Showing papers by "Yanju Liu published in 2009"

Fiber reinforced shape-memory polymer composite and its application in a deployable hinge

Abstract: This paper investigates the shape recovery behavior of thermoset styrene-based shape-memory polymer composite (SMPC) reinforced by carbon fiber fabrics, and demonstrates the feasibility of using an SMPC hinge as a deployable structure. The major advantages of shape-memory polymers (SMPs) are their extremely high recovery strain, low density and low cost. However, relatively low modulus and low strength are their intrinsic drawbacks. A fiber reinforced SMPC which may overcome the above-mentioned disadvantages is studied here. The investigation was conducted by three types of test, namely dynamic mechanical analysis (DMA), a shape recovery test, and optical microscopic observations of the deformation mechanism for an SMPC specimen. Results reveal that the SMPC exhibits a higher storage modulus than that of a pure SMP. At/above Tg, the shape recovery ratio of the SMPC upon bending is above 90%. The shape recovery properties of the SMPC become relatively stable after some packaging/deployment cycles. Additionally, fiber microbuckling is the primary mechanism for obtaining a large strain in the bending of the SMPC. Moreover, an SMPC hinge has been fabricated, and a prototype of a solar array actuated by the SMPC hinge has been successfully deployed.

Review of electro-active shape-memory polymer composite

Abstract: Shape-memory polymers (SMPs) have been one of the most popular subjects under intensive investigation in recent years, due to their many novel properties and great potential. These so-called SMPs by far surpass shape-memory alloys and shape-memory ceramics in many properties, e.g., easy manufacture, programming, high shape recovery ratio and low cost, and so on. However, they have not fully reached their technological potential, largely due to that the actuation of shape recovery in thermal-responsive SMPs is normally only driven by external heat. Thus, electro-activate SMP has been figured out and its significance is increasing in years to come. This review focuses on the progress of electro-activate SMP composites. Special emphases are given on the filler types that affect the conductive properties of these composites. Then, the mechanisms of electric conduction are addressed.

Shape-Memory Polymers—A Class of Novel Smart Materials

Abstract: Shape-memory polymers (SMPs) offer a number of potential technical advantages that surpass other shape-memory materials such as shape-memory metallic alloys and shape-memory ceramics. The advantages include high recoverable strain (up to 400%), low density, ease of processing and the ability to tailor the recovery temperature, programmable and controllable recovery behavior, and more importantly, low cost. This article presents the state-of-the-art regarding SMPs. First, the architecture, type, and main properties of the traditional and recently developed SMPs are introduced. Second, structural and multifunctional SMP composites are summarized and discussed. These composites greatly enhance the performance of the SMPs and widen their potential applications. Finally, current applications of SMP materials in aerospace engineering, textiles, automobiles, and medicine are presented.

Infrared light‐active shape memory polymer filled with nanocarbon particles

Abstract: In addition to the fabrication of thermoset styrene-based shape memory polymer (SMP) and its nanocomposite filled with nanocarbon particles, this study presents the effect of nanocarbon particles on infrared light-active shape recovery behaviors of this type of SMP material. The experimental results reveal that both pure SMP and SMP filled with nanocarbon par- ticles can be actuated by infrared light in vacuum, while shape memory effect shown by the composite is stronger than that of in pure SMP. Shape memory effect is eval- uated by shape recovery ability and shape recovery speed in detail. Moreover, factors which would influence the infrared light-active shape memory effect in SMP with/without nanocarbon particles are explored by scan- ning electron microscopy (SEM), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and infrared absorption characteristic tests. The better shape memory effect of the nanocomposite attributes to its higher storage modulus and higher infrared absorp- tion capability. V

Electromechanical stability of dielectric elastomer

Abstract: Electromechanical instability may occur in dielectric elastomer films due to the coupling between mechanical forces and electric fields. According to Zhao and Suo [Appl. Phys. Lett. 91, 061921 (2007)], free-energy in any form, which consists of elastic strain energy and electric energy, can be used to analyze the electromechanical stability of dielectric elastomer. By taking the permittivity as a variable depending on the deformation in a free energy function, a relationship is established among critical nominal electric field, critical real electric field, nominal stress, and principal stretch ratios. The accurate expressions of these parameters are presented for a special equal biaxial stretch case. All the results obtained by utilizing the single material constant neo-Hookean elastic strain energy model coincide with the conclusions by Zhao and Suo.

