碳纳米管薄膜层间改性复合材料 在不同应变率下的力学性能

将浮动催化化学气相沉积法制备的碳纳米管膜作为碳纤维层间改性材料,采用热压成型工艺制备了碳纳米管膜/碳纤维/环氧树脂混杂复合材料,并对其进行准静态II型断裂韧性实验以及准静态和动态压缩实验。碳纳米管膜层间改性后,复合材料的II型断裂韧性比未改性材料提高约60%,扫描电镜图像显示增韧机理主要是碳纳米管对基体的桥联。压缩实验结果表明,准静态压缩下碳纳米管膜改性材料在面内和面外两个方向的压缩强度都得到一定提高,动态面外压缩时改性后材料的压缩强度提高约9%,但是动态面内压缩时压缩强度没有提高,断口形貌显示这主要是碳纳米管膜内的分层所致。     

具有不同界面相厚度的混凝土动态特性

在细观层次上,可将混凝土看作由水泥砂浆、粗骨料和界面相组成的三相复合材料。为探讨界面相对混凝土动态力学特性的影响,编写了含界面相的圆形骨料随机分布程序,并对具有不同界面相厚度的混凝土动态破坏过程进行模拟,揭示界面相厚度、粗骨料大小、粗骨料体积分数和试件尺寸对混凝土动态特性的影响规律。研究表明:与普通混凝土相同,含界面相的混凝土表现出明显的尺寸效应;随着界面相厚度的增大,混凝土承载能力逐渐减小;保持界面相厚度和粗骨料尺寸不变时,随着粗骨料体积分数的增加,混凝土承载能力呈先增大后减小趋势;保持粗骨料最小粒径和界面相厚度不变时,随着粗骨料最大粒径的增大,混凝土承载能力呈先增大后减小趋势。     

二维骨料随机分布混凝土的动态力学性能数值模拟北大核心CSCD

将混凝土看成粗骨料和水泥砂浆组成的二相非均质复合材料。根据富勒级配曲线和瓦拉文平面转化公式,编写了二维圆形骨料随机分布程序,对混凝土在冲击荷载作用下的动力响应进行了数值模拟。分析冲击速度、试件尺寸、粗骨料大小及分布和粗骨料体积分数对混凝土动态力学性能的影响,讨论了混凝土的冲击破坏模式。数值模拟表明:混凝土的峰值应力随着冲击速度的增大而增大,具有明显的率效应;随着模型尺寸的增大而减小,表现出明显的尺寸效应;随着粗骨料体积分数增大,冲击荷载作用下混凝土的峰值应力呈现先增大后减小的趋势,粗骨料体积分数为40%时混凝土峰值应力最大;保持粗骨料最大粒径不变,随着粗骨料最小粒径的增大,混凝土的峰值应力逐渐减小;保持粗骨料最小粒径不变,随着粗骨料最大粒径的增大,混凝土的峰值应力呈现先增大后减小的趋势。数值模拟结果为混凝土的工程应用提供了理论依据和技术支撑。     

不同应变率下两种岩石的压缩破碎特征试验研究

为了研究不同岩石在不同应变率下压缩时裂纹的产生规律及破坏模式,将石灰岩和红砂岩制成试件,研究其在不同应变率和受力模式下裂纹的形成模式。开展了两种岩石的准静态压缩和动态压缩试验,采用高速摄影机记录了裂纹的产生和破坏模式。对两种岩石试件的裂纹形态进行对比,基于岩石的物理性质、受力状态、能量演化分析,得到了在不同应变率下压缩时产生差异性的原因。结果表明:准静态压缩下岩石试件受压的破坏模式也会因应变率的不同而存在差异,并且破坏模式的差异对岩石试件的抗压强度将产生显著的影响;从能量演化的角度分析,入射能量的大小将会决定岩石试样动态抗压强度曲线是否出现起伏;动态压缩时,裂纹的周向扩展速度与岩石抗压强度呈正相关。     

二维骨料随机分布混凝土的动态力学性能数值模拟

将混凝土看成粗骨料和水泥砂浆组成的二相非均质复合材料。根据富勒级配曲线和瓦拉文平面转化公式,编写了二维圆形骨料随机分布程序,对混凝土在冲击荷载作用下的动力响应进行了数值模拟。分析冲击速度、试件尺寸、粗骨料大小及分布和粗骨料体积分数对混凝土动态力学性能的影响,讨论了混凝土的冲击破坏模式。数值模拟表明:混凝土的峰值应力随着冲击速度的增大而增大,具有明显的率效应;随着模型尺寸的增大而减小,表现出明显的尺寸效应;随着粗骨料体积分数增大,冲击荷载作用下混凝土的峰值应力呈现先增大后减小的趋势,粗骨料体积分数为40%时混凝土峰值应力最大;保持粗骨料最大粒径不变,随着粗骨料最小粒径的增大,混凝土的峰值应力逐渐减小;保持粗骨料最小粒径不变,随着粗骨料最大粒径的增大,混凝土的峰值应力呈现先增大后减小的趋势。数值模拟结果为混凝土的工程应用提供了理论依据和技术支撑。     

