碳纤维增强形状记忆聚合物复合材料的热-力学行为建模与影响因素碳纤维增强形状记忆聚合物复合材料的热-力学行为建模与影响因素

采用变形梯度分解的方法,基于热力学内变量理论,构建了适用于描述碳纤维增强形状记忆聚合物（CF/SMP）复合材料热-力学行为的热黏弹性本构模型。模型中考虑了材料的结构松弛效应和应力松弛效应,且适用于有限变形条件。依据该模型研究了一种CF/SMP薄板在受到单向均布载荷作用且处于平面应力状态时的碳纤维有效应变影响因素。理论上证实了虽然碳纤维的容许应变很小,但合理取向的纤维分布形式能使其应用于有限变形条件下而不被破坏。此外,分析了该CF/SMP形状记忆热-力学循环过程中形状记忆效应（SME）的影响因素。结果表明,碳纤维含量的增大和纤维倾斜角的减小会导致CF/SMP刚度增大,从而降低其形状固定率。此外,碳纤维体积含量和温度变化率对升温回复阶段也存在一定影响。上述研究方法和结果能对单向连续CF/SMP的设计与应用提供一定理论指导。      

Effect of fiber arrangement on shape fixity and shape recovery in thermally activated shape memory polymer-based composites

In the present study, we conducted periodic-cell simulations of the thermomechanical cycle of thermally activated shape memory polymer (SMP)-based composites. The present simulation utilizes a micromechanical model for reproducing the discontinuous fibers and SMP. We analyzed the effect of fiber volume fraction, fiber aspect ratio, and fiber end position on the shape fixity and shape recovery of the composite. The simulated results revealed that fiber elasticity is a key factor for the shape fixity of the composite, while both strain concentration near the fiber ends and fiber elasticity play important roles in the shape recovery properties of the composite.     

Synergistic effect of siloxane modified aluminum nanopowders and carbon fiber on electrothermal efficiency of polymeric shape memory nanocomposite

The present study reports an effective approach of significantly enhancing electrothermal efficiency and shape recovery performance of shape memory polymer (SMP) nanocomposite, of which shape recovery was induced by electrically resistive heating. Metallic aluminum (Al) nanopowders synthesized from Al³⁺ solution were chemically grafted onto carbon fiber. Siloxane groups were grafted onto surfaces of the Al nanopowders to enhance the interfacial bonding between the carbon fiber and SMP matrix via van der Waals force and covalent bond, respectively. The siloxane modified Al surfaces could improve both the electrically induced shape recovery performance and electrothermal efficiency through facilitating the electrically resistive heating from carbon fiber into the SMP matrix. Effectiveness of the synergistic effect between siloxane modified Al surface and carbon fiber was demonstrated to achieve the electrical actuation for SMP nanocomposites at a low electrical voltage below 4.0 V.     

Significantly improving infrared light-induced shape recovery behavior of shape memory polymeric nanocomposite via a synergistic effect of carbon nanotube and boron nitride

The present work studies the thermomechanical properties and infrared light-induced shape memory effect (SME) in shape memory polymer (SMP) nanocomposite incorporated with carbon nanotube (CNT) and boron nitride. The combination of CNT and boron nitride results in higher glass transition temperature, mechanical strength and thermomechanical strength. While CNTs are employed to improve the absorption of infrared light and thermally conductive property of SMP, boron nitrides facilitate heat transfer from CNTs to the polymer matrix and thus to enable fast response. A unique synergistic effect of CNT and boron nitride was explored to facilitate the heat transfer and accelerate the infrared light-induced shape recovery behavior of the shape memory polymeric nanocomposite.     

形状记忆基聚合物复合材料的研究进展

形状记忆聚合物是一种新型智能材料,广泛应用于医疗和航天领域。但是传统的形状记忆聚合物由于力学强度低、形变回复力小等缺点使其发展应用受到限制。主要介绍了以各种形状记忆聚合物为基体,添加增强填料来制备形状记忆聚合物复合材料,并且描述了复合材料的形状记忆效应与力学性能和填料的体积分数或质量分数之间的关系,以及形状记忆聚合物复合材料的应用领域与前景。     

The effective properties and local aggregation effect of CNT/SMP composites

A micromechanics model of the thermomechanical constitutive behavior and micro-structural inhomogeneity of carbon nanotubes (CNTs)/shape memory polymer (SMP) composites is presented. It is assumed that the CNTs are elastic and the SMP obeys a thermomechanical constitutive law. The effective properties of CNT/SMP composites are examined using a micro-mechanics method. The effect of CNT aggregation in the composite, frequently encountered in real engineering situations, is studied. The degree of aggregation is described by an aggregation coefficient, and the effective properties of SMP composites with aggregated CNTs are calculated using a stepping scheme. It is shown that the degree of CNT aggregation dramatically influences the effective properties of the CNT/SMP composites. A homogeneous microstructure leads to maximum levels of effective composite properties.     

