Article

High performance carbon fibre reinforced epoxy composites with controllable stiffness

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Abstract

The mechanical properties of polystyrene-interleaved carbon fibre reinforced epoxy composites, which exhibit controllable stiffness, have been investigated. DMTA and flexural tests showed that the storage modulus and flexural stiffness of these composites could be reduced by up to 98% when heated from 20 °C to 120 °C and the stiffness was fully recoverable on cooling. The flexural stiffness of the interleaved composites at room and elevated temperatures were predicted using simple beam theory and were found to be in good agreement with the measured values. Compressive and tensile properties were significantly reduced at 120 °C due to the presence of the softened polystyrene interleaves. Flexural strength tests at 20 °C indicate that there is a need for improvement of the adhesion between polystyrene and carbon fibre reinforced epoxy plies.

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... Ply slip can subsequently be controlled by the dielectric constant of the material (ε r ), its thickness (d), the applied electric potential (V) and coefficient of friction (μ). σ = ε r ε 0 V 2 2d 2 (4) τ = μσ (5) Conversely, structural morphing composites utilising active slip typically incorporate thermoplastic (TP) layers as interleaves between composite plies to induce slip when the service temperature exceeds T g of the interleaf during the phase change from stiff to flexible [145,199,[236][237][238][239][240][241][242][243][244][245][246] (Fig. 8). Balsa wood plies interleaved with a hot glue (TP) layer and aluminium plies interleaved with TP polymers embedded with a nichrome wire heating element and electric heating blankets, respectively, are simple examples that can illustrate the potential of structural morphing composites with controllable stiffness [246,247]. ...
... Balsa wood plies interleaved with a hot glue (TP) layer and aluminium plies interleaved with TP polymers embedded with a nichrome wire heating element and electric heating blankets, respectively, are simple examples that can illustrate the potential of structural morphing composites with controllable stiffness [246,247]. More recently, extensive studies investigating CFPR plies as interleaves with polystyrene (PS) or poly(styrene-co-maleic anhydride) (SMP) TPs identified possible flexural modulus reductions of up to 98% resulting from ply slip [243][244][245]248,249]. ...
... No data was used for the research described in the article. [243] and Kuder, Arrieta, Raither and Ermanni [175]. ...
Article
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From plants tracking the sun to the aerodynamics of bird wings, shape change is key to the performance of natural structures. After years of reliance on mechanical joints, human engineering now focuses on improving aerodynamic efficiency through smooth, full form changes in material geometry, achieved using technologies such as morphing composites. Promising improved power generation and efficiency in wind turbines and safer more sustainable aircraft and cars, these materials can achieve both large geometric changes with low energy requirements by cycling between several stable physical states and more gradual changes in geometry by exploiting coefficient of thermal expansion mismatch and structural anisotropy, shape memory polymers and 4D printing. The merits and limitations of these various shape change systems are the subject of extensive and ongoing academic research and both commercial and defence industry trials to improve the viability of these technologies for widespread adoption. Shape change capabilities are often associated with problems in material cost, mass, mechanical properties, manufacturability, and energy requirements. Nonetheless, the considerable and rapid advances in this technology, already resulting in successful trials in advanced civilian and military aircraft and high-performance cars, indicate that future research and development of this materials platform could revolutionise many of our most critical power generation, defence and transport systems.
... Controllable stiffness materials possess stiffness that can be changed on demand. The mechanism for the stiffness control in the interleaved configuration, which is the focus of the current paper, was described in our previous work [8,9]. Thermoplastic interleaved carbon fibre reinforced polymer (cfrp) composites are used and when the interleaf material is heated above its glass transition temperature (T g ), the loss in shear stiffness of the interleaf layers allows the unsoftened cfrp plies to slide relative to each other resulting in a reduced flexural stiffness. ...
... McKnight and Barvosa-Carter also patented various variable stiffness structure concepts which included the interleaved configuration [13]. Maples et al. [8,9] conducted experimental investigations of a polystyrene-interleaved carbon fibre reinforced epoxy composite which showed that the flexural stiffness was reduced by over 90% when the interleaved composite was heated to 120 C. (The T g of the polystyrene is approximately 100 C.) In addition to controlling the flexural stiffness, Raither and colleagues were able to demonstrate that the bend-twist coupling could be reduced by a factor 10 in a cfrp multidirectional laminate containing elastomer interleaf layers when heated above the T g of the elastomer [14]. ...
... Flexural stiffness tests were performed on initially straight, polystyrene-interleaved carbon epoxy composite laminates by Maples et al. [8]. It was observed that when testing at 120 C (i.e. higher than the T g of polystyrene) a deflection imposed in a 3-point bend test would be almost fully retained by the specimen if it was cooled down in the deflected state i.e. the material can be re-shaped. ...
Article
Trials have been conducted to investigate the shape memory capability of an interleaved composite consisting of carbon fibre reinforced epoxy laminae and polystyrene interleaf layers. It has been shown that the composite can be readily re-shaped by deforming it at an elevated temperature and then cooling the composite in the deformed state. On re-heating, the composite almost fully returns to its original shape. One potential application of the shape memory capability of the interleaved composite is in deployable structures and a simple structure has been manufactured to demonstrate this possibility.
... The thermoplastic interleaf material is chosen to have glass transition temperature (T g-t ) less than that of the fibre reinforced composite plies (T g-c ). At temperatures lower than T g-t the laminate is in a high flexural stiffness state but when the temperature is increased to above T g-t (but less than T g-c ) the loss of shear stiffness of the interleaf layers allows the constant stiffness plies to slide relative to each other and this results in a reduced flexural stiffness [7,8]. ...
... Maples et al. [7,8] have conducted preliminary experimental investigations of a polystyreneinterleaved carbon fibre reinforced epoxy hybrid composite which have shown that large reductions of over 90% in flexural stiffness are possible when the interleaved composite was heated to 120°C (the T g of the polystyrene was 100 °C). In addition to simply controlling the flexural stiffness, Raither and colleagues [9] were able to demonstrate that the bend-twist coupling could be reduced by a factor 10 in a cfrp multidirectional laminate containing elastomer interleaf layers when heated above the glass transition temperature of the elastomer. ...
... In the flexural stiffness tests on polystyrene interleaved carbon epoxy composite laminates conducted by Maples et al [7,8], it was observed that when testing at 120°C (i.e. higher than the T g of the polystyrene) a deflection imposed in a 3-point bend test would be retained by the specimen if it was cooled down in the deflected state i.e. this material can be re-shaped. When this specimen, now unloaded, was reheated to 120°C it recovered its original straight form. ...
Article
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A hybrid composite, which consists of carbon fibre reinforced epoxy laminae and polystyrene interleaf layers, has been shown to exhibit a significant loss in flexural stiffness on heating. Trials have been performed to demonstrate that the cured hybrid composite can be readily reshaped and that the re-shaped specimens can recover their original shape on heating. One potential application of this shape memory capability is in deployable structures and a simple structure has been manufactured to illustrate this possibility.
... Carbon fibre reinforced epoxy polymer (CFRP) composites interleaved with Polystyrene (PS) have been shown to exhibit controllable flexural stiffness properties [1]- [3]. A significant reversible reduction in flexural stiffness is produced when the composite is heated above the glass transition temperature of PS. ...
... In the current work, we For layups with embedded SS meshes, the meshes were attached to a DC power supply to allow Joule heating of the composites. A DC power input of 30 W was chosen as the power input for Joule heating necessary to reach a surface temperature of around 120°C for controllable stiffness and shape memory studies (120°C is the temperature used in previous works [1]- [4]). In layup A (which was heated in an oven) and in layups B and C (subjected to Joule heating), the temporary loss in flexural stiffness (measured by ASTM D7264-07 standard) due to heating was over 98.5%. ...
... Recently this concept has been expanded into active control via thermal loading [20,21] to obtain controllable stiffness epoxy composites by alternating the plies with a thermoplastic layer [20], as well as by directly coating the fibers with a thermoplastic layer before embedding them into the hosting matrix [21]. The actuation system was modified by applying a controlled force when the configuration change is required (to induce snap-through) or with shape memory alloy wires [22,24] and with piezocomposite actuators [25,26]. ...
