Mechanical behaviour of SMC composites with toughening and low density additives

A new type of SMC material (Flex-SMC) developed for automotive exterior body panels has been investigated. Flex-SMC contains hollow glass micro-spheres and thermoplastic toughening additives. A conventional SMC (Std-SMC) was used as a reference material. Materials were tested in monotonic tension and compression. Stiffness degradation with strain as well as fracture toughness was determined. In situ SEM was used to study failure mechanisms. Flex-SMC has a density almost 20% lower than Std-SMC and has higher impact resistance. The damage threshold strain of the Flex-SMCs is higher than for Std-SMC. Flex-SMCs have more than twice the fracture toughness of Std-SMC. The major reason identified is that Flex-SMCs shows extensive fibre pullout.     

Bridging law and toughness characterisation of CSM and SMC composites

This work presents an experimental investigation of the fracture properties of three different short-fiber-reinforced composites [one chopped strand mat (CSM) and two sheet molding compound (SMC) materials]. Fracture tests are performed on double-cantilever beam (DCB) specimens loaded with pure bending moments. In this experimental configuration, the bridging law for the material can be derived directly from measurements. No significant dependency on specimen height was observed in our results. The bridging laws determined can, therefore, be considered as material properties. The coupling between microstructure and fracture behaviour is discussed through the measured bridging laws. The beneficial effect (in terms of fracture energy) of increasing tendency for pull-out is confirmed for one SMC, referred to as Flex-SMC, which shows remarkably high fracture energy, J_c=56.0 kJ/m², compared to a standard SMC, termed Std-SMC, J_c=25.9 kJ/m². This increasing tendency for pull-out is observed to shift the bridging law towards larger crack openings. On the basis of our observations we find the concept of characterising the failure behaviour in terms of bridging laws attractive since it can be used as a tool for the tailoring of the microstructure towards desired fracture behaviour.     

Modelling of inter-laminar toughening from chopped Kevlar fibers

Inter-laminar toughening from sparsely distributed chopped fibers is analyzed in this paper. A detailed micromechanical model describing the special toughening effects of those chopped Kevlar fibers lying within the delamination plane has been developed. To determine bridging forces of the Kevlar fibers lying in the matrix near a delamination-crack face, a simplified model based on coupling of matrix spalling (matrix fracture following by separation of fiber away from the matrix) and fiber pull-out is proposed. The model is then used to study toughening of carbon-fiber/epoxy composite laminates with chopped Kevlar fibers. The model shows that the energy dissipation in the pull-out process of chopped Kevlar fibers is much more than that in the matrix spalling process. The numerical results indicate that interfacial properties, interactions among Kevlar fibers and percentage of Kevlar fibers involved in crack bridging play important roles in delamination toughening. The modelling results are in good agreement with the experimental results available in the literature.     

Cracking and toughening of concrete and polymer-concrete dispersed with short steel wires

A fibre-strengthened brittle solid can crack and fracture in a number of ways and simple models can be used to describe quantitatively the fracture processes. This paper discusses some of these models and compares experimental measurements of cracking stress and toughness for two brittle fibrous composites with the theoretical predictions. The two brittle matrices are concrete and concrete impregnated with polymethylmethacrylate reinforced by discontinuous (short) high strength steel wires. It involved extracting a single steel wire from each brittle matrix to evaluate the debonding stress and pull-out stress as a function of fibre embedded length. These key material parameters and the energetics of cracking determined in three-point flexural experiments, together with the cracking and toughening equations are then used to characterize the fracture behaviour of fibrestrengthened concrete and polymer-concrete composites.     

短切炭纤维的CVI处理及其在CFRC中的分散性

采用CVI法对短炭纤维进行表面处理, 借助超声波对其进行预分散, 用新型分散剂羟乙基纤维素(HEC)和超细粉硅灰对其进行分散, 并研究了其在水泥基体中的分散性; 在SEM电镜下观察了短炭纤维增强水泥基复合材料(CFRC)的断口形貌, 用炭纤维质量变动系数定量评价了短炭纤维在CFRC中的分散性。结果表明, 采用CVI预处理和超声波预分散, 在分散剂HEC和硅灰不同掺量下, 炭纤维的分散性均得到显著改善。炭纤维的分散性随HEC掺量的增加而提高, 当HEC掺量为水泥质量的0.6%、 硅灰掺量为水泥质量的10%时, 两种分散剂的协同作用使炭纤维质量变动系数最小, 此时炭纤维在水泥基体中的分散性最理想。      

