felt layer needled perform

NEW CARBON MATERIALS Volume 26, Issue 2, Apr 2011 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2011, 26(2):109–116.

RESEARCH PAPER

Performance analysis of a carbon cloth/felt layer needled perform JI A-lin1, CUI Hong2*, LI He-junl*, CHENG Wen2, JI Ling-ling2 1

School of Materials Science, Northwestern Polytechnic University, Xi'an 710072, China

2

Xi'an Aerospace Composites Research Institute, Xi'an 710025, China

Abstract: The structural characteristics of a carbon cloth/felt layer needled preform are investigated. X–Y direction tensile strength, Z direction peel-off stress, and the Naval Ordnance Laboratory (NOL) ring-integrated tensile strength were tested. The X-Y direction tensile strength decreased with the increase of needling density. The law of surface density of fiber felt affecting the Z direction stress is not obvious. Peel-off stress of a 3 K carbon cloth/felt needled preform is higher than that of 6 K and 12 K cloth/felt, and the peel-off stress of a diagonal carbon cloth/felt needled preform is better than that of a satin one. There are three different damage modes for the NOL-integrated tensile ring, namely complete rupture, incomplete rupture, and interlayer peel-off. For 3K satin carbon cloth/felt needling preform, its NOL ring-integrated tensile strength is the lowest and only 3 MPa, and it shows complete rupture. For the 12 K satin carbon cloth/felt needled preform, the damage mode is interlayer peel-off , and for the 6K satin carbon cloth/felt needled preform, the damage mode is incomplete rupture, and its integrated mechanical performance is excellent. NOL ring-integrated tensile strength is much higher than X–Y tensile intensity for the preforms made by same process. Key Words: Needling; Preform; X-Y direction tensile strength; Z direction Peel-off stress; Integrated tensile strength of NOL ring

1

Introduction

Needling technology is a kind of carbon fiber preform-forming technology with high automaticity. According to the structure, preform can be divided into two kinds: integral type felt and carbon cloth/felt layer needling preform. This technology has many advantages, such as short production period, low production cost, diversified design, and so on. Needling technology is suitable for the preparation of divergent section[1] of nozzle, exit cone[2-3], chamber, gas vane[4], airplane brake[5], throat[6], and other carbon/carbon(or carbon/ceramic)[7] composite components, and it is a multiuse, high-tech technology for carbon fiber preform preparation. Some thin-wall structure part of rocket motor, such as chamber, divergent section, and so on, should have excellent integrated mechanical performance to endure a certain inner pressure[8]. The integrated performance of structure parts is determined by layer mode and needling process. In this article, the structural characteristics of carbon cloth/felt layer needling preform are investigated and the X-Y direction tensile strength and Z direction peel-off stress of different preforms are compared and analyzed. NOL ring test has been used to analyze the integrated performance of needling preform. The rationality of needling preform performance characterization is discussed.

2 2.1

Experiment Experiment process

Raw materials: 3 K diagonal carbon cloth, 3K satin carbon cloth, 6K satin carbon cloth,12 K satin carbon cloth, carbon fiber felt, and preoxidized fiber felt. The flat and cylindrical preforms are prepared by using a certain needling process. The density of carbon cloth/felt needle preform is 0.40 g/cm3. 2.2

Performance test

(1) X-Y direction tensile strength and Z direction peel-off stress Test criterion is GJB1867-94. The test equipment for integral type felt is DSS-10T-S electric universal material tester. The dimension of X-Y direction tensile strength sample is 120 mm×25 mm×10 mm, and thickness is 10 mm. The dimension of Z direction peel-off sample is 150 mm×50 mm×12 mm, and thickness is 12 mm. The pre-open length of peel-off sample is 40 mm, and load rate is 200 mm/min. Fig.1 shows the basic principle of peel-off test. (2) Preform-integrated performance test The test method refers to the criterion GB1458-88, and equipment is DSS-10T-S electric universal material tester. The

Received date: 22 October 2010; Revised date: 3 April 2011 *Corresponding author. E-mail: [email protected]; [email protected] Copyright©2011, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(11)60070-X