超声动载荷下三维随机骨料混凝土的损伤

混凝土是一种由粗骨料与水泥砂浆组成的非均质复合材料。本研究利用APDL语言程序编写三维水泥混凝土骨料随机投放程序并导入ABAQUS中,同时赋予各相材料塑性损伤本构关系来研究混凝土动态加载下的破坏规律,运用超声波在混凝土破碎中的作用机理对混凝土动态损伤破坏过程进行模拟研究。结果表明:随着超声动态载荷的增大,粗骨料体积分数为40%的混凝土始终能够承受最大应力载荷;随着超声应力波幅值增大,混凝土在动载荷下的损伤值逐渐增大,且粗骨料体积分数为40%时,其抗损伤能力最优;当粗骨料最大粒径逐渐增大,或者当粗骨料最小粒径增大,混凝土级配不合理导致性能不稳定,更易损伤破坏。     

树脂基复合材料在连续激光作用下的损伤

 采用热压工艺制备了碳纤维布和高硅氧纤维布增强的环氧树脂和酚醛树脂基复合材料,研究了不同功率密度连续激光辐照下,复合材料的破坏形式及其组织结构与力学性能的变化。结果表明：当激光辐照功率密度大于0.1 kW/cm²后,树脂基体产生燃烧,碳纤维没有明显的损伤,而玻璃纤维布开始熔融,复合材料的拉伸性能降低30%~40%;当功率密度达到1 kW/cm²以后,除基体燃烧外,碳纤维复合材料产生明显的鼓泡分层,表层碳纤维有少量破断,而高硅氧纤维产生明显的熔融烧损,复合材料的拉伸性能降低80%以上。采用有限元计算方法,对碳纤维增强环氧树脂复合材料在连续激光辐照下的温度场进行了研究,计算结果与实验中复合材料的损伤行为相吻合。     

SiC纳米纤维/C/SiC复合材料拉伸行为的分子动力学研究

本文采用分子动力学计算方法和Tersoff作用势研究了无定型碳(amorphous carbon, a-C) 涂层厚度对SiC纳米纤维/SiC纳米复合材料断裂方式及力学性能的影响. 分析结果发现, 随着涂层厚度的增加, 纳米纤维的平均应力集中系数下降, 即足够厚度涂层可以同时起到增强和补韧的作用. 当a-C涂层厚度t ≤ 0.3 nm时, 裂纹直接穿透纤维, 纳米复合材料表现出典型的脆性断裂方式; t = 4.0 nm时, 裂纹发生偏转, SiC纳米纤维发生拔出现象, 此时纳米复合材料的拉伸强度约为无涂层纳米复合材料的4倍, 断裂能则提高一个数量级. 计算结果表明, a-C涂层的厚度是SiC纳米纤维/SiC纳米复合材料中产生韧性机理的重要因素, 即传统微米级陶瓷基复合材料的增韧理论在纳米复合材料中仍适用. 研究结果可望为设计同时具有高强度、高韧性的陶瓷基纳米复合材料提供理论基础.     

环氧树脂玻璃钢的动静态拉伸力学特性

为系统地研究环氧树脂玻璃钢在静、动态拉伸载荷作用下的力学性能，采用材料测试系统和分离式霍普金森拉杆对材料进行拉伸试验，获得0.001～0.1 s^-1及1 128～1 840 s^-1应变率下的应力-应变曲线和相应的力学参数。结果表明，动态加载下环氧树脂玻璃钢的应变率增强效应较为明显。为此，引入动态增强因子描述环氧树脂玻璃钢在高应变率下力学性能的增强。采用扫描电镜对损伤断面进行观测，发现动态加载下纤维束平整断裂，而非静态加载下纤维拔出失效。相较于静态加载，动态拉伸载荷作用下玻璃钢的基体-纤维界面断裂韧度更高。基于环氧树脂玻璃钢在动态拉伸下的力学响应，引入宏观损伤累积量，建立一种考虑损伤的非线性拉伸本构模型。拟合结果表明，该模型整体上可以反映环氧树脂玻璃钢在动态拉伸载荷作用下的力学响应。     