Effect of a linear monomer on the thermomechanical properties of epoxy shape-memory polymer

Abstract: The effect of a linear monomer on thermomechanical properties and shape recovery behavior of an epoxy shape-memory polymer is studied. These shape-memory polymers are prepared from epoxy base resin, hardener and linear epoxy monomer. As the content of the linear monomer increases, the glass transition temperature (Tg) determined using differential scanning calorimetry ranges from 37 to 96 °C. A decrease in rubber modulus is seen from dynamic mechanical analysis for the polymers, which reveals decreasing crosslink density with increasing linear monomer content. Tensile test results show that the elongation at break and strength depends on the content of linear monomer at Tg or Tg−20 °C, while the linear monomer content has minor influence on elongation at break and strength at Tg+20 °C. Finally, investigation on shape recovery behavior reveals that full recovery can be observed for each polymer when the temperature is equal to or above Tg. Also, increasing the linear monomer content results in a decrease in both shape recovery ratio (below Tg) and shape recovery speed (at Tg). These results are interpreted in terms of various crosslink densities attributed to the increasing linear monomer content.

Dielectric elastomer film actuators: characterization, experiment and analysis*

Abstract: Silicone is a common dielectric elastomer material. Actuators made from it show excellent activation properties including large strains (up to 380%), high energy densities (up to 3.4 J g−1), high efficiency, high responsive speed, good reliability and durability, etc. When a voltage is applied on the compliant electrodes of the dielectric elastomers, the polymer shrinks along the electric field and expands in the transverse plane. In this paper, a silicone dielectric elastomer is synthesized and the area strains are tested under different electric fields. Pre-strain and a certain driving electric field are applied to the film and the induced large strain by the Maxwell stress is measured. Barium titanate (BaTiO3) was incorporated into the silicone to fabricate a new dielectric elastomer: the experimental results show that the elastic modulus and dielectric constant were significantly improved. The experimental results coincide well with those of finite element analysis at a large deformation. Also, a theoretical analysis is performed on the coupling effects of the mechanical and electric fields. A nonlinear field theory of deformable dielectrics and hyperelastic theory are adopted to analyze the electromechanical field behavior of these actuators. Also the mechanical behavior of the dielectric elastomer undergoing large free deformation is studied. Finally, the constitutive model of a dielectric elastomer composite under free deformation and restrained deformation is derived.

Qualitative separation of the effect of the solubility parameter on the recovery behavior of shape-memory polymer

Abstract: For the thermal response of shape-memory polymer (SMP) it has been experimentally demonstrated that the actuation can be achieved using interactive solvent. In this paper, the effect of the solubility parameter of the interactive solvent on the shape recovery behavior and glass transition temperature of polystyrene SMP was investigated experimentally. The effect of the solubility parameter on the activation energy was separated and qualitatively identified, as expected from the relaxation theory and Eyring equation. This approach should be applicable to other SMPs and their composites and many extension applications and achievements could be based on this outcome.

Electroactive thermoset shape memory polymer nanocomposite filled with nanocarbon powders

Abstract: This paper concerns an electroactive thermoset styrene-based shape memory polymer (SMP) nanocomposite filled with nanosized (30?nm) carbon powders. With an increase of the incorporated nanocarbon powders of the SMP composite, its glass transition temperature (Tg) decreases and storage modulus increases. Due to the high micro-porosity and homogeneous distributions of nanocarbon powders in the SMP matrix, the SMP composite shows good electrical conductivity with a percolation of about 3.8%. This percolation threshold is slightly lower than that of many other carbon-based conductive polymer composites. Consequently, due to the relatively high electrical conductivity, a sample filled with 10?vol% nanocarbon powders shows a good electroactive shape recovery performance heating by a voltage of 30?V above a transition temperature of 56?69??C.

An investigation on electromechanical stability of dielectric elastomers undergoing large deformation

Abstract: Dielectric elastomers are one of the important electroactive polymers used as actuators in adaptive structures due to their outstanding ability to generate very large deformations when subjected to an external electric field. In this paper, the Mooney–Rivlin elastic strain energy function with two material constants is used to analyze the electromechanical stability performance of a dielectric elastomer. This elastic strain energy together with the electric energy incorporating linear permittivity are the main items to construct the free energy of the system. Particular numerical results are also calculated for a further understanding of the dielectric elastomer's typical stability performance. The proposed model offers great help in guiding the design and fabrication of actuators featuring dielectric elastomers.