... Recently this concept has been expanded into active control via thermal loading [20,21] to obtain controllable stiffness epoxy composites by alternating the plies with a thermoplastic layer [20], as well as by directly coating the fibers with a thermoplastic layer before embedding them into the hosting matrix [21]. The actuation system was modified by applying a controlled force when the configuration change is required (to induce snap-through) or with shape memory alloy wires [22,24] and with piezocomposite actuators [25,26]. ...
Conference Paper
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A shape-adaptable Carbon Fiber Reinforced Composite (CFRC) is proposed to derive a material with tunable mechanical properties in order to optimize its response to external excitations. The composite is bi-stable thanks to internal stresses arising in the manufacturing process and is characterized by a built-in heating system that can control the temperature of the material. This approach allows to gradually change the actual curvature of the material as well as tuning its natural frequencies and damping properties.
... Interleaved composites have previously been shown to display controllable stiffness and shape memory characteristics. In these works, the composites were re-shaped (or programmed) at an elevated temperature after curing, cooled to retain the new shape and then deployed by heating the composites to return them to their cured state [1,2]. A mesh made of interleaved composite which can be deployed from a compact as-cured state into an expanded planar mesh (see Figure 1a) has been previously developed and investigated using FE analysis [3,4]. ...
... By weighing the samples and using the areal weight and density of the reinforcements, the fibre volume fraction of reinforcements in the interleaf was calculated and the theoretical modulus of reinforced thermoplastic films (ERTP in Eq 2) was predicted using the mosaic model discussed by Byström et al [3]. Table 2 Theoretical modulus prediction of reinforced films and interleaved composites Using the simple beam theory technique presented by Maples et al [4] but including the stiffness of the interleaf for the control case at room temperature, the apparent flexural moduli of the interleaved composites in RT and HT states were predicted, and are listed in Table 2. ...
Conference Paper
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Interleaving the plies of carbon fibre reinforced epoxy composites with thermoplastic interleaves have previously been shown to enable these composites to display controllable stiffness and shape memory properties. However, the incorporation of unreinforced thermoplastic interleaves leads to a decrease in flexural modulus of the interleaved composites. In this study, the flexural modulus of composites with reinforced polystyrene interleaves was investigated. The reinforcements used in this study were: (1) stainless steel mesh (SS), (2) unidirectional carbon fabric (UD), (3) woven carbon fabric, (4) woven carbon fabric with epoxy coating and (5) non-woven short carbon fibre mesh. The flexural moduli of the interleaved composites with reinforced interleaves were predicted theoretically and determined experimentally. Among these composites, significant increases in the flexural modulus were achieved in the interleaves with UD, woven and woven+epoxy reinforcements. Additionally, these interleaved composites were shown to retain their controllable stiffness and shape memory properties.
... particles), and they have applied particle dispersion and dissolution based sample preparation processes not lamination, the most conventional composite manufacturing process. On the other hand, Robinson et al. [46,47] applied thick interleaves of polystyrene (PS), which has a similar chemical character to ABS, for developing controllable stiffness FRPC laminates. They confirmed sufficient bonding between the carbon/epoxy and the thermoplastic layers during repeated shape-memory experiments therefore we expect good adhesion between ABS and fibre reinforced epoxy layers. ...
Article
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Improvement of the interfacial fracture toughness of the layer interfaces is one way to increase the performance of interlayer hybrid laminates containing standard thickness carbon/epoxy plies and make them fail in a stable, progressive way. The layer interfaces were interleaved with thermoset 913 type epoxy or thermoplastic acrylonitrile-butadiene-styrene (ABS) films to introduce beneficial energy absorption mechanisms and promote the fragmentation of the relatively thick carbon layer under tensile loads. Carbon layer fragmentation and dispersed delamination around the carbon layer fractures characterised the damage modes of the epoxy film interleaved hybrid laminates, which showed pseudo-ductility in some cases. In the ABS film interleaved laminates, a unique phase-separated ABS/epoxy inter-locking structure was discovered at the boundary of the two resin systems, which resulted in a strong adhesion between the fibre-reinforced and the thermoplastic layers. As a result, the delamination cracks were contained within the ABS interleaf films.
... With the development of current technology and productivity, composite materials are widely employed and gradually becoming a critical composing for structures in the aerospace eld [1,2]. Most structures consist of assembly of individual components, and mechanical bolted joints are required to transfer loads between lots of parts to meet engineering applications. ...
Preprint
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An experimental approach of 5-harness satin woven silicon carbide modification carbon/carbon composites arranged in various geometrical configuration is presented in this paper. Seven types of samples divided into three groups were tested under pin-loading to examine the effects of width-to-hole diameter ratio (W/D), edge distance -to- hole diameter ratio (E/D) and hole diameter-to- thickness ratio (D/t) on the failure mode. To further enhance the understanding of failure propagation, damage mechanism was observed and assessed combining acoustic emission monitoring. From the experimental results and observations, it follows that the net tension and shearing out failure respectively switch to the bearing failure with the increasing ratio of W/D and E/D, while D/t hardly affect the failure mode. Major features of damage mechanism include matrix cracking, fiber buckling and pulling-out, interface debonding, delamination and fiber fracture corresponding to different acoustic emission signal ranges.
... The stretching length is different for an individual material and improves the effectivity of material utilization in a product. Based on these findings, the effective composite module from a macroscopic view can be optimized [72]. ...
Thesis
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This dissertation thesis deals with the topic of external fixators for the healing process of long bone fractures of lower extremity. Nowadays, the most important disadvantages in term of state of the art are high weight, X-ray impermeability during the surgery and too difficult adjustability of external fixator. During the thesis preparation, the research of this topic has been proceeded from the biomechanical, material and engineering point of view. Further, the individual goals have been established directionaly to solve different disadvantages, designed osteosynthesis external fixator using composite material, the unified test has been created. This test serves as a complex and established method of the new fixator design evaluation with the analytical and experimental method application, using deformation analysis, external fixator and composite samples loading during the cyclic tests and gradual loading by the pressure. Results of the unified test indicate, that the new fixator design from the perspective of stress tests of unified method is convenient for the attestation process of this orthopaedic product. These results also confirm that the problems defined from the surgeon perspective are minimalized. The last important finding is an application of this new unified testing method, that can be used even for another fixator development in the future.
... Relatively recently, various promising concepts for material systems with variable stiffness have been presented in the literature (Bachinger, 2015;Bachinger et al., 2014Bachinger et al., , 2015Gandhi and Kang, 2007;Henke and Gerlach, 2016;Maples et al., 2014;Rivas and Barbero, 2016;Tridech et al., 2013;Yuen et al., 2016). Among these are active stiffness control (ASC) approaches for high-performance carbon fiber reinforced polymer (CFRPs; Bachinger, 2015;Bachinger et al., 2014Bachinger et al., , 2015. ...
Article
Recently, novel material concepts for high-performance carbon fiber–reinforced composites with active stiffness control were presented in the literature. Although this new class of intelligent, smart, and responsive materials has wide application potential, actual design concepts using active stiffness control are still rare. The integration of smart materials into conventional products often requires radically new design concepts. This communication presents an innovative automotive hood design concept, which integrates active stiffness control composites in order to achieve improved design performance trade-offs in terms of structural weight reduction and vulnerable road user safety. The integration of active stiffness control composites in the hood structure aims to enable active stiffness reduction of the hood or bonnet structure in order to reduce head impact injuries in case of a collision, while satisfying the structural stiffness requirements and lightweight objectives under normal operating conditions. The design concept is investigated using simulation-based evaluation of static, dynamic, and lightweight design criteria. The results are promising, and the presented concept design is a step toward the realization of lightweight smart hood structures for head impact mitigation. Several design features could also be of interest for the integration of active stiffness control composites, in other applications.
... Composite materials, which are taken from the English term composition materials or shortened to composite materials, is a material made of two or more materials making up mutually having different physical and chemical properties, that will produce a material characteristic different from the materials constituent. Carbon fiber composites are one type of composite material using carbon fiber as one constituent [11][12][13]. ...
