1、文章编号: 1007- 8827( 2014) 03- 0169- 07弯曲疲劳后 4D-C /C 复合材料的抗弯强度及热膨胀性能Wajed Zaman1， 李克智1， 李伟1， Hira Zaman2， Khurshid Ali2( 1 西北工业大学 材料学院， 陕西 西安 710072;2 Institute of Chemical Sciences，University of Peshawar，Khyber Paktunkhwa，Pakistan)摘要: 采用液相浸渍炭化技术， 在压力为 75MPa 下制备出 4D- C/C 复合材料， 并进行高温热处理。研究静态和动态加载条件下， 材料
2、沿厚度方向的弯曲性能及断裂行为。结果表明， 循环次数达到 10 105次、 频率为 10 Hz 时， 材料的临界弯曲疲劳极限是静态弯曲强度的 80%。静态弯曲加载情况下， C/C 复合材料失效机制取决于试样底层炭纤维的取向。循环疲劳载荷作用下， 其失效机制包括基体开裂、 纤维 基体界面弱化及纤维断裂。复合材料在循环加载过程中界面结合强度降低， 并释放内应力， 故增强了纤维拔出以及复合材料的假塑性， 疲劳加载后其剩余弯曲强度增加 10% 左右， 而模量降低。疲劳载荷引起材料基体缺陷和裂纹数量的增加及纤维断裂， 削弱了长度方向上的热膨胀， 使材料热膨胀系数降低。关键词: C/C 复合材料; 疲劳;
3、 抗弯强度; 微观结构; 热膨胀中图分类号:TB332文献标识码:A基金项目: 高等学校创新引智计划( B08040) ; 国家重点基础研究发展计划( 973) 项目( 2011CB605806) 通讯作者: 李克智， 教授 E- mail:likezhi nwpu edu cn作者简介: Wajed Zaman， 博士研究生 E- mail:wajed_zaman yahoo comFlexural strength and thermal expansion of 4Dcarbon/carbon composites after flexural fatigue loadingWajed
4、Zaman1， LI Ke- zhi1， LI Wei1， Hira Zaman2， Khurshid Ali2( 1 School of Materials Science and Engineering，Northwestern Polytechnical University，Xi an710072，China;2 Institute of Chemical Sciences，University of Peshawar，Khyber Paktunkhwa，Pakistan)Abstract:A four directional carbon/carbon ( 4D C/C)compos
5、ite was fabricated by first using liquid phase impregnation carboni-zation ( LPIC) ，followed by hot isostatic pressure impregnation and carbonization ( HIPIC)at 75MPa，and finally high temperaturetreatment The flexural properties and fracture behavior of the composite were investigated in the through
6、- thickness direction understatic and fatigue loading The critical fatigue limit of the composite was 80% of the static flexural strength for one million loadingcycles at 10Hz The failure mechanism of the composite under static flexural loading was dependent on the orientation of the carbonfibers in
7、 the tested specimen Cyclic fatigue loading decreased the interfacial bonding strength and released the inherent stresses inthe composite，which increased fiber pull- out，enhanced pseudo- ductility and increased the residual static flexural strength at the ex-pense of a decrease in the flexural modul
8、us The fatigue loading increased the number of noncritical matrix cracks，increased interfa-cial debonding， and caused the fracture of filaments in the surviving fatigued C/C composite These features of the fatigued compos-ite internally accommodated expansion in long direction as the temperature was
9、 increased，which resulted in a decrease in its residualthermal expansionKeywords: Carbon/carbon composites;Fatigue;Flexural strength;Microstructure;Thermal expansioneceived date: 2014- 02- 04; evised date: 2014- 06- 05Foundation item:Programme of Introducing Talents of Discipline to Universities ( B
10、08040) ;State Key Development Program forBasic esearch of China ( 2011CB605806) Corresponding author:LI Ke- zhi，Professor E- mail:likezhi nwpu edu cnAuthor introduction:Wajed Zaman，Ph D candidate E- mail:wajed_zaman yahoo comEnglish edition available online ScienceDirect (http: www sciencedirect com
11、sciencejournal18725805 ) DOI: 10 1016/S1872- 5805( 14) 60132- 31IntroductionCarbon- carbon(C /C ) compositesarewellknown for their high specific strength and excellentablation and thermophysical properties C /C compos-ites retain their strength and structural integrity at ele-vated temperature like
12、3000 K in vacuum or inert en-vironment1， 2 ThesecharacteristicsdeclareC /C第 29 卷第 3 期2014 年 6 月新型炭材料NEW CABON MATEIALSVol 29No 3Jun 2014composites as suitable materials for aerospace andaeronautical applications and they are used to makenozzles of solid rocket motors ( SM) ，reentry tipsand leading e
13、dges for space shuttles and other hyper-sonic vehicles3， 4 Today，the brake discs of almostall of the military and passenger aircrafts are made upof C /C composites because of their high thermal sta-bility and long lifeFour directional ( 4D)C /C composites are re-garded as the ideal materials for aer
