Fiber-reinforced Composite Parts Research Articles

A vat photopolymerization process for structures reinforced with spatially steered flexible fibers

We present a modified vat photopolymerization process for realizing novel fiber-reinforced composite parts wherein continuous fibers are selectively positioned and oriented across multiple layers along the build direction. This is in contrast to laminates and fiber-wound parts made by repeating fiber patterns; and additively manufactured parts with fibers inserted a posteriori or co-extruded selectively. Our process endows two benefits to the parts made with it: (i) spatially steered fibers embedded along specific paths inside to increase stiffness and strength; and (ii) in-situ fiber-matrix interface concurrently cured with matrix material as opposed to the matrix-matrix interface that exists in co-extruded fibers. The process is enabled by a robot manipulator that guides the fiber in each layer by incrementally orienting it along the tangential direction to the bespoke path. The robot arm and the end-effector do not obstruct the UV light source as they pull the fiber taut from afar in multiple directions to alleviate the undesirable effects of shadows. The robot arm is integrated with the build platform and the UV projector, and the entire process is automated. Examples that illustrate making parts with single and multiple fibers are presented to show the efficacy of the process and the machine. An example is shown with continuous fibers embedded along a few spatially varying principal-stress-flow paths inside a part to improve its stiffness and strength. An increase of 71 % in stiffness and 90 % in strength of the part is observed at fiber volume fraction as small as 0.39 %.

Application of micro-computed tomography for authentication of 3D printed composite parts

• Tomography analysis is used to study 3D printed fiber-reinforced composite parts. • The samples are different in infill printing direction and the presence of an outer wall. • Part identification methods are developed based on fibers, porosity, and printing features. • Frequency domain analysis captures distinguishing differences in CT datasets. • The result can be used for part authentication procedures for AM composite parts. Additive Manufacturing (AM) can apply unique customized printing patterns for each layer, which govern the microstructural features in a fiber-reinforced composite part. Non-destructive evaluation methods are used extensively to understand the defects and the microstructure of 3D printed composite parts. Considerable intellectual property is involved in designing and manufacturing composite parts, which needs to be protected using innovative methods. One of the concerns in this area is counterfeit parts made on high quality 3D printers, which need to be identified. This study aims to investigate and provide a part-authentication methodology for 3D printed composite parts by using the micro-computed tomography ( μ CT) scans of discontinuous fiber-reinforced polymer composite (FRPC) parts. The microstructure of FRPC contains fibers, matrix and porosity that are unique to the specimen and is a result of the printing parameters. Specimen A is printed with an infill direction of 90 ∘ and wall count of 3 and specimen B has the same infill direction but does not contain a wall boundary. A comparison between the different μ CT datasets of the specimens can identify the distinguishing features of the 3D printed composite part. The correlation developed across different μ CT datasets from extracted features and the metrics formulated can be used to quantitatively differentiate the composite specimens. Frequency domain analysis shows the most promising results by correctly identifying 3 out of the 6 datasets belonging to the same AM. A part identification and authentication method for AM composites will be useful for identifying genuine or counterfeit parts and protect intellectual property.

Determination of Fiber Content in 3-D Printed Composite Parts Using Image Analysis

Fiber-reinforced composite parts used in drones, automobiles, and sports equipment are now being manufactured by additive manufacturing (AM), where the material parameters such as fiber direction can be changed within a layer or from one layer to the other. Nondestructive evaluation methods are required to assess the quality of such customized printed parts. In this work, a microcomputed tomography ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> CT) dataset is analyzed to determine the fiber content in a 3-D printed composite material part using a digital binary image processing method. The existing literature on binary image analysis methods to measure the fiber volume fraction is limited to continuous fiber reinforced composites. Discontinuous fiber reinforced 3-D printing filaments are popular in manufacturing parts with increased strength. The methods developed in this work expands the binary image process to scans that show fibers embedded lengthwise in different directions in the 3-D printed layers. An optimized thresholding method is trained on the filament sample and then applied to 3-D printed samples. The results show fiber volume fraction measurements with standard deviations below 0.15%. The results in this work will be useful for product quality validation.

