Fiber Composite System Research Articles

Micromechanical modeling of discontinuous long fiber reinforced composites

The performance of discontinuous fiber composites is intricately linked to the microstructure of the composite material. Therefore, micromechanics prove valuable for the analysis and design of these materials. Two micromechanics theories, the high fidelity generalized method of cells and Carrera Unified Formulation, are used to model a repeating unit cell (RUC) of the Tailorable Universal Feedstock for Forming, a highly aligned discontinuous fiber carbon fiber composite system. The models are evaluated for their ability to predict the effective properties and local stress fields within the RUC. The stress fields between the fiber ends and the fiber-matrix interfacial stresses are of particular interest. The models are used to predict the effective properties of single fiber and multiple fiber RUCs with varying fiber-to-fiber end gap lengths, as well as the local stresses in the fiber and matrix. Both models produced similar results and demonstrated that they could capture the complex local load transfer. A statistical study is also undertaken to understand the effect of gap clustering on the effective properties and local stress fields.

Experimental and statistical study on the effect of process parameters on the quality of continuous fiber composites made via additive manufacturing

Ongoing research in additive manufacturing towards structural and industrial application has led to the use of commingled roving as a manufacturing feedstock for printing high fiber volume fraction composites. The prospects of using this technology for high performance applications necessitates the need for a comprehensive experimental investigation into the effects of processing parameters on the quality of an additively manufactured composite printed from commingled roving feedstock. In this work, transverse flexure and void fraction matrix pyrolysis testing are both performed to evaluate composite quality. The transverse flexure test is a testing approach that evaluates the quality of the interfacial fiber-matrix bond while the void fraction test estimates the void content in the printed composite. A full observational study consisting of 27 different test combinations is done to investigate the effects of three different process parameters namely, temperature, pressure, and print speed across three different levels. Composite samples were made from commingled roving of E-glass and amorphous PET using an in-house built continuous fiber composite digital manufacturing system. Least squares regression analysis is performed to study the main, interaction and quadratic effects of process parameters. A statistical regression model having an R2 adjusted value of 80.1% is generated from the transverse flexure study, which is used to explain main and interaction effects and also predict performance. Response surface plots are also generated and are used to optimize process parameters which can subsequently be of help in scaling up composite manufacturing. Results show that all three process parameters are highly statistically significant at the 0.01 level of significance. Pressure * Temperature and Pressure * Printspeed are significant interaction terms. Pressure plays a weightier role when print speed is increased or temperature is decreased as it closes more voids that would ordinarily have been introduced because of drop in polymer melt viscosity. Micrographic analysis is also performed.

Numerical implementation of a modified cohesive zone model for HCF behavior of adhesively bonded composite laminates under mixed mode loading

In this paper, the cohesive zone model for high-cycle fatigue (HCF) behavior of adhesively bonded composite laminates subjected to mixed mode loading is implemented in Abaqus/Standard 6.14. The cyclic cohesive zone model takes into account mode-dependent penalty stiffness and fracture process zone size in adhesively bonded orthotropic composite laminates. Unlike earlier cyclic cohesive-based models, the developed UEL/UMAT is made available to the readers and end users for their specific needs. The model is validated extensively against the available experimental results in both load as well as displacement control. The model exhibited superior accuracy, positioning it as a valuable research tool. Furthermore, a novel parametric study was conducted to investigate the impact of fracture toughness and strength on HCF behavior. Finally, numerical simulation on a single fiber composite system is performed to explore micro-scale debonding using the developed model.

Adaptive iterative learning control and inverse compensation of macro fiber composite system with hysteresis

Macro fiber composite (MFC) actuators, which exhibit excellent flexibility and strong deformability, are widely used in aerospace exploration, flexible robotics, and other applications. However, the rate-dependent hysteresis nonlinearity of the flexible beam driven by MFCs significantly affects positioning accuracy. This study utilizes a new Hammerstein model to describe the hysteresis of an MFC system and proposes a control strategy that combines adaptive iterative learning control and inverse compensation. The adaptive iterative learning control allows the learning gain parameters to be adjusted based on the tracking error. A high gain ensures the convergence speed of iterative learning, whereas a small learning gain provides high robustness and convergence accuracy. Trajectory tracking experiments are conducted to verify the effectiveness of the proposed control strategy. Compared with the case of direct inverse compensation, after the sixth iteration, the tracking maximum absolute error, relative error, and corresponding root mean square error of the proposed control strategy reduce by 73% on average. Moreover, compared with the case of P-type iterative learning control combined with inverse compensation, the metrics above reduce by 12% on average. The proposed control strategy offers higher tracking accuracy and greater practicality in the trajectory tracking of MFC systems.