JI A-lin et al. / New Carbon Materials, 2011, 26(2): 109–116

Fig.l

The principle of peel-off test

Fig.4 "Pin" structure of needling preform

Fig.2

Test process of NOL ring

direction (namely interlayer)[9-10]. Every layer must be needled after it is stacked to ensure that the fiber quantity of every part of preform in thickness direction is same and uniformity of preform is good[11]. There are four different kinds of structures in X–Y and Z directions of needling perform. They are the carbon cloth structure with regular distributed longitude and woof fiber damaged by needling in X-Y direction (namely plane), the disorderly distributed felt fibers in X-Y direction (namely plane) without being transformed to Z direction, the felt fibers transformed by needling in Z direction (namely interlayer) and the transition section of carbon cloth and felt by needling[12]. From Fig.3, it can be seen that disorder distributed felt fiber is transformed to Z direction. The Z direction fiber with the certain length has linked carbon cloth layer and felt layer tightly. Carbon cloth and felt have been restricted each other to form a three-dimensional net structure. As shown in Fig.4, the monolayer structure of composite fabric looks like regularly distributed "pin" structure. While needling, the felt fiber of same layer can be hooked and inducted to interlayer by prick repeatedly, forming a “pin” structure. In the “pin” structure, one end or two ends of the most of felt fibers are still in plane. This kind of “pin” structure increases the integrity of preform and the particular quasi-3D network is formed. (2) Z direction structure of needling preform

Fig.3 The filament state in the needling preform

dimension of NOL ring sample is D150innermm×50 mm×12 mm, and thickness is 12 mm. The load rate is 0.5 mm/min, and the test process of needling NOL ring is shown in Fig.2.

3 3.1

Preform performance analysis Preform structure analysis

(1) Structure model for carbon cloth/felt layer needling preform Carbon cloth/felt layer preform is needled with carbon cloth and carbon fiber (or preoxidized fiber) felt layer alternately. By prick hook, felt fiber has been transformed to Z

For needling preform, Z direction structure consists of two types: a. Z direction fiber transformed from felt by needling; b. transition section of carbon cloth and felt by needling. Z direction filament exists as perpendicular or oblique fiber bundle by needling from felt fiber in the plane[l3] and is not single. The Z direction filament bundle not only enhances tangle of carbon cloth fiber and felt fiber and prevents planar felt fiber from slipping reciprocally under tensile stress but also makes the preform to hold better strength, and it mends weak interlayer performance of 2D preform because Z direction filament has impenetrated carbon cloth layer and felt layer. For the transition section of carbon cloth and felt by needling, the mid-structure has been formed when planar felt and ruptured carbon fiber by needling entwists each other. In virtue of the transition section, the stabilized composite between felt

JI A-lin et al. / New Carbon Materials, 2011, 26(2): 109–116

volume content directly influence on the preform tensile stress[14]. Carbon cloth fiber provides X-Y direction tensile strength of preform mainly. Because of discontinuity and limited entwisting of felt fiber, it does affect the X-Y tensile strength. X-Y tensile strength of pre-oxidized felt needle preform is only 0.15MPa, and the whole carbon felt is only 0.12 MPa[15], so for carbon cloth/felt layer needle preform, X-Y tensile strength has direct relation to carbon cloth performance. While needling, the prick inevitably damages carbon cloth fiber, and the damaged mode includes two kinds: rupture and scratch. As shown in Fig.5, the fiber around needle eye had been cut off and formed a hole. Fig.5 Damaged fibers around needle eye

Fig.6 The effect of needling density on X-Y direction tensile strength

Fig.7 Z direction fibers in preform

and carbon cloth in the plane has been formed. If only carbon cloth layer is needled, the Z direction filament and reinforcement effect on the interlayer will be very little because of the low toughness of carbon cloth fiber and the restriction of its structure. 3.2

X-Y direction tensile strength analysis For continuous fiber preform, fiber strength and fiber

