【作者】 朱俊；

【导师】 王继辉；

【作者基本信息】 武汉理工大学 ， 固体力学， 2016， 博士

【摘要】 先进复合材料（ACMs）具有比模量高、比强度高的显著优点,近年来取得了飞速发展和广泛应用。作为最典型的先进复合材料,纤维增强聚合物基复合材料（FRP）是由增强纤维和树脂基体经过特定的成型工艺复合而成,其使用比例和应用范围在整个先进复合材料领域占有绝对优势,代表了未来新材料的发展方向。然而,在纤维增强复合材料制造过程中,由于主观或客观原因,缺陷总是难以避免。缺陷的存在,导致纤维增强复合材料的力学性能大打折扣,例如强度下降、提前失效等,并给复合材料结构带来很多潜在的安全问题。增强纤维、树脂基体、模具之间热膨胀系数（CTE）的不匹配、层间压力分布不均匀、操作失误等,都会引起纤维的偏离、变形、弯曲,专业术语称之为波纹。波纹缺陷是最常见的缺陷之一,普遍存在于复合材料结构中。从客观上讲,CTE不匹配、层间压力分布不均匀是固有诱因,因此,复合材料中的波纹缺陷不可能避免和消除。同时,由于结构设计的需要,经向纬向纤维束交错编织,形成了一种独特的纤维弯曲结构,使波纹成为纺织复合材料的固有特征。波纹形成机理、保证结构安全运行的波纹接受标准、含波纹缺陷的复合材料的力学性能评价及其失效机制研究等问题已成为复合材料领域研究热点之一。本文以定量评估波纹对纤维增强复合材料力学性能的影响为目的,从多个角度对含波纹缺陷的传统层合板复合材料和平衡平纹机织复合材料的力学性能开展理论研究,并对含波纹缺陷的复合材料层合板进行了实验研究,主要工作及结论如下:首先,对面外波纹和面内波纹的几何特征进行了阐述,指出了二者的相似之处和独有特征,在此基础上,分别采用不同的三角函数对其弯曲曲线进行描述。以经典层合板理论（CLT）为基础,针对面外波纹缺陷的特点,在计算含面外波纹的层合板基本单元——单层板的刚度时,引入与面外波纹有关的波纹比、纤维偏转角、波纹区域大小、波纹类型等参数,运用等应力模型建立含面外波纹缺陷的层合板刚度预测多参数模型;针对面内波纹缺陷的特点,在计算含面内波纹的层合板基本单元——单层板的柔度时,引入与面内波纹有关的波纹比、纤维偏转角、波纹类型等参数,运用等应变模型建立基本模型;以此为基础,在计算层合板整体刚度时,引入与面内波纹有关的波纹位置、含面内波纹的单层板层数等参数,最终建立含面内波纹缺陷的层合板刚度预测多参数模型。其次,基于组分材料性能和有限元模型,运用均匀化方法预测了不同纤维体积分数的AS4/3501-6和E-glass/8084单层板力学性能,以此作为下文研究波纹缺陷对复合材料层合板力学性能影响的基础。以单向层合板为例,运用含面外波纹缺陷的层合板刚度预测多参数模型讨论了与面外波纹有关的参数对AS4/3501-6单向层合板弹性性能的影响,并进一步研究了面外波纹对不同纤维体积分数的AS4/3501-6单向层合板、不同材料体系（AS4/3501-6和E-glass/8084）的单向层合板弹性性能的影响。结果表明:波纹比、波纹区域大小对AS4/3501-6单向层合板E_x、E_z、G_yz、G_xz、v_yx、v_zx产生了较大影响,对E-glass/8084单向层合板E_x、G_xz、v_yx、v_zx、v_zy产生了较大影响;梯度型面外波纹对层合板弹性性能的影响小于均一型面外波纹;波纹参数对不同纤维体积分数的层合板相对弹性性能的影响几乎相同。第三,以铺层为[±45,（0,90）₂]_s的AS4/3501-6层合板为例,首先运用经典层合板理论预测了该铺层结构的标准层合板弹性性能。基于含面内波纹缺陷的层合板刚度预测多参数模型,建立适合铺层为[±45,（0,90）₂]_s的AS4/3501-6层合板的多参数模型,并讨论了与面内波纹有关的参数对该多向层合板弹性性能的影响。结果表明:面内波纹比大于0.5时,层合板力学性能几乎不再随波纹比增大而变化;含波纹的单层板越靠近几何中心平面,层合板力学性能受面内波纹的影响越小;在单层板力学性能对整个层合板贡献越多的方向上,当面内波纹出现在该层时,层合板在这个特定方向上的力学性能下降越显著。第四,开展实验研究,对前期的预测结果进行验证,并对层合板拉伸强度、弯曲强度等进行了前瞻性研究。基于干的单向纤维纤维布,本文设计了一种sandwich结构,提出了“两步法”制备含面外波纹缺陷的玻璃纤维增强单向层合板。利用VARI工艺,首先制备“夹芯”结构,然后在其上下面铺设一定数量的干纤维布,再用VARI工艺进行二次成型,由此制备了含梯度型面外波纹缺陷的单向层合板及标准层合板。在此基础上,制备拉伸、弯曲实验试样并进行测试。结果表明:“两步法”制备的标准层合板和正常VARI成型的标准层合板力学性能基本相同,即“两步法”是可行的;含梯度型面外波纹缺陷的层合板刚度预测模型得到的E_x、v_yx均与实验值吻合很好,含均一型面外波纹缺陷的层合板刚度预测模型得到的E_x、v_yx均与实验值吻合较好;面外波纹导致层合板拉伸和弯曲强度显著下降;对于弯曲实验试样,当波纹比为0.02左右且凹面朝下时,小角度的纤维弯曲能够起到“强化”作用,引起试样挠度和弯曲强度略微增大;拉伸和弯曲试样在面外波纹区域极易发生层间剪切破坏和分层。第五,机织复合材料的特殊结构——经向纬向纤维束交织,使“波纹”即纤维弯曲成为这种复合材料固有特征。针对平衡平纹机织复合材料的几何结构特征,本文提出了一种新的描述经纬纤维束弯曲形态的数学表达式,采用与传统层合板中的波纹缺陷相似的研究方法,并考虑与这种特殊波纹有关的参数,如沿经向（或纬向）纤维束波纹比、经向（或纬向）纤维束横截面波纹比、单胞内弯曲的经向（或纬向）纤维束长度、单胞内相邻纤维束间距等,建立了含波纹的平衡平纹机织复合材料刚度预测多参数模型,系统讨论了波纹参数对其弹性性能的影响。更多还原

【Abstract】 Advanced Composite Materials（ACMs）can offer high specific modulus and strength,and has rapid development and widespread applications.Fiber reinforced plastic composite（FRP）constituted of fibers and polymeric matrix is one of the most typical ACMs,which has a large proportion and wide applications.Moreover,FRP represents the future development of new materials.However,defects are inevitable due to the subjective and/or objective reasons during the process of FRP manufacture.Defects have greatly adverse influence on the mechanical performance of FRP such as strength reduction,pre-failure,and lead to many potential safety problems.Mismatch of coefficient of thermal expansion（CTE）,non-uniform distribution of the interlaminar pressure,and operation mistakes would cause the deviation,deformation and/or curvature of