行波型杆式超声电机定子的参数化有限元法优化设计

摘要：利用参数化有限元优化方法,对行波型杆式超声电机定子进行优化设计。首先,在确定电机定子初始结构的基础上,建立其参数化有限元模型。其次,对定子有限元模型进行模态分析,求解工作模态频率对各结构参数的灵敏度,选取灵敏度高的结构参数为设计变量,并以反映电机输出性能的重要参数作为目标函数。同时,设计了定子结构的优化方案,采用了零阶优化方法,对其结构进行优化设计。最后,根据优化结果,制作了定子样机。试验表明：定子工作模态和端面质点的振幅都满足了预期的设计要求,试验结果与优化设计结果相符。研究表明,利用该优化设计方法能有效地缩短超声电机设计周期。     

Improvement of plant based natural fibers for toughening green composites—Effect of load application during mercerization of ramie fibers

Effect of mercerization to tensile properties of a ramie fiber was explored. Load application technique during mercerization has been employed in order to improve mechanical properties of the fiber. A chemical treatment apparatus with tensile loading portion for applying monofilaments was newly developed. The ramie fiber was alkali-treated by 15% NaOH solution with applied loads of 0.049 and 0.098 N. The results showed that tensile strength of the treated ramie fiber was improved, 4–18% higher than that of the untreated ramie fiber, while Young’s modulus of the treated fibers decreased. It should be noted that fracture strains of the treated ramie fiber drastically increased to 0.045–0.072, that is, twice to three times higher than those of the untreated ramie fiber. It was considered that such property improvements upon mercerization were correlated with change of morphological and chemical structures in microfibrils of the fiber. Finally, the plastic deformation behavior and fracture mechanism of the mercerized fibers under tensile loading process was explained using a schematic model.     

Corrected optimum design of experiments on exponential absorption for materials in short supply

A corrected general expression is obtained for the statistical error in the absorption coefficient measured in an experiment on exponential absorption with n observations, making an allowance for a background measurement for each observation and using a fixed volume of absorber and a variable cross-sectional area for the incident beam. For n = 2 the design is optimised without constraint on the weighting of the points and also for equal weighting. With equally spaced values of absorber thickness, and with equal weighting on each point, the values of thickness interval and all the counting times are optimised for n = 3, 4, 6, 8 with relative backgrounds of zero and 0.001–100. For zero and very large backgrounds simplified formulae are obtained and optimised for any value of n. The results are compared with those for the simplest design with equal counting times for the transmitted beam plus background, equal counting times for each corresponding background count, and optimised absorber-thickness interval. For small backgrounds the loss of accuracy for this simplest design is relatively small, but is more significant for larger backgrounds and larger n values. For comparison, the optimum design is computed for unlimited absorber volume with equal counting times. It is shown that the unconstrained optimum design with limited absorber volume for two or more absorber thicknesses reduced to that for one thickness. The advantages of using the maximum available beam are are explored.     

Toughening of denture base resin with short deformable fibers

Fracture resistance of polymer reinforced with short fibers consists of a sum of contributions from matrix and fiber fracture, fiber debonding and pull-out. The existing models for predicting dependence of fracture toughness on structural variables were derived for the commercially important fiber volume fractions, i.e., for v_f ? 0.1. In this contribution, modification of the existing model for the dependence of the critical strain energy release rate, G_IC, on the fiber type, length and aspect ratio, interfacial adhesion and volume fraction has been attempted to allow predictions at low v_f < 0.10. The predictions based on the modified model were compared with experimental data on fracture toughness of lightly x-linked PMMA used to manufacture base of removable dentures toughened with short randomly oriented deformable fibers. The composite toughness was measured under impact loading to simulate typical mode of fracture of removable dentures. The G_IC for composites containing short Kevlar 29, S2-glass and poly(vinyl alcohol) (PVOH) fibers were obtained using instrumented Charpy impact tests at room temperature and impact speed of 1.0 m/s. Theoretical prediction based on the proposed model and experimental results agreed reasonably well.     

短纤维增强聚丙烯泡沫复合材料的研究新进展

详细介绍了天然纤维、短玻璃纤维、碳纳米纤维及晶须等在增强PP泡沫复合材料中的应用;重点阐述了短纤维的种类和含量对发泡行为、微观结构及力学性能等的影响规律,并总结了相关增强机理;展望了短纤维/PP泡沫复合材料的发展趋势。     

Flammability and tensile properties of polylactide nanocomposites with short carbon fibers

Nanocomposites of polylactide (PLA) with aluminum hydroxide (ATH), short carbon fibers (CF), and montmorillonite (MMT) were prepared via direct melt blending. The exfoliated and intercalated clay structures with some aggregations in the PLA matrix were observed. The tensile strength and elongation at break of the PLA composite caused by the high content of the retardant ATH were improved by adding modified MMT and CF to replace a portion of ATH in the PLA matrix. The thermal degradation temperatures and char residue of the PLA/ATH/MMT/CF nanocomposites as determined by thermogravimetric analysis were higher than without MMT. Furthermore, a novel method was proposed to analyze the flammability of composite using an infrared camera, which could capture the apparent thermal image of the sample during UL 94 V test. It was found that, with addition of the MMT and short CF, a more effective insulation layer could be formed on the ablating surface of the PLA/ATH composite, and the high thermal conductivity of the CF might increase the release rate of heat from the surface composite during burn, thus the PLA/ATH/MMT nanocomposite containing short carbon fibers having a V-0 rating without flaming dripping could be obtained.     

