Continuous Fiber Composites Research Articles

Reconstruction of mesostructural material twin models of engineering textiles based on Micro-CT Aided Geometric Modeling

Engineering textiles are used as fibrous reinforcements in high performance polymer composites. The mechanical properties of composite materials depend on their dual-scale porous structure: long and elongated microscopic open spaces (micropores) appear between the filaments of fiber tows, and up to two orders of magnitude larger mesoscopic spaces (mesopores) exist between yarns. Because of the complex structure of composites, it is difficult to establish a connection between fiber architecture and mechanical behavior. To achieve this goal, X-ray microtomography (Micro-CT) can be used to provide detailed information on the geometric mesostructure of continuous fiber composites. The objective of this paper is to present a new approach based on “Micro-CT Aided Geometric Modeling” (Micro-CT AGM) to construct detailed geometric models of engineering textiles from Micro-CT three-dimensional (3D) images. The obtained geometric models are called “material twins” because of three distinctive features: (1) they are representative of the material variability; (2) their accuracy can be evaluated; and (3) they can be used to carry out computer simulations of mechanical behavior. This new method is applied to two industrial reinforcements: a 2D plain woven fabric and a 3D orthogonal glass textile. By introducing a new multiple factor morphological accuracy criterion, the quality of the “material twin” reconstruction can be compared to models obtained by standard image processing techniques or textile modeling software.

An Overview of Research on FDM 3D Printing Process of Continuous Fiber Reinforced Composites

Continuous fiber reinforced composites have excellent mechanical properties, but traditional processes are costly, complex to form, have a long production cycle, and are only suitable for relatively simple structures, which limits the wider application of the material. Fuse Deposition Modeling(FDM) 3D printing has the advantages of simple process, fast molding, low cost, and the ability to manufacture high-complexity structures, which provides the possibility of low-cost and rapid prototyping of composite materials. In this paper, several FDM 3D printing processes for continuous fiber composites are reviewed, and the effects of some related process parameters on the properties of the composites are analyzed. Finally, the problems of FDM 3D printing continuous fiber reinforced composites are pointed out.

Reinforcing Efficiency of Micro and Macro Continuous Polypropylene Fibers in Cementitious Composites

The effect of the microstructure of hydrophilic polypropylene (PP) fibers in the distribution of cracking associated with the strengthening and toughening mechanism of cement-based composites under tensile loading was studied. Using a filament winding system, continuous cement-based PP fiber composites were manufactured. The automated manufacturing system allows alignment of the fiber yarns in the longitudinal direction at various fiber contents. Composites with surface-modified hydrophilic macro-synthetic continuous polypropylene fibers and monofilament yarns with different diameters and surface structures were used. Samples were characterized using the tensile first cracking strength, post-crack stiffness, ultimate strength, and strain capacity. A range of volume fractions of 1–4% by volume of fibers was used, resulting in tensile first cracking strength in the range of 1–7 MPa, an ultimate strength of up to 22 MPa, and a strain capacity of 6%. The reinforcing efficiency based on crack spacing and width was documented as a function of the applied strain using digital image correlation (DIC). Quantitative analysis of crack width and spacing showed the sequential formation and gradual intermittent opening of several active and passive cracks as the key parameters in the toughening mechanism. Results are correlated with the tensile response and stiffness degradation. The mechanical properties, as well as crack spacing and composite stiffness, were significantly affected by the microstructure and dosage of continuous fibers.

Hybrid composites of aligned discontinuous carbon fibers and self-reinforced polypropylene under tensile loading

Highly aligned discontinuous fiber composites have demonstrated mechanical properties comparable to those of unidirectional continuous fiber composites. However, their ductility is still limited by the intrinsic brittleness of the fibers and stress concentrations at the fiber ends. Hybridization of aligned discontinuous carbon fibers (ADCF) with self-reinforced polypropylene (SRPP) is a promising strategy to achieve a balanced performance in terms of stiffness, provided by the ADCF, and ductility, delivered by SRPP. The current work focuses on interlayer hybridization of these materials and their tensile behavior as a function of different material parameters. Effects of the carbon layer thickness, carbon/SRPP layer thickness ratio, layer dispersion and interface adhesion are investigated. The carbon fiber misalignment is characterized using X-ray computed tomography to predict the modulus of the aligned discontinuous carbon fiber layer. The hybrids exhibit a gradual tensile failure with high pseudo-ductile strain of above 10% facilitated by multiple carbon layer failures (layer fragmentation) and dispersed delaminations. At the microscopic scale, the carbon layer fails mainly through interfacial debonding and fiber pull-out.