A glass transition model for shape memory polymer and its composite

Abstract: As novel smart materials, shape memory polymer (SMP) and its composite (SMPC) have the ability to regain its original shape after undergoing significant deformation upon heating or other external stimuli such as light, chemic condition and so on. Their special behaviors much depends on the glass transitions due to the increasing of material temperature. Dynamic Mechanical Analysis (DMA) tests are performed on the styrene-based SMP and its carbon fiber fabric reinforced SMPC to investigate their glass transition behaviors. Three glass transition critical temperatures of SMP or SMPC are defined and a method to determine their values from DMA tests is supposed. A glass transition model is developed to describe the glass transition behaviors of SMP or SMPC based on the results of DMA tests. Numerical calculations illustrate the method determining the glass transition critical temperature is reasonable and the model can well predict the glass transition behaviors of SMP or SMPC.

Design and analysis of morphing wing based on SMP composite

Abstract: A new concept of a morphing wing based on shape memory polymer (SMP) and its reinforced composites is proposed in this paper. SMP used in this study is a thermoset styrene-based resin in contrast to normal thermoplastic SMP. During heating, the wing curled on the aircraft can be deployed, providing main lift for a morphing aircraft to realize the stable flight. Aerodynamic characteristics of the deployed morphing wing are calculated by using CFD software. The static deformation of the wing under the air loads is also analyzed by using the finite element method. The results show that the used SMP material can provide enough strength and stiffness for the application. Finally, preliminary testing is conducted to investigate the recovery performances of SMP and its reinforced composites. During the test, the deployment and the wind-resistant ability of the morphing wing are dramatically improved by adding reinforced phase to the SMP.

Stability analysis of dielectric elastomer film actuator

Abstract: Dielectric elastomer (DE) is the most promising electroactive polymer material for smart actuators. When a piece of DE film is sandwiched between two compliant electrodes with a high electric field, due to the electrostatic force between the two electrodes, the film expands in-plane and contracts out-of-plane so that its thickness becomes thinner. The thinner thickness results in a higher electric field which inversely squeezes the film again. When the electric field exceeds the critical value, the dielectric field breaks down and the actuator becomes invalid. An elastic strain energy function with two material constants is used to analyze the stability of the dielectric elastomer actuator based on the nonlinear electromechanical field theory. The result shows that the actuator improves its stability as the ratio k of the material constants increases, which can be applied to design of actuators. Finally, this method is extended to study the stability of dielectric elastomers with elastic strain energy functions containing three and more material constants.

Comment on: On electromechanical stability of dielectric elastomers

Abstract: †Appl. Phys. Lett. 93, 101902 „2008...‡ Yanju Liu, Liwu Liu, Shouhua Sun, Liang Shi, and Jinsong Leng Department of Astronautical Science and Mechanics, Harbin Institute of Technology (HIT), P.O. Box 301, No. 92 West dazhi Street, Harbin 150001, People’s Republic of China Centre for Composite Materials, Science Park of Harbin Institute of Technology (HIT), P.O. Box 3011, No. 2 YiKuang Street, Harbin 150080, People’s Republic of China

Comment on “On electromechanical stability of dielectric elastomers” [Appl. Phys. Lett. 93, 101902 (2008)]

Abstract: We would like to thank Diaz-Calleja et al. [Appl. Phys. Lett.93, 101902 (Year: 2008)] for their insight and help on “On electromechanical stability of dielectric elastomers;” unstable domain of electromechanical coupling system of neo-Hookean-type silicone was analyzed by Diaz-Calleja et al. Different from that given in the paper of Diaz-Calleja, in the current work, the elastic strain energy function with two material constants was used to analyze the stable domain of electromechanical coupling system of Mooney–Rivlin-type silicone, and the results seem to support the theory of Diaz-Calleja.

A macro-mechanical constitutive model of shape memory alloys

Abstract: It is of practical interest to establish a precise constitutive model which includes the equations describing the phase transformation behaviors and thermo-mechanical processes of shape memory alloy (SMA). The microscopic mechanism of super elasticity and shape memory effect of SMA is explained based on the concept of shape memory factor defined by the author of this paper. The conventional super elasticity and shape memory effect of SMA are further unified as shape memory effect. Shape memory factor is redefined in order to make clear its physical meaning. A new shape memory evolution equation is developed to predict the phase transformation behaviors of SMA based on the differential relationship between martensitic volume fraction and phase transformation free energy and the results of DSC test. It overcomes the limitations that the previous shape memory evolution equations or phase transformation equations fail to express the influences of the phase transformation peak temperatures on the phase transformation behaviors and the transformation from twinned martensite to detwinned martensite occurring in SMA. A new macro-mechanical constitutive equation is established to predict the thermo-mechanical processes realizing the shape memory effect of SMA from the expression of Gibbs free energy. It is expanded from one-dimension to three-dimension with assuming SMA as isotropic material. All material constants in the new constitutive equation can be determined from macroscopic experiments, which makes it more easily used in practical applications.

