Flexible Composite Structures Research Articles

Theoretical modeling and vibration analysis of composite laminated wing-box structures of hydrogen-electric aircraft under hygrothermal environment

For the sake of achieving the goal of ultra-low or even zero carbon emissions during flight, hydrogen-electric aircraft has been receiving increasing emphasis. Nevertheless, the hydrogen-electric aircraft owns an all-composite fuselage configuration and is subjected hygrothermal environment, which have a significant effect on the aircraft dynamics, especially for wing structure. In this study, an improved orthogonal polynomial solution based on the Rayleigh-Ritz method is developed for the dynamic analysis of composite laminated wing-box structures (CLWBSs) of hydrogen-electric aircraft with humid and hot environment and elastic boundary. In this solution approach, the potential energy, kinetic energy, elastic potential energy and hygrothermal potential energy of CLWBSs are derived from the framework of Kirchhoff thin plate hypothesis. A set of artificial springs is imported to the free edges of CLWBSs to simulate the elastic boundary and connection between two plates. The convergence, accuracy and computational efficiency of the proposed formulation are validated by experiment and finite element analysis. The effect of the coupling parameters and the elastic boundaries due to flexible composite fuselage in hygrothermal environment created by the hydrogen fuel cells are systematically investigated which have been confirmed by theoretical research, finite element analysis and experiment. The related findings indicate that the present approach provided an effective formulation for the vibration analysis and optimization of flexible composite wing-box structures. It can be applied in the field of aerospace and navigation industry at the same time.

Flexible composite structures with controlled physical and mechanical characteristics

Flexible composite structures with controlled physical and mechanical characteristics

Theoretical Study on Design of Composite Lining Structure with Rigid–Flexible Combination for Trenchless Pipeline Rehabilitation

Trenchless rehabilitation lining can be divided into flexible solid wall lining structures, rigid lining structures and rigid–flexible composite lining structures. This paper reviews the research status of the design theory of trenchless rehabilitation lining pipes, analyzes the theory of rigid–flexible composite lining structures, and points out that the value of the factor determined from non-grouting angle K1 is not clear in the design formula of the rigid–flexible composite lining structure mentioned in the ASTM ‘Standard Practice for Installation of Machine Spiral Wound Poly (Vinyl Chloride) (PVC) Liner Pipe for Rehabilitation of Existing Sewers and Conduits (ASTM F 1741)’. The value of K1 was obtained by the numerical analysis method with MATLAB software. Through the analysis of the variation trend of the K1 value, it is concluded that the design formula in the ASTM standard is only applicable to the case of incomplete grouting filling between the original pipe and the lining pipe, and is not applicable to the composite lining pipe with complete grouting filling. Finally, the explicit formula of the non-grouting angle is obtained by fitting, which is more convenient to use.

Simplified Analytical Model for Predicting Neutral Cross-Section Position of Lenticular Deployable Composite Boom in Tensile Deformation.

Foldable and deployable flexible composite thin-walled structures have the characteristics of light weight, excellent mechanical properties and large deformation ability, which means they have good application prospects in the aerospace field. In this paper, a simplified theoretical model for predicting the position of the neutral section of a lenticular deployable composite boom (DCB) in tensile deformation is proposed. The three-dimensional lenticular DCB is simplified as a two-dimensional spring system and a rigid rod, distributed in parallel along the length direction. The position of the neutral cross-section can be determined by solving the balance equations and geometric relations. In order to verify the validity of the theoretical model, a finite element model of the tensile deformation of a lenticular DCB was established. The theoretical prediction results were compared with the finite element calculation results, and the two results were in good agreement.

Heterogeneous Wafer Bonding Technology and Thin-Film Transfer Technology-Enabling Platform for the Next Generation Applications beyond 5G.