A stochastic model based on fiber breakage and matrix creep for the stress-rupture failure of unidirectional continuous fiber composites

Stress rupture is a time-dependent failure mode occurring in unidirectional fiber composites under sustained tensile loads, resulting in highly variable lifetimes. Stress-rupture is of particular concern in composite overwrapped pressure vessels (COPVs) since it is unpredictable, and has catastrophic consequences. At the micromechanical level, stress rupture begins with the breakdown of individual fibers at random flaws, followed by local load-transfer to intact neighbors through shear stress in the matrix. Over time, the matrix creeps in shear causing lengthening overload zones around fiber breaks, resulting in even more fiber breaks, and eventually, formation of a catastrophically unstable break cluster. Current reliability models are direct extensions of classic stochastic breakdown models for a single fiber, and do not reflect such micromechanical activity. These models are adequate for modeling composite stress rupture under a constant load, however, they may be unrealistic under more complex loading profiles, such as a constant load that follows a brief ‘proof test’ at a load level up to 1.5 times this constant load. For carbon fiber/epoxy COPVs, current models predict a reliability, conditioned on survival of a proof test, that is always higher than the reliability without such a proof test. Concern exists that this is incorrect, and that a proof test may result in reduced reliability over time. While the failure probability during a proof test may be very low, overwrap damage occurs nonetheless in the form of a large number of fibers breaks that would not occur otherwise based on fiber Weibull strength statistics. This phenomenon of increased fiber breakage during a proof test is captured in the model we develop and that specifically builds on the micromechanical failure process described above. For typical proof-test load ratios, the model predicts conditional reliabilities for lifetime that are typically much lower than those calculated in the absence of a proof test.

Quasi-static indentation analysis on three-dimensional printed continuous-fiber sandwich composites

Fiber reinforced sandwich structures are widely used as structural elements due to their lightweight and high load-bearing capacity. In this paper, effort is taken to fabricate fiber facesheet (0°/90° orientation) and corrugated cores (trapezoidal vertical pillared, core-1 and sinusoidal vertical pillared, core-2) of the sandwich structure using fused filament fabrication and Inkjet printing techniques, respectively. Firstly, the quasi-static indentation properties of the additively manufactured facesheet, cores, and sandwich structures were investigated. In addition, acoustic emission signals were monitored during quasi-static indentation testing and the resulting hits data were correlated with quasi-static indentation test results. The quasi-static indentation test results showed that sinusoidal sandwich structures have the highest load-bearing capacity of 1.79 kN until crack initiation and a total energy-absorption capacity of 20.05 J. Acoustic emission hits recorded in the range of 1–30, 30–100, and above 100 represented crack initiation, crack propagation, and specimen failure, respectively. Lastly, micro-computed tomography analysis was performed on the sandwich structures at intermediate indentation displacements, thus leading to the identification of hard to detect failure points and damage propagation. Fracture surface morphology of the sandwich structures showed that fiber pullout and core shear were the dominant damage mechanisms.

Molding process and properties of continuous carbon fiber three-dimensional printing

Currently, carbon fiber composite has been applied in the field of three-dimensional printing to produce the high-performance parts with complex geometric features. This technique comprise both the advantages of three-dimensional printing and the material, which are light weight, high strength, integrated molding, and without mold, and the limitation of model complexity. In order to improve the performance of three-dimensional printing process using carbon fiber composite, in this article, a novel molding process of three-dimensional printing for continuous carbon fiber composites is developed, including the construction of printing material, the design of printer nozzle, and the modification of printing process. A suitable structure of nozzle on the printer is adjusted for the continuous carbon fiber composites. For the sake of ensuring the continuity of composited material during the processing, a cutting algorithm for jumping point is proposed to improve the printing path during process. On this basis, the experiment of continuous carbon fiber composite is performed and the mechanical properties of the printed test samples are analyzed. The results show that the tensile strength and bending strength of the sample printed by polylactic acid–continuous carbon fiber composites increased by 204.7% and 116.3%, respectively compared with pure polylactic acid materials, and those of the sample printed by nylon–continuous carbon fiber composites increased by 301.1% and 17.4% compared with pure nylon materials, and those of test sample by nylon–continuous carbon fiber composites under the heated and pressurized treatment increased by 383.6% and 233.2% compared with pure nylon material.