Comparing Performance of 3D-Printed and Injection-Molded Fiber-Reinforced Composite Parts in Ring-Spinning Traveler Application

Fiber-reinforced 3D printing (3DP) technology is a recent addition to the material extrusion-based 3DP process unlocking huge potential to apply this technology for high-performance material fabrication with complex geometries. However, in order to take the full advantage of this technology, a comparative analysis with existing technologies targeting a particular application is necessary to understand its commercial applicability. Here, an applied composite part, ring-spinning travelers, has been developed using the unique design features of fiber-reinforced 3DP technology that is beyond the capability of the currently used technology; the injection molding, quality, and performance of the printed and molded travelers were investigated and compared. The results demonstrated that fiber-reinforced 3DP is a promising technology that offers a lot of flexibility regarding reinforcement patterns and materials including both short and continuous fibers to tailor the performance, although the printed travelers showed poorer surface characteristics and wear resistance than the molded travelers. Based on the present analysis, a number of recommendations have been proposed on the design of the traveler to apply the technology effectively and use the printer to improvise and manipulate the performance of the travelers.

Tensile Performance of 3D-Printed Continuous Fiber-Reinforced Nylon Composites

Fused Filament Fabrication (FFF) is a promising technology for production of fiber-reinforced composite parts with complex geometries. Continuous Fiber Reinforced Additively Manufactured (CFRAM) parts are becoming more prominent due to their mechanical performance, light weight, and recyclability. CFRAM components are lighter, yet they are strong materials with a wide range of potential applications in the automotive industry, aerospace, medical tools, and sports goods. The wide range of applications of these novel materials justifies the need to study their properties. Tensile is one of the most important tests to evaluate the mechanical performance of CFRAM parts. In this paper, a comprehensive study is conducted on tensile properties of CFRAM components. The composite parts are printed using a dual nozzle 3D printing machine and their tensile performance is investigated. Furthermore, the effect of fiber type, fiber content, infill density, infill pattern, and layer thickness on tensile properties was studied. Nylon was used as the matrix and Carbon fiber (CF), fiberglass (FG), and Kevlar were used as reinforcing agents. Microstructural analysis was conducted to investigate the fracture mechanism, internal morphology, interlayer adhesion, and the printing quality of specimens. Finally, a comparative study is conducted on the price and printing time of CFRAM parts. It is observed that fiber inclusion increases the tensile strength up to 2200%; moreover, increasing the fiber content improves the tensile performance of composite. The results obtained demonstrate that CF-reinforced parts have better performance compared to FG and Kevlar-reinforced components. The results show that CFRAM parts have potential to replace metals and conventional composites for engineering applications like the automobile industry.

Mechanisms of Origin and Classification of Out-of-Plane Fiber Waviness in Composite Materials—A Review

Out-of-plane fiber waviness, also referred to as wrinkling, is considered one of the most significant effects that occur in composite materials. It significantly affects mechanical properties, such as stiffness, strength and fatigue and; therefore, dramatically reduces the load-carrying capacity of the material. Fiber waviness is inherent to various manufacturing processes of fiber-reinforced composite parts. They cannot be completely avoided and thus have to be tolerated and considered as an integral part of the structure. Because of this influenceable but in many cases unavoidable nature of fiber waviness, it might be more appropriate to consider fiber waviness as effects or features rather than defects. Hence, it is important to understand the impact of different process parameters on the formation of fiber waviness in order to reduce or, in the best case, completely avoid them as early as possible in the product and process development phases. Mostly depending on the chosen geometry of the part and the specific manufacturing process used, different types of fiber waviness result. In this study, various types of waviness are investigated and a classification scheme is developed for categorization purposes. Numerous mechanisms of wrinkling were analyzed, leading to several recommendations to prevent wrinkle formation, not only during composite processing, but also at an earlier design stage, where generally several influence factors are defined.