14、ospace applicationsbecause of their inherent quasi- isotropic properties，and excellent mechanical， thermophysical and anti- ab-lative performance5- 8 Because of their excellent per-formance，4D C /C composites using both pitch andCVI as matrix have been used in the SM of Ariane 5space shuttles7- 9 4D
15、 C /C composites are capable ofhigh geometric stability and low thermal expansion，the important properties required when they are usedin engine turbine blades，rocket nozzles and seal ringsin liquid propellant rocket engines3 A lot of workhas been conducted in investigating the mechanicalbehavior of
16、one directional to multi- directional C /Ccomposites10- 17 These studies show that preform ar-chitecture，defects in fibers，and fiber/matrix interfa-cial bonding strongly influence the strength and frac-ture toughness of C /C compositesLiterature review shows that despite the highstrength and fractur
17、e toughness at elevated tempera-ture，C /C composites have not been used as primaryload bearing structures，probably because of the lackof reliable and sufficient data regarding their applica-tions in long term uses Fatigue properties of any ma-terial are crucial in considering the designing of theloa
18、d bearing structures for long term use Fatigue be-havior of 1D and 2D C /C composites have been ad-dressed by few18- 20 ， but to the best of our informa-tion，there is no report in the open literature regardingthe flexural fatigue behavior of 4D C /C compositesIn this study，the 4D C /C composite was
19、fabricatedby liquid phase impregnation carbonization ( LPIC)technique using coal tar pitch as a carbon matrix pre-cursor The aim of the study is to investigate the frac-ture behavior of 4D C /C composite under a static andfatigue loading in a flexural mode，and the influenceof the fatigue loading on
20、the flexural strength andthermal expansion of 4D C /C composites2Experimental2 1Preform formation4D C /C composite was fabricated using a medi-um strength polyacrylonitrile based carbon fibers andcoal tar mesophase pitch The 4D preform was pre-pared by placing pultru- rods of carbon fibers in fourdi
21、rections，that is，the three directions in the in- plane( xy- direction)and the fourth direction was perpendic-ular to xy plane in through- the- thickness ( t- t- t)direc-tion ( z- direction) A fiber volume fraction in eachdirection was maintained at about 9%- 10%，makingthe total fiber volume fraction
22、 to 40% Schematicsof the top and side views of the 4D carbon preformare shown in Fig 1Fig 1Schematic representation of the 4D preform:( a)top view and ( b)side view2 2DensificationThe 4D carbon preform was densified with a coaltar pitch using pressure impregnation followed by va-rious cycles of hot
23、isostatic pressure impregnation car-071新型炭材料第 29 卷bonization ( HIPIC)at around 75MPa High tempera-ture treatments ( HTT)of the composite were conduc-ted many times after carbonization or impregnationdepending on the densification stage at around 2300-2400 to achieve a high density，required for excel
24、-lent thermo- mechanical performance A detailed flow-chart for the fabrication of the 4D C /C composite isshown in Fig 2Fig 2Flow diagram for the fabrication offour directional pitch based C /C composites2 3Characterization2 3 1Density and porosityThe density of the 4D C /C composite of rectan-gular
25、 blocks of 15 15 10mm3after the final HTTwas measured by the Archimedes water- immersionmethod using Sartorius cp224 s densitometer at roomtemperatureFor measuring the open porosity，the same speci-mens ( used for measuring the density)were usedThe specimens were dried in oven at 120 foraround 24 h u