Tensile properties and failure behavior of chopped and continuous carbon fiber composites produced by additive manufacturing

The use of additive manufacturing (AM) is rapidly expanding in many industries mostly because of the flexibility to manufacture complex geometries. Recently, a family of technologies that produce fiber reinforced components has been introduced, widening the options available to designers. AM fiber reinforced composites are characterized by the fact that process related parameters such as the amount of reinforcement fiber, or printing architecture, significantly affect the tensile properties of final parts. To find optimal structures using new AM technologies, guidelines for the design of 3D printed composite parts are needed. This paper presents an evaluation of the effects that different geometric parameters have on the tensile properties of 3D printed composites manufactured by fused filament fabrication (FFF) out of continuous and chopped carbon fiber reinforcement. Parameters such as infill density and infill patterns of chopped composite material, as well as fiber volume fraction and printing architecture of continuous fiber reinforcement (CFR) composites are varied. The effect of the location of the initial deposit point of reinforcement fibers on the tensile properties of the test specimens is studied. Also, the effect that the fiber deposition pattern has on tensile performance is quantified. Considering the geometric parameters that were studied, a variation of the Rule of Mixtures (ROM) that provides a way to estimate the elastic modulus of a 3D printed composite is proposed. Findings may be used by designers to define the best construction parameters for 3D printed composite parts.

Study on the flexure performance of fine concrete sheets reinforced with textile and short fiber composites

In order to study the influences of the contents of short fiber on the mechanical properties of concrete matrix, the properties of compressive, flexure and splitting of concrete matrix reinforced by alkali resistant glass fiber and calcium carbonate whisker were tested. To study the reinforced effect of different scale fibers on the flexure behavior of fine concrete sheets, the flexural tests of concrete sheet of fine concrete reinforced with basalt fiber mesh and short fiber composites were carried out. The results show that the properties of the compressive, flexure and splitting of fine concrete reinforced with appropriate amount of alkali resistant glass fiber and carbonate whisker are improved compared with that of concrete reinforced by one type of fiber. The flexure properties of the concrete sheets are improved obviously when continuous fiber textile and short fiber composite are adopted to reinforce.

Bending fracture rule for 3D-printed curved continuous-fiber composite

Three-dimensional (3D) printing is an attractive technology to produce complex structures without the need for expensive tools and molds. Additives are usually incorporated with the plastic materials used in 3D printing to increase their strength and rigidity. In particular, carbon fiber-reinforced plastic (CFRP) shows promise as a material for use in 3D printing. However, the strength of CFRP after printing is still unclear, although it is known that its strength is affected by the plastic melting during printing. In this study, we analyzed the fracture behavior of CFRP specimens before and after bending to different curvature radii. From the experimental results, a fracture criterion that described the behavior of the materials by considering tensile and compressive loads was developed. The fracture mechanism was the same for CFRP specimens with different curvature radii. These results increase our understanding of the mechanical properties of CFRP materials used in 3D printing.

An experimental protocol to measure the parameters affecting the compressive strength of CFRP with a fibre micro-buckling failure criterion

Understanding axial compressive failure mechanisms and estimating the related strength in continuous fibre composites is of paramount importance in the design of their parts. The mechanism at stake is the micro-buckling instability of fibres, which is contained by the matrix. In experimental measurements, compressive strength in bending is consistently higher than in axial compression. Indeed, the induced strain gradient provides an additional containment, namely structural effect. The characterisation of all geometrical and materials properties is usually lacking for models describing these combined mechanisms. A comprehensive experimental protocol is proposed in this paper to measure all the input parameters involved in a design oriented failure criterion. An epoxy matrix/high-modulus carbon fibre composite material illustrates this protocol. The influence of some key parameters, including the initial misalignment of the fibre, is discussed, thanks to additional experimental results.

3D Printing of Ultrahigh Strength Continuous Carbon Fiber Composites

3D printing of continuous carbon fiber reinforced thermoplastic (CFRTP) composites is increasingly under development owing to its unparalleled flexibility of manufacturing 3D structures over traditional manufacturing processes. However, key issues, such as weak interlayer bonding, voids between beads and layers, and low volume ratio of carbon fiber, in the mainstream fuse deposition modeling (FDM) and extrusion suppress the applications of these techniques in mission‐critical applications, such as aerospace and defense. This communication reports a novel approach, inspired by laminated object manufacturing (LOM), for 3D printing of continuous CFRTPs using prepreg composite sheets. The prepreg sheets are successively cut based on the sliced CAD geometry, and then bonded layer upon layer using a CO2 laser beam and a consolidation roller system. The void content is low by computed tomography scans and interlaminar bonding strength is as high as conventional autoclave method by lap shear strength tests. The excellent interfacial bonding strength and high volume ratio of continuous carbon fiber contribute to the highest reported tensile strength (668.3 MPa) and flexural strength (591.16 MPa) to date, for all 3D printed CFRTPs. Moreover, the proposed technique also enables controlling the alignment of carbon fiber in printed layers. CFRTPs with unidirectional [0°]s, cross‐ply [0/90°]s, and [0/‐45/0/45°]s fiber reinforcement are produced and evaluated for mechanical properties. The proposed 3D printing method is broadly beneficial for industries requiring high performance and lightweight structural materials with complex geometries.