Article
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Bulletproof vest serves as a barrier and simultaneously absorbing the impact energy of a projectile shot from a firearm so it could not injure the users. Manufacture of lighter bulletproof vests with good absorbent of impact energy, is expected, because it supports the mobility and safety of its users. In this study, a composite composed of an epoxy matrix with a 16% Hollow Glass Microsphere (HGM) and carbon fiber reinforce to be implemented in a bulletproof vest. The objective of this research is to analyze the bullet-proof vests made of epoxy matrix composites with reinforcement in the form of HGM and carbon fiber through simulation with finite element method. Simulations with Ansys conducted in accordance with NIJ Standard 0101.06 from the U.S. Department of Justice, where the projectile initial velocity of 426 m/s for the category IIIA class weapon with a kinetic energy of projectile amounted to 528.37 Joules. The simulation with Ansys was performed by varying the thickness of bulletproof vests to obtain optimum thickness. The outcome of this research is a bulletproof vest that absorbs the impact energy of the projectile, so that the energy transmitted to the body is smaller than 170 Joules. Having obtaining the optimum thickness of bulletproof vest, an experimental verification will be performed to validate the simulation results. In the simulation results showed that a bullet proof vest with a thickness of 20 mm has been able to meet the standards of Major General Julian S. Hatcher, a U.S. Army ordnance expert with great energy generated at 138.77 Joules.
... Epoxy matrix has been increasingly used in various fields as it has got superior strength for its light in weight and exceptional functional characteristics at elevated temperatures. Composites differ from alloys in a way that they are made from two or more separate materials bonded in such a way as to form one solid piece of material, on the other hand alloys are mixtures of primarily metal atoms which form a continuous solid solution [5][6][7]. ...
Article
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The primary purpose of this study is to analyze, evaluate and compare the mechanical and thermal properties of epoxy based composites with different fiber reinforcements. Fabrication of Glass fiber, carbon fiber and hybrid composites was one by Hand lay-up technique. Tensile test, Three point bend test, Inter-laminar shear test and Compression Test were done on the composite laminates as per the ASTM Standards to obtain mechanical properties such as tensile strength, transverse strength, peak load, compressive strength. Thermal properties such as Glass Transition temperature, melting and decomposition peaks were investigated by Differential Scanning Calorimeter (DSC); Thermo Gravimetric Analysis (TGA) was done to study the decomposition behavior of the composites. Results are tabulated and the comparison is drawn between the laminates.
... [9,10] In FRP systems, adhesion at the fiber/matrix interface is often enhanced via liquid-phase oxidations [11], plasma treatments [12,13], dry gaseous oxidation [14] or electrochemical oxidation [15], in order to improve the composite strength and stiffness [16][17][18]; however, toughness is typically reduced. Interleaved architectures have also been investigated as a means to rebalance composite stiffness [19] and toughness. [1] ...
Article
Conventional fiber-reinforced composites suffer from the formation of critical clusters of correlated fiber breaks, leading to sudden composite failure in tension. To mitigate this problem, an optimized “brick-and-mortar” nanostructured interphase was developed, in order to absorb energy at fiber breaks and alleviate local stress concentrations whilst maintaining effective load transfer. The coating was designed to exploit crack bifurcation and platelet interlocking mechanisms known in natural nacre. However, the architecture was scaled down by an order of magnitude to allow a highly ordered conformal coating to be deposited around conventional structural carbon fibers, whilst retaining the characteristic phase proportions and aspect ratios of the natural system. Drawing on this bioinspiration, a Layer-by-Layer assembly method was used to coat multiple fibers simultaneously, providing an efficient and potentially scalable route for production. Single fiber pull out and fragmentation tests showed improved interfacial characteristics for energy absorption and plasticity. Impregnated fiber tow model composites demonstrated increases in absolute tensile strength (+15%) and strain-to-failure (+30%), as compared to composites containing conventionally sized fibers.
... Interestingly, bistable and interleaved structures have been applied to produce responsive composites by mechanical and thermal activation. [29][30][31][32] Mechanical energy has been used to rupture particulate fillers leading to self-healing and dissipation of stress to inhibit crack propagation. 33,34 More recently, mechanochemically activated polymers that display selfstrengthening behaviour when subjected to shear forces have been developed. ...
Article
Recent developments in smart responsive composites have utilized various stimuli including heat, light, solvents, electricity, and magnetic fields to induce a change in material properties. Here, we report a thermodynamically driven mechanically responsive composite, exploiting irreversible phase-transformation (relaxation) of metastable undercooled liquid metal core shell particle fillers. Thermal and mechanical analysis reveals that as the composite is deformed, the particles transform from individual liquid droplets to a solid metal network, resulting in a 300% increase in Young’s modulus. In contrast to previous phase change materials, this dramatic change in stiffness occurs autonomously under deformation, is insensitive to environmental conditions, and does not require external energy sources such as heat, light, or electricity. We demonstrate the utility of this approach by transforming a flat, flexible composite strip into a rigid, 3D structure that is capable of supporting 50x its own weight. The ability for shape change and reconfiguration are further highlighted, indicating potential for multiple pathways to trigger or tune composite stiffness.
... Recently, interphase approaches have been more and more focused on both the improvement of the mechanical performance of composites as well as the implementation of additional functions in order to create multifunctional composites. Bismarck et al., for example, reported on responsive fiber coatings with the objective to achieve composites with variable and/or controllable stiffness [4,5]. In our previous work based on glass fibers, many sensor utilities have switched to highly sensitive multifunctional carbon nanoparticle systems that can realize response properties due to their novel electronic properties. ...
Article
Chemical vapor deposition (CVD) is used as a method for the synthesis of carbon nanotubes (CNT) on substrates, most commonly pre-treated by a metal-catalyst. In this work, the capability of basalt fiber surfaces was investigated in order to stimulate catalyst-free growth of carbon nanotubes. We have carried out CVD experiments on unsized, sized, and NaOH-treated basalt fibers modified by growth temperature and a process gas mixture. Subsequently, we investigated the fiber surfaces by SEM, AFM, XPS and carried out single fiber tensile tests. Growth temperatures of 700 °C as well as 800 °C may induce CNT growth, but depending on the basalt fiber surface, the growth process was differently affected. The XPS results suggest surficial iron is not crucial for the CNT growth. We demonstrate that the formation of a corrosion shell is able to support CNT networks. However, our investigations do not expose distinctively the mechanisms by which unsized basalt fibers sometimes induce vertically aligned CNT carpets, isotropically arranged CNTs or no CNT growth. Considering data from the literature and our AFM results, it is assumed that the nano-roughness of surfaces could be a critical parameter for CNT growth. These findings will motivate the design of future experiments to discover the role of surface roughness as well as surface defects on the formation of hierarchical interphases. Fibers 2016, 4, 28; doi:10.3390/fib4040028 Full-text available --> https://www.mdpi.com/2079-6439/4/4/28
... Carbon fiber reinforced polymer (CFRP) has found wide applications, such as load-carrying structures in aircrafts, because of its outstanding performance [1][2][3]. Unfortunately, defects and damage are inevitable during its production and service in harsh environment, which deteriorates the performance of CFRP structures [4,5]. Therefore, nondestructive testing (NDT) is important for guaranteeing the quality and reliability of CFRP structures. ...
Article
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The paper studies the characteristics of eddy current (EC) distribution in carbon fiber reinforced polymer (CFRP) laminates so as to guide the research and operation of eddy current testing of CFRP. To this end, an electromagnetic field computation model of EC response to CFRP based on the finite element method is developed. Quantitative analysis of EC distribution in plies of unidirectional CFRP reveals that EC changes slowly along the fiber direction due to the strong electrical anisotropy of the material. Variation of EC in plies of multidirectional CFRP is fast in both directions. The attenuation of EC in the normal direction in unidirectional CFRP is faster than that in isotropic material due to faster diffusion of EC. In multidirectional CFRP, EC increases near the interfaces of plies having different fiber orientations. The simulation results are beneficial to optimizing sensor design and testing parameters, as well as damage detection and evaluation.
... Following classical lamination theory this effect, affected by the supramolecular polymer's mechanical properties and the thickness of the interleaf, can be quantified. This is confirmed in fact by studies where thermoplastic interleaves are used to design composites with controllable stiffness [32]. SP provide a unique combination of properties as a material system. ...
Article
Full-text available
This study focuses on the transfer of the healing functionality of supramolecular polymers (SP) to fibre reinforced composites through interleaving. SPs exhibiting self-healing based on hydrogen bonds were formed into films and were successfully incorporated into carbon fibre composites. The effect of the SP interleaves on in-plane fracture toughness and the subsequent healing capability of the hybrid composites were investigated under mode II fracture loading. The fracture toughness showed considerable increase since the maximum load (Pmax) of the hybrid composite approximately doubled, and consequently the mode II interlaminar fracture toughness energy (GIIC) exhibited an increase reaching nearly 100% compared to the reference composite. The healing component was activated using external heat. Pmax and GIIC recovery after activation were measured, exhibiting a healing efficiency after the first healing cycle close to 85% for Pmax and 100% for GIIC, eventually dropping to 80% for Pmax while GIIC was retained around 100% even after the fourth healing cycle. Acoustic Emission activity during the tests was monitored and was found to be strongly reduced due to the presence of the SP.
