不同结构UHMWPE纤维纬编针织复合材料弯曲性能

以超高分子量聚乙烯(UHMWPE)纤维为原料,在电脑横机上编织出较理想的UHMWPE纤维纬平针、罗纹、畦编针织结构织物.采用VARTM工艺、(0°,90°)3s铺层方式成功制备了六层纬平针、六层罗纹以及六层畦编UHMWPE纤维纬编针织复合材料板.对三种复合材料板的弯曲性能及其影响因素进行研究,比较并分析其弯曲应力-挠度变化曲线和破坏形式.结果表明:三种UHMWPE纤维纬编针织结构增强复合材料的弯曲应力-挠度曲线具有非线性特征,曲线均类似于抛物线;其中,纬平针织结构复合材料的弯曲强度最大,罗纹次之,畦编最小;承受弯曲破坏的主要是树脂基体,没有出现增强体断裂、撕开等现象,表明由高强聚乙烯纤维制成的增强体材料具有较强的韧性与较高的强力.     

聚酰亚胺纤维纬编织物增强橡胶基复合材料冲击性能

针对固体火箭发动机绝热层复合材料在冲击作用下因增强织物和橡胶基体变形不协调导致的破坏问题,基于罗纹纬编结构,设计制备了两种纤维细度、三种线圈长度和两种铺层结构的聚酰亚胺纤维纬编织物增强丁腈橡胶(NBR)复合材料,测试并分析了增强结构对复合材料低速冲击性能的影响。结果表明：选用大丝束纤维、长线圈及正交铺层的复合材料具有更高的冲击峰值载荷和吸能。采用轮廓仪和显微镜观测冲击损伤形貌,在20.1 J冲击能量下试样均未被穿透,基体沿纤维方向产生裂纹并沿厚度方向产生塑性变形是复合材料主要的损伤模式。     

飞机用复合材料斜胶接修补结构的冲击损伤

高传力效率的斜面式胶接在飞机复合材料传力接头和修补中被广泛使用,但该结构的低速冲击损伤阻抗和损伤容限未在飞机结构设计中考虑。本文研究了低速冲击下的较厚的复合材料斜胶接板的力学性能及损伤失效。在胶接区域布置不同冲击点,寻找最敏感位置,在该位置进行冲击能量变化研究,通过冲击响应（冲击载荷、挠度、能量等）及冲击损伤两个方面获取其规律和失效机制。小能量和大能量冲击结果表明,胶接区域5个典型冲击位置中,中心位置冲击损伤最大,冲击敏感性最高,因此中心点为冲击损伤阻抗最小位置。中心点不同能量冲击时,冲击响应研究揭示了冲击过程中冲击载荷具有典型的4阶段行为。冲击载荷还具有双峰值力的现象。冲击后沿试样中心线切开的显微损伤图揭示了该结构有两种损伤模式,包括复合材料损伤及胶层损伤。复合材料的损伤包含90°和45°层基体的开裂和0°与90°层之间的层间损伤。胶层损伤出现在试样冲击点正下方背部的复合材料斜接尖端部位。进一步通过考虑复合材料层内、层间损伤及胶层损伤的渐进损伤模型对试验进行仿真研究,找出导致第Ⅱ阶段冲击载荷突降的主要原因为复合材料层间损伤,第Ⅳ阶段冲击载荷再一次突降是由于胶层出现了损伤。     

UHMWPE纤维纬编针织复合材料的制备工艺研究

以超高分子量聚乙烯(UHMWPE)纤维为原料,实现其在电脑横机上的编织,将6层纬平针织物、6层罗纹织物以及6层畦编织物分别通过真空辅助树脂传递模塑成型技术(VARTM)复合成复合材料板。VARTM工艺的最大优点是模具简单、成本低、模具抽真空形成负压使纤维与树脂分布更均匀。综合考虑UHMWPE纤维特点及VARTM工艺的工艺条件,实验用树脂为粘度低、浸润性好的环氧树脂;为使制备的复合材料均衡对称,采用(0°,90°)3s铺层方式;成功制备出了比较理想的纬平针、罗纹和畦编UHMWPE纤维纬编针织增强复合材料,为后续UHMWPE纤维纬编针织复合材料力学性能的研究及其拓展领域的应用奠定了一定的理论和实验基础。     

