Manufacturing Of Composite Structures Research Articles

Characterization and analysis of filled knitted fabric formworks for advanced manufacturing of composite structures

An alternative construction methodology has been raising interest, consisting of employing fabrics as formworks. The implementation of flexible formworks allows the production of highly optimized organic structures, avoiding the use of unnecessary materials and consequently reducing construction loads, costs and waste. This study aims to integrate the experimental characterization of the mechanical behaviour of the knitted textiles used as formworks, the mechanical and deformational behaviour of the formworks while being filled with fresh mortar, and the numerical simulation of the multi-step process leading to the final structure. The integration of these different processes is essential for a sound design of this composite system as a construction strategy. To this end, this investigation identified several aspects, related to either experimental or computational mechanics that are relevant to the topic and should be further investigated, with the aim of a future integrated material and structural design approach.

Topology optimisation for robust design of large composite structures

The manufacturing of composite structures can lead to material defects. Some of these defects, due to their small size, can go undetected by current non-destructive testing techniques; especially in very large and geometrically complex components. Therefore, it is of paramount importance to design composite components to resist defects by accounting for them as early as possible. Topology optimisation is a methodology whereby complex features, such as cracks and debonds, can be accounted for in the design. Therefore, this work proposes a new approach for the design of very large composite structures. This new approach is enabled by a solid shell element formulation with boundary tracking capabilities (through the floating node and the level-set methods) and optimisation capabilities (through design sensitivity analysis of an energy release rate objective function). Furthermore, this work proposes a new algorithm and modelling workflow that has the ability to handle large multi-scale models. The proposed methodology shows advantages as the ability to track the topology explicitly, and the ability to increase defect tolerance of the design. This works shows the new methodology is capable of handling complex structures, such as a multi-scale stringer run-out model from an aircraft wing-box, while a kissing bond defect is present.

3D-printing of ceramic filaments with ductile metallic cores

The additive manufacturing of composite structures can open possibilities in the design and fabrication of devices but also demands new approaches. In this work, we use thermally reversible pastes to fabricate ceramic matrix (Al2O3) composites reinforced with continuous metallic fibres (steel). The approach is based on the micro-extrusion of ceramic–metal filaments with core–shell and layered arrangements. These filaments are employed in the additive manufacturing of dense and light-weight cellular structures that combine the stiffness and strength of the ceramic shell with the fracture resistance and energy absorption capabilities provided by the metal core. The approach can be used to extrude filaments with thin porous interlayers separating the core and the shells to create a weak fibre/matrix interphase. The cellular structures fabricated with these filaments exhibit higher compressive strengths and energy absorption capabilities than those fabricated from pure ceramics. The works of fracture of the dense composites are one to two orders of magnitude above those of the ceramic matrix (103 J/m2) while the bending strengths remain comparable to those of 3D printed alumina (200–350 MPa). The technique could be easily extended to other material combinations opening new opportunities in the additive manufacturing of multi-material parts and devices.

High temperature and toughened bismaleimide structural film adhesive for high performance CFRP bonding over 300 °C

Structural adhesive play an important role in the manufacturing of composite structures. Here, a high-temperature resistant bismaleimide (BMI) structural film adhesive was prepared by using a chemically modified BMI resin as the matrix with poly(phthalazinone ether nitrile ketone) (PPENK) as the film-forming agent. The toughness of the adhesive was greatly enhanced by adding vinyl-terminated butadiene acrylonitrile (VTBN). The performance of the high-temperature resistant BMI structural film adhesive was optimized with 15 phr of PPENK and 6 phr of VTBN, which exhibits outstanding heat resistance, excellent bonding and peeling performance. Its glass transition temperature is as high as 272 °C while maintaining the shear strength of 10.8 MPa at 316 °C. The synergistic toughening effects of VTBN and PPENK significantly improve the toughness with a high peel strength of 41.7 N mm/mm. The co-cured composite joint formed between BMI prepregs and modified BMI adhesive exhibits shear strength of 10.7 MPa at 316 °C and peel strength of 21.8 N mm/mm, indicating an effective bonding between the modified BMI adhesive and BMI prepreg. The phase at the bonding interface is proved to be an important factor for high-performance at the bonding joint.