Silicone dielectric elastomers filled with carbon nanotubes and actuator

Abstract: Dielectric elastomers (DEs) are one particular type of electroactive polymers. The excellent features of merit possessed by dielectric elastomers make them the most performing materials which can be applied in many domains: biomimetics, aerospace, mechanics, medicals, etc. In order to maximize actuator performance, the dielectric elastomer actuators should have a high dielectric constant and high dielectric breakdown strength. In this paper, multi-walled carbon nanotube (MWNT) is used to develop a particulate composite based on silicone elastomer matrix, with dielectric permittivity improved. And the composite is designed to a new configuration of dielectric elastomer actuator to show electrically activated linear contractions. Prototype samples of this folded actuator, along with the fabrication and analysis is discussed here.

Study on Thermo-Mechanical Behaviors of Shape Memory Polymer

Abstract: Dynamic mechanical analysis (DMA) tests are conducted on styrene-based shape memory polymer(SMP) to investigate its glass transition behaviors. The tensile tests at various temperatures are operated to detect the stress-strain relationship of styrene-based SMP. The material elastic moduli and yielding limits at 25oC, 30oC, 40oC, and 50oC are determined according to the results of tensile test. A new material parameter function is supposed to express the glass transition behavior of styrene-based SMP. The shape memory thermo-mechanical cycle of styrene-based SMP is numerically simulated by Tobushi’s constitutive equation coupled with the new material parameter function. Numerical results show the new material parameter function can express the thermo-mechanical properties of styrene-based SMP effectively.

Stability analysis of dielectric elastomer using the elastic strain energy function with two material constants

Abstract: Dielectric elastomers (DE) are the most promising electroactive polymer materials capable of being applied in smart actuators. When the DE film sandwiched between two compliant electrodes is applied high electric field, due to the electrostatic force between two electrodes, the film expands in-plane and contracts out-of-plane such that its thickness becomes thinner. The thinner thickness results in higher electric field which inversely squeezes the film again. This positive feedback induces a mode of instability, known as electromechanical instability or pull-in instability. When the electric field exceeds certain critical value, the DE film collapses. In this paper, the elastic strain energy function with two material constants is applied to analyze the stability of dielectric elastomers, which facilitates to understand fully Suo's nonlinear theory. The results verify again the truth of this theory and exploit larger application spectrum. The method is capable of analyzing the stability of different dielectric materials with different values of k and the result can be useful on design of the dielectric elastomer actuator.

Modeling the shape memory effect of shape memory polymer

Abstract: Dynamic mechanical analysis (DMA) tests are conducted on the styrene-based shape memory polymer (SMP) to investigate its state transition behaviors. Tensile tests at various constant temperatures are carried out to reveal the stressstrain- temperature relationship of the styrene-based SMP. A new mechanical constitutive equation is developed to describe the stress-strain-temperature relationship of the styrene-based SMP. Numerical calculations illustrate the proposed theory well describes the thermo-mechanical cycle of shape memory of styrene-based SMP, such as deformation at high temperature, shape fixity, unloading at low temperature and shape recovery.

Dielectric properties of carbon nanotube/silicone elastomer composites

Abstract: Dielectric elastomers have received a great deal of attention recently for effectively transforming electrical energy to mechanical work. Their large strains and conformability make them enticing materials which can be applied in many domains: biomimetics, aerospace, mechanics, medicals, etc. In order to maximize actuator performance, the dielectric elastomer actuators should have a high dielectric constant and high dielectric breakdown strength. Here we have investigated the increase in permittivity of a commercial silicone elastomer by the addition of carbon nanotube. The percolation threshold of the composites is obtained to be low. Experimental results suggest that for the case of conductive filler particle-elastomer matrix interaction, actuation strain increases with increasing carbon nanotube content.

Novel deployable morphing wing based on SMP composite

Abstract: In this paper, a novel kind of deployable morphing wing base on shape memory polymer (SMP) composite is designed and tested. While the deployment of the morphing wing still relies on the mechanisms to ensure the recovery force and the stability performance, the deploying process tends to be more steady and accurate by the application of SMP composite, which overcomes the inherent drawbacks of the traditional one, such as harmful impact to the flight balance, less accuracy during the deployment and complex mechanical masses. On the other hand, SMP composite is also designed as the wing's filler. During its shape recovery process, SMP composite stuffed in the wing helps to form an aerofoil for the wing and withstand the aerodynamic loads, leading to the compressed aerofoil recovering its original shape. To demonstrate the feasibility and the controllability of the designed deployable morphing wing, primary tests are also conducted, including the deploying speed of the morphing wing and SMP filler as the main testing aspects. Finally, Wing's deformation under the air loads is also analyzed by using the finite element method to validate the flight stability.

