Fiber-reinforced Composite Structures Research Articles

Coupling design and fabrication of continuous carbon fiber-reinforced composite structures using two-material topology optimization and additive manufacturing

Coupling design and fabrication of continuous carbon fiber-reinforced composite structures using two-material topology optimization and additive manufacturing

Experimental and theoretical study on the response of the X-frame CFRP sandwich beam under the local impulsive loading

The impact resistance of the carbon fibre reinforced X-frame sandwich beam and its foam reinforced structure was investigated experimentally and theoretically. To simulate the blast loading on the composite sandwich beams, the aluminum foam projectiles were utilised to impact the composite specimens. Under the local impulsive loads of the different intensities, the core shear failure, core crushing and skin fracture of the sandwich beams were observed by using a high-speed camera. A theoretical model for the dynamic response of the fibre-reinforced composite sandwich beam under the impact loads was constructed. Three phases, namely the local compression, the propagation of global bending/shearing wave and global stretching were considered in this theory. The theoretical model can provide the reasonable deflection history and failure modes of the sandwich beam compared with the experimental and simulated results. The impact pressure between the foam projectile and the beam were obtained by the theoretical and numerical predictions. The results indicated that the decreasing of the structural density can reduce the transmitted impulse. The optimal design charts were constructed by introducing the normalised variables based on the theoretical model. The minimum weight design revealed that the composite sandwich beams with low-density cores generally had the better specific impact resistance, while the foam-filled sandwich beams with the higher density had the outperformed impact resistance at the low impulse. It is expected that the theoretical model can provide a useful reference for predicting the impact resistance of the fibre-reinforced composite sandwich structures.

INFLUENCE OF PLY ORIENTATION ON INTERLAMINAR FRACTURE TOUGHNESS OF CARBON/EPOXY COMPOSITES

Delamination is a concern for the structural integrity of fiber-reinforced polymer composite structures. The decisive material parameter for delamination is fracture toughness (G<sub>Ic</sub>). The main objective of this study was to investigate the in-plane, quasi-static fracture toughness (both initiation and propagation) of two laminate composites. Both composites contain carbon fibers in an epoxy matrix. The tests were performed using double cantilever beam specimens with different ply orientations according to ASTM standards. The effect of ply orientation was evaluated using data reduction methods to obtain fracture toughness. The experiments revealed that the ply sequence changed the crack propagation mechanism, increasing the initiation and propagation fracture toughness values for 90/90 layup during mode I interlaminar tests, by 80% and 69%, respectively. In addition, the mode I fracture toughness increased by 40% after 50 mm crack extension.

Damage Detection in Carbon Fiber-Reinforced Composite Structures Using Eddy Current Pot-Core Sensor

Damage Detection in Carbon Fiber-Reinforced Composite Structures Using Eddy Current Pot-Core Sensor

An experimental study on 3D-printed continuous fiber-reinforced composite auxetic structures

Auxetic structures have negative Poisson’s ratios (NPR). Due to the unique deformation mechanism, auxetic structures possess extraordinary mechanical properties, such as indentation resistance, shear resistance, fracture toughness, and energy absorption capability. However, the stiffness and load-bearing capacity are the weak points for auxetic structures. 3D printing of continuous fiber-reinforced composite enables the fabrication of lightweight and highly stiff complex structures, providing a perfect manufacturing method to remedy the shortcomings of auxetic structures. This work investigated the mechanical properties of 3D-printed continuous fiber-reinforced composite auxetic structures. In this study, we utilized continuous fiber-reinforced composite 3D printing to fabricate two types of auxetic structures. The fiber path configurations were varied among the test specimens to explore the effect of fiber distribution on mechanical properties. A uniaxial tensile test was performed to evaluate the tensile properties and Poisson’s ratio of continuous fiber-reinforced composite auxetic structures. Results showed that the tensile modulus and strength have been dramatically improved with a minor mass increase. The auxetic behavior can be strengthened by properly allocating the reinforcing fibers. However, the addition of continuous fiber led to different performances on the selected auxetic structures. In summary, two out of the five specimens demonstrated simultaneous improvements in stiffness, strength, and auxeticity across the conducted tests.