低相干动态光散射研究浓悬浮液中布朗运动粒子的扩散特性

利用低相干动态光散射方法研究了悬浮液中布朗运动颗粒的扩散特性。利用单散射理论,考虑悬浮液的折射率随分数的变化,通过解析检测到的单散射光的光谱得到不同分数悬浮液中颗粒的有效扩散系数。讨论了三种不同粒径颗粒的有效扩散系数随悬浮液体积分数的变化关系。结果表明,浓悬浮液中布朗运动颗粒的扩散特性受到静态结构因子及颗粒间流体动力学相互作用的影响,随体积分数的增加而减小。在0.01～0.2的体积分数范围内,有效扩散系数测量值与采用Percus-Yevick近似和Cohen-de Schepper近似得到的理论值吻合较好。证明了低相干动态光散射法可用来研究不同体积分数的悬浮液中颗粒的扩散特性。     

A Numerical Investigation on the Influence of Steel Fiber Shape and Interface Strength in Reinforced Concrete

Not all steel fiber reinforced concrete composites are equally effective in enhancing structural performance. Their mechanical behaviour strongly depends upon the reinforcement morphology as well as the properties of the interface lying between steel reinforcement and concrete matrix. Using bone-shaped short (BSS) steel fibers, instead of conventional straight short (CSS) steel fibers, to reinforce concrete has demonstrated their potential in improving toughness, ductility and energy absorbing capacity under impact significantly and simultaneously. Accomplishing a strong steel–concrete interface leads to a slight increase in composite strength but simultaneously to a significant decrease in its toughness. Due to the sensitivity of steel reinforced concrete performance on these complex geometric and material parameters, the development of a numerical tool capable of simulating accurately the composite mechanical behaviour and thus leading to optimized design solutions is desirable. The physical problem of the present work involves a typical concrete composite uniformly reinforced with steel fibers subjected to tensional loading. A micromechanical non-linear finite element formulation is utilized in order to predict the load transfer characteristics and the failure process. A linear material behaviour is assumed for the steel component; a non-linear multi-crack material response is used to describe concrete while a mix-mode bilinear behaviour is utilized for the interface providing separation of primary material phases. Numerical results are presented in terms of the global design parameters. The influence of the fiber end shape, the interface strength and the fiber volume fraction on the composite strength and toughness is addressed and consequently optimized design preferences arise.     

钢纤维混凝土板在冲击与爆炸荷载下的K&C模型

钢纤维混凝土(Steel fiber reinforced concrete,SFRC)具有优异的延性、韧性及能量吸收能力,被广泛应用于各类防护结构。K&C模型已成为研究普通混凝土构件动力响应的常用材料模型,但仍无法准确表征SFRC的动力特性。为了提高K&C模型在冲击及爆炸荷载作用下预测SFRC板动力响应的能力,对K&C模型进行了改进:基于大量三轴压缩实验数据,建立了新的失效强度面参数模型;采用反复试验法,建立了新的损伤演化模型,并校准了拉、压损伤参数;基于大量高应变率下SFRC的单轴压缩实验数据,建立了新的受压动力增强因子模型。通过LS-DYNA显式有限元动力分析软件模拟了SFRC板的动力响应,模拟结果验证了上述改进的有效性与可靠性。     

A probabilistic mechanical model for prediction of aggregates’ size distribution effect on concrete compressive strength

To predict aggregates’ size distribution effect on the concrete compressive strength, a probabilistic mechanical model is proposed. Within this model, a Voronoi tessellation of a set of non-overlapping and rigid spherical aggregates is used to describe the concrete microstructure. Moreover, aggregates’ diameters are defined as statistical variables and their size distribution function is identified to the experimental sieve curve. Then, an inter-aggregate failure criterion is proposed to describe the compressive-shear crushing of the hardened cement paste when concrete is subjected to uniaxial compression. Using a homogenization approach based on statistical homogenization and on geometrical simplifications, an analytical formula predicting the concrete compressive strength is obtained. This formula highlights the effects of cement paste strength and aggregates’ size distribution and volume fraction on the concrete compressive strength. According to the proposed model, increasing the concrete strength for the same cement paste and the same aggregates’ volume fraction is obtained by decreasing both aggregates’ maximum size and the percentage of coarse aggregates. Finally, the validity of the model has been discussed through a comparison with experimental results (15 concrete compressive strengths ranging between 46 and 106 MPa) taken from literature and showing a good agreement with the model predictions.     

Mechanical and Thermal Properties of Amine Functionalized Multi-walled Carbon Nanotubes Epoxy-Based Nanocomposite

This study is focused on fabrication and characterization of multi-walled carbon nanotubes (MWCNTs) reinforced epoxy composites to understand variation of mechanical and thermal properties. Samples were prepared by infusing both amine functionalized and non-functionalized MWCNT into commercially available EL-350 epoxy resin. Flexure, quasi-static compression and high strain rate (HSR) tests were performed with 0.1%, 0.2% and 0.3% of MWCNTs to observe variation in mechanical properties. The optimum loading was found at 0.2%. Nanocomposites with amine functionalized MWCNTs showed better properties compared to that with unmodified MWCNTs. Scanning electron microscopy (SEM) analysis was performed to confirm an improvement in dispersion with functionalization. Thermal properties were investigated by thermogravimetric analysis (TGA).     