Prediction of Damage in Non-Crimp Fabric Composites Subjected to Transverse Crushing: A Comparison of Two Constitutive Models

Non-crimp fabrics (NCFs) are increasingly used in industry for manufacturing of composite structures due to a combination of high mechanical properties and excellent manufacturability. As with other composites, in-service damage can be a cause for severe reduction in load-carrying capacity of NCF-reinforced plastics. In this experimental and numerical study, two constitutive material models previously used only for damage prediction in unidirectional (UD) tape and woven fabric-reinforced materials (LS-DYNA’s *MAT_ENHANCED_COMPOSITE_DAMAGE—MAT54 and *MAT_LAMINATED_COMPOSITE_FABRIC—MAT58) were evaluated for simulating transverse crushing of composite parts processed from a non-crimp carbon fabric. For this purpose, UD NCF components of tubular shape were subjected to transverse crushing through a controlled indentation of a metallic cylinder, and results of the experiment were compared with numerical modeling. Considered verification metrics included the observed and the predicted patterns of interlaminar damage, the extent of delamination, as well as the ability of the models to replicate force-displacement response exhibited by the tested specimens.

A DMA-Based Approach to Quality Evaluation of Digitally Manufactured Continuous Fiber-Reinforced Composites from Thermoplastic Commingled Tow

Direct digital manufacturing of continuous fiber-reinforced thermoplastics exhibits the potential to relieve many of the constraints placed on the current design and manufacture of composite structures. At present, the additive manufacturing of continuous fiber-reinforced thermoplastics is demonstrated to varying extents; however, a comprehensive investigation of manufacturing defects and the quality of additively manufactured high fiber volume fraction continuous fiber-reinforced thermoplastic composites is limited. Considering the preliminary nature of the additive manufacturing of continuous fiber-reinforced thermoplastics, composites processed in this manner are typically subject to various manufacturing defects, including excessive void content in the thermoplastic matrix. Generally, quality evaluation of processed composites in the literature is limited to test methods that are largely influenced by the properties of the continuous fiber reinforcement, and as such, defects in the thermoplastic matrix are usually less impactful on the results and are often overlooked. Hardware to facilitate the direct digital manufacturing of continuous fiber-reinforced thermoplastic matrix composites was developed, and specimens were successfully processed with intentionally varied void content. The quality of the additively manufactured specimens was then evaluated in terms of the measured maximum storage modulus, maximum loss modulus, damping factor and the glass transition temperature by means of dynamic mechanical analysis (DMA). DMA allows for thermomechanical (i.e., highly matrix sensitive) evaluation of the composite specimens, specifically in terms of the measured elastic storage modulus, viscous loss modulus, damping factor and the glass transition temperature. Within the tested range of void contents from roughly 4–10%, evaluation by DMA resulted in a distinct reduction in the maximum measured storage modulus, maximum loss modulus and glass transition temperature with increasing void content, while the damping factor increased. Thus, the results of this work, which focused on the effect of void content on DMA measured properties, have demonstrated that DMA exhibits multi-faceted sensitivity to the presence of voids in the additively manufactured continuous fiber-reinforced thermoplastic specimens.

Strategies in the analysis of samples of composites for the quantification of porosity by image analysis from optical microscopy

Porosity and voids are a type of defect that may appear during the manufacturing of composite structures. Its possible presence is checked in production parts by different non-destructive techniques based on ultrasonic inspection. It is important to perform this evaluation  considering that a high content of these defects decrease the mechanical properties compared to the original design. In recent years, optical microscopy of composite materials has become a very powerful tool, which is  available for the industry to inspect and look at the internal structure and help to reach conclusions in the field of quality, failure analysis and I+D+I. In addition, the use of automatic scanning techniques of wide cross sections has opened the possibility of quantifying the percentage of porosity/voids, as well as become an alternative to more laborious testing procedures such as acid digestion of the resins. This study contributes to existing test standards within the aeronautical sector, in the sample preparation strategy. For this purpose, samples of epoxy matrices with carbon fibers, both unidirectional and fabric tapes have been analyzed to evaluate the influence of the quality of sample preparation, the stacking sequence and the initial porosity levels, the repeatability of the measurements and the quality of the results. An attempt has been made to correlate the results with standar physical / chemical analysis.

A parametric level set method for the optimization of composite structures with curvilinear fibers

This paper presents a parametric level set method for the optimization of composite structures with curvilinear fibers. We use iso-contours of a scalar function, also called the level set function, to represent the fibers, and the level set function is constructed by a set of radial basis functions with compact support. The orientation of fiber at any arbitrary point is defined by the orientation of the tangent vector of iso-contour. In order to prevent gaps and overlaps between neighboring fiber tows from arising during the manufacturing of composite structures, a constraint is formulated that the norm of gradient vector of the level set function is equal to 1 almost everywhere. Furthermore, these gradient constraints are aggregated into a single one by using the p-norm method to improve the efficiency of optimization. The compliance minimization problem is considered, and the coefficients of radial basis functions are taken as the design variables. The results of optimizations proved that the proposed method can provide an optimized composite structure with equally spaced curvilinear fibers.