... Following classical lamination theory this effect, affected by the supramolecular polymer's mechanical properties and the thickness of the interleaf, can be quantified. This is confirmed in fact by studies where thermoplastic interleaves are used to design composites with controllable stiffness [32]. SP provide a unique combination of properties as a material system. ...
Article
Full-text available
This study focuses on the transfer of the healing functionality of supramolecular polymers (SP) to fibre reinforced composites through interleaving. SP exhibiting self-healing based on hydrogen bonds were formed into films and were successfully incorporated into carbon fibre composites. The effect of the SP interleaves on in-plane fracture toughness and the subsequent healing capability of the hybrid composites were investigated under mode II fracture loading. The fracture toughness showed considerable increase since the maximum load (Pmax) of the hybrid composite approximately doubled, and consequently the mode II interlaminar fracture toughness energy (GIIC) exhibited an increase reaching nearly 100% compared to the reference composite. The healing component was activated using external heat. Pmax and GIIC recovery after activation were measured, exhibiting a healing efficiency after the first healing cycle close to 85% for Pmax and 100% for GIIC, eventually dropping to 80% for Pmax while GIIC was retained around 100% even after the fourth healing cycle. Acoustic Emission activity during the tests was monitored and was found to be strongly reduced due to the presence of the SP.
... Simple beam theory was used to predict the apparent flexural modulus and strength of the interleaved material using equations described previously [15]. Room temperature (RT) predictions (where, RT < T g of CFRP and thermoplastic) and high temperature (HT) predictions (where, T g of thermoplastic < HT < T g of CFRP) were calculated. ...
Article
Polystyrene-interleaved carbon fibre reinforced epoxy composites exhibiting controllable stiffness have been manufactured. These composites undergo reductions in flexural stiffness of up to 99% when heated above the glass transition temperature Tg of the interleaf layers. Potential applications for such materials include their use in morphing and deployable structures. Flexural tests at room temperature indicated that improvements in adhesion between the polystyrene and CFRP layers are required to prevent premature failure of the composites at low shear stresses. Here we investigate how modification of the interleaf layer improves the interlaminar shear strength of the laminates without affecting the stiffness loss at elevated temperatures. Two poly(styrene-co-maleic anhydride) (SMA) films with different maleic anhydride content were prepared and used as interleaf films. Thick adherend shear tests showed that the adhesion strength more than doubled, while flexural tests showed that composites containing SMA interleafs had more than twice the apparent flexural strength of composites containing pure polystyrene layers at 25 °C and yet still undergo significant reductions in stiffness at elevated temperature.
... However, considering previous works that incorporated various fillers, such as rubber [8,9], thermoplastic [10,11], organic [12,13] and inorganic [14,15] particles, in epoxy, reported in recent decades to improve the fracture toughness of the epoxy matrix. Particularly, the demand of advanced nanotechnology in recent years has attracted the attention of researchers to modify epoxy resin with nanoparticles, such as carbon nanotubes [16,17] or nanofibers [18][19][20][21] to form superior properties of nanocomposites. It has been verified that many CNT based epoxy nanocomposite studies have been explored [22][23][24]. ...
Article
This paper presents a multi-scale hybridization of carbon nanotube (CNT) with clay in polymers, which offers improvement in the mechanical properties of the composites. In this study, hybrid filler, comprised of CNT grown directly on muscovite particles by chemical vapour deposition using methane as the carbon precursor, is prepared. This CNT–Muscovite Hybrid compound is incorporated into the epoxy matrix at various filler loadings (1–5 wt.%) and compared with physically mixed CNT–Muscovite. The tensile strength, tensile modulus, and micro-hardness of Epoxy/CNT–MUS HYB nanocomposites are determine to exhibit an improvement of up to 86.58%, 134.59% and 14%, respectively, compared to neat epoxy. These improvements are mainly attributed to by the good dispersion of CNT–Muscovite Hybrid in the epoxy composites.
... Another study found that adding carbon nano tubes, indeed, improved the impact strength of CFREP composites material but made it stiffer (Rahman et al. 2015). Another approach is introducing layers of polystyrene in CFREP composites which give the possibility of controlling flexural stiffness of the composites by controlling its temperature (Maples et al. 2014). Recently, interaction between thermoplastics and thermosets polymers have been sought to have great potential applications in the aircraft industry (Deng et al. 2015). ...
Article
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In this work, a hybrid polyethylene/epoxy combination matrix is reinforced with carbon fiber fabric. The objective is to develop a composite material with better tensile and impact properties. Three ratios of high molecular weight polyethylene powder were used as an additive to the epoxy matrix system. These composite materials were manufactured using hand-layup and vacuum bagging technique. The study carried out here is to find the effect of polyethylene weight fraction on tensile and impact properties of the carbon fabric reinforced epoxy composite material. The results show the tensile strength has been improved by the lowest used ratio of polyethylene additive while, all the hybrid composites exhibit higher tensile ductility. On the other hand, the Izod impact strength shows degradation in impact properties for the hybrid composites. Several suggestions are made about ways to improve the behavior of such materials.
... Materials studied by the group reduce their stiffness upon application of an electric current through the carbon fibres. Two different concepts relying on this mechanism were studied: (i) a thermoplastic interphase between a thermoset matrix and its carbon fibre reinforcement [3] and (ii) thermoplastic layers in a carbon fibre reinforced epoxy laminate [4]. These concepts were designed to combine the good mechanical performance of thermoset resins with the large potential for stiffness-reduction of thermoplastics. ...
Conference Paper
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In this study different concepts to attain a material that can reduce its stiffness upon external stimulation were evaluated regarding their suitability for traffic safety applications. All concepts rely on resistive heating of a carbon fibre reinforcement upon application of electric current through the fibres. The stiffness reduction is achieved by a phase transformation due to heating of the material. The phase transformation takes place either in a thermoplastic interphase, in a thermoplastic matrix or in a thermoset matrix, depending on the concept. The different concepts were studied regarding their thermomechanical and processing properties and their ability to reduce their stiffness upon application of an electric current was tested. Moreover, the materials were evaluated regarding their potential for fast activation, which is crucial for applications in traffic safety. Stiffness-reduction was achieved upon application of an electric current, where the activation temperature was between 60 and 120°C and the extent of stiffness-reduction varied between 50 and 90%, depending on the material. The response time was found to depend to a large extent on the amount of material, which leads to the conclusion that smart design solutions are required for larger parts. It is concluded that the concepts vary in their thermal, mechanical and processing properties as well as in their extent of stiffness-reduction upon activation. The results presented in this work prove the feasibility of the studied materials for traffic safety applications and the concepts allow further optimization of the materials for specific applications.
Article
Flexural properties of 3D-printed carbon fibre (CF) composites are investigated, experimentally and numerically. A series of 3-point bending experimental tests are conducted on CF composite specimens, and a series of 3-point bending virtual tests are simulated in LS-DYNA using a composite modelling approach, which is user defined integration points through the composite thickness. The flexural stress-strain results are compared, and they show good agreement within the elastic region. In the plastic region, the experimental flexural modulus reduces significantly, which remain constant for a certain range of flexural strain, before structural failure. A new hypothesis on the flexural properties of the 3D-printed CF composite is suggested, which is the specimen behaves as two stacks of individual beams, after yielding/delamination. The hypothesis is supported by another series of FE simulations on the composite, modelled as two stacked composite beams, and subjected to 3-point bending load. Then the capability of the composite 3D-printing to manufacture a structure with complex geometries and to manufacture several parts as a single structure, are exploited to fabricate a CF composite corrugated structure with a trailing edge section. The structure which represents an internal structure of a morphing aerofoil is actuated by a NiTi shape memory alloy (SMA) wire with a 1.6% recoverable strain. A trailing edge deflection of 6.0 mm is obtained, which is measured using an IMETRUM optical system. It is reasonably close to the predicted deflection of 7.3 mm shown in the FE simulation, using a newly developed UMAT for SMA-actuation, in an explicit LS-DYNA.