Thermal cycling influences on compressive deformations of laminate composites

Composite structures always undergo temperature variations in service. In this procedure, the external loading coupled with thermal cycling will lead to their accelerated failures. In addition, the different loadings and microscopic structural parameters also play the important role in nonlinear deformation behaviors. This article presents a coupled thermo‐mechanical micromechanical model for investigating the compressive deformations of composite laminates with consideration of thermal residual stress. To this end, an effective microscopic model with consideration of coupled thermo‐mechanical behaviors for continuous fiber composites was established. The presented micromechanical method was verified by comparing with experimental data. On this basis, thermal cycling and compressive loading effect on effective deformations of unidirectional lamina composites, as well as laminate composites are both considered. It is indicated that thermal cycling tends to decrease stiffness behaviors first then increase stiffness behaviors under compressive loading. POLYM. COMPOS., 40:2908–2918, 2019. © 2018 Society of Plastics Engineers

3D printing of discontinuous and continuous fibre composites using stereolithography

3D printing technology has revolutionized the field of machinery, aerospace, and electronics. To address the shortcomings of previous studies on improving the poor mechanical properties of the resin used in 3D printers, this study presents a technology for fabricating short fibres or a continuous fibre-composite material using stereolithography 3D printing. Glass powder and fibreglass fabric were used as the discontinuous and continuous fibre reinforcement of light-cured resin material. The tensile strength and Young’s modulus showed a marked increase: these were 7.2 and 11.5 times higher than those of the resin specimen, respectively.

Simulation of prepreg platelet compression molding: Method and orientation validation

Prepreg platelet molding compounds (PPMCs) offer improved performance potential as compared to short-fiber composites and increased geometrical complexity as compared to continuous fiber composites. The increased adoption of PPMCs in automotive and aerospace applications necessitates the development of predictive tools for determining the final orientation state in complex geometries produced through molding flows. Due to the large geometric scale of platelets as compared to molded geometry scales, the orientation structure of PPMCs is spatially nonsmooth, thereby eliminating the requisite separation of scales utilized by typical molding simulation approaches. Additionally, the large fiber volume fraction and fiber length in platelets yield highly anisotropic flow behavior, as manifested in the resistance to stretching deformations along platelet fiber directions. Thus, the goal of this work is to present the development, implementation, and validation of a flow simulation approach for PPMCs accounting for fiber direction and platelet normal direction evolution with coupled anisotropic viscous behavior. The simulation approach is implemented using the smoothed particle hydrodynamics method. To validate the proposed approach, a pin bracket geometry is manufactured from a charge with the orientation state measured by computed tomography (CT). The final orientation state determined by the flow simulation predictions and by measuring the orientation state of the final bracket by CT is compared to validate the predictions.

Effect of fiber lay-up configuration on the electromagnetic interference shielding effectiveness of continuous carbon fiber polymer-matrix composite

Continuous carbon fiber polymer-matrix multifunctional structural composites capable of electromagnetic interference (EMI) shielding are needed for electronics and radiation sources. The laminate's fiber lay-up configuration affects the shielding effectiveness, as shown in this work for unmodified conventional carbon fiber polymer-matrix composite laminates with high-strength PAN-based carbon fiber and a polyamide thermoplastic matrix. The radiation is normal-incidence unpolarized plane wave, as commonly used. The shielding is dominated by absorption rather than reflection – more so for crossply composites than unidirectional composites. Due to the electrical conductivity longitudinal-to-transverse ratio of 930 for a lamina, the absorption-loss/thickness longitudinal-to-transverse ratio is 30 at 1.0 GHz. This factor of 30 means that the contribution of the fibers transverse to the electric field to absorption is negligible compared to that of the fibers parallel to the electric field. The ratio of absorption-loss/thickness for the crossply composite to that for the unidirectional composite with the same number of laminae is ∼4, and the ratio of the reflection loss for the crossply composite to that for the unidirectional composite is ∼2. The values of these ratios are consistent with electromagnetic theory for unpolarized radiation. This work strengthens the science base for the design of continuous fiber composites for shielding.