Accurate Stress Analysis of Variable Angle Tow Shells by High-Order Equivalent-Single-Layer and Layer-Wise Finite Element Models.

New concepts of lightweight components are conceived nowadays thanks to the advances in the manufacture of composite structures. For instance, mature technologies such as Automatic Fibre Placement (AFP) are employed in the fabrication of structural parts where fibres are steered along curvilinear paths, namely variable angle tow (VAT), which can enhance the mechanical performance and alleviate the structural weight. This is of utmost importance in the aerospace field, where weight savings are one of the main goals. For that reason, shell structures are commonly found in the aerospace industry because of their capabilities of supporting external loadings. Straight-fibre composite shell structures have been studied in recent decades and, now, spatially varying composite shells are attracting the attention of manufacturers. This work analyses the mechanical behaviour of VAT composite shells subjected to different external loadings and boundary conditions. The Carrera Unified Formulation (CUF) is employed to obtain the different structural models in a systematic and hierarchic manner. The outcomes of such numerical models are discussed and compared with commercial software Abaqus.

Space-Filling Curve Resistor on Ultra-Thin Polyetherimide Foil for Strain Impervious Temperature Sensing.

Monitoring process parameters in the manufacture of composite structures is key to ensuring product quality and safety. Ideally, this can be done by sensors that are embedded during production and can remain as devices to monitor structural health. Extremely thin foil-based sensors weaken the finished workpiece very little. Under ideal conditions, the foil substrate bonds with the resin in the autoclaving process, as is the case when polyetherimide is used. Here, we present a temperature sensor as part of an 8 µm thick multi-sensor node foil for monitoring processing conditions during the production and structural health during the lifetime of a construction. A metallic thin film conductor was shaped in the form of a space-filling curve to suppress the influences of resistance changes due to strain, which could otherwise interfere with the measurement of the temperature. FEM simulations as well as experiments confirm that this type of sensor is completely insensitive to the direction of strain and sufficiently insensitive to the amount of strain, so that mechanical strains that can occur in the composite curing process practically do not interfere with the temperature measurement. The temperature sensor is combined with a capacitive sensor for curing monitoring based on impedance measurement and a half-bridge strain gauge sensor element. All three types are made of the same materials and are manufactured together in one process flow. This is the key to cost-effective distributed sensor arrays that can be embedded during production and remain in the workpiece, thus ensuring not only the quality of the initial product but also the operational reliability during the service life of light-weight composite constructions.

Experimental investigation and optimization of delamination factors in the drilling of jute fiber–reinforced polymer biocomposites with multiple estimators

Currently, the manufacture of composite structures often requires material removal operations using a cutting tool. Indeed, since biocomposites are generally materials that do not conduct electricity, electro-erosion cannot be utilized. As a result, the processes that can be used not only the unconventional method of abrasive water jet but also conventional machining, such as drilling. Delamination factors (Fd) are widely recognized for controlling the damaged area (delamination) induced by drilling in industry. As discussed in the literature, several approaches are available to evaluate and quantify the delamination surrounding a hole. In this context, the objective of this study is to compare the three Fd evaluation methods that have been most frequently used in previous investigations. To this end, three spindle and feed speeds and three Brad and Spur drills (BSD) tool diameters were selected (L27) for drilling 155-g/m2 density jute fabric–reinforced polyester biocomposites. The desirability function (DF) was further made to optimize the drilling parameters. The response surface methodology (RSM) and artificial neural networks (ANNs) were applied to validate the results obtained experimentally as well as to predict the behavior of the structure depending on the cutting conditions.

Porosity detection and localization during composite cure inside an autoclave using ultrasonic inspection

