第14卷第6期 船舶力学 Vo1.14 No.6 2010年6月 Journal of Ship Mechanics Jun.2010 Article ID:l007—7294(2010)06—0641—08 Effects of Different Young’s Modulus on Stress Around Cracks in Composite Material Plate LI Cheng,TIE Ying (School of Mechanical Engineering,Zhengzhou University,Zhengzhou 450001,China) Abstract:Different Young’s Modulus has considerable effects on the stress field.Taking aim at the stress state of composite material plate with erack.the boundary condition problem of crack js s0lved with the heterogeneous anisotropic elastic theory and complex variable function.The boundary inte. gral equation based on exact boundary conditions is established.The accurate analytic solution of stress field around the crack in the composite materials plate is obtained.Moreover.based on the calculation model,effects of different Young’s Modulus on the s ̄ess field around the crack in the composite matefials plate are investigated. Key words:composite material;conformal mapping;crack;different Young’s Modulus; stress field around crack CLC number:034 Document code:A 1 Introduction The spread and expand of cracks are very important to design and analyze composite materials structure.There are three phases in fracture process:forming of micro-crack;steady spreading of micro—crack,up to size of macro-crack;unsteady spreading of macro-crack.The main characteristic of composite materials is the high resisting force against crack spread due to the ductility of the matrix.Fracture is induced by the defect of the stress around materials or the high stress around crackst”. Researches show that effects,which anisotropy property of materials and orientation func— tion of crack with respect to material principal directions have on the stress distribution of crack—tip in the composite materials,are not negligible【2J.In theories,solving and analyzing of crack problem in structures can mathematically be come down to establishing and solving of boundary value problemt31.To composite material structure,establishing of crack boundary con. dition is more complex than that of metal materia1.Whether the establishing of boundary con— dition is accurate and reasonable,and also has a great effect on the result,even it induces an essential difference.A large number of research findings have made great progress in technol— ogy level,which apply the fracture mechanics to composite materia1.Ref.[4]calculates the crack—tip stress intensity factor with method of smal1 parameter.the limit of this method is ob— Received date:2010—01—17 Foundation item:Supported by the National Natural Science Foundation of China(50975261) Biography:LI Cheng(1 962一),male,Ph.D.,professor of Zhengzhou University,the main research direction is the composite material structural strength. 642 船舶力学 第14卷第6期 vious.To crack problem,finite element method and boundary element method are wjde1v usedts-‘o】.However,owing to existing of the singular element in the crack-tip,eITors of the re. suh are often bigger.Because of the anisotropic property of composite materia1.researches on effects of different Young’s Modulus on the stress field around crack is not enough.In 0rder to further research this problem,establishing a set of efifcient method to ana1vze stress distribu— tion around crack is very necessary. The method,we use in this paper,is that,firstly,directly map the ellipse fthe length of the long axis is accord with the length of crack)to a unite circlew一121,then,make the sh0rt ax. 1s zero.So,the map function which maps the crack to a unite circle is obtained.It is certain that,if the crack has other shapes,it also can be mapped to a unite circle directly.The prob. 1em is not discussed in details.In order to explain the introduction method further.in this pa- per,stress in composite material plate with crack is analyzed,the accurate analytic soluti0n 0f stress around crack is obtained.Moreover,according to the established ealeulation mode1.the complete analysis of effects of different Yong’s Modulus on the stress field aIDund the erack is performed. 