Experimental and statistical investigation on synergistic effect of nano based epoxy hybrid FRP on strength and durability of circular concrete columns

Strengthening and retrofitting of concrete structures using fiber reinforced polymer (FRP) wrapping is a promising technique in construction sector. Epoxy is the commonly used matrix which possess a higher rate of degradation under exposure to harsh environment. The evaluation of strength and durability of concrete cylinders confined by multi walled carbon nanotube (MWCNT) incorporated epoxy with hybrid sisal and basalt fiber composite systems exposed to various environmental conditions forms focus of this paper. Specimens are subjected to various environmental exposure such as elevated temperature, acidic, alkaline and sea water conditions. Two varieties of epoxy viz. neat epoxy and MWCNT modified epoxy systems are considered. Mechanical and durability properties are analyzed based on axial compressive behavior, stress strain response, visual inspection and modes of failure. The MWCNT incorporated epoxy based hybrid FRP confined specimens exhibited a strength reduction less than 10% when compared with unexposed confined specimens under aggressive environmental conditions, while unconfined specimens showed strength reduction by 40% when compared to unexposed unconfined specimens. The MWCNT modified epoxy based hybrid confinement showed an energy absorption of 6.24 times that of unconfined specimens upon chemical exposure. Ultrasonic Pulse Velocity test revealed efficacy of confinement system in protecting concrete core from a sudden failure, which in turn increases the compressive strength of system. A statistical analysis using ANOVA was employed to find significance of these factors and confirmed with experimental results. The effect of MWCNT incorporation is significant in FRP confinement and shows the possibility of the FRP system to be adopted as a major retrofitting material in alkaline and sea water environments.

Sol-gel derived porous aluminum oxide cryogel – Fiber composite systems

The main aim of present research was to develop a new low-cost synthesis route to produce ultralight Al2O3 cryogel – fiber composite materials for high temperature insulation. The Al2O3 fibers give the matrix in these composites, instead of the typical reinforcement phase. The synthesis of precursor systems of fiber drawing and freeze drying is based on sol–gel chemistry. The starting materials (inorganic Al salts, organic solvents, and additives) were the same for both type of porous Al2O3 systems. The length of fibers (5–10 cm) is sufficient to form a matrix in the composites. The average diameter of fibers is 10 µm; the size of cryogel particles is generally 20–30 µm in the composites. The composite systems were characterised by 2D and 3D SEM, N2 sorption analysis, FTIR spectroscopy, thermal conductivity measurements, thermo-vacuum, vibration, and satellite tests.

Influence of Seawater Absorption on the Hardness of Glass Fiber/Polyester Composite

In the marine industry, glass fibers are commonly used to reinforce polyesters for ship hulls, submarine components, and other marine structures. Isophthalic polyesters are a feasible alternative due to their superior mechanical qualities and added end-of-life scenarios compared to orthophthalic polyesters. However, like other fiber composite systems, glass fiber reinforced polymer (GFRP) composites are also water sensitive. Here, GFRP composites of three different thicknesses are aged under three different immersion periods in seawater (20, 40 and 60 days). All samples are reconditioned and evaluated for hardness following aging. Significant emphasis is placed on the presence of calcium carbonate, over which increases in moisture content irrevocably reduce the composite’s hardness. Compared to untreated material, the hardness of 6, 8, and 10 mm composites decreased by 25.64, 10.92, and 4.63% after the 60-day aging period. This drop is mostly the result of microstructure evolution manifesting as an increase in porosity. Consequently, fiber deterioration, fiber cracks, and degradation of polymer-fiber bonding emerge in the composite, decreasing hardness.