Composite materials offer unique benefits in aerospace applications such as increased strength-to-weight ratio and improved fatigue properties. They are increasingly being used in major commercial aircraft programs. However, current processing methods can lead to defects in composite parts, which must be detected using post-manufacturing inspection methods. Porosity (i.e., pores, voids) is a critical defect from the cure process that is detrimental to performance of the composite part. It is therefore necessary to understand and eliminate the formation of porosity defects. At NASA Langley Research Center, an in-situ cure monitoring system was developed to detect porosity defects as they form in real-time inside an autoclave. A capability to directly detect and localize porosity within a composite during cure did not exist before.This study is focused on an elevated-temperature ultrasonic inspection system to detect porosity defects in composites during autoclave cure. The ultrasonic inspection system operated inside an autoclave within an enclosure cooled by intermittent liquid nitrogen (LN2) injections. A high-temperature, 2.25 MHz, transducer transmitted ultrasonic waves through the tool plate and into the composite part and measured the amplitude and time of flight of the reflected waves with a 1 mm × 1 mm step size/areal resolution. Porosity was observed via the ultrasonic reflections, which experienced increased attenuation in regions of high porosity. Distinct regions of increased porosity were present due to uneven pressure across the panel, which was driven by an intentional misfit between the flat caul plate and the tapered composite panel with ply drops. The results were validated by post-cure ultrasonic inspection and micrographs. The in-situ inspection system was able to successfully provide porosity detection and localization in the tapered ply-drop panel. The successful results indicate the promise of this system for future implementation in the manufacturing of composite structures for aerospace applications.

A Mini-Review and Perspective on Current Best Practice and Emerging Industry 4.0 Methods for Risk Reduction in Advanced Composites Manufacturing

The manufacturing of composite structures is a highly complex task with inevitable risks, particularly associated with aleatoric and epistemic uncertainty of both the materials and processes, as well as the need for in-situ decision-making to mitigate defects during manufacturing. In the context of aerospace composites production in particular, there is a heightened impetus to address and reduce this risk. Current qualification and substantiation frameworks within the aerospace industry define tractable methods for risk reduction. In parallel, Industry 4.0 is an emerging set of technologies and tools that can enable better decision-making towards risk reduction, supported by data-driven models. It offers new paradigms for manufacturers, by virtue of enabling in-situ decisions for optimizing the process as a dynamic system. However, the static nature of current (pre-Industry 4.0) best-practice frameworks may be viewed as at odds with this emerging novel approach. In addition, many of the predictive tools leveraged in an Industry 4.0 system are black-box in nature, which presents other concerns of tractability, interpretability and ultimately risk. This article presents a perspective on the current state-of-the-art in the aerospace composites industry focusing on risk reduction in the autoclave processing, as an example system, while reviewing current trends and needs towards a Composites 4.0 future.

Progressive Damage Numerical Modelling and Simulation of Aircraft Composite Bolted Joints Bearing Response.

Finite element numerical progressive damage modelling and simulations applied to the strength prediction of airframe bolted joints on composite laminates can lead to shorter and more efficient product cycles in terms of design, analysis and certification, while benefiting the economic manufacturing of composite structures. In the study herein, experimental bolted joint bearing tests were carried out to study the strength and failure modes of fastened composite plates under static tensile loads. The experimental results were subsequently benchmarked against various progressive damage numerical modelling simulations where the effects of different failure criteria, damage variables and subroutines were considered. Evidence was produced that indicated that both the accuracy of the simulation results and the speed of calculation were affected by the choice of user input and numerical scheme.

Material properties and manufacture technologies analysis of combined ship power-plants from different steels

The peculiarities of the structure of welded joints of different steels caused by their pronounced chemical, structural and mechanical heterogeneity must be taken into account when designing materials of a structure and choosing the technology of its manufacture. In the manufacture of composite structures of different steels, most existing welding methods can be used. The search for the most rational method of manufacturing ship auxiliary mechanisms of different steels, as well as for welding units of the same alloy, is determined by the conditions for obtaining a welded structure of high quality. It is not necessary to take into account the cost minimization and optimization of the type of connection, the grade of the alloying elements as well as the total cost of the construction. When welding steels of the same structural class, any additional requirements for the choice of a welding method and its mode, with consideration for the heterogeneity of the joint, can usually be ignored. Within one and the same structural class, the thermophysical properties of steels do not significantly differ. The properties of the transition layers of the seam due to the penetration of the base metal in such compounds in most cases are not significantly different from the properties of the weld metal either. The consideration of the choice of welding materials and the base metal for the compounds to be tested is the most difficult due to the significant difference between the welding steels. Therefore, the question of preserving properties in the transition areas of the seam, as well as the minimizing the diffusion layers in the alloy zone, are of particular importance for such compounds. It is important to compare the properties of welded joints using thermally-hardened and dispersion-hardened austenitic steels. For the steels of this class, heat treatment after welding is required to restore the properties of the surrounding area. The choice of the welded joint heat treatment mode is of particular value, it depends on the level of alloying both steel and the welded joint. The magnitudes of operational loads on the ship’s power plants are also taken into account

Formation of letters in the alphabet using 4D printing of composites

This paper presents the procedure for the manufacturing of composite structures with complex geometries using the method of 4D printing of composites (4DPC). In 4DPC, the complex structures are made without the use of complex molds. Flat layers of composites are deposited on a flat mold. Utilizing the anisotropy of the lay up sequence in the laminates, when these laminates are cured and cooled to room temperature, the flat laminates become curved. In this paper, the complex configurations of a few letters in the alphabet are made using the method of 4DPC. Specifically the formations of the letters C, O, N, R, D, I, A are made. These letters form the basic letters for the word CONCORDIA. The lay up sequence for each of the letters is presented. The final configurations of the letters after curing and cooling are presented.