2 Fracture problem of composite material Since composite material has a lot of distinct characteristics from isotropic materia1.it brings dififculties to the application of linear elastic fracture.In view of the fiber directi0n dif- ferent from the matrix,the anisotropic and inhomogeneous property is the main problem.It is simple to apply in homogeneous anisotropy,because the basic theories in fracture mechanics do not change.So,it is often that the homogeneous anisotropic material is used as the appI.0xi. marion of the general composite materials.Then,it can use the conformal mapping theory 0f the complex variable function to calculate the stress field around erack in anisotropic mate打. a1.Thus,to study the orthotropic plate with crack,the orthotropic plane pr0blem must be learned.If considering the general anisotropic elastic body,there are no elastic symmetrica1 factors,generalized Hooke’s law can be described as: f= f ( j,k,Z=l,2,3) f11 where,S z is softness factor tensor,with 8 1 components,due to the symmetric property 0f the elastic body,in general state,the independent constant is 2 1.To the orthotropic plate,gener. alized Hooke’s law can be simpliifed as: { }=[S】{ ) (2) here,the I S]is the 6 x6 flexibility matirx.The consistent equation can be obtained by sub. stituting Hooke’s law into deformation compatibility equation and equilibrium equation8.The main difference of consistent equation between orthotropic and isotropic plates is element 0f the softness factor tensor and material engineering constants. For the orthotropic plate,here 第6期 LI Cheng et al:Effects of Different Young’S Modulus… 643 击 一 , 等 +去ory, 毒 Equation of strain compatibility is as follows: +等一鲁=。 Substituting equation(3)into(4),the consistent equation can be obtained. In practical engineering,the common composite material often owns some symmetry. Thus,in many states,anisotropic problem can be changed to orthotropic problem to discuss. To plane stress problem,the stress component can be expressed as: 誓 豢 一等 Using the complex variable:Z1=%+s Y and Z2=%+s 2Y where,51= 1+ 1,s,= ,+ , thus,in formula(5),the partial derivative of ,Y can be written as: 暑=毒。 O_z3l_+。 丢・, ’ , 0= (\ 0一1 0)/ thus,stress function also can be written as the following bi—harmonic function: 4 :0 1 2 the above formula is integrated by partsthe expression of stress function can be obtained: ,(x+/ztY)+ (x+lx2y)+ (x+/x3Y)+ ( ),) (7) Substituting formula(7)to(5),after series of calculating and simplifying,the general ex— press formula of hole edge stress in the anisotropic plate can be obtained=. 2Re【s (z ) ( 2)], f8 8)O'=y2Re f ( 1)+ ( 2)], —2Rej sso ( 】)+ 2 ( 2)J where, ( 1), (z2)are two analytic functions which are introduced in order to simplify the calculation.Thus,the problem of plane stress in anisotropic material can be transformed into the problem that searches two analytic functions which satisfy the corresponding boundary conditions.To determine those two stress functionsit must be based on the boundary condi. ,tions. Under the given external force X,Y,from the expression(7)of the stress functi0n,the 船舶力学 boundary conditions can be written as: 第14卷第6期 ( 1)+ ( 1)+砂( 2)+砂( 2)=一 。 , 1 ( 1)怕1 ( 1)+ 2砂( 2)怕2 (Z2)=一 Jf + SO (9) To the method adopted in this paper,the following problem is searching the mapping function that maps the elliptic,which has the crack length a as the length of the long axis,to a unite circle.Only finding this function can the crack boundary problem be solved.Further, he crack stresst distribution in the anisotropic plate with crack of length a can be solved. According to Riemann theory,this kind of mapping function exits,moreover,with respect to each hole shape,it is unique.However,to search this function,the course is complex,ad— ditional,the calculating content is large,here,the course is not introduced,in the following case,the mapping function will be given directly. 3 Stress calculation of the orthotropic plate with crack The orthotropic plate with elliptical hole which is symmetrical about the middle surface is discussed here.Then,its short axis is made zero in the formula.The stress calculation of elliptical hole edge is transformed to that calculation of crack as following figure l: b=O > (a) (b) Fig.1 The sketch map of the transformation from elliptical hole to the crack First step,the conformal mapping is used,through Schwarz-Christoffel integration,the mapping function which maps the elliptic hole boundary in the z planum to a unite circle in the planum can be obtained.The calculating content is large,here,the mapping function win be given directly. ( )=华 譬 Thus,through transformation by relation formula Z1--: ̄1+ 1,Z2--3C2+ 2,the relative position in l, z 2 planum of the point in z planum can be obtained.With the boundary condition(9)and Cauchy integration,the two )=赢 analytic functions are determined. f[ : ]术 -第6期 LI Cheng et ah Effects of Different Young’S Modulus。・・ 645 z1"r/+——. +A 叼 1 叩 z)= .』 ‘ ]术 rl+zz——盟+A 7"1 叼-z2 Then,the short axis of the elliptic is made zero in above two analytic functions.At last,from stress component expression(8),the stress distribution around the crack in the orthotropic plate can be obtained. 