Tools for precision diamond machining of spherical heads of endoprostheses from pure titanium

Abrasive composites based on modified epoxy resins and synthetic diamonds, which are three-level adaptive systems that make it possible to machine technically pure titanium without seizure, have been developed. The tools containing these composites have allowed us to elaborate an efficient technology of manufacturing heads of hip joints from technically pure titanium.     

一种自适应抽样的代理模型构建及其在复材结构优化中的应用

提出了基于一种自适应抽样和增强径向基插值的自适应代理模型方法，这种自适应抽样方法以确定适量的样本点数量和提高代理模型自适应能力为目的，使新增样本点位于设计空间的稀疏区域并确保所有的样本点均匀分布于设计空间以提高代理模型精度。标准误差用来判断代理模型的精度大小并决定是否对代理模型进行更新。一种条件随机抽样被用来对比本文的自适应抽样方法。经过对比验证发现，采用自适应抽样方法的代理模型精度比条件随机抽样方法的代理模型精度高。这种自适应代理模型结合多岛遗传算法被用来优化旋翼臂的碳纤维增强环氧树脂复合材料铺层角度使得旋翼臂的一阶模态频率最大。优化结果表明，不同的碳纤维增强环氧树脂复合材料铺层角度对旋翼臂的一阶模态频率值影响较大，优化结果获取了最优铺层角度，旋翼臂的一阶模态频率值被提高以远离激励频率而避免旋翼飞机的共振。     

形状记忆合金丝增强复合材料的热机特性——智能复合材料研究

在形状记忆合金丝增强复合材料中,形状记忆合金丝作为分布式驱动器使复合材料具有自适应功能.本文根据形状记忆合金的一维本构关系,对形状记忆合金丝增强复合材料的温度与应力、应变关系进行理论探索;对形状记忆合金丝增强复合材料的拉伸性能、温度变化时的自由回复和受限回复性能进行了实验,并与模拟计算结果进行了比较.     

Ultrasonic consolidation for embedding SMA fibres within aluminium matrices

For this paper, adaptive composites will be considered as structural materials for advanced aerospace applications which have the ability to measure and respond to external stimuli by adapting the structure accordingly, through embedded active or passive functional elements. The intention of this project was targeted at the fabrication of adaptive composites using a novel layered manufacturing technique called ultrasonic consolidation (UC). This paper details the initial study of this research to identify plastic deformation of the matrix material around shape memory alloy (SMA) fibres, and bond quality, based on the microscopic observation and mechanical test results obtained. The embedding method, considered during this study, has successfully produced laminate specimens, with full consolidation, within seconds, using low oscillation amplitude and low contact pressures (<300 kPa). This work will report on the identification of the bonding characteristics for these SMA fibres when embedded in aluminium alloy 3003 specimens.     

Stimuli–Responsive Mechanoadaptive Elastomeric Composite Materials: Challenges,Opportunities, and New Approaches

Stimuli–responsive mechanoadaptive materials, capable of reversibly changing their mechanical properties when exposed to an external stimulus, are the next generation of smart materials with immense transformative potential for various technological applications. Although the concept of adaptive mechanical properties has been proven for some materials, it remains very challenging for soft elastomeric materials. The aim of this review is to provide new ideas and strategies for the development of mechanoadaptive elastomeric composites using commercial rubber as the matrix polymer. The fundamental question addressed here is as follows: How do the phase-responsive functional fillers alter the mechanical properties? For a given physical network environment, what is the mechanism that gives rise to the stimuli–responsive properties of the resulting composites? Herein, the preparation, structure, and properties of recently developed mechanoadaptive elastomeric materials are summarized. Furthermore, based on their structure–property relationships, plausible applications of these smart materials in various technology-specific applications such as soft robotics, actuators, sensors, smart tires, automotive design, aerospace, etc. are demonstrated with representative examples. Finally, the article critically discusses the existing challenges in the field of mechanoadaptive elastomers in order to provide valuable insights in this area.     

Micromechanics and damping properties of composites integrating shear thickening fluids

Damping is an important parameter for vibration control, noise reduction, fatigue endurance or impact resistance of composite materials. In this study, a micromechanical model was used to predict the damping of a composite material containing shear thickening fluids (STFs) at the fibre–matrix interfaces. Predictions of the model and dynamical mechanical analysis results are in concert. The damping of the composites was improved significantly. The dynamic properties exhibited a strong dependence on both frequency and applied external load amplitude. Damping peaks appeared which coincided with the thickening of the STF at the fibre–matrix interface. The location of the peaks depends on the onset of thickening and post-thickening rheological behaviour of the STF. This work shows that a micromechanics approach can be useful for an appropriate choice of microstructural design and properties of STFs in order to control the stiffness and damping behaviour of composites. STFs can be integrated at the microscale of polymer composites to create new materials with load-controlled adaptive dynamic stiffness-damping properties.     