Influence of Pre-consolidation parameters on the preforming deformation mechanisms of polymeric composites

The manufacture of composite structures with prepreg materials is typically performed over the desired shape using different methods, such as double diaphragm process. The double diaphragm forming process requires preheating and compaction pressure, which causes some degree of consolidation within the composite sheets. This initial step, pre-consolidation, has an effect on the deformation mechanisms, including in-plane and out-of-plane properties, that controls the material’s formability. This paper aims to investigate the effect of pre-consolidation parameters, specifically: processing temperature and compaction pressure, on the in-plane deformation mechanisms. The research is mainly focused on the deformation mechanisms of 8-harness satin (8HS) weave carbon/epoxy prepreg under in-plane shearing using picture frame test. The results of the study demonstrate that an increase in pre-consolidation temperature and pressure, consequences in decreased in-plane shear deformability of woven textile prepreg. However, optimum pre-consolidation parameters must be attained to enhance resin flow within fiber tows and to avert micro-voids.

Laser writing of Cu/CuxO integrated structure on flexible substrate for humidity sensing

Efficient manufacturing of composite structure is one of the key challenges in the fabrication of flexible devices. Laser direct writing provides a solution due to its area selectivity, controllability and rapid processing. In this study, one-step patterning of Cu/CuxO structure with tunable composition on flexible substrate has been realized with laser direct writing on Cu precursor film. An integrated humidity sensor that combines a CuxO-rich porous sensing structure with a conductive Cu-rich structure has been fabricated. The one-step-written device showed high sensitivity to human breathing. This work highlights the promise of this one-step mask-less laser writing as an efficient, rapid and low-cost composite structure fabrication process for flexible devices.

Additive manufacturing of composite structures for the restoration of bone tissue

Bone defects of critical size are still challenging to repair and significant effort has been put into the development of composite structures fabricated by additive manufacturing (AM) as bone restoring materials. The controlled and fully interconnected porosity is one of the features that makes AM especially attractive for the reconstruction of bone defects. However, developments over the past years have yielded additional valuable findings on the optimization of additive manufactured composites for their application in bone restoration. In this review we discuss different AM techniques that can be applied for composite fabrication. We demonstrate the important parameters for composites processing and aim to illustrate how AM has facilitated the development of improved structures for bone repair.

Optimization of Multi-Component Topology and Material Orientation Design of Composite Structures Using AI

Composite materials offer a wide range of advantages in terms of strength, weight, and versatility, making them a popular choice for structural applications. The performance of composite structures depends not only on the material constituents but also on the topology and arrangement of these components. However, designing an optimal configuration that maximizes the desired performance metrics can be a challenging task due to the large design space and complex interactions between the components. It presents a novel approach for optimizing the multi-component topology and material orientation design of composite structures using artificial intelligence (AI) techniques. The objective is to develop an automated and efficient methodology that can identify the most optimal configuration for a given set of performance requirements. The proposed methodology combines AI algorithms, such as genetic algorithms and machine learning, with advanced computational modeling techniques. The genetic algorithms are employed to explore the design space and search for the best combination of topology and material orientations. Concurrently, machine learning techniques are utilized to model the complex relationships between the design variables and performance metrics, enabling the identification of design patterns and accelerating the optimization process.
 To achieve this, a comprehensive framework is established, encompassing the generation of an initial population of candidate designs, evaluation of their performance using numerical simulations or experimental testing, application of genetic algorithms to iteratively evolve the population by selecting the fittest designs and introducing variations, and utilization of machine learning models to predict the performance of new designs and guide the optimization process. The effectiveness of the proposed methodology is demonstrated through a case study involving the design of a composite aerospace structure. The results reveal that the AI-based optimization approach significantly outperforms traditional trial-and-error methods, leading to improved performance metrics such as strength, stiffness, and weight.
 The findings have significant implications for the design and manufacturing of composite structures in various industries, including aerospace, automotive, and civil engineering. The automated optimization process enabled by AI techniques can efficiently explore the design space, identify innovative solutions, and ultimately enhance the overall performance of composite structures.