不同结构UHMWPE纤维纬编针织复合材料压缩性能

针对高性能超高分子量聚乙烯（UHMWPE）纤维的特性,成功编织出纬平针、罗纹、畦编3种针织物。采用真空辅助树脂传递模塑成型（VARTM）技术,分别制备出6层纬平针、6层罗纹以及6层畦编织物复合材料板。对3种结构复合材料进行压缩试验,并比较分析了比压缩强度-应变曲线、比压缩能量-应变曲线、比压缩能量-应变拟合曲线,分析了压缩过程中的能量吸收情况及材料的破坏形式。结果表明:纬平针结构复合材料的比压缩强度和比压缩能量均最大,其次是罗纹,畦编最小;且3种结构复合材料的压缩破坏过程均不属于脆性破坏;由于材料表现出较好的柔韧性,试样的比压缩能量与压缩应变呈线性相关;相同结构复合材料纵、横向比压缩强度-应变曲线和比压缩能量-应变曲线几乎重合。基体沿增强结构呈分层现象的破坏和树脂的塑性变形是材料的主要破坏形式。      

混杂比对碳/芳纶纤维混杂纬编双轴向多层衬纱织物增强复合材料力学性能的影响

利用层内混杂的方式制备碳/芳纶纤维混杂纬编双轴向多层衬纱织物,通过对材料进行拉伸、三点弯曲等实验研究该织物增强复合材料的力学性能及混杂比对其力学性能的影响。结果表明,按照一定的混杂比加入芳纶纤维后复合材料的拉伸性能提高,表现出积极的混杂效应。由于延伸性好的芳纶纤维的加入,使复合材料的拉伸断裂伸长率明显提高,材料破坏模式出现了完全脆性断裂模式(C12材料破坏形式)和“扫帚”形纤维断裂模式(C8A4,C6A6材料破坏形式)。此外,按照一定的混杂比加入芳纶纤维也有效改善了碳纤维增强复合材料的破坏韧性,碳/芳纶纤维混杂MBWK织物增强复合材料的弯曲强度和弯曲模量随混杂比的提高而呈下降趋势,当复合材料中芳纶含量从42%(体积分数,下同)(C6A6)到59.2%(C4A8)的变化过程中,弯曲强度和弯曲模量的降低率较高。0°试样在混杂比为59.2%(C4A8)时,弯曲挠度最大,达到7.49 mm,远高于纯芳纶纤维或纯碳纤维增强的复合材料。所有90°混杂复合材料试样的弯曲挠度均高于纯芳纶纤维或纯碳纤维增强的复合材料,表现出积极的混杂效应。     

复合材料加筋板低速冲击损伤的数值模拟

建立了复合材料加筋板在横向低速冲击载荷作用下的渐进损伤有限元模型.该模型考虑了复合材料加筋板受低速冲击时的纤维断裂、基体开裂及分层脱粘等五种典型的损伤形式,在层内采用应变描述的失效判据,结合相应的材料性能退化方案,通过编写VUMAT用户自定义子程序以实现相应损伤类型的判断和演化.在层间以及筋条与层板间加入界面元,模拟层间区域的情况,结合传统的应力失效判据和断裂力学中的能量释放率准则来判断分层损伤的起始和演化规律.通过对数值模拟结果与实验数据的比较,验证了模型的合理性和有效性.同时探讨了不同位置、不同冲击能量以及含初始损伤(脱粘)等因素对复合材料加筋板低速冲击性能的影响.     

复合材料加筋板低速冲击损伤的数值模拟

建立了复合材料加筋板在横向低速冲击载荷作用下的渐进损伤有限元模型。该模型考虑了复合材料加筋板受低速冲击时的纤维断裂、基体开裂及分层脱粘等五种典型的损伤形式, 在层内采用应变描述的失效判据, 结合相应的材料性能退化方案, 通过编写VUMAT用户自定义子程序以实现相应损伤类型的判断和演化。在层间以及筋条与层板间加入界面元, 模拟层间区域的情况, 结合传统的应力失效判据和断裂力学中的能量释放率准则来判断分层损伤的起始和演化规律。通过对数值模拟结果与实验数据的比较, 验证了模型的合理性和有效性。同时探讨了不同位置、不同冲击能量以及含初始损伤(脱粘)等因素对复合材料加筋板低速冲击性能的影响。     

T300/QY8911层合板低速冲击试验及有限元模拟

对T300/QY8911复合材料层合板进行了低速冲击试验研究及数值仿真模拟。通过自由落体装置对层板进行冲击,并使用超声C扫描技术检测了层板冲击后的损伤状态,获得了不同能量下层板内部的损伤面积。建立了用于预测复合材料层合板在低速冲击作用下损伤演化的3D有限元模型,模型包含了用于模拟分层损伤的界面元和用于模拟纤维断裂、纤维挤压、基体开裂、基体挤裂等面内损伤形式的3D实体单元。该模型考虑了面内基体损伤对层间强度的影响。本文中的数值仿真结果和试验结果的对比验证了模型的合理性和有效性,文中还分析了影响低速冲击后层板内部分层面积的主要因素。     

