Magnetically-sensitive shape memory polyurethane composites crosslinked with multi-walled carbon nanotubes

Magnetically-sensitive polyurethane composites, which were crosslinked with multi-walled carbon nanotubes (MWCNTs) and were filled with Fe₃O₄ nanoparticles, were synthesized via in situ polymerization method. MWCNTs pretreated with nitric acid were used as crosslinking agents. Because of the crosslinking of MWCNTs with polyurethane prepolymer, the properties of the composites with a high content of Fe₃O₄ nanoparticles, especially the mechanical properties, were significantly improved. The composites showed excellent shape memory properties in both 45 °C hot water and an alternating magnetic field (f = 45 kHz, H = 29.7 kA m⁻¹). The shape recovery time was less than one minute and the shape recovery rate was over 95% in the alternating magnetic field.     

Preparation and characterization of water-borne epoxy shape memory composites containing silica

Shape memory silica/epoxy composites were successfully prepared by hydrolysis of tetraethoxysilane (TEOS) within the epoxy matrix via latex, freeze-drying, and hot-press molding method. The silane coupling agent 3-triethoxysilylpropylamine (KH550) was introduced to improve the interfacial properties between the in-situ generated silica particle and epoxy matrix. The morphology structure and the effect of the content of the in-situ formed silica on the mechanical and shape memory properties of the silica/epoxy composites were studied. The experimental results indicated that the silica particles were homogenously dispersed and well incorporated into the epoxy matrix. Significant improvements were achieved in the mechanical property of the organic–inorganic hybrid materials. The silica/epoxy composites exhibited high shape recovery and fixity ratio approximately 100% even after 10 thermo-mechanical cycles.     

Dynamic analysis of sandwich cement-based piezoelectric composites

Theoretical analysis of a sandwich cement-based piezoelectric composite is presented based on the theory of piezo-elasticity. The steady-state responses of two kinds of this composite under different loading cases are obtained by the use of displacement method. The effects of piezoelectric phases on the performance of this kind of devices are simulated and discussed. The solutions are compared with both the numerical and experimental results, and good agreements are found. Sandwich cement-based piezoelectric composites have great application potential in civil structure health monitoring. The results obtained in this paper are beneficial to the design of this kind of smart devices.     

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.     

Static and dynamic thermo-electro-mechanical analysis of angle-ply hybrid piezoelectric beams using an efficient coupled zigzag theory

This work presents static and dynamic electro-thermo-mechanical analysis of angle-ply hybrid piezoelectric beams using a recently developed efficient coupled zigzag theory. In this theory, the displacement approximations account for the thermoelectric strain in the thickness direction and the shear traction-free conditions at the top and the bottom of the beam and the shear continuity conditions at the layer interfaces are exactly satisfied. The theory is assessed against two-dimensional exact piezo-thermo-elasticity solution and compared with the first and third order shear deformation theories. The effect of the span-to-thickness ratio, type of loading and the orientation of the angle-plys on the accuracy of the theories is investigated. It has been concluded that, in general, the new zigzag theory is more accurate than the other theories considered.     

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.     

Bond behavior of superelastic shape memory alloys to carbon fiber reinforced polymer composites

In this research pull-out specimens were tested to investigate the bond behavior of superelastic NiTi (Nitinol) SMA wires to carbon fiber reinforced polymers (CFRP). A total of 45 pull-out specimens were tested monotonically up to failure. The test parameters considered include the wire diameter and embedment length. A digital image correlation (DIC) system was used to identify the onset and propagation of debonding. Based on the experimental observations two debonding mechanisms were observed: complete debonding after the onset of martensitic transformation of SMA wire, and complete debonding before the onset of wire transformation. The former mechanism predominated, while the latter mechanism governed for larger diameter wires with shorter embedment lengths. A 3-D non-linear finite element model (FEM) was developed to predict the pull-out behavior. A cohesive zone model (CZM) was used to model the interface. A parametric study was conducted using the FEM to quantify the parameters of the cohesive zone model. The results demonstrate that the proposed modeling approach can be used to characterize the bond behavior of superelastic SMA wires embedded in FRP composites.     