Wafer bonding technology is one of the most effective methods for high-quality thin-film transfer onto different substrates combined with ion implantation processes, laser irradiation, and the removal of the sacrificial layers. In this review, we systematically summarize and introduce applications of the thin films obtained by wafer bonding technology in the fields of electronics, optical devices, on-chip integrated mid-infrared sensors, and wearable sensors. The fabrication of silicon-on-insulator (SOI) wafers based on the Smart CutTM process, heterogeneous integrations of wide-bandgap semiconductors, infrared materials, and electro-optical crystals via wafer bonding technology for thin-film transfer are orderly presented. Furthermore, device design and fabrication progress based on the platforms mentioned above is highlighted in this work. They demonstrate that the transferred films can satisfy high-performance power electronics, molecular sensors, and high-speed modulators for the next generation applications beyond 5G. Moreover, flexible composite structures prepared by the wafer bonding and de-bonding methods towards wearable electronics are reported. Finally, the outlooks and conclusions about the further development of heterogeneous structures that need to be achieved by the wafer bonding technology are discussed.

Stress States in Highly Flexible Thin-Walled Composite Structures by Unified Shell Model

This work deals with highly flexible laminated shell structures. The main focus is to provide an advanced model for the accurate prediction of the interlaminar three-dimensional stress state of shells subjected to large displacements/rotations, buckling, and snap-through phenomena. In this context, a two-dimensional shell finite element based on the Carrera unified formulation (CUF) is formulated in an orthogonal curvilinear reference system. Thanks to CUF, the governing equations are expressed in terms of fundamental nuclei, which are invariant of the shell theory approximation order. Thus, classical theories of structures to layerwise approaches can be implemented with ease and in a unified manner. The full Green–Lagrange strain tensor is employed because far nonlinear regimes are investigated. Furthermore, the geometrical nonlinear equations are written in a total Lagrangian framework and solved with an opportune Newton–Raphson method along with a path-following approach based on the arc-length constraint. The results demonstrate that classical theories may bring to wrong and unconservative stress predictions, especially in nonlinear equilibrium states, where the use of advanced and layerwise approaches shall be recommended.

Fatigue damage tracking and life prediction of fiberglass composites using a laser induced graphene interlayer

Fiberglass-reinforced composite materials are commonly used in engineering structures subjected to dynamic loading, such as wind turbine blades, automobiles, and aircraft, where they experience a wide range of unpredictable operating conditions. The ability to monitor these structures while in operation and predict their remaining structural life without requiring their removal from service has the potential to drastically reduce maintenance costs and improve reliability. This work exploits piezoresistive laser induced graphene (LIG) integrated into fiberglass-reinforced composites for in-situ fatigue damage monitoring and lifespan prediction. The LIG is integrated within fiberglass composites using a transfer-printing process that is scalable with the potential for automation, thus reducing barriers for widespread application. The addition of the conductive LIG within the traditionally insulating fiberglass composites enables direct in-situ damage monitoring through simple passive resistance measurements during tension-tension fatigue loading. The accumulation and propagation of structural damage are detected throughout the fatigue life of the composite through changes to the electrical resistance measurements, and the measurement trends are further used to predict the onset of catastrophic composite failure. Thus, this work results in a scalable and multifunctional composite material with self-sensing capabilities for potential use in high-performing, dynamic, and flexible composite structures.

Analysis and verification of a biomimetic design model based on fish skin

Many biological architectures are Bouligand structures, which comprise uniaxial fiber layers stacked in a periodic helical arrangement and are characterized by high damage resistance. As an effective flexible protective structure, fish skin is a Bouligand structure that protects the body while ensure flexibility during swimming and predation. In this paper, an analytical model inspired by fish skin is established based on previous studies, and the parameters for describing crack growth are determined. Then, mathematical expressions for the local stress intensity factors and plastic zone are used to predict how the helical stacking angle α influences the crack propagation. The results show that crack deflection and twisting improve the fracture toughness of the composite structure greatly, with the optimal fracture toughness being that for α = 60° – 70°. Moreover, biomimetic flexible composite structures inspired by fish skin are produced using silicone and Kevlar fibers. Scanning electron microscopy is used to observe the cross-sectional morphology of the composite structures, showing that the interfaces between the silicone and Kevlar fibers are highly compact. Results from experimental impact tests agree well with the predicted results.