碳/芳纶混杂纤维增强波纹夹芯结构低速冲击性能

采用碳纤维和芳纶纤维增强复合材料对波纹夹芯结构的面板进行层间混杂铺层设计,通过真空辅助树脂灌注(VARI)成型工艺制备混杂波纹夹芯结构。在60 J、80 J和100 J三种不同冲击能量下,研究了面板混杂铺层方式对波纹夹芯结构低速冲击性能及冲击后压缩强度的影响,并利用超声C扫和工业CT断层成像两种无损检测技术对波纹夹芯结构的冲击损伤机制进行了分析。结果表明：冲击能量较低时,波纹夹芯结构的吸收能量基本不受面板的混杂铺层方式影响,而凹坑深度随表层碳纤维层数增加而减少。冲击能量较高时,面板为分层式混杂(碳/芳纶纤维单层交替铺层)的波纹夹芯结构的抗冲击性能最好,纤维断裂损伤和层间分层主要发生在试样表层,但损伤面积较大;面板为夹层式混杂(以碳纤维为蒙皮、芳纶纤维为芯材)的波纹夹芯结构具有较高的吸收能量,整个上面板的纤维都发生了断裂破坏,但损伤面积较小。碳/芳纶混杂波纹夹芯结构的面板采用分层式和夹层式的混杂铺层设计时,具有较高的冲击后压缩强度。     

Investigation of knitting architecture on the impact behavior of glass/epoxy composites

In this study, the effect of impact and post-impact behavior of E-glass/epoxy composite plates having different knitted fabrics were investigated by considering energy profile diagram and the related load–deflection curves. Different impact energies (5–25 J) were subjected to the plates consisting of eight layers of Plain [P]₈, Milano [M]₈, and Rib [R]₈ knitted fabrics. The impact tests were continued until complete perforation of layer fabrics. The damage modes and damage processes of layer fabrics under varied impact energies were also discussed. At the end of the impact tests, the damaged samples were mounted into a compression apparatus to determine the Compression after Impact (CAI) strength of layer fabric samples. The results of these impact and post-impact tests showed that the maximum contact force was observed in the [R]₈ fabric and the minimum contact force was observed in the [P]₈ fabric and the CAI strength reduced by increasing the impact energy.     

On the impact response of sandwich composites with cores of balsa wood and PVC foam

This paper presents an experimental investigation on impact response of sandwich composite panels with PVC foam core and balsa wood core. A number of tests were performed under various impact energies. Damage process of the sandwich composites is analyzed from cross-examining load–deflection curves, energy profile diagrams and the damaged specimens. The primary damage modes observed are; fiber fractures at upper and lower skins, delaminations between adjacent glass–epoxy layers, core shear fractures, and face/core debonding. After visual inspection of the top and bottom face-sheets, initial examination, damage mechanisms at the interior layers and cores were ascertained through destructive analysis, i.e. sectioning by an abrasive water-jet machine, of samples. In addition to the single impacts, repeated impact response of the samples is also investigated.     

Impact and post impact behavior of layer fabric composites

In this study, the effect of impact and post impact behavior of E-glass/epoxy composite plates having different layer fabrics were investigated by considering energy profile diagram and the related load–deflection curves. Different impact energies (5 J–60 J)were subjected to the plates consisting of eight layers of plain weave (1D), double (2D) and triple (3D) layer fabrics. The impact tests were continued until complete perforation of layer fabrics. The damage modes and damage processes of layer fabrics under varied impact energies were also discussed. At the end of the impact tests, the damaged samples were mounted into a compression apparatus to determine the Compression After Impact (CAI) strength of layer fabric samples. The results of these impact and post impact tests showed that contact force occurring between the impactor and the composite specimen increased and the CAI strength reduced by increasing the impact energy. The objective of this study was to determine the perforation threshold of E-glass/epoxy composite plates having different layer fabrics as plain weave (1D), double (2D), and triple (3D) layer fabrics.     