Structure, Thermal, and Mechanical Behavior of the Polysulfone Solution Impregnated Unidirectional Carbon Fiber Yarns.

The paper is devoted to the study of thermal and mechanical behavior and structural features of the polysulfone solution impregnated unidirectional carbon fiber yarns depending on fabrication conditions and appearance for optimum production method of the composites. The effect of producing conditions, such as polysulfone solution concentration, drying and post-heating temperatures, and the residual solvent content on the structure, mechanical, and thermal properties of the carbon fiber-reinforced composites was studied. The polysulfone solution impregnated carbon fiber yarns show relatively high mechanical properties, realizing up to 80% of the carbon fibers' tensile strength, which can be attributed to good wettability and uniform polymer matrix distribution throughout the entire volume of the composites. It was found that the composites impregnated with 40 wt.% of the polysulfone solution showed lower porosity and higher mechanical properties. The results of a dynamic mechanical analysis indicate that residual solvent has a significant effect on the composites' thermal behavior. The composites heated to 350 °C for a 30 min showed higher thermal stability compared to ones dried at 110 °C due to removal of residual solvent during heating. The impregnated carbon fiber yarns can be used for the further producing bulk unidirectional composites by compression molding and the proposed method can be easily transformed to continuous filament production, for example for further use in 3-D printing technology.

Experimental Investigation of Mechanical Properties of Additively Manufactured Fibre-Reinforced Composite Structures for Robotic Applications

AbstractThis study explores the variation in mechanical properties of additively manufactured composite structures for robotic applications with different infill densities and layer heights using fused deposition modelling (FDM). Glass fibre-reinforced polyamide (GFRP), and carbon fibre-reinforced polyamide (CFRP) filaments are used, and the specimens are printed with 20%, 40%, 60% and 100% infill density lattice structures for tensile and three-point bending tests. These printed samples are examined in the microscope to gain more understanding of the microstructure of the printed composites. To characterise the mechanical properties, a set of tensile and three-point bend tests are conducted on the manufactured composite samples. Test results indicate the variations in tensile strength and Young’s modulus of specimens based on the printing parameters and reveal the tensile and bending behaviour of those printed composite structures against varying infill ratios and reinforcing fibres. The experimental findings are also compared to analytical and empirical modelling approaches. Finally, based on the results, the applications of the additively manufactured structure to the robotic components are presented.

On Impact Damage and Repair of Composite Honeycomb Sandwich Structures

This study is conducted on glass fiber-reinforced composite honeycomb sandwich structures by introducing delamination damage through low-velocity impact tests, establishing a three-dimensional progressive damage analysis model, and evaluating the delamination damage characteristics and laws of honeycomb sandwich structures under different impact energies through experiments. Repair techniques and process parameters for delamination damage are explored. It is found that as the impact energy increases, the damage area of honeycomb sandwich panels also increases, and the delamination damage extends from the impact center to the surrounding areas, accompanied by damage such as fiber fracture and matrix cracking. The strength recovery rates of sandwich panels at impact energies of 5 J, 15 J, and 25 J after repair are 71.90%, 65.89%, and 67.10%, respectively, which has a considerable repair effect. In addition, a progressive damage model for low-velocity impact on the composite honeycomb sandwich structure is established, and its accuracy and reliability are verified.

Spatial 3D Printing of Continuous Fiber-Reinforced Composite Multilayer Truss Structures with Controllable Structural Performance.

Continuous fiber-reinforced composite truss structures have broad application prospects in aerospace engineering owing to their high structural bearing efficiency and multifunctional applications. This paper presents the design and fabrication of multilayer truss structures with controlled mechanical properties based on continuous fiber-reinforced thermoplastic composite 3D printing. Continuous fiber composite pyramid trusses fabricated by 3D printing have high specific stiffness and strength, with maximum equivalent compression modulus and strength of 401.91 MPa and 30.26 MPa, respectively. Moreover, the relative density of a truss structure can be as low as 1.45%. Additionally, structural units can be extended in any direction to form a multilayer truss structure. Structural performance can be controlled by designing the parameters of each layer. This study offers a novel approach for designing a multifunctional multilayer truss structure, a structure with low-density needs and unique load-bearing effects.