For carbon cloth/felt layer needle preform, X-Y tensile strength reflects needling density. Seen from Fig.6, X-Y tensile strength decreases as needling density increases because high needling density increases the damaged degree of X-Y direction fiber. As far as preform of 3 K diagonal carbon cloth/carbon fiber felt, when needling density increases from 26 pin/cm2 to 32 pin/cm2, X-Y tensile strength decreases from 2.84 MPa to 1.88 MPa，and the decreasing rate is 33.8%. The increasing of needling density will not help for the retaining of X-Y tensile strength of preform. When peel-off stress of needle preform meets requirement, the needling density can be decreased properly to improve X-Y direction tensile strength of preform. If the amount of X-Y direction continuous fiber is large, namely the needling density is less, only fraction of carbon cloth fiber has been ruptured or scratched, so the strength of continuous fiber bundle can be considered as X-Y tensile strength of needling preform. The average strength of fiber bundle is related to the fiber length, distribution parameters, and so on, and the tensile damaging mode is mostly shown as fiber rupture[16]. If the amount of X-Y direction continuous fiber is small, namely the needling density is high, a majority of carbon cloth fiber has been cuffed or scratched, so it can be considered that the X-Y fiber contains short fiber. In this instance, X-Y tensile strength is related to the force of fiber tangle reciprocally, and the tensile breakage behaves fiber rupture. "The ability of fiber entwisted reciprocally" reflects the magnitude of fiber friction force, the fiber-entwisted degree, and so on. The X-Y tensile strength of preform is related to fiber length, diameter, surface roughness, section shape, disorder degree of fiber distribution, interaction force between fibers, fiber volume content, and so on. 3.3

Z direction peel-off stress analysis

Z peel-off stress indicates the force of separating two layers in preform in unit width, and scales the "joint" force between two layers. Fig.7 shows the Z direction fiber in preform after felt being peeled-off. If entwisting force is big enough in Z and X-Y direction fiber, the fiber is likely to be ruptured, and the peel-off stress is higher because the support

JI A-lin et al. / New Carbon Materials, 2011, 26(2): 109–116

of needle holes and increases damage probability of fiber, which will influence joint force between fibers, at the same time; the peel-off stress is decreased. Thus, it is observed that peel-off stress will be decreased although increasing of needling density can improve Z direction fiber content, so the needling density should be chosen properly so as to improve the interlayer performance.

Fig.8 The effect of needling density on Z direction peel-off stress

Owing to the different type of carbon cloth and the different content of felt, peel-off stress of needling preform is diverse although the needling process is same. As the primary provider of Z fiber, the planar of felt greatly affects the peel-off stress of needling preform. In a certain score, the content of Z direction fiber and peel-off stress of preform increases with an increase in the felt planar density. But impacted on the uniformity of felt planar density, fiber toughness, and so on, the planar density of felt has no distinct rule influence on peel-off stress. In Table1, the preform has been needled by 3K diagonal carbon cloth and the preoxidized felt. The planar densities of preoxidized felt are 100 g/m2, 200 g/m2, and 300 g/m2. Table l The peel-off stress of needling preform for different planar density felt Planar density of felt ρ/g·m-2

Peel-off stress γ/N·m-1

100

71.2

200

88.2

300

112.0

Fig.9 Effect of carbon cloth type on Z direction peel-off stress

fiber is more. So peel-off stress shows tensile rupture strength of Z direction fiber. However, if entwisting force is lower in Z and X-Y direction fiber, the fiber is likely to be pulled-off, and peel-off stress shows the entwisting force in Z and X-Y direction fiber[16]. The effect of needling density on peel-off stress of preform is obvious. Using the same needling depth and different needling density, the preform peel-off stress of 3K diagonal carbon cloth/carbon fiber felt is shown in Fig.8. From Fig.8, peel-off stress increases following with the increase in needling density in suitable range. But, when needling density is too high, peel-off stress will decrease. The change of needling density leads to change in the Z fiber content in the preform. In certain range, Z direction fiber content increases because of the increasing of needling density, so the compactness of preform interlayer increases. And the peel-off stress increases as a result of increase in entwisting force in Z and X-Y fiber. While needling density is too high, however, the distance between border of needle eyes gets short and felt fiber has been fixed, so the free fiber isn’t easy to be catched in felt and the Z direction fiber to be transformed is influenced; however, a large number of X-Y fiber is cut, and the entwisting force in X-Y and Z fiber is decreased, it’s easy to pull off Z direction fiber with small force, and the peel-off stress is in a low level. Otherwise, extremely high needling density leads to repetition