Advances in shape-memory polymer actuation

Abstract: Shape memory polymer (SMP) is a promising smart material, which is able to perform a large deformation upon applying an external stimulus, such as heat, light and moisture, etc. In recent years, many investigations have been advanced in thermo-responsive SMP actuation, and several novel actuations have been applied in SMP. In this paper, the mechanism and demonstration of three types of SMP actuations (infrared laser, physical swelling effect and electricity) are presented. These novel actuation approaches may help SMP to fully reach its potential application. Firstly, for the infrared laser-activated SMP, it is concerned about the drive of SMP by infrared light. The infrared laser, transmitted through the optical fiber embedded in the SMP matrix, was chosen to drive the SMP. The working frequency of infrared laser was installed in 3-4μm. Moreover, this paper presents a study on the effects of solution on the glass transition temperature (T g ). It shows that the hydrogen bonding of SMP was aroused by the absorbed solution that significantly reduces transition temperature of polymer. In this way, the shape memory effect (SME) can undergo solution-driven shape recovery. Finally, the actuation of two types of electro-active SMP composites filled with electrically conductive powders (carbon black, nickel powers) have been carried out, and the SMP composite can be driven by applying a relatively low voltage.

Electrically conductive shape-memory polymer filled with Ni powder chains

Abstract: The electrical resistivity of a thermoset styrene-based shape-memory polymer (SMP) filled with Ni powders is investigated in this paper. We demonstrate a simple approach to improve the electrical conductivity of thermo-responsive shape-memory polymers (SMPs), so that they can be easily triggered for shape recovery by Joule heating at a low electrical voltage. After adding a small amount of Ni particles into a styrene-based SMP, the electrical resistivity is reduced. Furthermore, if these Ni particles are aligned into chains, by applying a low magnetic field on SMP/Ni solution and then drying to fix the conductive chains, the drop of electrical resistivity is more significant. Due to the improvement of electrical conductivity of the SMP composite, it is more suitable for Joule heat induced shape recovery.

A novel embedded fiber optic acoustic emission sensor and its applications for monitoring failures of composite laminates

Abstract: A fiber optic acoustic emission sensor based on fused-tapered coupler and its applications in structural health monitoring are proposed in this paper. The sensor was embedded into the Carbon Fiber Reinforced Polymer (CFRP) laminates and tested using pencil lead break tests compared with the commercial acoustic emission sensor (R15 PZT). It successfully detected the AE signals. FOAES was applied in the Structural Health Monitoring (SHM) of CFRP materials. Failures of carbon fiber/epoxy composite laminates during three-point- bending test were monitored embedded FOAES. Results identified that the sensor embedded into composite structures successfully monitored failures of composite laminates online.

Swelling effect actuation of shape-memory polymer: mechanism and demonstration

Abstract: Recently, there is increasing interest in triggering shape recovery of shape-memory polymers (SMPs) by novel inductive effect In this paper, many hard works have been carried out to make SMP induced while along with swelling effect Based on the Free-volume theory, Rubber Elasticity Theory and Mooney-Rivlin Equation, it is theoretically and experimentally demonstrated the feasibility of SMP activated by swelling effect The mechanism behind it is solvent acting as plasticizer, to reduce the glass transition temperature (Tg) and melting temperature (Tm) of polymers, make them softer and more flexible, facilitating the diffusion of the molecules to polymer chains, and then separating them In addition to this physical action, the intermolecular interactions among the chains are weakened, because interactions are hindered at the points where the plasticizer is located Finally, the Dynamic mechanical analysis (DMA), FTIR study and glass transition temperature measurement tests were used to exemplify the feasibility of SMP driven by swelling effect And it is qualitatively identified the role of swelling effect playing in influencing the transition temperature Swelling effect occurs due to the interaction between macromolecules and solvent molecules, leading to free volume of polymeric chains increasing (namely the flexibility of polymer chains increasing), resulting in the Tg decreasing All above mentioned investigation can be used to confirm that the shape recovery is induced by swelling effect This actuation almost is applicable for all the SMP and SMP composite, as the swelling theory is almost applicable for all the polymeric materials