Free Vibration and Dynamic Analysis on Free-Constrained Layer of Graphene Based on Composite Conical Shell via Jacobi–Ritz Method

Excessive vibration has always been a serious problem for conical shell structures, while the application of the graphene-based free-constrained layer (GFCL) based on carbon fiber-reinforced composite (CFRC) structure is a novel way to improve structural performance. An analytical model for vibration and dynamic characteristics of the GFCL-CFRC conical shell resting on the Winkler–Pasternak elastic foundation with arbitrary boundary conditions is constructed, and four types of GFCL porosity distribution and GFCL dispersion pattern are considered in this model. The multi-segment technique and virtual spring technique are utilized to simulate arbitrary boundary conditions. Then, the first-order shear deformation theory (FSDT) and Hamilton’s principle are employed to obtain the motion equation of the GFCL-CFRC conical shell, and the motion equation of the GFCL-CFRC conical shell is solved by the Ritz method. In conclusion, the dispersion mode of GFCL, thickness ratio of GFCL, and fiber angle have influence on the dynamic performance. With a reasonable design, the dynamic performance of the GFCL-CFRC conical shell can be further improved.

Fabrication of a 3D Printed Continuous Carbon Fiber Composite Grid Stiffened Structure Using Induction Heating.

In aerospace applications, composite grids have been widely utilized to enhance the strength of large thin-shell components. Recently, a growing focus has been on the research of 3D printing continuous fiber-reinforced thermoplastic composites. The 3D printing method offers various advantages over traditional molding processes, including a simpler process, higher material utilization, and lower manufacturing costs. However, the use of 3D printing for manufacturing continuous fiber-reinforced composite structures presents challenges, such as a high occurrence of defects within the structure and insufficient mechanical properties. These limitations hinder its widespread application. To address these issues, this study proposes a method for treating 3D-printed composite grid structures using induction heating. Initially, the induction heating mechanism of 3D-printed composite grids was analyzed by studying the impedance at the junction, including direct contact resistance and dielectric hysteresis loss. Subsequently, the impact of induction heating treatment on internal defects was explored by observing micro morphologies. The results show that the combination of induction heating and vacuum pressure effectively reduces porosities within the 3D-printed carbon fiber composite grids. Additionally, 3D-printed composite grid-stiffened PLA structures were fabricated with induction heating, and the bending and impact tests were conducted to evaluate their mechanical properties. The results indicate that using a grid-unit size of 4 mm leads to significant increases in bending strength and modulus of the grid-stiffened structure, with improvements of 137.6% and 217.8%, respectively, compared to the neat PLA panel. This demonstrates the exceptional mechanical enhancement efficiency of the 3D-printed lightweight composite grids.

Role of fiber dosages and lengths on the mechanical strength, shrinkage, and pore structure of CO2-cured amorphous metallic-fiber-reinforced belite-rich cement composites

The impact of varying dosages and lengths of amorphous metallic fiber (AMF) on the mechanical strength, shrinkage, and pore structure of fiber-reinforced cementitious composites cured in water and in a carbonation chamber were assessed. This study evaluated fourteen mortar mixes, i.e., seven mixes for traditional water curing and seven for CO2-curing (60% relative humidity, 20 °C, and 10% CO2 concentration). The outcomes reveal that the compressive and flexural strength are significantly augmented for the mixes reinforced with AMFs as compared to the control mix (fiber-free mix), and this behavior is more pronounced with increased dosages and lengths of the AMF. Furthermore, compared to the control mix, significant mitigation in the development and magnitude of shrinkage was recorded with increased dosages and lengths of the AMF. The porosity and voids were all increased in the samples with AMF as compared to the control mix. The carbonation-cured specimens exhibited significantly higher mechanical strength, fewer voids, and higher shrinkage than those cured in water. Based on the experimental findings, it is recommended to use 0.50% volume of AMF and a length of 20 mm to produce fiber-reinforced cement composites, as this combination provides outstanding performance among all mixes tested here.