Towards Embroidered Circuit Board From Conductive Yarns for E-Textiles

In this study, polymeric/metallic yarns were fabricated by using the micrometric copper multifilament and the polymer multifilament to create electrically conductive, thin, flexible composite yarn structures. The main aim is to realize the circuit board by using fabricated conductive yarns in the needle position of the embroidery machine and to integrate electronic components on textile structures by the soldering process. In the embroidery machine, the usage of the yarn in the needle gives a chance to create a tailored design according to the specified application. Mechanical and electrical properties of fabricated yarns were investigated. Meanwhile, a benchmark test has been done by using other commercial conductive yarns. Their embroidery performances were tested by investigating the possible harms during the stitching process. Finally, several embroidered circuit boards have been realized to show the versatility of fabricated yarns for different circuit designs.

3D manufactured, water-powered soft actuators for orthodontic application

Soft actuators with embedded power sources are critical for applications where space is limited but safety and functionality are demanded. We have successfully demonstrated (1) a 3D design/fabrication/packaging scheme that can readily upgrade conventional hydraulic actuators into autonomous osmotic actuators, and (2) integrated soft actuators that can yield desired locomotion and force outputs for orthodontic application. Flexible composite structures with semipermeable interfaces are employed to realize the desired actuation, including elongation, contraction, bending, and twisting. More specifically, thin-walled polymer bellows are utilized to control and amplify the actuation caused by osmosis. In the prototype demonstration, 25 mm long soft bellow structures with outer diameters up to 4 mm and wall thicknesses down to 250 μm were designed, 3D printed, shrink wrapped with semipermeable membranes, assembled, and characterized. When placed in aqueous media, the actuators were osmotically pressurized to function in steady and preprogrammed manners. The constant water flow rate across the semipermeable interface (with an effective working area of 30 mm2) into each presented osmotic actuator was measured to be 156 μl h−1, while the inner pressure can reach 200 kPa in few hours. It was demonstrated that both the inner/outer pressure difference and locomotion/force outputs are proportional to the actuation time. When applied on teeth, the osmotic actuators are expected to steadily overcome the rising resistance from surrounding tissues, and therefore achieve constant-rate and long-lasting movement of teeth. In addition to pumping, it is demonstrated for the first time that osmotic actuation can be tailored to accomplish sophisticated locomotion functions.

Three-Dimensional Electrohydrodynamic Printing and Spinning of Flexible Composite Structures for Oral Multidrug Forms.

A simple method to rapidly customize and to also mass produce oral dosage forms is arguably a current bottleneck in the development of modern personalized medicine. Specifically, delayed-release mechanisms with well-controlled dosage profiles for combinations of traditional Chinese herbal extracts and Western medications are not well established. Herein, we demonstrate a novel multidrug-loaded membrane sandwich with structures infused with ibuprofen (IBU) and Ganoderma lucidum polysaccharide (GLP) using three-dimensional electrohydrodynamic printing and electrospinning techniques. The resulting flexible membrane consists of microscaled, multilayered cellulose acetate (CA) membranes loaded with IBU in the shape of either concentric squares or circles, as the top and bottom layers of a sandwich structure. In between the CA-IBU layers are randomly electrospun polyvinyl pyrrolidone (PVP) layers loaded with GLP. The complete fibrous membrane sandwich can be folded and embedded into a 0-size capsule to achieve oral compliance. Simulated in vitro testing of gastric and intestinal fluids demonstrated a triphasic release profile. There was an immediate release of GLP after gastric juices dissolved the capsule shell and the PVP, followed by the short-term release of 60% of the IBU within an hour afterward, and the remaining IBU was released in a sustained manner following a Fickian diffusion profile. In summary, this multidrug (both hydrophilic and/or hydrophobic) oral system with precision-designed structures should enable personalized therapeutic dosing.