Article
In this paper, four groups of shape memory composites with carbon fiber mass fraction of 28.09%, 41.87%, 53.20%, and 63.80% were prepared by vacuum infiltration hot-press forming experimental system. The effect of carbon fiber mass fraction on the shape memory properties of shape memory composites was studied. The results showed that the shape fixed rate and the shape recovered speed of composite decreased with the increase of carbon fiber mass fraction. The shape recovered rate and the maximum recovered force first increased and then decreased, and the shape recovered time increased gradually.
Article
Carbon fiber-reinforced plastics (CFRPs) made of uni-directional carbon fibers (UDCFs) are used in various applications such as construction, aerospace, and automobiles. Therefore, their structural health monitoring (SHM) and non-destructive evaluation (NDE) are important to ensure safety during operation. While there is literature on self-sensing of CFRPs to realize various properties, there is no information on their impact self-sensing properties. Therefore, in this study, CFRPs in several orientations were investigated in terms of their mechanical fracture and electromechanical behavior. Changes in their electrical resistance due to impact damage can be utilized for SHM using the corresponding electrically equivalent circuit models. The circuit models constructed consisted of electrical resistors that described the UDCFs. In addition to converting CFRPs into 2D circuits, 3D electrical routes between electrodes were proposed for NDE. Calculating the detour length of the electrical routes using the proposed models helps in assessing the severity of the impact damage. Therefore, the models for CFRPs developed in this study not only provide support for SHM but also for NDE using electrical resistance.
Article
Functional epoxy composites reinforced with cotton fabric layers coated with conducting polypyrrole (ppy) were prepared. Oxidative vapor phase polymerization was used to obtain thin coating of ppy. When embedded within the epoxy resin (2, 4 and 6 layers), the resulting composites showed improved flexural strength (up to 20.7 %, 26.9 % and 19.7 % for respective layers) and highly reduced water absorption (up to 61.2 %) after 24 hours at 35 °C than those containing uncoated cotton layers. The conducting fabric layers allowed composites to heat upon voltage application via Joule’s effect. Statistical modelling revealed good correlation with experimental data and the influence of number of fabric layers, electrode area and applied voltage on peak temperature was studied. Composite with 6 layers showed highest peak surface temperature (92 °C) at 24 V and demonstrated a uni-directional shape recovery (greater than 60 %) when subjected to electric-heating after being deformed previously under load.
Article
An experimental approach of three-dimensional woven carbon/carbon composites arranged in various geometrical configuration is presented in this paper. Seven types of samples divided into three groups were tested under pin-loading to determine the effects of width-to-hole diameter ratio (W/D), edge distance -to- hole diameter ratio (E/D) and hole diameter-to- thickness ratio (D/t) on the failure mode. For an advanced understanding of failure propagation, damage mechanism was observed and assessed combining acoustic emission monitoring. The experimental results and observations indicated that net-tension failure and shearing-out failure switched to bearing failure with the increasing ratio of W/D and E/D, respectively. while D/t hardly affected the failure mode. Major features of the damage mechanisms included matrix crashing and cracking, fiber micro-buckling and bending, interface debonding and fiber breakage with different acoustic emission signal ranges.
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During the last decades, fiber reinforced composites (FRPs) are year by year replacing metals due to their high stiffness and high strength in combination with low specific weight and corrosion resistance. However, during their service life, these composites due to their laminated structure (no fibers are present in transverse direction) appear matrix cracking and delaminations between the reinforcing plies. A primary limitation of these composites is the poor interlaminar toughness and strength. The mismatch of anisotropic mechanical and thermal properties in between plies of deferent principal directions promotes out of plane stresses at the edges of the structures as well as in the case of stringer run out, thickness variation, holes and structural stiffeners joined to composite skin, and are only some of the candidate areas for delamination under in plane and out of plane loadings. Delaminations are among the most frequent modes of failure encountered in laminated composites and are resulted either from fatigue loadings or low velocity impact events. Conventional repair techniques of composites have a lot of drawbacks; are expensive, require extensive human work and cannot repair defects deep inside the material. Self-healing polymers is an approach which has not yet been incorporated to commercial composites but promises to face some principal weak points. This smart technology aims to in-situ repair matrix cracks and matrix/reinforcement debonding and thus to extend the effective life-span of the composites, to reduce the maintenance needs and costs and to improve the damage tolerance and reliability of composite structures. Self-healing composites have previously been developed by embedding healing agents into the matrix using microcapsules or vascular networks, that will release the healing agent upon crack damage. A different approach towards self-healing composites is matrices that comprise reversible polymers that are able to proceed with multiply healing cycles at the same damaged site. In the present investigation, the utilization of three different reversible polymeric systems (based on their chemistry) as healing agent into CFRPs was studied. More precisely, common thermoplastics such as PET and Polyamides (Nylon-66) based on reversible covalent bonds, Bis-maleimide polymers (pure and blends) based on Diels Alder (DA) and Retro-DA reactions through special covalent bonding and finally Supramolecular polymers based on hydrogen bonds were integrated into aerospace-grade CFRPs. A variety of methodologies (i.e. blending, interleaving, sieving and pre-preging) was utilized for the modification. The assessment of potential knock down effects and the healing capability of the resulting composites were investigated under mode I and mode II fracture tests, low velocity impact (LVI), compression after impact (CAI) and three-point bending (3PB) tests. Optical microscopy, SEM examinations and acoustic emission activity (AE) of the samples was monitored and led to qualitative conclusions regarding the involved failure and healing mechanisms. According to experimental campaign conducted, it was shown that by the incorporation of all these SHAs to the composites the mode I and II fracture toughness characteristics were significantly increased with samples containing supramolecular interleaves to exhibit dramatically increased fracture toughness characteristics (e.g., GIC increased with more than one order of magnitude at approximately 1550%). These modified composites exhibited healing efficiency values from 60% to 100% after the application of the first healing cycle. In addition the effect of the curing regime on the toughening and healing behaviour of CFRPs containing bis-maleimide polymers was investigated. It was shown that curing temperatures lower than the melting point of the healing agent slightly decreased the fracture toughness characteristics while increased the healing capabilities of these samples. LVI tests revealed that samples containing supramolecular prepregs or MWCNT doped nylon electrospun veils as interleaves between the primary layers of the composite exhibited higher resistance to delamination and increased CAI characteristics after the application of the healing cycle. Finally, AE recordings showed that by the incorporation of a ductile phase (i.e., healing agent) into the composite the AE activity in terms of hits is typically reduced while both AE characteristics (hits and energy) was reduced after the application of the healing cycles.
Article
In one way or the other, all of the world’s major scientific inventions have been dependent on the materials available at that time. Edison was able to invent light bulb only because Tungsten, a material capable to sustain high temperature, was available to him. Wright brothers were able to make their airplane fly because their engine was made out of aluminium and not steel, which kept their aircraft light weight. Materials play an important part in defining the function of the structure, they are used for making. In the 21st century entire world is moving towards automation and artificial intelligence so it becomes necessary that the structures, that are made, be intelligent and smart so as to adapt to their surrounding thus increasing efficiency and reducing the complexity in designing. There is a need to develop smart structures for aerospace application which can suffice the demands of this expanding industry. Smart materials like Shape Memory Alloys (SMA), piezoelectric materials, Carbon Fiber Reinforced Polymer (CFRP), Shape Memory Polymer (SMP) etc. are the materials that make up the backbone for latest aerospace application. Developing materials that can be used for morphing application is of strategic and economic importance for both civil and military application. For morphing application, the element of the material should be one that can possess variable stiffness and allow the shape change to be a reversible process. This paper reviews different types of smart materials that are used in the field of aerospace and explores their application. This paper will help future students and researchers gain a concise knowledge of smart materials used in the world of aerospace and will pave a way for future work that needs to be done in this field.
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2D-T700/E44 composites are prepared by natural curing process, heating curing process and improved compression moulding process (ICM) severally. Test shows the bending strengths of the composites prepared by three processes are 260, 390 and 605 MPa respectively. By observing the microstructure, the composite prepared by the natural curing process has many infiltration voids and cracks in the bending fracture, so the bending strength is low. The composite prepared by the heating curing process also has a small number of voids and cracks in the bending fracture. However, the defects are less than the former. This is due to the high-temperature environment enhances the fluidity of the resin during the period of heating curing. When the ICM is used, the composite is infiltrated in high temperature and high pressure. The voids and cracks are well controlled, so the bending strength reaches 605 MPa. The ICM is beneficial to prepare high-performance materials.