Shape memory and thermo-mechanical properties of shape memory polymer/carbon fiber composites

In this study, chopped carbon fiber reinforced trans-1, 4-polyisoprene (TPI) was developed via a proposed new manufacturing process with the aim of improving weak mechanical properties of bulk TPI bulk. Specimens of the developed shape memory polymer (SMP) composites were fabricated with carbon fiber weight fraction of 5%, 7%, 9%, 11% and 13%, respectively. Measured are the effects of chopped carbon fiber and temperature on: (a) shape recovery ratio and rate; (b) stress–strain relationship; (c) maximum tensile stress, strain and Young’s modulus; and (d) maximum stress and residual strain under a constant strain cyclic loading. In addition, SEM micrographs were also presented to illustrate the fracture surface. The present experimental results show that the SMP with 7% carbon fiber weight fraction appears to perform best in all the tests. This indicates that the 7% carbon fiber weight fraction could be the optimum value for the SMP developed using the proposed manufacturing process.     

A dual-functional polymeric system combining shape memory with self-healing properties

With the aim of seeking a convenient way for integrating functional materials, a polymeric system, presenting both self-healing property and shape memory behavior, was proposed and constructed based on epoxy based shape memory polymer (ESMP) and poly (ε-caprolactone) (PCL). The synthesis principle of PCL–ESMP composite was based on phase separation phenomenon between the two ingredients. Such phase separated PCL–ESMP composite reserved melting transition of PCL and glassy transition of ESMP, respectively, which was the crucial mechanism for achieving self-healing performance and shape memory behavior. A bending-recovery experiment demonstrated that PCL–ESMP composite possessed excellent thermal-induced dual-shape memory effect. Meanwhile, single edge notched bend testing revealed that such composite exhibited desirable self-healing performance as well. This article introduced a simple contrivable concept and exhibited some experimental results of the PCL–ESMP dual-functional composite system. The promising applications are expected to more widely, such as functional composite matrix and intelligent structures.     

Structural capacitor materials made from carbon fibre epoxy composites

In this paper, an approach towards realising novel multifunctional polymer composites is presented. A series of structural capacitor materials made from carbon fibre reinforced polymers have been developed, manufactured and tested. The structural capacitor materials were made from carbon fibre epoxy pre-preg woven laminae separated by a paper or polymer film dielectric separator. The structural capacitor multifunctional performance was characterised measuring capacitance, dielectric strength and interlaminar shear strength. The developed structural CFRP capacitor designs employing polymer film dielectrics (PA, PC and PET) offer remarkable multifunctional potential.     

An investigation on shape memory behaviours of hydro-epoxy/glass fibre composites

Shape-memory polymers (SMPs) have the capacity to return large strains by external stimuli. Among various SMPs, shape-memory epoxy has received considerable attention because of its superior mechanical and thermal properties as well as excellent shape-memory performance. In this study, short glass fibre-reinforced shape-memory hydro-epoxy composites are developed to improve further the mechanical property of shape-memory epoxy resin. The thermomechanical and shape-memory properties of the developed composite materials are investigated by dynamic mechanical analysis, bend test and shape recovery test. The results indicate that the glass modulus and bend strength of the developed composite materials initially increase and then slightly decrease with increasing short glass fibre content. The glass transition temperature of the developed composite materials does not change with increasing short glass fibre. When the short glass fibre content is less than 4.5 wt.%, full recovery can be observed after only several minutes at different temperatures. The shape-memory property of the composite materials is not affected greatly. However, when the short glass fibre content is more than 4.5 wt.%, the material would be destroyed after deformation.     

Shape memory polymer based self-healing syntactic foam: 3-D confined thermomechanical characterization

In this study, the thermomechanical behavior of a shape memory polymer (SMP) based syntactic foam under three-dimensional (3-D) confinement was investigated through strain-controlled programming and fully confined shape recovery tests. The 3-D confinement was created by encasing the foam in circular confining tubes and subjecting the foam cylinder to uniaxial compression. The parameters investigated included two programming temperatures, three types of confining tubes with varying lateral confinements, three prestrain levels, and one fully-confined recovery condition. A three-layer plane-stress analytical model was also developed to estimate the volume change of the specimen by prestressing. It is found that the stress recovery ratio is the highest with rubber liner and the recovered stress is the highest with nylon liner. The stress recovered in the foam specimen which is confined by the nylon liner is as high as 26 MPa, making it possible as actuators. While volume reduction during programming is the key for the foam to self-close cracks, the volume reduction must be within a certain limit; otherwise, the foam loses its shape memory functionality.     