Dynamic responses of composite structures coupled through fluid medium

Dynamic analysis was conducted for two flexible composite structures coupled by a fluid medium. As the dynamic loading is applied to one structure, the other structure responds to the dynamic loading resulting from the load transfer through the fluid medium. To investigate such a coupled interaction between two structures, both experimental and numerical studies were conducted to supplement each other. First, a series of experiments were undertaken for two separate composite plates. In the designed test setup, water was filled between two composite plates. The water level between the two plates was varied and strain gages were attached to the composite plates. An impact load was applied to the front composite plate, and the dynamic responses of the both plates were measured using the strain gages. The cellular automata (CA) technique was also used for the numerical study. The results showed that the fluid–structure interaction (FSI) played an important role for the structural coupling depending on the water level between the plates.

Femtosecond-laser-based fabrication of metal/PDMS composite microstructures for mechanical force sensing

We experimentally demonstrate the fabrication of silver/polydimethylsiloxane (PDMS) composite microline structures based on the simultaneous induction of the photoreduction of silver ions and the photopolymerization of PDMS using a femtosecond laser. We show that the fabricated line structures exhibit electrical conductivity and that the resistance of the structures increases when they are subjected to mechanical force. Demonstration of sensitivity to an external mechanical force obtained by exploiting flexible composite structures fabricated by a femtosecond laser is reported for the first time. The present technique using a femtosecond laser is promising for the fabrication of flexible optical/electrical devices.

Numerical Evaluation of Dynamic Response for Flexible Composite Structures under Slamming Impact for Naval Applications

The deformable composite structures subjected to water-entry impact can be caused a phenomenon called hydroelastic effect, which can modified the fluid flow and estimated hydrodynamic loads comparing with rigid body. This is considered very important for ship design engineers to predict the global and the local hydrodynamic loads. This paper presents a numerical model to simulate the slamming water impact of flexible composite panels using an explicit finite element method. In order to better describe the hydroelastic influence and mechanical properties, composite materials panels with different stiffness and under different impact velocities with deadrise angle of 100 have been studied. In the other hand, the inertia effect was observed in the early stage of the impact that relative to the loading rate. Simulation results have been indicated that the lower stiffness panel has a higher hydroelastic effect and becomes more important when decreasing of the deadrise angle and increasing the impact velocity. Finally, the simulation results were compared with the experimental data and the analytical approaches of the rigid body to describe the behavior of the hydroelastic influence.

Deformations of flexible and foldable electro-active composite structures

This paper presents numerical analyses of elastic and viscoelastic smart flexible and foldable composite structures under electric field actuation. The studied composites comprise of multiple distributed piezoelectric patches bonded to the surfaces of in-active thin planar structures (substrates). Upon applications of electric field input, the planar structures can undergo three-dimensional large rotational deformations while their strains and stretches remain relatively small. A nonlinear time-dependent electro-mechanical coupling relation for the piezoelectric patches is considered to simulate more precisely response of piezoelectric materials when subjected to large magnitude of electric field. Co-rotational Lagrangian finite element approach is used for solving the governing equations of the deformations of flexible and foldable electro-active composite structures. Various three-dimensional shape changes of originally planar structures are achieved with different arrangements of integrated patches and subjected to different magnitude of electric fields. The effect of viscoelastic substrates and time-dependent electro-mechanical coupling of piezoelectric materials on the deformed shapes is also studied. This analysis can help designers in simulating desired deformed shapes and determining external stimuli to be prescribed prior to fabricating smart and flexible composites.

Size effects on the magnetoelectric response on PVDF/Vitrovac 4040 laminate composites

Tri-layered and bi-layered magnetoelectric (ME) flexible composite structures of varying geometries and sizes consisting on magnetostrictive Vitrovac and piezoelectric poly(vinylidene fluoride) (PVDF) layers were fabricated by direct bonding. From the ME measurements it was determined that tri-layered composites structures (magnetostrictive–piezoelectric–magnetostrictive type), show a higher ME response (75 V cm−1 Oe−1) than the bi-layer structure (66 V cm−1 Oe−1). The ME voltage coefficient decreased with increasing longitudinal size aspect ratio between PVDF and Vitrovac layers (from 1.1 to 4.3), being observed a maximum ME voltage coefficient of 66 V cm−1 Oe−1. It was also observed that the composite with the lowest transversal aspect ratio between PVDF and Vitrovac layers resulted in better ME performance than the structures with higher transversal size aspect ratios. It was further determined an intimate relation between the AreaPVDF/AreaVitrovac ratio and the ME response of the composites. When such ratio values approach 1, the ME response is the largest. In addition the ME output value and magnetic field response were controlled by changing the number of Vitrovac layers, which allows the development of magnetic sensors and energy harvesting devices.

