three-dimensionally reinforced carbon/carbon...

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