fibers,which is termed as waviness defect.Waviness defect is one of the frequently encountered defects in composite structures.Essentially,mismatch of CTE and non-uniform distribution of the interlaminar pressure may be the inherent inducements.Waviness defect is thus impossible to be avoided and eliminated,completely.Furthermore,fill and warp strands are interlaced to fulfill the structure design of textile composites,which forms a particular fiber curvature.Consequently,waviness becomes the inherent characteristic of textile composite.Formation mechanism of waviness,waviness acceptance/rejection criteria for ensuring structure safety,assessment of mechanical properties and investigation of failure mechamism for composite materials with waviness,and so on,are one of the hot topics in the FRP field.In this paper,effects of waviness on the mechanical properties of FRP are accurately quantified and assessed.Theoretical prediction and experimental investigation are employed to study the mechanical performance of the traditional laminated composite and the balanced plain weave fabric composite embedded waviness.Firstly,the geometric characteristics of out-of-plane and in-plane waviness are presented and distinguished,and the trigonometric function is employed to describe the characteristics of waviness defect.Based on classical lamination theory（CLT）,the iso-stress assumption are used to establish the model of stiffness prediction for composite laminates with out-of-plane waviness（MPM-OP）.The parameters associated with the out-of-plane waviness such as the waviness ratio,the off-axis angle,the waviness zone and the waviness pattern,are introduced during the derivation of stiffness matrix of the wavy lamina.Similarly,the parameters associated with the in-plane waviness such as the waviness ratio,the off-axis angle,and the waviness pattern are introduce dring the derivation of compliance matrix of the wavy lamina.By use of iso-strain assumption,the basic model is established.Then the position and the number of wavy lamina are included in the basic model,and the model of stiffness prediction for composite laminates with in-plane waviness（MPM-IP）.Secondly,based on the elastic properties of fibers and matrix and finite element model,the homogenization method is employed to evaluate the elastic constants of AS4/3501-6 and E-glass/8084 lamina at a range of fiber volume fractions.With employment of the MPM-OP to the AS4/3501-6 unidirectional laminates embedding out-of-plane waviness,the effects of waviness on the elastic properties studied.For different fiber volume fractions and material systems,the unidirectional laminates with out-of-plane wavienss are also investigated.The results show that the waviness ratio and the waviness zone have large effects on E_x,E_z,G_yz,G_xz,v_yx,v_zx of AS4/3501-6 laminates with uniform waviness of out-of–plane（UW-OP）as well as E_x,G_xz,v_yx,v_zx,v_zy of E-glass/8084 laminates with graded waviness of out-of–plane（GW-OP）.The GW-OP has smaller effects on laminates than UW-OP.In-plane waviness has smaller effects on mechanical properties of wavy laminates for the closer distance between the wavy ply and the geometric central plane.The loss of mechanical performance is significant if a ply