On the dynamic characteristics of biological inspired multicellular fluidic flexible matrix composite structures

The objective of this research is to explore the dynamic characteristics of a multicellular fluidic flexible matrix composite (F2MC) structure. F2MC is a novel composite idea inspired by the fibrillar organizations of plant cell walls. Previous work on F2MC has mostly focused on single cell studies and on its static or quasi-static characteristics. The F2MC dynamic characteristics with interaction between cells through a flow circuit have not yet been investigated. When under external load, a network of F2MC cells with different fiber angles will generate pressure gradient and induce internal fluid flows. Therefore, the working fluids and flow port can be selected/designed for new types of functionality. An analytical model, incorporating the flow port characteristics with the cell structural dynamics, is developed and analyzed. Experimental investigations are also performed. It is shown that a dual F2MC cellular structure can be used as a vibration absorber and as an enhanced actuator with higher actuation authority compared to a single F2MC cell in a designated frequency band. These features are studied in correlation to the various system parameters, such as the fiber composite parameters, flow port parameters, and working fluid effective bulk modulus.

Highly Organized Two- and Three-Dimensional Single-Walled Carbon Nanotube–Polymer Hybrid Architectures

Single-walled carbon nanotube (SWCNT) network architectures combined with flexible mediums (especially polymers) are strong candidates for functional flexible devices and composite structures requiring the combination of unique electronic, optical, and/or mechanical properties of SWCNTs and polymer materials. However, to build functional flexible devices with SWCNTs, it is required to have abilities to assemble and incorporate SWCNTs in desired locations, orientations, and dimensions on/inside polymer substrates. Here, we present unique two- and three-dimensional SWCNT network-polymer hybrid architectures by combining unprecedented control over growth, assembly, and transfer processes of SWCNTs. Several SWCNT architectures have been built on polymer materials ranging from two-dimensional suspended SWCNT microlines on PDMS microchannels to three-dimensional "PDMS-vertically aligned SWCNTs-PDMS" sandwich structures. Also a combined lateral SWCNT microline and vertically aligned SWCNT flexible device was demonstrated with good electrical conductivity and low junction resistance. The results reported here open the pathway for the development of SWCNT-based functional systems in various flexible device applications.

Force Tracking Control of Fluidic Flexible Matrix Composite Variable Stiffness Structures

Active valve control is investigated for force tracking of fluidic flexible matrix composite (F2MC) variable stiffness structures through analytical and experimental studies. F2MC structures are based upon fluid-filled flexible matrix composite tubes, and previous investigations have shown that several orders of magnitude change in stiffness can theoretically be achieved by opening and closing the inlet valve to the tubes. In this article, a simple analytical model of the F2MC system is developed that captures the dynamics of the composite tube, the servovalve, and the flow of the fluid through the valve. A combined observer/ regulator control system is determined using linearized equations of motion and is applied to the non-linear system. Analysis and experimental results demonstrate that the F2MC system can track a desired force—displacement curve using active valve control.

An original pure bending device with large displacements and rotations for static and fatigue tests of composite structures

As composite structures, flexible structures have, for pure bending loading, significant displacements and rotations. The design and the dimensioning of these structures require good material characterization beyond the small deformations domain. This paper thus proposes a new original device making pure bending loading possible in large transformations. The sample is embedded at each end using a sleeve with a rigid body movement. The applied bending moment is perfectly controlled during the loading. A behavior model of the bench based on the beam theory for large transformations is developed, making it possible to dimension and design this bench. An experimental campaign using glass/epoxy composite samples has validated this new device in static and fatigue states. This device was used for identification of a fatigue model dedicated to flexible composite structures.