Platelet-reinforced polymer matrix composites by combined gel-casting and hot-pressing. Part I: Polypropylene matrix composites

Platelet-reinforced polymer matrix composites were fabricated by a combined gel-casting and hot-pressing method. Submicrometer-thin alumina platelets were dispersed in a highly diluted grafted maleic anhydride polypropylene solution. Upon cooling, the polymer formed a gel which trapped the platelets in their well separated positions. During subsequent solvent evaporation, the polymer–platelet gel densified and the platelets were oriented horizontally. The dried composites were hot-pressed to further improve the platelet orientation and increase the density of entanglements in the polymer. This method combines several advantages of large scale and lab-scale fabrication methods in that it is fast, simple but also versatile. Composites with platelet volume fractions up to 0.5 were easily fabricated. The maximal achieved yield strength and elastic modulus of the composites were 82% and 13 times higher, respectively, than the values of the polymer alone. The enhancement in the composites mechanical properties was caused by classical load transfer into the platelets as the crystallinity of the polymeric matrix was not affected by the platelets. Alumina platelets with an aspect ratio below the critical value allowed for the ductile platelet pull-out fracture mode enabling large plastic deformation of the composites prior to fracture. At high concentrations of platelets, the strength and stiffness decreased again and the ductility was almost lost due to out-of-plane misalignment of platelets and the increasing number and size of voids incorporated during the fabrication. The designing principles and fabrication method described in this work can potentially be extended to other types of polymers and platelets to create new composites with tailored properties.     

Platelet-reinforced polymer matrix composites by combined gel-casting and hot-pressing. Part II: Thermoplastic polyurethane matrix composites

A combined gel-casting and hot-pressing method was used to fabricate platelet-reinforced polymer matrix composites. Submicrometer thin alumina platelets were dispersed in a highly diluted polymer solution. A thermoplastic polyurethane elastomer was used as matrix for its high elasticity and excellent adhesion to the platelets. After dissolution of the polymer and casting, quick evaporation of the solvent triggered the formation of a polymer gel trapping the platelets in their well dispersed positions. The polymer–platelet gel densified during drying and the platelets were oriented horizontally due to the capillary forces and the large decrease in the thickness of the gel. The dried composites were hot-pressed to further improve the platelet orientation along the shear flow and close potential pores in the polymer. While the ultimate tensile strength of the composites gradually decreased with increasing platelet volume fractions, the increase in the elastic modulus and the stress necessary to deform the composite 10% was more than 100 and 18 times higher than the respective values of the pure polymer. The use of alumina platelets with an aspect ratio below the critical value allowed for the ductile platelet pull-out fracture mode. Since the polymer had to deform more to achieve identical deformation of the composite at higher platelet volume fraction, the strain at rupture steadily decreased. The incorporation of voids towards high platelet concentrations and the thereby triggered crack initiation and growth during straining lead to an additional decrease in the elasticity of composites with increasing platelet volume fractions. However, the extremely high extensibility of a polymer matrix allowed for the fabrication of composites that still deformed up to 162 ± 19% at platelet volume fractions as high as 0.33. When compared to other platelet-reinforced elastomers, the achieved platelet volume fraction is much higher and the relative increase in elastic modulus and stress at low strains is therefore much larger at the expense of a decrease in the strain at rupture. The fabrication method and designing principles employed in this study are transferable to other types of polymers and platelets and potentially allow the creation of new composites with tailored properties.     

Thermal post-buckling and flutter characteristics of composite plates embedded with shape memory alloy fibers

It is investigated that the composite plate embedded with shape memory alloy (SMA) fibers is subject to the aerodynamic and thermal loading in the supersonic region. The nonlinear finite element equations based on the first-order shear deformation plate theory (FSDT) are formulated for the laminated composite plate embedded with SMA fibers (SMA composite plate). The von Karman strain–displacement relation is used to account for the large deflection. The incremental method considering the influence of the initial deflections and initial stresses is adopted for the temperature-dependent material properties of SMA fibers and composite matrix. The first-order piston theory is used for modeling aerodynamic loads. This study shows the effect of the SMA on the critical temperature, thermal post-buckling deflection, natural frequency and critical dynamic pressure of the SMA composite plate.     