contributes more stiffness in the laminates with in-plane waviness.Thirdly,CLT is employed to determine the elastic constants of the AS4/3501-6laminates with lay-up of[±45,（0,90）₂]_s.Based on the developed stiffness model of laminates with in-plane waviness（MPM-IP）,the influence of waviness parameters on the AS4/3501-6 laminates with lay-up of[±45,（0,90）₂]_s is discussed.It is shown that the elastic properties of the multi-direction laminates nearly change when the waviness ratio is larger than 0.5.In-plane waviness has smaller effects on mechanical properties of wavy laminates for the closer distance between the wavy ply and the geometric central plane.The loss of mechanical performance is significant if a ply contributes more stiffness in the laminates with in-plane waviness.Fourthly,the validation of the predictions of MPM-OP above and a prospective study of tensile and bending of laminates with out-of-plane waviness are carried out by experiments.Based on the dry E-glass preforms,a“sandwich”configuration and the“two-step method”are developed for fabrication of laminates with out-of-plane waviness.First,the“core”is fabricated by the vacuum assisted resin infusion（VARI）process.Second,the dry performs viewed as the“skin”are laid up and down of the“core”.Then the“sandwich”is infused again by VARI and the laminates with graded waviness are manufactured.Additionally,the tensile and bending specimens are fabricated and tested.Experimental data shows that the mechanical properties of the laminates fabricated by“two-step method”is the same as the laminates by normal VARI.The predictions of E_x、v_yx agree well the experimental data for GW-OP and UW-OP.Out-of-plane waviness leads to the large reduction of tensile and bending strength.The fiber curvature at a small angle induces“stiffing effect”concave-up laminates with a/λ=0.02.Interlaminar shear damage and delamination tend to occur in the vicinity of waviness and cause the final failure of wavy laminates.Fifth,the fiber curvature,also called waviness in this study,is the inherent characteristic for plain weave composite where warp and fill strands are interlaced in two mutually orthogonal directions to one another.As a consequence,the similar methodology employed in the investigation of the traditional laminated composites with waviness is adapted to study the waviness within plain weave composite.First of all,a particular plain weave composite,the balanced plain weave fabric composite is selected and a new mathematical description method is developed for characterizing the geometric morphology of the balanced plain weave fabric composite.Second,based on CLT and iso-stess assumption,the morphology of warp and fill strands and pure resin are respectively considered,then the 3D model of plain weave fabric composite is developed.The elastic constants of the balanced plain weave composite are predicted and the results agree well with others’theoretical values and experimental data.The influence of waviness parameters such as waviness ratio of the warp（or fill）strand and the the warp（or fill）strand cross section,the length of the udulation,and the gap between the adjacent warp（or fill）strands on the elastic constants of the balanced plain weave fabric composite are predicted.更多还原