Three-scale asymptotic homogenization of short fiber reinforced additively manufactured polymer composites

In this research, prediction of mechanical properties of short fiber-reinforced composites manufactured with the help of fused filament fabrication (FFF) process is investigated. Three-scale formulation of asymptotic homogenization is employed to upscale the properties from microscale to mesoscale and from mesoscale to macroscale. Since generating microscale representative volume element (RVE) infused with short fibers requires sophisticated modeling tools, the algorithm for the microscale RVE generation is presented and discussed. Homogenization was performed for microscale RVEs with random and aligned (fibers aligned with the beads on mesoscale) fiber orientations, and for mesoscale RVEs with unidirectional and 0/90 layup formation. Tensile tests were performed for different short carbon fiber concentrations 5, 7.5 and 10% (by volume) to validate predicted homogenized properties. Moreover, to analyze the morphology of 3D printed specimens, microstructural analysis using SEM was performed on all the printed specimens. Surface morphology helped to gain more insight into the bead structure and fiber distribution. It was concluded that Young's modulus prediction using random fiber orientation has low relative errors tested in bead direction. Overall, this study has unique contribution to mechanical property prediction for FFF-made short fiber-reinforced composite parts.

A methodology to design a 3D graphic editor for micro-modeling of fiber-reinforced composite parts

Modern design process is a complex work which includes many aspects and so CAD/CAM/CAE are widely used now. The next special generation of such software which combines complex geometrical design facilities, micro-models of composite structures and FE analysis, and tools for additive technologies has to be developed. This paper proposes a design methodology of a new 3D raster/vector drawing program to make micro-models with crowding or coarsening of fiber structures. The definitions of the new terms, principles, and algorithms to design such structure as a set of domains are given. Files for additive technology machines and finite element analysis were created. This approach is the convenient means of micro-modeling of FRC complex shape parts.

A Method of Improving Fiber-reinforced Composite Workpiece Surface Quality During the Machining on 5-axis CNC Machines

Fiber-reinforced composite parts are often finally processed by machining. Complex-shaped surfaces are often cut using the CNC machines. The resulting composite workpiece surface quality is essentially dependent on angles between two vectors: the resultant cutting direction and he fiber direction. As 5-axis CNC machines support every imaginable tool movement it is reasonable to guide these movements so that the specified angles were the most favorable to obtain the best surface quality. The proposed method enables to secure the required tool movements. Computer software has been developed and a cutting simulation has been executed. The verification of the designed CNC-program with CAM systems has proven the efficiency of the method.

Forming of thermoplastic plain woven carbon composites

By combining carbon woven fabric with thermoplastics, a thermostamping process was proposed to form composite parts with complex double curvatures in one step and to implement affordable application of fiber-reinforced composites in automotive industry. In the proposed process, a laminated carbon woven fabrics with thermoplastic grains were heated, then transferred rapidly to a preheated mold for thermostamping, and cooled down to form the composite part. Various thermoplastics including polyamide 6, polypropylene, and acrylonitrile butadiene styrene were used as matrix materials. It is demonstrated that high-quality parts can be achieved with the proposed forming process and defects are controllable. The experiment results show that the rheological characteristics of thermoplastic resin, the fixed method of mold, and the yarn orientation of the composite blank are very important factors in the thermostamping process. With the proposed process, it is feasible to implement the high-volume and low-cost manufacturing of fiber-reinforced composite parts.

Prediction and optimization of cure cycle of thick fiber-reinforced composite parts using dynamic artificial neural networks

Curing of thermoset-based composites experience substantial temperature overshoot, especially at the center of thick parts and large temperature gradient exists through the whole part due to large amount of heat released and low conductivity of the composite. This leads to non-uniformity of cure, residual stress and consequently composite cracks and possibly degradation of the polymer. The scope of this work is to optimize the cure cycle in order to improve the properties and gaining a relatively uniform part of composite, using trained recurrent artificial neural networks purposed for speeding up the repetitious model re-calls during the optimization process. Numerical results obtained based on the three-dimensional finite volume method is used to train the network. The optimization problem is aimed to develop multi-linear-stage cure cycles by minimizing the objective function that includes maximum temperature difference through the cure cycle with the constraints of maximum allowable temperature considering degradation temperature of the polymer, minimum temperature of cure initiation, and finally reaching a maximum final conversion. The results showed the effectiveness of the approach used in this study in terms of the optimization computation time.