As shown in Table 2, the peel-off stress of 3K carbon cloth/ carbon fiber felt needling preform is higher than 6K and 12K. The planar density of carbon fiber felt is 100 g/m2. The monolayer thick of 3K carbon cloth is only 0.34mm, and the monolayer thick of 12K arrives at 0.7mm. In case of steady Z direction fiber performance, increase in the thickness of unit layer is equal to decrease in the length and quantity of Z direction fiber, and the Z direction fiber can be easy to be pulled out and the interlayer peel-off stress is lowered. Table 2 The peel-off stress of needling preform fordifferent type carbon cloth performs Carbon clothes type

Peel-off stress γ/N·m-1

3K carbon clothes

194

6K carbon clothes

150

12K carbon clothes

152

As observed from Fig.9, irrespective of the type of needling process followed, the peel-off stress of 3K diagonal carbon cloth preform is always higher than that of 3K satin carbon cloth preform. For diagonal carbon cloth, there are lots of joints of longitude and weft, and the structure is compact. After felt fiber had been transformed to Z direction (namely interlayer), and following with the needle-squeeze function vanishing, the carbon cloth fibers tightly compress Z direction fiber and form great entwisting force between Z direction

JI A-lin et al. / New Carbon Materials, 2011, 26(2): 109–116

Fig.10 NOL ring (D =150 mm) tensile breakage mode. (a) Completion rupture; (b) Single-draped incompletion breakage; (c) Symmetrical-draped incompletion breakage; (d) Interlayer peel-off

fibers and X-Y direction fibers, so the Z direction fiber isn't easy to be pulled out and the peel-off stress of preform is greater. For satin carbon cloth, there are few of joints of longitude and weft, and the structure is loose; it's easy to be deformed while needling if the entwisting force is weaker in X–Y carbon cloth fiber and Z direction felt fiber. The Z direction fiber is easy to be pulled out, and the peel-off stress of preform is lower than the diagonal carbon cloth preform. 3.4

Integrated performance analysis

As needling preform is used as a thin-wall structure parts, the integrated mechanical performance has always been concentrated by material researchers and designers. NOL ring is used to reveal the integrated mechanical performance of needling preform. Although the NOL ring is enduring tensile stress, because it is close to tensile disc, the inner carbon cloth seam is weak, so the breakage initially takes place in the seam and then from inner to outer. First, the inner carbon cloth fiber has endured pull load and produced the big deformation, and finally, carbon fiber is pulled out to break. Along with sustaining the increasing of load, outer layer fiber begin to endure pull load, carbon fiber persist to be pulled out. Outer layer fiber isn't completely pulled out or ruptured after unloading, so different rupture modes are formed. For NOL ring of carbon cloth/carbon fiber felt layer needling preform, there are three kinds of rupture modes: completion rupture, draped incomple-

tion breakage, and interlayer peel-off, as shown in Fig.10. The completion breakage mode of NOL ring is shown in Fig10 (a), breakage section is at the carbon cloth seam. For the low performance or high damaged needing preform, when NOL ring is tested, it can only stand a low circle pull load and completion breakage will occur. Draped incompletion breakage consists of two kinds of mode: single-draped mode and symmetrical-draped mode, as shown in Fig.10(b) and Fig10(c). Single fiber drape exist in one distinct drape and another inconspicuous drape in a general way. The symmetrical drape mode exists in two distinct fiber drapes. Fiber draped breakage occur because there is no matrix to transfer stress. Because of friction force among NOL ring interlayer layers, there is stress loss when tensile stress transfers from inner layer to outer layer fiber. The stress endured by outer layer carbon cloth fiber is lower than that endured by inner layer fiber, so the preform deforms and fiber drape breakage mode occurs after unloading. Drape incompletion breakage shows that NOL ring stands uniformity load, and carbon cloth fiber stands circle stress fully without damaging interlayer at the same time. The drape breakage mode is an ideal mode, which shows the preform can endure the higher load, and its integrated mechanical performance is better. As shown in Fig.10(c), interlayer peel-off breakage mode illuminates that preform can support stronger circle pull load, but interlayer performance is weaker, so the sample appears to detach at one seam in the inner layer and go on to