Nondestructive sensing evaluation of surface modified single-carbon fiber reinforced epoxy composites by electrical resistivity measurement

Nondestructive sensing of a single-carbon fiber reinforced epoxy composites was evaluated by the measurement of electrical resistivity under reversible cyclic loading. For the strain–stress sensing, the strain up to the maximum load of a bare carbon fiber itself is larger than that of carbon fiber composite. As curing temperature increased, apparent modulus up to the maximum load increased and the elapsed time became shorter. Higher residual stress might contribute to the improved interfacial adhesion. The strain up to the maximum load at low temperature was larger than that at higher temperature. The strain of electrodeposition (ED) treated carbon fiber was smaller than that of the untreated carbon fiber composite until the maximum load reached. This could be due to higher apparent modulus of composite based on the improved interfacial shear strength (IFSS). Since the electrical resistivity was responded well quantitatively with various parameters, such as matrix modulus, the fiber surface modification, the electrical resistivity measurement can be a feasible method of nondestructive sensing evaluation for conductive fiber reinforced composites inherently.     

Piezoresistive and compression resistance relaxation behavior of water blown carbon nanotube/polyurethane composite foam

The in-situ bulk polycondensation process in combination with a ball milling dispersion process was used to prepare the water blown multiwall carbon nanotubes (CNT)/polyurethane (PU) composite foam. The mechanical properties, piezoresistive properties, strain sensitivity, stress and resistance relaxation behaviors of the composite foams were investigated. The results show that the CNT/PU composite foam has a better compression strength than the unfilled polyurethane foams and a negative pressure coefficient behavior under uniaxial compression. The resistance response of CNT/PU nanocomposites foam under cyclic compressive loading was quite stable. The nanocomposite foam containing a weight fraction of carbon nanotubes close to the percolation threshold presents the largest strain sensitivity for the resistance. The characteristic of resistance relaxation of CNT/PU composite foam is different from the stress relaxation due to the different relaxation mechanism. During compressive stress relaxation, the CNT/PU foam composites have excellent resistance recoverability while poor stress recoverability.     

Electroactive shape memory polymer based on optimized multi-walled carbon nanotubes/polyvinyl alcohol nanocomposites

The development of shape memory polymers (SMPs) has gained remarkable attention due to their wide range of applications, from biomedical to electromechanical. In this work, we have developed and optimized an electroactive SMP based on polyvinyl alcohol/multi-walled carbon nanotubes (PVA/MWNTs) composites. When a constant voltage of 60 V was applied to the optimized sample, the polymer shape could be recovered to the original form within 35 s. Different weight fractions of MWNT/PVA composites were prepared by using a simple solution blending and transitional solution casting method, and their microstructures, electrical conductivities, thermal conductivities, and electroactive shape memory properties were investigated. According to our systematic analysis, the enhanced performance can be attributed to the reinforcement of MWNTs that led to the improved electrical and thermal conductivities of the PVA matrix.     

Effect of fiber arrangement on mechanical properties of short fiber reinforced composites

The present paper developed a three-dimensional (3D) “tension–shear chain” theoretical model to predict the mechanical properties of unidirectional short fiber reinforced composites, and especially to investigate the distribution effect of short fibers. The accuracy of its predictions on effective modulus, strength, failure strain and energy storage capacity of composites with different distributions of fibers are validated by simulations of finite element method (FEM). It is found that besides the volume fraction, shape, and orientation of the reinforcements, the distribution of fibers also plays a significant role in the mechanical properties of unidirectional composites. Two stiffness distribution factors and two strength distribution factors are identified to completely characterize this influence. It is also noted that stairwise staggering (including regular staggering), which is adopted by the nature, could achieve overall excellent performance. The proposed 3D tension–shear chain model may provide guidance to the design of short fiber reinforced composites.     