Influence of Seawater Absorption on the Hardness of Glass Fiber/Polyester Composite

In the marine industry, glass fibers are commonly used to reinforce polyesters for ship hulls, submarine components, and other marine structures. Isophthalic polyesters are a feasible alternative due to their superior mechanical qualities and added end-of-life scenarios compared to orthophthalic polyesters. However, like other fiber composite systems, glass fiber reinforced polymer (GFRP) composites are also water sensitive. Here, GFRP composites of three different thicknesses are aged under three different immersion periods in seawater (20, 40 and 60 days). All samples are reconditioned and evaluated for hardness following aging. Significant emphasis is placed on the presence of calcium carbonate, over which increases in moisture content irrevocably reduce the composite’s hardness. Compared to untreated material, the hardness of 6, 8, and 10 mm composites decreased by 25.64, 10.92, and 4.63% after the 60-day aging period. This drop is mostly the result of microstructure evolution manifesting as an increase in porosity. Consequently, fiber deterioration, fiber cracks, and degradation of polymer-fiber bonding emerge in the composite, decreasing hardness.

Effects of interfacial bonding strength on the formation of conductive anodic filament in polyphenylene oxide/glass fiber laminates

The formation of conductive anodic filament (CAF) inside printed circuit boards (PCBs) is one of the major failure modes related to the long-term reliability of electronic devices. In this paper, the relationship between susceptibility to CAF formation and glass fiber/resin interfacial bonding strength of a thermosetting polyphenylene oxide (PPO)/glass fiber composite system was investigated. Laminates with different interfacial bonding strengths were prepared by coating glass fibers with different silane coupling agents. Interlaminar shear strength (ILSS) of the PPO composites with silane-treated glass fibers has been improved by around 30 %, compared to the PPO/glass fiber composite without silane treatment. An accelerated response to CAF susceptibility was realized by substantially reducing the hole wall to hole wall distance of PCB to 5mil and 7 mil. By correlating the test results of interlaminar shear strength, haloing effect and susceptibility to CAF formation, it is confirmed that the CAF resistance of PCBs is positively correlated with the glass fiber/resin interfacial bonding strength.

Melt Spinning of Elastic and Electrically Conductive Filament Yarns and their Usage as Strain Sensors

Electrically conductive fibers are required for numerous fields of application in modern textile technology. They are of particular importance in the manufacturing of smart textiles and fiber composite systems with textile-based sensor and actuator systems. Elastic and electrically conductive filaments can be used as strain sensors for monitoring the mechanical loading of critical components. In order to produce such sensorial filaments, thermoplastic polyurethane (TPU) is compounded with carbon nanotubes (CNT) and melt spun. The mechanical performances of filaments produced at different spinning speeds and containing different amounts of CNT were tested. Furthermore, the correlation between the specific electrical resistance of the filaments and the mechanical strain were analyzed depending on the CNT-content and the spinning speed.

Ceramic-Based Piezoelectric Material for Energy Harvesting Using Hybrid Excitation.

This paper analyzes the energy efficiency of a Micro Fiber Composite (MFC) piezoelectric system. It is based on a smart Lead Zirconate Titanate material that consists of a monolithic PZT (piezoelectric ceramic) wafer, which is a ceramic-based piezoelectric material. An experimental test rig consisting of a wind tunnel and a developed measurement system was used to conduct the experiment. The developed test rig allowed changing the air velocity around the tested bluff body and the frequency of forced vibrations as well as recording the output voltage signal and linear acceleration of the tested object. The mechanical vibrations and the air flow were used to find the optimal performance of the piezoelectric energy harvesting system. The performance of the proposed piezoelectric wind energy harvester was tested for the same design, but of different masses. The geometry of the hybrid bluff body is a combination of cuboid and cylindrical shapes. The results of testing five bluff bodies for a range of wind tunnel air flow velocities from 4 to 15 m/s with additional vibration excitation frequencies from 0 to 10 Hz are presented. The conducted tests revealed the areas of the highest voltage output under specific excitation conditions that enable supplying low-power sensors with harvested energy.