电磁屏蔽涂料用改性短碳纤维的研究

利用阻抗匹配原理和电容器通交流断直流的频率特性,提出了在导电填料表面包覆不导电层设计的构想,来实现对表面DC电导率随填料含量的提高而增大、从而形成回路而短路等问题的改善.采用原位聚合法在短碳纤维(SCF)的表面均匀包覆一层不导电的聚苯胺(PANI),使用扫描电子显微镜(SEM),热分析(TG)以及傅里叶变换红外光谱(FTIR)等表征了改性SCF的表观形貌和结构,分析并探讨了SCF微结构与电磁波的交互作用机制.研究表明:在聚合温度为5℃,聚合反应时间为12h,苯胺与引发剂的摩尔比为1:1,碱性溶液摩尔浓度为1mol/L时,可制得表面均匀包覆不导电PANI的改性SCF;应用改性填料制得电磁屏蔽涂料,通过同轴法兰装置测试,在频率＜1.5GHz下,相对使用碳纤维体系而言,其屏蔽效能(最大值)提高了3倍多.     

Fabrication and properties of short carbon fibers reinforced copper matrix composites

Short carbon fibers (SCFs) reinforced copper matrix composites have been produced by a new electrodeposition plus cold press and sintering technique. SCFs were copperized directly by the new method, and the electrodeposit had a loose porous structure. The coating thickness is uniform, and can be controlled by appropriate parameters. A model representing the growth process of these electrodeposits was presented. SCFs were distributed homogeneously, and no defects were found in the Cu/SCFs composites. The effects of SCFs volume fraction on mechanical, physical, thermal, and tribological properties of the composites were discussed.     

基体结构与短纤维对硅橡胶耐烧蚀绝热材料性能的影响

采用甲基硅橡胶、乙烯基硅橡胶、甲基苯基乙烯基硅橡胶作为耐烧蚀绝热材料的基体,对其性能进行了研究,结果表明:甲基苯基乙烯基硅橡胶属于综合性能较好的成碳型绝热材料基体。以甲基苯基乙烯基硅橡胶为基体,制备短切玻璃纤维、短切碳纤维和聚芳砜纶浆粕增强耐烧蚀绝热复合材料材料,对其物理性能、导热性能及耐烧蚀性能进行了系统研究。结果表明不同纤维的增强效果差别较大,相同用量条件下,PSA浆粕增强制得的复合材料线烧蚀率较低(0.086mm.s-1);从综合性能及碳层结构方面考虑,玻璃纤维与PSA浆粕可以作为苯基硅橡胶耐烧蚀绝热材料的成碳增强材料。     

Vibration design of laminated fibrous composite plates with local anisotropy induced by short fibers and curvilinear fibers

The present paper studies an optimum design method for proposing new types of fiber reinforced composite plates with locally anisotropic structure. A finite element program is developed to analyze vibration of such locally anisotropic plates, and the fundamental frequency is taken as an object function to be maximized. First, for demonstrating the effectiveness of local anisotropy, the optimum distributions of short fibers are calculated without directional constraints using a simple genetic algorithm (GA), and the layerwise optimization (LO) concept is used to reduce the computation time in the finite element calculation. Secondly, optimum arrangements of continuous curvilinear fibers are obtained under the continuity constraints where fiber directions are considered as projections of contour lines of a cubic polynomial surface. Numerical results show that the local anisotropy successfully improves frequency property and the optimum directions of short fibers indicate physically reasonable orientations. Also, the plates with optimally shaped continuous fibers yield higher fundamental frequencies than the conventional plates with parallel fibers.     

导电导磁聚苯胺包覆短碳纤维的研究

采用磁性物质结合导电聚苯胺(PANI)层包覆短碳纤维(SCF)制备出导电导磁的短碳纤维,以改善SCF的电磁性能。采用扫描电子显微镜(SEM)结合能谱(EDS)表征了改性SCF的表面形貌、结构,利用PNA-L N5230A微波网络测试仪测定了其在频率2GHz以下的复介电常数和复磁导率。结果表明:在SDBS和油酸为1.0g处理磁流体,HCl用量为0.2mol/L,Fe3O4含量为3.2g时,聚合温度为10℃,反应时间为12h,可制得包覆层均匀、致密的导电聚苯胺/磁性物质包覆改性的短碳纤维。改性后,其介电性能有所下降,磁损耗有所增大。