Article
Morphing structures, defined as body panels that are capable of a drastic autonomous shape transformation, have gained importance in the aerospace, automotive, and soft robotics industries since they address the need to switch between shapes for optimal performance over the range of operation. Laminated composites are attractive for morphing because multiple laminae, each serving a specific function, can be combined to address multiple functional requirements such as shape transformation, structural integrity, safety, aerodynamic performance, and minimal actuation energy. This paper presents a review of laminated composite designs for morphing structures. The trends in morphing composites research are outlined and the literature on laminated composites is categorized based on deformation modes and multifunctional approaches. Materials commonly used in morphing structures are classified based on their properties. Composite designs for various morphing modes such as stretching, flexure, and folding are summarized and their performance is compared. Based on the literature, the laminae in an n-layered composite are classified based on function into three types: constraining, adaptive, and prestressed. A general analytical modeling framework is presented for composites comprising the three types of functional laminae. Modeling developments for each morphing mode and for actuation using smart material-based active layers are discussed. Results, presented for each deformation mode, indicate that the analytical modeling can not only provide insight into the structure's mechanics but also serve as a guide for geometric design and material selection.
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This article discusses the process of projectile impact resistance on polyester fiberglass composite panel with ash rice husk as filler. The composite panel consists of polyester resin (unsaturated polyester BQTN 157) and reinforced woven roving S-glass with 3, 5, 7, and 9 wt% rice husk ash variations. Then it was observed the decrease of panel performance into projectile ballistic impact. The structure characterizations of materials were carried out by Fourier transform infrared (FTIR) spectroscopy and X-ray diffractometer (XRD). The impact tests were carried out with calibre ammunition of 9 mm bullet FN gun. The result showed rice husk additions decreased the panel density. There is a chemical interaction influence between the filler and polyester, the composite becomes more crystalline. Then it also showed significant influence of rice husk ash on the strength of composite panels against the high impact of a bullet.
Article
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2D-T700/E44 composite materials were prepared by improved compression molding process (ICM) then microstructure and properties of the composites were analyzed and summarized by scanning electron microscope (SEM) and electronic universal testing machine. It is found that defects will occur when the process parameters are not controlled properly and the main defects of composite materials include inadequate resin impregnation, weak interlaminar binding force, fiber displacement warping, hole and brittle fracture. Moreover, there are significant differences in the infiltration microstructure, bending properties, and fracture morphology of the composite materials with different defects. When the defects of weak interlaminar binding force and brittle fracture occur, bending properties of composite materials are relatively low, and they are 220 MPa and 245 MPa, respectively, which reach 34.9% and 38.9% of the bending strength of composite material whose defects are effectively controlled. When the process parameters are reasonable and the defects of the composite materials are effectively eliminated, the bending strength can reach 630 MPa. This will lay a foundation for the preparation of 2D-T700/E44 composite materials with ideal microstructures and properties by ICM.
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In this paper, the effect of processing temperature on the elastic and viscoelastic properties including storage modulus, loss modulus and damping value of PVC/plain weave fiberglass composites laminates was investigated. For this, composite samples with [0/90]10 lay ups were produced in three different temperatures including 160 ᵒC, 200 ᵒC and 230 ᵒC using film stacking procedure. Firstly, the flexural strength and modulus of the samples were measured using three points bending test according to ASTM D790-07 standard. Then, viscoelastic properties of the samples were measured in the temperature range of 25 ᵒC up to 220 ᵒC using Dynamic Mechanical Thermal Analysis (DMTA) and the effect of temperature on the viscoelastic properties was studied. Also, the effect of fiber/ matrix impregnation quality on the thermal and dynamic properties of the samples was evaluated using optical microscope images. It was concluded that the temperature of 230 ᵒC is proper to achieve high quality impregnation, according to both DMTA and three points bending test. Also, it was seen that increase of processing temperature up to 230 ᵒC increases the storage modulus; however, processing temperature doesn’t affect the glass transition temperature of the samples.
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Shape memory deployable composite structures have exciting applications for spacecraft, unmanned Earth based applications and more. In this project, shape memory hinges, a box section and parts of a beam were manufactured and tested for their deployment performance. The initial theoretical and FE results were show to be in good agreement with each other with a maximum percentage error of just over 10%. All the shape memory composites tested exhibited recovery percentages above 90% and recovery times lower than two minutes maximum. There is a lot of scope for further work with this project. A full beam section can be manufactured and tested, the FE model can be updated for the spring-back phenomenon and mechanical tests can be performed on the structures to validate their performance for real life engineering applications.
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Reconfigurable and morphing structures can potentially provide a range of new functionalities including system optimization over broad operational conditions and multi-mission capability. Previous efforts in morphing surfaces have generally focused on small deformation of high stiffness structural materials (e.g. aluminum, CFRP) or large deformation of low stiffness non-structural materials (e.g. elastomers). This paper introduces a new approach to achieving large strains in materials with high elastic moduli (5 to 30+ GPa). The work centers on creating variable stiffness composite materials which exhibit a controllable change in elastic modulus (bending or axial) and large reversible strains (5-15%). Several prototype materials were prepared using a commercial shape memory polymer, and measurements on these materials indicate a controllable change in stiffness as a function of temperature along with large reversible strain accommodation. We have fabricated and tested several design variations of laminar morphing materials which exhibit structural stiffness values of 8-12 GPa, changes in modulus of 15-77x, and large reversible bending strain and recovery of 2% area change in specific sample types. Results indicate that significant controllable changes in stiffness are possible.
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A review of morphing concepts with a strong focus on morphing skins is presented. Morphing technology on aircraft has found increased interest over the last decade because it is likely to enhance performance and efficiency over a wider range of flight conditions. For example, a radical change in configuration, i.e. wing geometry in flight may improve overall flight performance when cruise and dash are important considerations. Although many morphing aircraft concepts have been elaborated only a few deal with the problems relating to a smooth and continuous cover that simultaneously deforms and carries loads. It is found that anisotropic and variable stiffness structures offer potential for shape change and small area increase on aircraft wings. Concepts herein focus on those structures where primary loads are transmitted in the spanwise direction and a morphing function is achieved via chordwise flexibility. To meet desirable shape changes, stiffnesses can either be tailored or actively controlled to guarantee flexibility in the chordwise (or spanwise) direction with tailored actuation forces. Hence, corrugated structures, segmented structures, reinforced elastomers or flexible matrix composite tubes embedded in a low modulus membrane are all possible structures for morphing skins. For large wing area changes a particularly attractive solution could adopt deployable structures as no internal stresses are generated when their surface area is increased.
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Morphing aerospace structures could benefit from the ability of structural elements to transition from a stiff load-bearing state to a relatively compliant state that can undergo large deformation at low actuation cost. The present paper focuses on multi-layered beams with controllable flexural stiffness—comprising polymer layers affixed to the surfaces of a base beam and cover layers, in turn, affixed to the surfaces of the polymer layers. Heating the polymer through the glass transition reduces its shear modulus, decouples the cover layers from the base beam and reduces the overall flexural stiffness. Although the stiffness and actuation force required to bend the beam reduce, the energy required to heat the polymer layer must also be considered. Results show that for beams with low slenderness ratios, relatively thick polymer layers, and cover layers whose extensional stiffness is high, the decoupling of the cover layers through softening of the polymer layers can result in flexural stiffness reductions of over 95%. The energy savings are also highest for these configurations, and will increase as the deformation of the beam increases. The decoupling of the cover layers from the base beam through the softening of the polymer reduces the axial strains in the cover layers significantly; otherwise material failure would prevent large deformation. Results show that when the polymer layer is stiff, the cover layers are the dominant contributors to the total energy in the beam, and the energy in the polymer layers is predominantly axial strain energy. When the polymer layers are softened the energy in the cover layers is a small contributor to the total energy which is dominated by energy in the base beam and shear strain energy in the polymer layer.