Hybrid specimens eliminating stress concentrations in tensile and compressive testing of unidirectional composites

Two novel approaches are proposed for elimination of stress concentrations in tensile and compressive testing of unidirectional carbon/epoxy composites. An interlayer hybrid specimen type is proposed for tensile testing. The presented finite element study indicated that the outer continuous glass/epoxy plies suppress the stress concentrations at the grips and protect the central carbon/epoxy plies from premature failure, eliminating the need for end-tabs. The test results confirmed the benefits of the hybrid specimens by generating consistent gauge-section failures in tension. The developed hybrid four point bending specimen type and strain evaluation method were verified and applied successfully to determine the compressive failure strain of three different grade carbon/epoxy composite prepregs. Stable failure and fragmentation of the high and ultra-high modulus unidirectional carbon/epoxy plies were reported. The high strength carbon/epoxy plies exhibited catastrophic failure at a significantly higher compressive strain than normally observed.     

Two-dimensional strain mapping in model fiber-polymer composites using nanotube Raman sensing

The feasibility of using Raman spectroscopy to map strain fields in model composites is demonstrated by means of two experiments. (1) The mapping of the stress concentration in the vicinity of a break in a strained E-glass fiber, using the Raman spectrum of single-wall nanotubes dispersed in the polymer around the fiber. (2) The mapping of strain using the Raman response of a strained high modulus carbon fiber and, simultaneously, from the surrounding polymer matrix using single-wall nanotubes dispersed in the polymer. The size and shape of the ‘zone of influence’ resulting from the stress concentration effects around the fiber breaks are evaluated in both cases. Model experiments of this type provide fundamental design information about the fiber–matrix stress transfer mechanisms in composites.     

Methods to measure interlaminar tensile modulus of composites

This work expands a recently developed short-beam method coupled with the Digital Image Correlation full-field surface deformation measurement technique to enable assessment of the interlaminar tensile stress–strain constitutive properties of polymer–matrix composite materials. This work also expands the American Society for Testing and Materials Standard D 6415 curved-beam method as another means for measurement of the interlaminar tensile stress–strain constitutive behavior. The interlaminar tensile modulus values resulting from both methods are compared for Hexcel IM7/8552 carbon/epoxy tape composite material system.     

Determination of tensile strength of electrospun single nanofibers through modeling tensile behavior of the nanofibrous mat

The mechanical properties of polyamide-6 (PA-6) electrospun nanofibrous mat samples were tested. With the aid of the previously developed modeling software the whole tensile process was analyzed. The structural changes under the tensile process were evaluated from the modeling results and also compared to scanning electron micrographs. It was found that above a critical stress value nanofibers are slipping on one another which plays an important role together the changes in the fiber orientation during the process. With the aid of the modeling software the tensile strength of single nanofibers under ideal axial stress and ideal gripping circumstances were estimated. It was found that single nanofibers have 48% higher tensile strength than the bulk PA-6 material has.     

Self-assembled multi-layered carbon nanofiber nanopaper for significantly improving electrical actuation of shape memory polymer nanocomposite

This study presents an effective approach to significantly improve the electrical properties of shape memory polymer (SMP) nanocomposites that show Joule heating triggered shape recovery. Carbon nanofibers (CNFs) were self-assembled to form multi-layered nanopaper to enhance the bonding and shape recovery behavior of SMP, respectively. It was found that both glass transition temperature (Tg) and electrical properties of the SMP nanocomposites have been improved by incorporating multi-layers of self-assembled nanopapers. The electrically actuated shape recovery behavior and the temperature profile during the actuation were monitored and characterized at a voltage of 30 V.     

Preparation and bending properties of three dimensional braided single poly (lactic acid) composite

New three dimensional (3D) braided single poly (lactic acid) composites (PLA–SPCs) were obtained by combining 3D and five (5)-direction braiding technique and hot-compression technical process. 3D and 5-direction braided preforms with different braiding angles, thicknesses and fiber volume fractions were prepared. Preforms were preheated in the specially designed die system in order to make all of the fibers partially melted. In the next stage, the preforms were consolidated under a certain pressure (from 7.8 to 10 MPa) at temperatures ranging from 130 up to 150 °C. Under the controlled processing conditions, one part of fiber body formed matrix while the other part retained its fibrous form.At the same consolidation temperature, the maximum bending stress values resulted to be substantially dependent on the fiber volume fraction of PLA–SPCs, while the bending modulus values were largely subjected to the fiber content in the length direction. The increases of consolidation pressure gave rise to better fusion of neighboring fibers with the result that the maximum stress and modulus were increased. As the consolidation temperature increases, the fusion bonding was improved, the bending failure feature was converted from plastic to brittle, both maximum bending stress and modulus values were increased. It is expected that this study could provide a new approach for the manufacture of high-performance single polymer composites (SPCs) by using thermoplastic polymer fibers.     