26、ntil their weight became constant Theopen porosity was measured using air displacementmethod by filling the pores with kerosene under vacu-um The density of the composite was found to be( 1 91 0 01)g /cm3，while the final open porositywas ( 5 5 0 2) %2 3 2Flexural fatigue analysisThe flexural fatigue
27、 tests were conducted usingthe smooth specimens ( Fig 3) ，in three point ben-ding configuration with a span of 60 mm The direc-tion of the flexural loading was kept perpendicular tothe direction of t- t- t fibers ( z- fibers) ，while the z- fi-bers were aligned parallel to the length of the speci-men
28、s The fatigue tests were conducted using a hy-draulic- loading testing machine ( Instron 8872)underload controlled sinusoidal loading in ambient atmos-phere at a frequency of 10Hz The stress ratio ( ) ，defined as the ratio of the minimum to the maximumapplied stress，was set to 0 12 3 3Static flexura
29、l analysisThe flexural strengths of the 4D C /C composite，before and after fatigue loading for one million cycles( 106cycles) ，were determined through three pointbending tests All static three point bending tests werecarried out with a span of 60mm on a computer con-trolled universal testing machine
30、 at a cross head speedof 0 50mm /min using ASTM C 1161- 94 standard testprocedure The static flexural analyses of the 4D C /Ccomposite specimens were conducted before and afterfatigue tests The specimens used to determine theflexural strength had the dimensions of 6 mm ( thick-ness) 12 mm ( width) 7
31、0 mm ( length) Thenominal bending stress ( )was calculated using thefollowing equation ( equation 1) Fig 3 Smooth specimens of 4D C /C compositesfor the static and flexural fatigue tests =3FL2bd2( 1)Where，F is the force，L the span，b the width and dthe specimen thickness2 3 4Coefficient of thermal ex
32、pansionThecoefficientoflinearthermalexpansion( CTE)of the composite before and after fatigue ana-lyses were measured in the in- plane directions Thespecimens had the dimensions of ca6 mm 25mm The CTE was measured in the temperaturerange of 25- 1 250 at a heating rate of 5 /min，using a Netzsch- DIL 4
33、02C TMA40 instrument2 3 5Microscopic examinationScanningelectronmicroscope， SEM(JEOLJSM- 6700f，field emission electron microscope)wasused to analyze the fracture behavior of the 4D C /Ccomposite The fracture behavior of the compositewas investigated before and after the failure of thecomposite speci
34、mens during the static and fatigue tes-ting171第 3 期Wajed Zaman et al:Flexural strength and thermal expansion of 4D carbon/carbon composites3esults and discussion3 1Fracture behaviorDuring the fracture analyses of the composites inthe three point bending mode，it is found that if thebottom layer of th
35、e 4D C /C composite specimensconsists of z- fibers oriented along the length of thespecimen，then the ultimate failure of the compositedue to the application of the load is mainly caused bythe fracture of the carbon fibers in the bottom layer( z- fibers)of the specimens in the tensile mode ( Fig4a) ，
36、and the crack is initiated along the width of thespecimen that grows deep with the time On the otherhand，when the lower most layer ( bottom layer)ofthe specimens consists of the xy- carbon fibers ( i e ，oriented along the width of the specimens)then firstthe fiber/matrix debonding in the lower most
37、layeroccurs with the applied load and then the cracks growinside through the matrix and causes the fracture ofthe z- fibers，as shown in Fig 4bFig 44D C /C composite specimens failed under flexural loading;when the lower most layer ( bottom layer)of the specimens consists of( a)z- fibers present alon
38、g the length of the specimen，and( b)xy- fibers present along the width of the specimensFig 5 shows the SEM micrograph of the frac-tured surfaces of the 4D C /C composite，obtainedduring the flexural static loading in three point ben-ding test The carbon matrix seems to be still attachedto the carbon