Piezoelectric Properties of PVDF-Zn2GeO4 Fine Fiber Mats

The current paper presents the development and characterization of polyvinylidene fluoride (PVDF)-Zn2GeO4 (ZGO) fine fiber mats. ZGO nanorods (NRs) were synthesized using a hydrothermal method and incorporated in a PVDF solution to produce fine fiber mats. The fiber mats were prepared by varying the concentration of ZGO NRs (1.25–10 wt %) using the Forcespinning® method. The developed mats showed long, continuous, and homogeneous fibers, with average fiber diameters varying from 0.7 to 1 µm, depending on the ZGO concentration. X-ray diffraction spectra depicted a positive correlation among concentration of ZGO NRs and strengthening of the beta phase within the PVDF fibers. The composite system containing 1.25 wt % of ZGO displayed the highest piezoelectric response of 172 V. This fine fiber composite system has promising potential applications for energy harvesting and the powering of wearable and portable electronics.

Melt Spinning of Highly Stretchable, Electrically Conductive Filament Yarns.

Electrically conductive fibers are required for various applications in modern textile technology, e.g., the manufacturing of smart textiles and fiber composite systems with textile-based sensor and actuator systems. According to the state of the art, fine copper wires, carbon rovings, or metallized filament yarns, which offer very good electrical conductivity but low mechanical elongation capabilities, are primarily used for this purpose. However, for applications requiring highly flexible textile structures, as, for example, in the case of wearable smart textiles and fiber elastomer composites, the development of electrically conductive, elastic yarns is of great importance. Therefore, highly stretchable thermoplastic polyurethane (TPU) was compounded with electrically conductive carbon nanotubes (CNTs) and subsequently melt spun. The melt spinning technology had to be modified for the processing of highly viscous TPU–CNT compounds with fill levels of up to 6 wt.% CNT. The optimal configuration was achieved at a CNT content of 5 wt.%, providing an electrical resistance of 110 Ωcm and an elongation at break of 400%.

Robust Three‐Component Elastomer–Particle–Fiber Composites with Tunable Properties for Soft Robotics

Materials with tunable properties, especially dynamically tunable stiffness, have been of great interest for the field of soft robotics. Herein, a novel design concept of robust three‐component elastomer–particle–fiber composite system with tunable mechanical stiffness and electrical conductivity is introduced. These smart materials are capable of changing their mechanical stiffness rapidly and reversibly when powered with electrical current. One implementation of the composite system demonstrated here is composed of a polydimethylsiloxane (PDMS) matrix, Field's metal (FM) particles, and nickel‐coated carbon fibers (NCCF). It is demonstrated that the mechanical stiffness and the electrical conductivity of the composite are highly tunable and dependent on the volume fraction of the three components and the temperature, and can be reasonably estimated using effective medium theory. Due to its superior electrical conductivity, Joule heating can be used as the activation mechanism to realize ≈20× mechanical stiffness changes in seconds. The performance of the composites is thermally and mechanically robust. The shape memory effect of these composites is also demonstrated. The combination of tunable mechanical and electrical properties makes these composites promising candidates for sensing and actuation applications for soft robotics.

Failure Analysis of Laminated Composite Plate Under Hygro-Thermo Mechanical Load and Optimisation

Failure analysis of laminated composite plates for different mechanical, thermo mechanical and hygro-thermo mechanical loads for different ply thicknesses, stacking sequences, fiber orientation angles and composite material systems is presented in the paper. A comparative study of different failure theories is also presented in the paper. The effect of fiber orientation angles on the first ply failure load is also studied. A hybrid composite laminate is developed based on the first ply failure load which minimizes weight and cost. The last ply failure load based on fully discounted method is calculated for different stacking sequences. An optimum composite material system and laminate layup is studied for a targeted strength ratio which minimizes weight.

Design of a multi-agent, fiber composite digital fabrication system.

Swarm-based fabrication of interwoven composite tubes via a fully autonomous, cooperative system can help create architectural-scale structures in effective and efficient ways, including in remote environments.

High-density polyethylene crystals with double melting peaks induced by ultra-high-molecular-weight polyethylene fibre.