Thermosetting epoxy reinforced shape memory composite microfiber membranes: Fabrication,structure and properties

The thermosetting epoxy-based shape memory composite microfibers are successfully fabricated by means of coaxial electrospinning. The PCL/epoxy composite fiber shows core/shell structure, in which epoxy as the core layer is for an enhancing purpose. By incorporating epoxy and PCL, the mechanical strength of composite fibers is greatly reinforced. The deformation is via the heating and cooling process, and the shape memory effect can be demonstrated from the micro level to the macro level. The whole shape recovery performance takes only 6.2 s when triggered by the temperature being at 70 °C. The porosity of woven microfibers changes in response to temperature. In addition, the PCL/epoxy composite microfiber membranes are analyzed in an in vitro cytotoxicity test, which proves that PCL as the shell layer provides the composite microfibers potential capabilities in biomedical science.     

The effect of ultra-thin graphite on the morphology and physical properties of thermoplastic polyurethane elastomer composites

Composites of thermoplastic polyurethane (TPU) and ultra-thin graphite (UTG) with concentrations ranging from 0.5 wt.% to 3 wt.% were prepared using a solution compounding strategy. Substantial reinforcing effects with increased loadings are achieved. Compared to neat TPU, values for storage modulus and shear viscosity are enhanced by 300% and 150%, respectively, for UTG concentrations of 3 wt.%. Additionally, an enhancement of thermal properties is accomplished. The crystallization temperature and thermal stability increased by 30 °C and 10 °C, respectively, compared to neat TPU. Furthermore, the use of oxidized UTG (UTGO) with its added functional oxygen groups suggests the presence of chemical interactions between UTG and TPU, which additionally impact on the thermal properties of the corresponding composites. Controlling the oxidation degree, thus offers further possibilities to obtain composites with tailored properties. The presented approach is straightforward, leads to homogeneous TPU-UTG composites with improved materials properties and is especially suitable for commercial UTG materials and further up-scaled production.     

Effect of reinforcing layer on shape fixity and time-dependent deployment in shape-memory polymer textile composites

The purpose of the present study is to model shape fixity and time-dependent deployment in shape-memory polymer composites (SMPCs) and to evaluate the effect of textiles’ tensile and bending moduli on these properties. We constructed an SMPC model by combining SMP layers and a reinforcing layer. We also considered the thermo-viscoelasticity of SMP and the difference in values between the tensile and bending moduli of the reinforcing layer. Employing this model, we simulated deployment under pure bending conditions. Comparison with experimental results confirmed that our proposed model is able to simulate shape fixity and time-dependent deployment in SMPCs. We also confirmed that the bending modulus is an important factor for shape fixity and time-dependent deployment, whereas the tensile modulus has nothing to do with these properties.     

Sensing of damage and healing in three-dimensional braided composites with vascular channels

With the rise of composite materials as replacements for traditional monolithic materials comes an increase in demand for multifunctionality. Prior studies have demonstrated the ability of an embedded, electrically percolating carbon nanotube network to respond electrically to the onset and progression of damage in composite structures. We build upon this work by incorporating healing functionality into braided composites through the use of a hollow channel resin delivery system. This study demonstrates the ability of a carbon nanotube network to sense crack filling during resin injection, thus providing the scientific basis required for sensing healing in advanced composites. With practical application in mind, a two-part healant system is employed in this study. Two methods for qualitatively assessing healing are employed and compared; these include elastic modulus/strain energy recovery and FTIR spectroscopy.     

Dynamic response and simulations of nanoparticle-enhanced composites

The dynamic modulus and damping, low-velocity impact and high-strain (Hopkinson bar) response of nanoparticle-enhanced composites and fly ash based fire resistant structural foams have been characterized. Molecular Dynamic (MD) simulations were also used for obtaining the elastic constants for different matrices reinforced with single and multi-wall carbon nanotubes (MWCNT). Experimental results on the dynamic and impact response of nylon 6,6 with MWCNT, vinyl ester with nanoclay and graphite platelets, and Eco-Core^® foams are presented; along with some of the predictions from molecular simulations.