39、fiber reinforcement，which indicates agood bondability between the matrix and the fibersAt the final stage of the failure，the fiber pull outtakes place，which is responsible for the pseudo- duc-tile behavior of the material during bending tests3 2S- N curveThe fatigue tests of the composite specimensw
40、ere conducted in three point bending configuration tostudy the relationship of maximum stress and numberof loading cycles ( S- N) During fatigue tests，the z-fibers of the specimens were aligned parallel to thelength of the specimen and perpendicular to the ap-plied fatigue loading The fatigue analys
41、es were con-ducted at different stress levels Stress level is the ra-tio of the applied flexural fatigue loading to the staticflexural loading corresponding to the ultimate flexuralstrength of the material The fatigue tests were con-ducted at the stress level of 100%，95%，90%，85%，80% and 75% correspo
42、nding to 133，126，120， 113， 106 and 100MPa，respectively The com-posite failed at the stress level of 100%，95% and90%，corresponding to 133， 126 and 120MPa respec-tively，during fatigue testing before the completion of106cycles Only one sample survived 106loading cy-cles at 113MPa，which corresponds to 8
43、5% of the o-riginal flexural strength，while all other samples failedat different stages before the completion of 106cyclesAt 106 MPa，which is 80% of the original strength，except two specimens all others reached to 106cycles，while none of them failed at 100 MPa，correspondingto 75% of the original str
44、ength，as shown in the S- Ncurve in Fig 6Fig 5SEM micrograph of the fractured surface of 4D C /Ccomposite failed during static flexural loading in 3- point bending modeFig 6S- N curve of the 4D C /C compositeunder flexural fatigue loading in 3- point bendingTable 1 shows the data of the static flexur
45、alstrength of the virgin 4D C /C composite specimensand the composite specimens survived after fatigueloading ( fatigued composite specimens) at 80%stress level ( 106 MPa) for 106loading cycles at10Hz It is observed that the standard deviation of thedata is not high，indicating the uniform distributi
46、on of271新型炭材料第 29 卷cracks and pores in the matrix The flexural strengthof the material is increased by 10% after fatigue load-ing，as shown in Table 1 Fig 7 shows that the fa-tigue loading causes a change in the load- displacementcurve of the composite The flexural modulus of thecomposite is decrease
47、d while the material becomesmore ductile and its strain to failure is increased Thiscan be attributed to the weakening of the fiber- matrixinterface and the increase in matrix microcracking dueto cyclic fatigue loading，which releases the inherentstresses in the composite material It is known thatwea
48、k fiber/matrix interfaces serve to hold cracks andresulting in the splitting in advancing cracks19 Thedecrease in the modulus of the composite is due to theweakening of the fiber/matrix interfacial strength andthe increase in the number and size of the noncriticalmatrix cracks The fiber/matrix inter
49、face is weakenedconsiderably by cyclic fatigue loading which is one ofthe important factors in the early increase in the strainand decrease in the modulus of the composite On theother hand，in the un- fatigued composite specimens，more cracks free matrix，fibers and comparativelystronger fiber/matrix i
50、nterface contribute towards theincrease in the modulus of the composite and theyhave therefore high flexural modulus than the fatiguedspecimensTable 1Flexural strength ofthe 4D C/C composites before and after fatigueloading in 3- point bending configurationS NoFlexural strength /MPaUnfatiguedFatigue