JI A-lin et al. / New Carbon Materials, 2011, 26(2): 109–116

Fig.11 Comparison of circle tensile strength and X-Y tensile strength

delaminating. In this breakage mode, carbon cloth fiber doesn't play a role completely, Z direction fiber tends to be pulled out and the preform interlayer begin to peel off because of shear function and friction function among interlayer layers. The breakage mode of 3K satin, 6K satin, 12K satin preform is similar. NOL ring tensile strength of 6K satin carbon cloth needling preform is best and 3K is lowest, because the tensile strength of 3K satin carbon cloth is weaker to stand circle pull load. Since the shear function in Z direction, NOL ring of 12K satin carbon cloth preform will, first, delaminate and then damage the circle direction before the biggest pull load is reached, and its circle tensile strength is lower. The carbon cloth tensile strength and interlayer peel-off performance of 6K satin carbon cloth needling preform is between 3K and 12K satin carbon cloth, and the preform can balance plane and interlayer performance, so it can stand a higher circle pull load. NOL ring tensile rupture mode reflects the integrated performance of preform. When interlayer performance is weak because of inferior felt quality or improper needling process, the tensile breakage shows the mode of interlayer peel-off rupture; when X-Y direction tensile strength of needling preform is lower because of inferior carbon cloth fiber quality or highly damaged by improper needling process, the tensile breakage shows completion rupture mode. Using NOL ring to estimate the preform performance, the tensile strength isn't the only criterion and the rupture mode should be combined to analyze the preform structure characteristics. 3.5 Comparison of circle tensile strength and X-Y direction tensile strength X-Y direction structure is plane structure, containing regular distributed carbon cloth fiber and disorder distributed felt fiber, which is not to be transformed to Z direction, namely interlayer. As shown in Fig.11, circle tensile strength and X-Y tensile strength of 3K diagonal carbon cloth /preoxidized felt needling preform is compared. Through comparison of circle tensile strength and X-Y

tensile strength, it can be concluded that circle tensile strength is higher than X-Y tensile strength greatly; the reason lies in the different pull load and breakage mode. NOL ring tensile rupture is a conjunct result of interlayer and plane function. While standing the pull load, carbon cloth fiber has been supported the uniformity stress, so the carbon cloth fiber of inner layer tends to be, first, deformed and then these pull load have been transformed to outer layer fiber, and NOL ring is damaged in succession. While NOL ring is standing circle pull load, the sample stands not only circle pull load but also interlayer cut stress, which makes Z direction fiber play a role to prevent the damage of interlayer. X-Y tensile load inflicts on the fiber of preform. Fiber stands not only parallel plane pull load but also vertical plane compression load, which leads to fiber break fast because of stress centralization, so X-Y direction tensile strength is weaker. Owing to uniformity load, NOL ring can stand stronger circle pull load and has better tensile strength.

4

Conclusions

(1) For carbon cloth/felt layer needling preform, carbon cloth provides planar mechanical property, and out-of-order distributed felt fiber can improve not only the entwisting probability of Z direction fiber but also the distribution uniformity of composite fabric in every direction of plane, so the isotropy of preform is improved. (2) NOL ring tensile strength, X–Y direction tensile strength, and Z peel-off stress are used to characterize the integrated mechanical performance, carbon cloth strength retention, and binding force between different layers of needling preform, respectively. (3) The law of surface density of fiber felt affecting the Z direction stress is not obvious. Peel-off stress of 3K carbon cloth/felt needling preform is higher than that of 6K and 12K cloth/felt, and peel-off stress of diagonal carbon cloth/felt needling preform is better than that of satin carbon cloth. (4) There are three kinds of damaging modes for NOL-integrated tensile loop, namely completion rupture mode, draped incompletion rupture mode, and interlayer peel-off mode. For 3K satin carbon cloth/felt needling preform, its NOL ring-integrated tensile strength is the lowest (only 3MPa) and its damaging mode belongs to completion rupture mode. For 12K satin carbon cloth/felt needling preform, its damaging mode belongs to interlayer peel-off, and for 6K satin carbon cloth/felt needling preform, its damaging mode belongs to draped incompletion rupture, and its integrated mechanical performance is the best. (5) NOL ring-integrated tensile strength is greatly higher than X-Y tensile intensity for the preforms made by same process. NOL ring tensile test can exactly reflect the integrated mechanical performance of axially symmetric parts, which provides a new way for characterization of preform performance.

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