Design of multistable composites for application in adaptive structures

During the manufacturing process and operating conditions of multilayered fibre-reinforced composites with variable fibre orientations, residual stresses build up due to the directional expansion of the unidirectionally reinforced single layers. Dependent on the laminate lay-up, these inhomogeneous residual stresses, which are primarily caused by thermal effects, moisture absorption and chemical shrinkage, can lead to large multistable out-of-plane deformations in the case of unsymmetric laminates. Instead of avoiding these laminate's curvatures, they can be advantageously used for technical applications such as novel adaptive structures. In order to adjust the laminate deformations to technical requirements, a dimensioning tool based on a modified stability analysis in combination with a novel optimisation procedure has been developed and experimentally verified. Based on the theoretical investigations, an adaptive prototype of a multistable composite with integrated smart alloys has been designed and manufactured.     

Quantitative characterization of multiple delaminations in laminated composites using the compton backscatter technique

A Compton X-ray backscatter technique was used to supplement ultrasonic pulse-echo C-scan imaging to quantitatively assess the impact damage in quasi-isotropic laminated composites which were impacted by a drop-weight tester. A Compton backscatter imaging system with a slit-type camera was developed to obtain a cross-sectional profile of impact-damaged laminated composites from the density variation of the cross-section. A nonlinear reconstruction model is introduced to overcome distortion of the Compton backscatter image due to attenuation effects, beam hardening, and irregular distributions of the fibers and the matrix in composites. An adaptive filter is used to reduce noise from many sources including quantum noise, especially when the SNR (signal-to-noise ratio) of the image is relatively low. Delaminations masked or distorted by the first few delaminations in an ultrasonic C-scan image are detected and characterized by the Compton back-scatter technique, both in width and location.     

Adaptive multi-scale modeling of high velocity impact on composite panels

A computationally efficient adaptive multi-scale methodology for modeling composites under high rates of loading is proposed. The physically based model relies on micromechanical properties of the constituents only. The adaptive algorithm switches between two different constitutive laws. Initially, the material response is calculated based on effective linear-elastic, orthotropic material properties at the ply scale which are calculated using the rule of mixtures. A modified Hashin–Rotem criterion is then used to identify the switch to a more accurate micromechanical analysis based on the generalized method of cells (GMC). The methodology is verified by simulating tensile tests on laminates with different stacking sequences. Finally the model validated against experimental data for high-velocity impact on quasi-isotropic composite targets taken from the literature in order to illustrate the efficiency and accuracy of the proposed methodology.     

A constitutive model for self-reinforced ductile polymer composites

Self-reinforced polymer composites are gaining increasing interest due to their higher ductility compared to traditional glass and carbon fibre composites. Here we consider a class of PET composites comprising woven PET fibres in a PET matrix. While there is a significant literature on the development of these materials and their mechanical properties, little progress has been reported on constitutive models for these composites. Here we report the development of an anisotropic visco-plastic constitutive model for PET composites that captures the measured anisotropy, tension/compression asymmetry and ductility. This model is implemented in a commercial finite element package and shown to capture the measured response of PET composite plates and beams in different orientations to a high degree of accuracy.     

Properties of squeeze cast Mg-6Zn-3Cu alloy and its saffil alumina short fibre reinforced composites

In the present work, Mg-Zn-Cu alloy (ZC63) and its saffil alumina short fibre reinforced composites produced using the squeeze casting technique were evaluated for their properties. The unreinforced base alloys and their composites were characterized for their microstructure, hardness, yield strength, impact strength, wear resistance and corrosion resistance. The dependence of the properties of composites was studied as a function of fibre volume fraction. Results showed that the composites exhibited improved hardness, yield strength at elevated temperature and wear resistance in comparison to the monolithic alloy. However, ductility, impact strength and corrosion resistance of the composites were inferior to that of the base alloy. The nature of the base alloy matrix in determining the properties of the composites was discussed based on fractographic analysis.     

Plant fibre composites – porosity and stiffness

Plant fibre composites contain typically a relatively large amount of porosity which influences their performance. A model, based on a modified rule of mixtures, is presented to include the influence of porosity on the composite stiffness. The model integrates the volumetric composition of the composites with their mechanical properties. The fibre weight fraction is used as an independent parameter to calculate the complete volumetric composition. A maximum obtainable stiffness of the composites is calculated at a certain transition fibre weight fraction, which is characterised by a best possible combination of high fibre volume fraction and low porosity. The model is validated with experimental data from the literature on several types of composites. A stiffness diagram is presented to demonstrate that the calculations can be used for tailoring and design of composites with a given profile of properties.     

Effect of deformation mode on the strength of deformation processed Cu-20%Nb composites

It was previously concluded that all f.c.c.-b.c.c. deformation processed composites have larger Hall-Petch slopes than layered composites (1). The present results on rolled versus wire drawn Cu-20%Nb show that the Hall-Petch slopes in deformation processed f.c.c.-b.c.c. composites depend on the mode of deformation processing and presumably the resulting filament morphology. Layered composites appear to be a good approximation to rolled deformation processed composites because of the filaments being essentially planar in both types of composites. Layered composites appear to be a poor approximation to axisymmetrically deformation processed composites because of the convoluted ribbonlike morphology of the filaments in the latter composites as compared to their planar nature in the former composites.     