Article
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This paper describes a method to vary the flexural bending stiffness of a multi-layered beam. The multi-layered beam comprises a base layer with polymer layers on the upper and lower surfaces, and stiff cover layers. Flexural stiffness variation is based on the concept that when the polymer layer is stiff, the cover layers are strongly coupled to the base beam and the entire multi-layered beam bends as an integral unit. In effect, we have a 'thick' beam with contributions from all layers to the flexural bending stiffness. On the other hand, if the shear modulus of the polymer layers is reduced, the polymer layers shear as the base beam undergoes flexural bending, the cover layers are largely decoupled from the base, and the overall flexural bending stiffness correspondingly reduces. The shear modulus of the polymer layer is reduced by increasing its temperature through the glass transition. This is accomplished by using embedded ultra-thin electric heating blankets. From experiments conducted using two different polymer materials, polymer layer thicknesses and beam lengths, the flexural stiffness of the multi-layered beam at low temperature was observed to be between two and four times greater than that at high temperature.
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One of the possibilities for the next generation of smart high-lift devices is to use a seamless morphing structure. A passive composite variable-stiffness skin as a solution to the dilemma of designing the structure to have high enough stiffness to withstand aerodynamic loading and low stiffness to enable morphing is proposed. The variable-stiffness skin is achieved by allowing for a spatial fibre angle and skin thickness variation on a morphing high-lift system. The stiffness distribution is tailored to influence the deformation of the structure beneficially. To design a realistic stiffness distribution, it is important to take aerodynamic and actuation loads into account during the optimization. A two-dimensional aero-servo-elastic framework is created for this purpose. Skin optimization is performed using a gradient-based optimizer, where sensitivity information is found through application of the adjoint method. The implementation of the aero-servo-elastic environment is addressed and initial optimization results presented. The results indicate that a variable-stiffness skin increases the design space. Moreover, the importance of taking the change in aerodynamic loads due to morphing skin deformation into account during optimization is demonstrated.
Article
Morphing, understood as the ability to undergo pronounced shape adaptations to optimally respond to a diversity of operational conditions, has been singled out as a future direction in the pursuit of maximised efficiency of lightweight structures. Whereas a certain degree of adaptivity can be accomplished conventionally by means of mechanical systems, compliance allowing for substantial reversible deformability exhibits far more potential as a morphing strategy. A promising solution to the inherent contradiction between high stiffness and reversible deformation capacity posed by morphing is offered by introducing variable stiffness components. This notion indicates the provision of a controllable range of deformation resistance levels in place of fixed properties, as required by real-time shape adaptation dictated by maximum efficiency under changing external conditions. With special emphasis on the morphing context, the current review aims to identify the main tendencies, undertaking a systematic classification of existing approaches involving stiffness variability. Four broad categories in which variable stiffness has been applied to morphing are therefore distinguished and detailed: material engineering, active mechanical design, semi-active techniques and elastic structural behaviour. Adopting a wide perspective, the study highlights key capabilities, limitations and challenges. The need for attention directed to the variable stiffness strategy is recognised and the significance of intensive research activities in a highly integrated and multidisciplinary environment emphasised if higher maturity stages of the concepts are to be reached. Finally, the potential of emerging directions of semi-active design involving electro-bonded laminates and multi-stable structures is brought into focus.
Article
High performance carbon fibre reinforced composites with controllable stiffness could revolutionise the use of composite materials in structural applications. Here we describe a structural material, which has a stiffness that can be actively controlled on demand. Such a material could have applications in morphing wings or deployable structures. A carbon fibre reinforced - epoxy composite is described that can undergo an 88% reduction in flexural stiffness at elevated temperatures and fully recover when cooled, with no discernible damage or loss in properties. Once the stiffness has been reduced, the required deformations can be achieved at much lower actuation forces. For this proof-of-concept study a thin polyacrylamide (PAAm) layer was electrocoated onto carbon fibres that were then embedded into an epoxy matrix via resin infusion. Heating the PAAm coating above its glass transition temperature caused it to soften and allowed the fibres to slide within the matrix. To produce the stiffness change the carbon fibres were used as resistance heating elements by passing a current through them. When the PAAm coating had softened, the ability of the interphase to transfer load to the fibres was significantly reduced, greatly lowering the flexural stiffness of the composite. By changing the moisture content in PAAm fibre coating, the temperature at which the PAAm softens and the composites undergo a reduction in stiffness can be tuned.
Article
Structures that can physically adapt to fulfill many roles can enable a new generation of high-performance military systems. The key to achieving substantial benefit from shape-changing operations is large changes in structural geometry and stiffness. In this study, we demonstrate variable stiffness cellular materials capable of large global changes in area through local buckling modes. Furthermore, stiffness properties and Poisson ratios may be tailored to provide desirable structural reconfiguration properties such as negative Poisson ratio and highly anisotropic stiffness. However, stiffness properties of cellular materials are two to three orders of magnitude below their constitutive materials properties. Their elastic properties can vary considerably as a function of the applied strain level due to the redistribution of structural material within the cells. Another complication is the difficulty in controlling the local buckling mode due to sensitivity to boundary conditions and loading conditions.
Article
Composite materials have increased the range of mechanical properties available to the design engineer compared with the range afforded by single component materials, leading to a revolution in capabilities. Nearly all commonly used engineering materials, including these composite materials, however, have a great limitation; that is, once their mechanical properties are set they cannot be changed. Imagine a material that could, under electric control, change from rubbery to rigid. Such composite "meta-materials" with stiffness and damping properties that can be electrically controlled over a wide range would find widespread application in areas such as morphing structures, tunable and conformable devices for human interaction, and greatly improved vibration control. Such a technology is a breakthrough capability because it fundamentally changes the paradigm of composite materials having a fixed set of mechanical properties. These electronically controllable composites may be the basis of discrete devices with tunable impedance. The composites can also be multifunctional materials: They can minimize size and mass by acting not only as a tunable impedance device, but also as a supporting structure or protective skin. Current approaches to controllable mechanical properties include composites with materials that have intrinsically variable properties such as shape memory alloys or polymers, or magnetorheological fluids, or composites that have active materials such as piezoelectrics, magnetostrictives, and newly emerging electroactive polymers. Each of these materials is suitable for some applications, but no single technology is capable of fast and efficient response that can produce a very wide range of stiffness and damping with a high elongation capability, that is, go from rubber to rigid. Such a material would be capable of a change in its maximum elastic energy of deformation of 50,000 J/cm3. No existing material is within three orders of magnitude of this value. Similarly, no material appears capable of going from a very lightly damped to a very heavily damped condition over a wide range of motion. We suggest an approach based on composites whose meso-scale structure can be changed with actuation or change in intrinsic properties. Passive composite meta-materials have been demonstrated, however, such active composite meta-materials have not yet been demonstrated.
Article
In this research, the capability of utilizing fluidic flexible matrix composites (F2MC) for autonomous structural tailoring is investigated. By taking advantage of the high anisotropy of flexible matrix composite (FMC) tubes and the high bulk modulus of the pressurizing fluid, significant changes in the effective modulus of elasticity can be achieved by controlling the inlet valve to the fluid-filled F2MC structure. The variable modulus F2MC structure has the flexibility to easily deform when desired (open-valve), possesses the high modulus required during loading conditions when deformation is not desired (closed-valve — locked state), and has the adaptability to vary the modulus between the flexible/stiff states through control of the valve. In the current study, a 3D analytical model is developed to characterize the axial stiffness behavior of a single F 2MC tube. Experiments are conducted to validate the proposed model, and the test results show good agreement with the model predictions. A closed/open modulus ratio as high as 56 times is achieved experimentally. With the validated model, an F2MC design space study is performed. It is found that by tailoring the properties of the FMC tube and inner liner, a wide range of moduli and modulus ratios can be attained. By embedding multiple F 2MC tubes side by side in a soft matrix, a multi-cellular F2MC sheet with a variable stiffness in one direction is constructed. The stiffness ratio of the multi-cellular F2MC sheet obtained experimentally shows good agreement with a model developed for this type of structure. A case study has been conducted to investigate the behavior of laminated [+60/0/-60] s multi-cellular F2MC sheets. It is shown that the laminate can achieve tunable, steerable, anisotropy by selective valve control.
Article
In this paper, the recent activity in conceptual design, prototype fabrication, and evaluation of shape morphing wing is concisely classified. Of special interest are concepts which include smart materials such as shape memory alloys (SMA), piezoelectric actuators (PZT), and shape memory polymers (SMP). We will also provide several concepts that have been developed and evaluated by the authors. Our work indicates that antagonistic SMA-actuated flexural structures form a possible enabling technology for wing morphing of small aircraft. The use of SMA-actuated structures in shape morphing wing designs reduces the weight penalty due to the actuation systems, because such SMA-actuated structures carry aerodynamic loads.