高温处理后活性粉末混凝土动力学行为及本构模型研究

 利用分离式霍普金森压杆系统,采用铅片作为整形器,分别对常温下及400、600、800 ℃高温处理后的活性粉末混凝土（Reactive Powder Concrete,RPC）试样进行单轴冲击压缩实验,研究高温后RPC材料的动态力学性能,建立高温处理后材料的率型本构模型。结果表明：经不同高温处理后的RPC材料的动态抗压强度和韧性指标均有较明显的应变率敏感性,而峰值应变、初始弹性模量受应变率影响不大;不同应变率下,400 ℃以上高温处理后RPC材料的单轴动态压缩力学性能有所降低。扫描电镜分析表明,高温处理后RPC材料微观结构的劣化是宏观力学性能降低的根本原因。对ZWT粘弹性本构模型进行了修正,修正后的模型适用于混凝土材料经高温处理后的率型本构关系的分析。     

高温SHPB冲击实验技术及其应用

 为研究高温下材料的动态力学性能,研制了一套适用于分离式霍普金森压杆（Split Hopkinson Pressure Bar,SHPB）高温冲击实验的温控系统。利用该温控系统和Φ100 mm常规SHPB装置,对混凝土在高温下的动态力学性能进行了实验研究,实验温度分别为20、200、400、600、800和1 000 ℃。结果表明：由管式实时加热装置和箱式预加热炉组成的温控系统操作方便,实验效率高,试件组装方法简便可行;热传导导致的试件温度分布不均匀和压杆局部温升对实验结果产生的影响可以忽略,实验技术可靠;高温下混凝土动态力学性能的温度效应十分明显,相同冲击速率下,随温度升高,平均应变率逐渐增大,动态应力-应变曲线逐渐表现出塑性特性,动态抗压强度随温度升高先增大后减小,动态峰值应变随温度升高不断增大。     

Modified flax fibers reinforced soy-based composites: mechanical properties and water absorption behavior

Flax fibers are often used in reinforced composites which have exhibited numerous advantages such as high mechanical properties, low density and biodegradablility. On the other hand, the hydrophilic nature of flax fiber is a major problem. In this study, we prepare the soybean oil based composites reinforced with protein coated and lipid acylated flax fibers and compare their water uptake properties. Results showed that water resistance properties of the composites are improved where treated flax fibers are used. The composite with lipid acylation of the flax fiber exhibited to enhance tensile strength and water resistance properties. Influences of fiber length, fiber loading and pressure on mechanical properties are also reported.     

Microstructure of multilayer interface in an Al matrix composite reinforced by TiNi fiber

A multilayer interface was formed in the Al matrix composite which was reinforced by 30% volume fraction of TiNi fiber. The composite was fabricated by pressure infiltration process and the interface between the TiNi fiber and Al matrix was investigated by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). When the TiNi fiber was pre-oxidized in the air at 773 K for 1 h, three layers have been found and characterized in the interface: TiNi–B₂ layer near the TiNi fiber, Ti–Al compound layer with Ti and granular TiO₂ near the Al matrix, and Ti–Ni compound layer between TiNi–B₂ and Ti–Al compound layers. The effect of the multilayer interface on the mechanical properties of the composite was also discussed. The result showed that the uniaxial tensile strength of the composite at room temperature was 318 MPa, which was very close to the theoretical calculation value of 326 MPa. Moreover, the composite with good ductility exhibited a typical ductile-fracture pattern.     

Relation between interfacial shear strength and compressive strength of carbon fiber/epoxy resin composite strands

The mechanism with which the fiber-matrix interfacial strength exerts its influence on the compressive strength of fiber reinforced composites has been studied by measuring the axial compressive strength of carbon fiber/epoxy resin unidirectional composite strands having different levels of interfacial shear strength. The composite strands are used for experiments in order to investigate the compressive strength which is not affected by the delamination taking place at a weak interlayer of the laminated composites. The interfacial strength is varied by applying various degrees of liquid-phase surface treatment to the fibers. The efficiency of the compressive strength of the fibers utilized in the strength of the composite strands is estimated by measuring the compressive strength of the single carbon filaments with a micro-compression test. The compressive strength of the composite strands does not increase monotonically with increasing interfacial shear strength but showes lower values at higher interfacial shear strengths. With increasing interfacial shear strength, the suppression of the interfacial failure in the misaligned fiber region increases the compressive strength, while at higher interfacial shear strengths, the enhancement of the crack sensitivity decreases the compressive strength.