3.1 Comparison of orthotropic plate with different Young’S Modulus Considering the orthotropic plate symmetic aboutr the middle surface with the crack as ifgure(1b),the plate bore tension on the two ends of edges uni ̄rmly deforms.The action point is far from the crack.The direction of the fiber is vertica1 to the force.Considering the ̄llow— mg three situations. (1)E11=650GPa 2=30GPa, G12=10GPa,b'12=O.325 4 Stress dist ribution (2)El1=275GPa E22=25GPa, G12=9.5GPa,/}12=O.3 : ——(3)E11=20GPa E22=190GPa, G12=7.5GPa,/212=0.25 4—-2 —兰 、 Crac ̄ boundary 2 With above method,the exact stress analytic solution around the crack can be obtained. 4 Owing to its powerful capacity in function dewing,the calculation and simulation are all inifshed in mathematics.Considering the paper length,the stress expression is not given here. Fig.2 Stress distribution around crack in the orthotropic plate with crack (material 1) Stress distr bution a Stress dist; ibution / 一.: 、 一~ 一4 —2 .二 、 Crack b undary —4 —2 —2 —]4 Crack boundary 2 —4 Fig.3 Stress distribution around crack in the Fig.4 Stress distribution around crack in the orthotropic plate with crack(material 2) orthotropic plate with crack(material 3) 船舶力学 第14卷第6期 To above three situations,from the analytic expression,the stress distibutrions are respective- ly as Figs.2-4. The above three figures are stress distribution figure under polar coordinate system,de— scribing the stress change th angle.Where,O- is the stress around the crack. is the ex— ternal load at the edge of the plate.From the three fiures,git can be seen that at crack tip,the stress is trend to infinite,which accords with the description of classical methods.Namely,the relationship between stress and厂radius of he smaltl area with center at the crack tip is:ort oC 1 ,when广+o, 。。. dlrtehancegt itoFhn igds至io rf2。e cth4ioe aint eborcifga tnhe 3sb t,ecw Yrhasoecnkeg n StlIph≤Mart oadl2s uterl ueasto Cehtoa myreo’ pumdanrid:e 0。. 5. 一, i’ sl . .厂‘f}L_ .—一 —T1L_—一,— T七.L ——.一下I— —.-T}1 ——. .]_{I 0.3 when the l entgh direction of the crack vertical to:Ef三/l三三F 三二一I J I j crack 吼 ctly different~nder the same load,the direction of the biggest Yong’s Modulus.he t.——._;——— —— —— —— the ati。。f;tw。princp1e direct 。n Y。ng’s M ulus rMadlutio。outwo pincp fe direc i。n Y。ng’s sE,E,,around crack in orthotropic plate is obtained.The derivation process is the same as above sec— crack in isotropic and orthotropic plate are tion and omitted.The stress distibutrions around given in the same coordinate system as Fig.6. -Cra5 —ck boundary ̄stress dist r.but 。 :5 5 stress djstributi。r Fig.6 Comparison of stress distribution around crack in isotropic and orthotropic plate Considering the plate symmetirc about the middle surface with the crack as figure(1b), 第6期 LI Cheng et ah Effects of Different Young’s Modulus… 647 under the same loading condition,the comparison of stress distribution around crack in or- thotropic and isotropic plate is shown as Fig.6.From Fig.6,it can be seen that at crack tip,the stress values have a significant difference in the two cases,while in the region away from the crack tip the stress values of orthotropic one are consistent with those of isotropic one gradu- ally. 4 Conclusions In this p印er,the analytic solution of stress distribution around crack in the orthopotic plate is brought forward.The stress analysis of analyitc method is established based on the complex variable stress function and integral function application.To composite material plate th crack,when with different Yong’s Modulus,under tension stress,the stress distributions around different materia1 crack are obtained respectively.Based on the results in the paper. the following conclusion can be drawn that under the same load.when the length direction of the crack vertical to the direction of the biggest Yong’s Modulus,the stress is the smallest. 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[1 2]Du Y H,Xu J G,Wen L J,et a1.Hyper-singular integral equation method for problem of crack parlaleled with inter. face in plane bi—material[J].Journal of Mechanical Strength,2003,25(2):175—177.(In Chinese) 船舶力学 第14卷第6期 不同的杨氏模量对含裂纹复合材料板 裂纹周围应力的影响研究 李 成,铁 瑛 (郑州大学机械工程学院,郑州450001) 摘要:不同的杨氏模量对于应力场有着相当的影响。针对含裂纹的复合材料板,根据非均质各向异性弹性理论和复变函 数理论解决了裂纹的边界条件问题。建立了基于准确边界条件的边界积分方程,得到了含裂纹复合材料板裂纹周围应 力场的精确解析解。并按照所建立的计算模型对不同的杨氏模量对裂纹周围应力场的影响进行了探讨。 关键词:复合材料;保角映射;裂纹;不同杨氏模量;裂纹周围应力场 中图分类号:034 文献标识码:A 基金项目:国家自然科学基金资助项目(50975261) 作者简介:李成(1962一),男,博士,郑州大学机械工程学院教授,email:chengli@zzu.edu.cn; 铁瑛(1978一),女,博士,郑州大学讲师。