Article
This paper focuses on the ability to introduce change in the flexural bending stiffness of a multilayered beam. The multilayered beam comprises abase layer with polymer layers on the upper and lower surfaces and stiff cover layers. The flexural stiffness can be reduced by effecting a reduction in the shear modulus of the polymer layers by heating through glass transition. Stiffer polymer layers strongly couple the cover layers to the base beam and the entire multilayered beam bends more as an integral unit. On heating, a reduction in the shear modulus of the polymer layer results in its undergoing shear deformation as the base beam undergoes flexural bending and results in the cover layers decoupling from the base beam. This reduces the overall flexural bending stiffness. A finite element analysis is developed for the multilayered beam, and after experimentally verifying its ability to predict change in flexural bending deflection under load with a change in the polymer-layer shear modulus, it is used to conduct parametric studies. The results of the parametric studies provide broad insights into how the achievable change in flexural bending stiffness with a change in the polymer-layer modulus varies with design parameters such as the modulus and thickness of both the cover layers and the polymer layers. Changes in flexural bending stiffness by a factor of over 70 for a clamped-free beam and by a factor of over 130 for a pinned-pinned beam were observed for certain configuration designs.
Article
Problems in measuring the compressive strength properties of continuous fibre composites are still encountered at present day even though ISO and ASTM standards exist, and the strength prediction remains an unresolved topic. The objective of the present work is to evaluate the compressive response of the T800/924C carbon fibre-epoxy composite system (currently available for aerospace structural applications) in hot-wet environments using a modified Celanese test rig. The weight gains, maximum moisture contents and through-thickness diffusion coefficients of unidirectional laminates immersed in boiling water (accelerated ageing) are reported. Data are also presented on the effects of moisture and temperature on the compressive strength and failure mode of these laminates. It is observed that failure of specimens tested in hot-wet conditions always occurs as a result of out-of-plane fibre microbuckling. This is attributed to the reduction in matrix strength arising from elevated temperatures and environmental conditioning. In addition, two recent microbuckling models are employed to predict the compressive stress-strain response and failure load of the composite; agreement between theory and experiment is acceptable.
Article
Performance and efficiency of morphing structures can be increased by the integration of variable-stiffness elements. In this context, the present work investigates a concept of adaptive bending-twist coupling stiffness of laminated composite plates based on variable shear stress transfer at laminate interfaces. These adaptive interfaces are put into practice by exploiting the change of mechanical properties that is characteristic of the glass transition of a polymeric material. Numerical simulation and experiment are performed to verify the effectiveness of the concept and to analyze the main influences on the elastic behavior of the adaptive laminates. The results show changes in coupling stiffness by about one order of magnitude.
Article
In this paper, a kind of morphing skin embedded with pneumatic muscle fibers is proposed from the bionics perspective. The elastic modulus of the designed pneumatic muscle fibers is experimentally determined and their output force is tested with internal air pressure varying from 0 to 0.4 MPa. The experimental results show that the contraction ratio of the pneumatic muscle fibers using the given material could reach up to 26.8%. Isothermal tensile tests are conducted on the fabricated morphing skin, and the results are compared with theoretical predictions based on the rule of mixture. When the strain is lower than 3% and in its linear-elastic range, the rule of mixture is proved to possess satisfying accuracy in the prediction of the elastic modulus of the morphing skin. Subsequently, the output force of the morphing skin is tested. It is revealed that when the volume ratio of the pneumatic muscle fibers is 0.228, the contraction ratio can reach up to 17.8%, which is satisfactory for meeting the camber requirement of morphing skin with maximum strain level below 2%. Finally, stress-bearing capability tests of the morphing skin on local uniformly distributed loads are conducted, and the test results show that the transverse stiffness of the morphing skin can be regulated by changing the internal air pressure. Under a uniformly distributed load of 540 Pa, the designed morphing skin is capable of varying by more than two orders of magnitude in the transverse stiffness by changing the internal air pressure.
Article
The suppression of vibrations in structures is commonly considered a useful measure for the extension of their lifetime, when high amplitude vibrations are observed. In the experiments presented in this work, the modification of the stiffness of a beam as a means to suppress vibrations due to resonance is proposed as an alternative to the introduction of discrete damping devices. The stiffness of a beam is modified by applying an electric field between the main element of the structure and additional stiffening elements applied to its surface, thus coupling the latter to the former by transfer of shear stresses. The effect of electrostatic tuning of the bending stiffness (and consequently of its eigenfrequencies) of a large size GFRP–CFRP beam is shown by the shift of the resonance peak for the first bending mode to higher frequencies. The discrete character of the stiffness increase in multi-layer beams (n≥3) is postulated.
Deformable variable-stiffness cellular structures
  • Henry C Mcknight
Henry C, McKnight G. Deformable variable-stiffness cellular structures. Patent US 7678440 B1; 2010.
Analysis and design of a morphing wing tip using multicellular flexible matrix composite adaptive skins
  • T L Hinshaw
Hinshaw TL. Analysis and design of a morphing wing tip using multicellular flexible matrix composite adaptive skins. Masters thesis. Virginia Polytechnic Institute; 2009.
Variable stiffness materials for reconfigurable surface applications. In: Smart structures and materials 2005: active materials: behavior and mechanics
  • G Mcknight
  • Henry
McKnight G, Henry C. Variable stiffness materials for reconfigurable surface applications. In: Smart structures and materials 2005: active materials: behavior and mechanics, San Diego, USA; March 2005.
Variable stiffness structure
  • H Mcknight
  • Barvosa
  • Carter
McKnight H, Barvosa-Carter W. Variable stiffness structure. Patent US 7892630 B1; 2011.
Carbon fibre reinforced epoxy composites with variable stiffness for use in morphing aerostructures
  • P Robinson
  • H A Maples
  • O Gaite
  • S Smith
  • A Bismarck
Robinson P, Maples HA, Gaite O, Smith S, Bismarck A. Carbon fibre reinforced epoxy composites with variable stiffness for use in morphing aerostructures. In: ICCM19, Montreal, Canada; July 2013.
Smart fibre coatings: stiffness control in composite structure
  • C Tridech
Tridech C. Smart fibre coatings: stiffness control in composite structure. Ph.D. thesis. Imperial College London; 2011.
The Imperial College method for testing composite materials in compression. London: Imperial College London
  • Häberle
  • Jg
  • Godwin
  • Ew
Häberle JG, Godwin EW. The Imperial College method for testing composite materials in compression. London: Imperial College London; 1999.
In search of the true interlaminar shear strength In: Nineteenth technical conference of the joint American society for composites/ American society for testing and materials committee D30
  • Feraboli Pj
  • Kedward
  • Kt
Feraboli PJ, Kedward KT. In search of the true interlaminar shear strength. In: Nineteenth technical conference of the joint American society for composites/ American society for testing and materials committee D30, Atlanta, USA; October 2004.
Influence of moisture on the mechanical properties of graphite epoxy system
  • A Ankara
  • D Weisgerber
  • J Vilsmeir
Ankara A, Weisgerber D, Vilsmeir J. Influence of moisture on the mechanical properties of graphite epoxy system. In: International conference on advanced composite materials and structures, Taipei, Taiwan; May 1986.
Deformable variable-stiffness cellular structures. Patent US 7678440 B1
  • C Henry
  • G Mcknight
Henry C, McKnight G. Deformable variable-stiffness cellular structures. Patent US 7678440 B1; 2010.
Variable stiffness structure. Patent US 7892630 B1
  • H Mcknight
  • W Barvosa-Carter
McKnight H, Barvosa-Carter W. Variable stiffness structure. Patent US 7892630 B1; 2011.
The Imperial College method for testing composite materials in compression
  • J G Häberle
  • E W Godwin
Häberle JG, Godwin EW. The Imperial College method for testing composite materials in compression. London: Imperial College London; 1999.
Mechanical testing of advanced fibre composites
  • S Turner
Turner S. General principles and perspectives. In: Hodgkinson JM, editor. Mechanical testing of advanced fibre composites. Cambridge: Woodhouse Publishing; 2000. p. 27.
In search of the true interlaminar shear strength
  • P J Feraboli
  • K T Kedward
Feraboli PJ, Kedward KT. In search of the true interlaminar shear strength. In: Nineteenth technical conference of the joint American society for composites/ American society for testing and materials committee D30, Atlanta, USA; October 2004.