Tensile properties of multilayer-connected biaxial weft knitted fabric reinforced composites for carbon fibers

Multilayered-connected biaxial weft knitted (MBWK) fabric reinforced composites have excellent tensile properties. Three kinds of different fabrics reinforced composites are used in this paper, which are three-layer-connected biaxial weft knitted fabric, four-layer-connected biaxial weft knitted fabric and five-layer-connected biaxial weft knitted fabric. The tensile properties of MBWK fabrics reinforced composites are studied with 0° and 90° directional testing with different carbon fiber volume fractions. The results show that the carbon fiber volume fraction has significant effect on tensile strength of MBWK fabrics reinforced composites. The linear correlation between tensile strength and carbon fiber volume fraction is very well in the certain range, and failure analyses are also available by means of sample debris examination to identify the failure modes and the fracture surfaces.     

Micromechanical modeling approaches for the stiffness and strength of knitted fabric composites: a review and comparative study

Knitted fabric reinforced composites have been investigated widely in recent years. A number of different micromechanical modeling schemes have been proposed in the published literature for various types of knitted fabric composites. However, to date, no comparative study has been made to evaluate the suitability of different modeling schemes to predict the stiffness and strength properties of knitted fabric composites. This paper presents a review of currently developed micromechanical modeling techniques for predicting the stiffness and strength of knitted fabric composites. Further, a comparative study of the predictive capabilities of various techniques is carried out based on a plain weft knitted glass fiber fabric reinforced epoxy matrix composite. Useful conclusions are drawn based on the comparative study.     

Low velocity impact and compression after impact characterization of woven carbon/vinylester at dry and water saturated conditions

The low velocity impact response and compression after impact strength of dry and water saturated plain weave carbon/vinylester composites have been determined. The composites employed T700 carbon fibers and vinylester 510A and 8084 resins. Quasi-static impact tests were conducted on dry C/VE510A and C/VE8084 to estimate the threshold impact force required to initiate damage in the composites. Falling-weight impact tests were conducted on the composites over a range of impact energies from 6.7 to 47 J. Destructive inspection of damaged panels revealed damage in the form of matrix cracks as well as delamination between fiber bundles. The quasi-static estimation of the threshold impact force was in reasonable agreement with that measured in the impact test. To examine structural degradation due to impact loading, impacted panels were tested in compression (CAI). The CAI strength decreased with increasing impact energy. Absorbed moisture caused further reductions of the CAI strength.     

Low-velocity impact and residual tensile strength analysis to carbon fiber composite laminates

In this paper, low-velocity impact characteristics and residual tensile strength of carbon fiber composite laminates are investigated by experimentally and numerically. Low-velocity impact tests and residual tensile strength tests are performed using an instrumented drop-weight machine (Instron 9250HV) and static test machine (Instron 5569), respectively. The finite element (FE) software, ABAQUS/Explicit is employed to simulate low-velocity impact characteristics and predict residual tensile strength of carbon fiber composites laminates. These numerical investigations create a user-defined material subroutine (VUMAT) to enhance the damage simulation which includes Hashin and Yeh failure criteria. The impact contact force and the tensile strength are accurately estimated using the present method. Two different tensile damage modes after different impact energies are observed. The degradation of residual tensile strengths can be divided to three stages for different impact energies, and amplitudes of degradation are affected by stacking sequences.     

Tensile properties and meso-scale mechanism of weft knitted textile composites for energy absorption

The investigation on large deformation tensile properties and the relevant meso-scale mechanisms of weft knitted textile composites is presented. The correlation between fabric structure (e.g. loop height and width, number of wale or course per unit length, etc.), matrix damage and material properties are described. Weft knitted fabrics with 1×1 interlock structure were used as the preform for the composites. The materials studied include knitted nylon fabric/unsaturated polyester resin and co-knitted polyethylene terephthalate (PET)/polypropylene (PP) textile composites. The results show that all the nylon/polyester thermoset textile composites samples displayed an ideal bi-linear character in their tensile stress–strain curves, whilst the tensile curves of PET/PP co-knitted thermoplastic samples along the wale, course and 45° directions are all significantly non-linear. The tensile behavior is superior in the wale direction to those in the course and 45° directions. The deformation mechanisms in meso-scale were identified experimentally by in-situ observation of large deformation process for both thermoset matrix and thermoplastic matrix textile composites. For the nylon/polyester composite samples, the non-linear properties mainly come from the change in the configuration of the fabric structure during extension. For the PET/PP co-knitted textile composite samples, the inelastic properties are attributed to the damage evolution in the matrix, interface damage between fiber bundle and matrix, sliding between the wales of the knitted fabric, as well as the change in the configuration of the fabric structure during loading.