Mechanical properties of poly(vinyl alcohol) fibres and composites

This paper describes the performance of poly(vinyl alcohol) (PVOH) fibres and their composites. PVOH fibres have been studied, with the emphasis on long-term properties. Results indicated that the long-term properties of PVOH fibres are superior to those of high-performance polyethylene (HP-PE) fibres. Mechanical tests on PVOH/epoxy composites showed that, in general, their structural performance is between those of plasma-treated HP-PE and aramid fibre-reinforced composites. Structural mechanical properties of PVOH composites are better than those of HP-PE based composites due to the combination of a strong interfacial bond strength and a less pronounced anisotropic fibre character. However, the strong bonding between PVOH and epoxy results in a rather brittle failure mode of the composite and consequently relatively poor impact properties.     

Overcoming the cohesive zone limit in composites delamination: modeling with slender structural elements and higher-order adaptive integration

Cohesive element (CE) is a well-established finite element for fracture, widely used for the modeling of delamination in composites. However, an extremely fine mesh is usually needed to resolve the cohesive zone, making CE-based delamination analysis computationally prohibitive for applications beyond the scale of lab coupons. In this work, a new CE-based method of modeling delamination in composites is proposed to overcome this cohesive zone limit on the mesh density. The proposed method makes use of slender structural elements for the plies, a compatible formulation with adaptive higher-order integration for the CEs, and the corotational formulation for geometrically nonlinear analysis. The proposed method is verified and validated on the classical benchmark problems of Mode I, II, mixed-mode delamination, a buckling-induced delamination problem and a double-delamination problem. The results show that elements much larger than the cohesive zone length can be used while retaining accuracy.     

Load carrying capacity of 2D FRP/strengthened masonry structures

An adaptive discontinuous finite element model is formulated for limit analysis of masonry structures strengthened by fiber composites. The model is able to predict the effects of fracture damage and delamination on the load carrying capacity of the reinforced structures. A numerical investigation on the collapse mechanisms of masonry structures under plain strain/stress is presented, accounting for different mechanical properties of FRP–masonry interface and different placements of the reinforcement in the masonry structures.     

A review of modeling and control during drilling of fiber reinforced plastic composites

Drilling induced damage in polymer–matrix composites (PMCs) is a research area of immense engineering importance. Various approaches have been tried worldwide to minimize drilling induced damage. In this study, a review of automated drilling operation has been done. Various mathematical modeling methods used for dynamic phenomenon of drilling in PMCs and conventional materials have been discussed. Drilling of fiber reinforced plastic composites can be modeled using empirical techniques, neural network/fuzzy-logic and transfer function modeling methods. This paper brings state-of-the-art in the control of drilling process. The drilling of fiber reinforced plastic composites can be controlled using neural network, fuzzy logic, supervisory, PI, PID, pole placement and adaptive controllers. Results indicate that thrust force and torque have not been controlled simultaneously for delamination free drilling in PMCs. Critical thrust force has also not been precisely tracked. There is a need to create a combined mathematical model consisting of thrust force, torque and feed rate coupled with a suitable control law for simultaneous control of thrust force as well as torque for delamination free drilling of composites.     

原位合成钛基复合材料的研究现状与展望

原位合成钛基复合材料以其高比强度、高比模量引起了人们的广泛关注,尤其是如何提高其高温性能更成为近年研究的热点.综述了原位合成钛基复合材料的主要制备方法、增强体与钛基体的选择、反应体系以及复合材料的显微组织与力学性能,提出了当前存在的主要问题和今后的发展方向.     

Calculation of effective moduli of fibrous composites with micro-mechanical damage

A micro-mechanics model for continuous fibrous composites was developed in order to determine the effective moduli of composites based on the material properties of their constituents, i.e. fiber and matrix materials. The model can calculate elastic or nonelastic effective moduli of composites depending on their constituents' behavior. Furthermore, micro-mechanical damage can also be considered in the present model to determine effective moduli. Predicted effective moduli from the present model compared very well with experimental data available elsewhere for both undamaged and damaged composites.     

Effects of CNT diameter on mechanical properties of aligned CNT sheets and composites

Drawing, winding, and pressing techniques were used to produce horizontally aligned carbon nanotube (CNT) sheets from free-standing vertically aligned CNT arrays. The aligned CNT sheets were used to develop aligned CNT/epoxy composites through hot-melt prepreg processing with a vacuum-assisted system. Effects of CNT diameter change on the mechanical properties of aligned CNT sheets and their composites were examined. The reduction of the CNT diameter considerably increased the mechanical properties of the aligned CNT sheets and their composites. The decrease of the CNT diameter along with pressing CNT sheets drastically enhanced the mechanical properties of the CNT sheets and CNT/epoxy composites. Raman spectra measurements showed improvement of the CNT alignment in the pressed CNT/epoxy composites. Research results suggest that aligned CNT/epoxy composites with high strength and stiffness are producible using aligned CNT sheets with smaller-diameter CNTs.