Tensile and flexural properties of snake grass natural fiber reinforced isophthallic polyester composites

Natural fiber composite materials are one such capable material which replaces the conventional and synthetic materials for the practical applications where we require less weight and energy conservation. The present paper, which emphasis the importance of the newly identified snake grass fibers which are extracted from snake grass plants by manual process. In this paper, the tensile properties of the snake grass fiber are studied and compared with the traditionally available other natural fibers. The mixed chopped snake grass fiber reinforced composite is prepared by using the isophthallic polyester resin and the detailed preparation methodology is presented. Fiber pull-outs on the fractured specimen during the physical testing of the composites are also investigated. The experimental evidence also shows that the volume fraction increases the tensile, flexural strength and modulus of the snake grass fiber reinforce composite.     

High mechanical performance SiC/SiC composites by NITE process with tailoring of appropriate fabrication temperature to fiber volume fraction

Unidirectional SiC/SiC composites are prepared by nano-powder infiltration and transient eutectic-phase (NITE) process, using pyrolytic carbon (PyC)-coated Tyranno-SA SiC fibers as reinforcement and SiC nano-powder with sintering additives for matrix formation. The effects of two kinds of fiber volume fraction incorporating fabrication temperature were characterized on densification, microstructure and mechanical properties. Densification of the composites with low fiber volume fraction (appropriately 30 vol%) was developed even at lower fabrication temperature of 1800 °C, and then saturated at 3rd stage of matrix densification corresponding to classic liquid phase sintering. Hence, densification of the composites with high volume fraction (above 50 vol%) became restricted because the many fibers retarded the infiltration of SiC nano-powder at lower fabrication temperature of 1800 °C. When fabrication temperature increased by 1900 °C, densification of the composites was effectively enhanced in the intra-fiber-bundles and simultaneously the interaction between PyC interface and matrix was strengthened. SEM observation on the fracture surface revealed that fiber pull-out length was accordingly changed with fabrication temperature as well as fiber volume fraction, which dominated tensile fracture behaviors. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailoring of appropriate fabrication temperature to fiber volume fraction as follows: (1) high ductility type and (2) high strength type.     

Improvement of SiCf/SiC density by slurry infiltration and tape stacking

SiC fiber-reinforced SiC–matrix ceramic composites (SiC_f/SiC) were fabricated by vacuum infiltration of a SiC slurry into Tyranno™-SA grade-3 fabrics coated with a 200 nm-thick pyrolytic carbon (PyC) layer followed by hot pressing using a transient eutectic-phase. The density of the composite was improved using a special infiltration apparatus with a pressure gradient and alternating tape insertion between fabrics. Their overall properties were compared with those of monolithic SiC and composite containing chopped fibers. Although the density of the composites decreased with increasing fiber fraction, SiC_f/SiC containing 50 vol.% fibers had a density of 3.13 g/cm³, which is the highest reported thus far. The composites containing continuous fibers had a maximum flexural strength of 607 MPa and a step increase in the stress–displacement behavior during the three-point bending test due to fiber reinforcement, which was not observed in the monolith.     

Controlled growth of CuO nanowires on woven carbon fibers and effects on the mechanical properties of woven carbon fiber/polyester composites

The effect of CuO nanowires on the improvement of the mechanical properties of woven carbon fiber (WCF)-based polyester resin composite was studied. The composite was manufactured by the vacuum-assisted resin transfer molding (VARTM) process. CuO nanowires were grown on woven carbon fiber sheets in subsequent steps of seeding followed by growth. Scanning electron microscopy (SEM) showed the growth of CuO nanowires on the surface of the carbon fibers; this growth increased with the number of seeding cycles and the length of the growth time. The concentration of the growth solution did not have a significant effect. The maximum amount of growth occurred for 8 seeding cycles with a 60 mM growth solution and a growth time of 8 h. An analysis of the percent weight change, along with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, supported the above findings. The crystalline peak height of the CuO nanowires increased with the nanowire growth. The new absorption peaks arising in the FTIR spectra also indicated growth of CuO nanowires on the WCF. The mechanical properties in terms of tensile strength, modulus, and impact resistance improved significantly after the growth of nanowires on the carbon fibers: the modulus and strength improved by up to 33.1% and 42.8%, while the impact energy absorption increased by 136.8% relative to bare WCF.