High-density polyethylene (HDPE)/ultra-high-molecular-weight polyethylene (UHMWPE) fibre composites were prepared via solution crystallization to investigate the components of epitaxial crystal growth on a highly oriented substrate. Scanning electron microscopy morphologies of HDPE crystals on UHMWPE fibres revealed that the edge-on ribbon pattern crystals that were formed initially on UHMWPE fibres converted afterwards to a sheet shape as crystallization progressed. Wide-angle X-ray diffraction confirmed that the polymer chain oriented along the fibre axis and the orthorhombic crystal form of HDPE remained unchanged in HDPE/UHMWPE fibre composite systems. The thermal behaviour of the fibre composites measured by differential scanning calorimetry showed double melting peaks, the nature of which, as disclosed by partial melting experiments, is ascribed to bilayer components existing in the induced crystals: the inner layer is composed of more regularly folded chain crystals induced by UHMWPE fibres, and the outer layer formed on the inner one with a thinner and lower ordered crystal structure.

State Diagram for a Waxy Rice Starch–Soluble Dietary Fiber Composite System

Soluble dietary fiber (SDF) significantly affects human health. The addition of SDF to food not only improves the health benefits, but also modifies its textural properties and shelf life. Moisture sorption isotherms and state diagrams are developed for waxy rice starch (WRS) and waxy rice starch–soluble dietary fiber (WRS‐SDF) to investigate the effects of SDF on the stability of WRS. The moisture sorption isotherms of WRS and WRS‐SDF are modeled using the Guggenheim–Anderson–de Boer (GAB) equation. The GAB monolayer moisture contents are calculated as 8.87 and 9.28 g/100 g (dry basis) for WRS and WRS‐SDF, respectively. For WRS, the maximal‐freeze‐concentration conditions at (characteristic solids content) = 0.75 g g−1 (dry basis) are (characteristic end point of freezing) = −16.8 °C and (characteristic glass transition) = −38.5 °C. For WRS‐SDF at =0.74 g g−1 (dry basis), = −13.5 °C and = −29.6 °C. The differences in values of the maximal‐freeze‐concentration conditions are attributed to the good hygroscopicity and water‐holding capacity of SDF. When combined with the Williams–Landel–Ferry (WLF) equation, the results indicated that the storage of WRS‐SDF is shorter than that of WRS under the same holding condition.

Comparative study on the behaviour of different infill materials for pre-fabricated fibre composite repair systems

Prefabricated fibre-reinforced polymer (FRP) composite jacket is now becoming an effective repair system for deteriorating piles and columns exposed to marine environment. This system works by providing grout infills between the annulus of the existing structure and the composite jacket. Few studies are however available on the optimal grouting materials that can effectively transfer the stresses between the existing structure and the FRP jacket. This study is investigating the effect of cementitious, concrete and epoxy-based grout infills on the structural behaviour of pre-fabricated glass-FRP (GFRP) tubes. The considered grouts have compressive strength and modulus of elasticity ranging from 10 MPa to 70 MPa and from 10 GPa to 35 GPa, respectively. The experimental results showed that the behaviour of the composite repair system is highly dependent on the modulus of elasticity and the compressive strength of the grout infill. The brittle failure behaviour of the cementitious and epoxy grouts led to localised failure in the FRP repair system while the progressive cracking and crushing of the concrete infill resulted in effective utilisation of the high strength properties of the composite materials. Theoretical analysis of the overall compressive behaviour has also been conducted and showed very good agreement with the experimental results.

On the translaminar fracture toughness of Vectran/epoxy composite material

The mode I fracture toughness associated with fibre tensile failure was investigated for a Vectran/MTM57 composite system. A modified compact tension specimen was designed and manufactured to mitigate compressive and buckling failure due to the low compressive properties which are an inherent characteristic of Vectran fibres. On average, the mode I translaminar fracture toughness for Vectran/MTM57 was found to be approximately 130–145 kJ/m2 for initiation and 250–260 kJ/m2 for propagation. In contrast with other composite systems such as carbon and glass fibre, the fracture toughness of Vectran/MTM57 was found to be relatively higher, with up to 48.26% and 95.27% for initiation and propagation, respectively for some carbon fibre composite system; 9.93% and 68.6% for initiation and propagation, respectively for S2-Glass/epoxy system.