Enhanced Bonding via Additive Manufacturing‐Enabled Surface Tailoring of 3D Printed Continuous‐Fiber Composites

Additive manufacturing (AM) of composites, particularly continuous fiber composites as studied here, is emerging as a new paradigm for creating advanced structures. Traditional machining and joining approaches will evolve or be fully displaced by AM approaches, and here the authors introduce a seminal investigation into AM‐enabled bonding/joining of continuous fiber composite laminates/multilayers. The AM‐enabled tailoring approach volumetrically increases surface area available for bonding by ∼150% by printing lines of continuous Nylon matrix filaments at an infill density of 50% to create a porous (∼50 vol%) bonding surface, rather than the conventional/baseline solid (100 vol%) surface approach. The AM‐tailored multilayers show improvements relative to the baseline single‐lap‐joint (SLJ) multilayer, and versus the industry‐standard surface preparation approach: 550% and 145% increases in ultimate strength, 10 000% and 800% increases in toughness, respectively, while maintaining joint stiffness. Benchmark mechanical performance data is established via the AM‐tailored bonding approach for carbon, glass, and Kevlar continuous fiber reinforced additively manufactured composite SLJ multilayers. AM‐enabled enhanced joining, such as the concept demonstrated here, is critical as AM components are increasingly being joined, both to each other and to other (non‐AM, different AM, etc.) structures in advanced applications.

Thermoelectric polymer-matrix structural and nonstructural composite materials

Abstract This paper reviews thermoelectric polymer-matrix composites, including nonstructural and structural materials. The structural materials are continuous fiber composites; the nonstructural materials are discontinuously reinforced composites and polymer-polymer composites. A thermoelectric structural composite uses the reinforcing continuous fibers to enhance the electrical conductivity. The directionality of the continuous fibers allows tailoring of the thermoelectric behavior in various directions, particularly the through-thickness direction and the fiber direction. The use of laminae with dissimilar fibers (with opposite signs of the thermoelectric power) that are in different directions provides an array of unmodified and ordinarily made interlaminar interfaces, which serve as an array of thermocouple junctions that are based on the fiber-dominated thermoelectric behavior in the fiber direction of each lamina. By modification through positioning fillers at the interlaminar interface, the filler-dominated through-thickness thermoelectric behavior can be tailored. By using dissimilar fillers at the interlaminar interface, dissimilar composites that exhibit opposite signs of the thermoelectric power are obtained. Nonstructural thermoelectric composites exhibit a large range of ZT values, depending on the composition, structure and method of preparation.

Microstructure and mechanical properties of three dimensional-printed continuous fiber composites

Additive manufacturing processes have a demonstrated capacity for flexible production of metal and polymer components. More recently, capabilities for three dimensional printing of continuous fiber reinforced composites were developed. As with printed metal and polymer materials, printed composites will exhibit a unique microstructure with morphological features and process artifacts that manifest on multiple length scales. The aim of this research was to investigate the microstructures of various printed continuous fiber composites and determine linkages to consequent mechanical properties such as stiffness and strength. Samples investigated in this study comprised unidirectional carbon fibers in nylon matrix, unidirectional Kevlar fibers in nylon matrix, and Kevlar fibers in nylon matrix aligned at ±45° directions. Tensile properties of the samples were evaluated along with comparison to expected properties. Fiber volume ratios were analyzed by thermogravimetric analysis. Scanning electron microscopy and optical microscopy were used to observe and characterize the hierarchical microstructure. Both strength and stiffness were approximately 30–40% weaker than traditionally produced composites, owing to features such as imperfect interfaces between printed layers, microvoids, incomplete fill density, and similar process artefacts. Future work will investigate mitigation of such effects through process modifications and post-processing to produce higher performance printed composites.

Predicting the axial crush response of CFRP tubes using three damage-based constitutive models

Availability of newly developed rapid manufacturing processes may in the near future enable the integration of continuous fiber composites into vehicles while maintaining the volume production rates typical for the automotive industry. In particular, polymer matrix composites reinforced with continuous carbon fibers are considered as substitutes for metals in the design of front rail components, owing to their exceptional impact energy dissipation capabilities. To support development of such structures, it is important to revise capabilities of available composite material models for prediction of axial crushing – a major loading mode experienced by front rails. In this study, predictive capabilities of three widely used LS-DYNA composite material models – MAT054, MAT058 and MAT262 – were investigated and compared with respect to modeling of axial crushing of CFRP energy absorbers. Results of crush simulations with non-calibrated material models were compared with available experimental data, and then parameter tuning was conducted to improve correlation with experiments. Furthermore, calibrated material models were used to conduct independent crash simulations with distinct composite layups. As a result, advantages and shortcomings of the considered material models, as well as directions for future developments, were identified.