Hydrolytically Stable Interphase on Alumina and Glass Fibers via Hydrosilylation

The poor hydrolytic stability of silane interphase greatly limits the use of fiber reinforced composites (FRC) in demanding applications in which the FRC part is permanently exposed to a moist environment such as in prosthetic dentistry and orthodontics. To improve hydrolytic stability of the interphase between the matrix composed of a blend of triethyleneglycol dimethacrylate (TEGMA) and bisphenol A glycidylmethacrylate (Bis–GMA) monomers and glass or alumina oxide fibers, a two-step hydrosilylation procedure was employed. The process consisted of creating hydride intermediate on the fiber surface followed by hydrosilylation reaction attaching the unsaturated organic monomer (Bis–GMA) forming stable –Si–C bonds. Infrared spectroscopy (FTIR) confirmed formation of the hydride intermediate on the surface and then, attachment of the appropriate organic compound in the second step. The amount of deposited interphase and its stability was significantly enhanced compared to standard silanization treatment. Fracture surfaces were observed by scanning electron microscopy (SEM) before and after environmental exposure proving that the most stable interfacial bond was obtained with the two-step treated fibers. It was concluded that hydrosilylation provides a viable alternative to silanization for both glass and ceramic fibers in composites intended for applications requiring enhanced hydrolytic stability of the composite parts.

Design and Testing of a New Injection Approach for Liquid Composite Molding

Resin Transfer Molding (RTM) is used to manufacture advanced fiber-reinforced composite parts. In this process, a fiber preform is placed into the mold cavity and sealed shut. Then, a liquid thermoset resin is injected into the mold through the injection gates and the gates are not closed until the resin arrives at the vent locations. One expects the resin to arrive last at the vents after having saturated the complete preform to obtain a successful part. However, the resin path to impregnate the preform is usually not repeatable due to the inherent nature of not being able to place the fiber preform exactly in the same location in the mold. A micron-size gap between the preform and the mold edge will introduce flow deviations and alter the flow pattern causing the resin to arrive prematurely at the vent locations. Thus, it is desirable to have multiple gate and vent locations, to aid in distributing the resin throughout the mold effectively and efficiently. In this paper, we present the design and validations of an automated RTM intelligent workstation. The workstation will have distributed injection gates and vents that will be triggered by the control methodology selected based on the on-line flow pattern of the resin during the impregnation process. Point sensors placed in the mold walls that will monitor the arrival of the resin will detect the on-line flow pattern. To automate and control the fluid flow during the RTM process it is also necessary to individually open or close various injection locations. A new method, which can deliver the resin to various locations within the mold, while still allowing any individual gate to be opened or closed at any time during the filling, was designed and tested. Development of such versatile injection scheme provides the potential to have better control on the distribution of the resin during impregnation and will lead to successful mold filling without any dry regions despite the presence of disturbances created by the quality and the placement of the perform.

Interfacial debonding and fiber pull-out stresses of fiber-reinforced composites Part V. With a viscous interface

A fiber-reinforced elastic composite with a viscous interface is considered. The time-dependent fiber pull-out behavior is analyzed for various boundary conditions at the loaded end and the embedded end of the fiber. Specifically, loading conditions of constant stress and constant displacement rate are considered. With these loading conditions, fiber pull-out is analyzed for several conditions at the embedded end of the fiber; the embedded end is free, fixed or spring like. The time dependence of the distributions of the stress and the displacement rate in the fiber are analyzed. The parameters, which control the time-dependent behavior and the steady state condition, are addressed.

Interfacial debonding and fiber pull-out stresses of fiber-reinforced composites Part VI. Interpretation of fiber pull-out curves

The stress-displacement relationship of the fiber during the fiber pull-out process in a fiber-reinforced composite ( i.e. the fiber pull-out curve) is considered. Understanding of both interfacial debonding and friction at the debonded interface is essential in interpreting the fiber pull-out curve. The behavior of interfacial debonding, which can be unstable, partially stable or stable, is characterized in the present study. The debonded interface can be either subjected to Coulomb friction, or free of friction which is contingent upon the comparison between the residual clamping stress and the interfacial radial tension induced by Poisson's contraction of the axially tensile-loaded fiber. The important parameters controlling the characteristics of interfacial debonding and friction are studied, and various fiber pull-out curves are elucidated.