Additive Manufacturing of Continuous Fiber-Reinforced Polymer Composite Sandwich Structures with Multiscale Cellular Cores

The use of composite sandwich structures with cellular cores is prevalent in lightweight designs owing to their superior energy-absorbing abilities. However, current manufacturing processes, such as hot-press molding and mold pressing, require multiple steps and complex tools, thus limiting the exploration of advanced sandwich structure designs. This study reports a novel multi-material additive manufacturing (AM) process that allows the single-step production of continuous fiber-reinforced polymer composite (CFRPC) sandwich structures with multiscale cellular cores. Specifically, the integration of CFRPC-AM and in situ foam AM processes provides effective and efficient fabrication of CFRPC panels and multiscale cellular cores with intricate designs. The cellular core design spans three levels: microcellular, unit-cell, and graded structures. Sandwich structures with a diverse set of unit-cell designs, that is, rhombus, square, honeycomb, and re-entrant honeycomb, were fabricated and their flexural behaviors were studied experimentally. The results showed that the sandwich structure with a rhombus core design possessed the highest flexural stiffness, strength, and specific energy absorption. In addition, the effect of the unit-cell assembly on the flexural performance of the CFRP composite sandwich structure was examined. The proposed design and fabrication methods open new avenues for constructing novel and high-performance CFRPC structures with multiscale cellular cores that cannot be obtained using existing approaches.

Improving the crashworthiness of carbon fiber-reinforced composite channel section structure through controllable failure mechanisms

To improve the crashworthiness of the composite channel section structure (CCSS), three external triggers including ditch plug trigger (DPT), trapezoidal groove trigger (TGT), and v-groove trigger (VGT) are proposed. A progressive damage model for CCSS is developed considering the damage initiation and evolution of intra- and inter-ply. Based on the model, the effects of the size of the external trigger and the friction coefficient between the external trigger and the structure on the axial energy absorption characteristics and failure modes of CCSS are investigated. The results show that the external trigger has a large influence on the failure process. The increase in ditch height of DPT is beneficial to improve the load-bearing capacity of the structure in the middle stage of crushing. Compared to DPT and TGT, VGT is more effective in decreasing the initial peak load and increasing the crushing load efficiency. The failure mechanisms of CCSS induced by the external trigger are revealed. The proposed external trigger can induce specific failure modes, and the structure tends to slide toward the middle after contact with TGT and VGT. The increase of the friction coefficient between CCSS and TGT or VGT can improve energy absorption. The results of the study can provide a reference for the design of external triggers for CCSS.

Topology optimization of continuous fiber-reinforced composite (CFRC) structures considering the residual stress in additive manufacturing

This work proposes a topology optimization method for continuous fiber-reinforced composite (CFRC) structures with additive manufacturing (AM), which realizes the concurrent optimization of material layout and fiber orientation. The residual stress caused by AM process is constrained to avoid the failure of warpage and distortion. The inherent strain method (ISM) is applied to simplify the calculation of residual stress, the Tsai–Wu criterion and P-norm are incorporated to realize structural failure prediction and local constraint aggregation. Several typical instances are adopted to verify the effectiveness of proposed method, which provides a novel solution for the optimization of CFRC structures fabricated by AM.

Static and dynamic mechanical behaviors of bamboo scrimber under combined tension-bending

Bamboo scrimber is a material with significant potential in the construction industry. In the context of its application as a beam structure, bamboo scrimber experiences combined tension-bending rather than single load. Furthermore, potential factors such as earthquakes, crosswinds, and progressive collapse events can induce static and dynamic deformations at varying loading rates in the structure. Up to now, the static and dynamic deformation behavior of bamboo scrimber under combined tension-bending has not been fully understood. The aim of this study is to investigate the effects of loading rate and pre-tension force on the mechanical behavior of bamboo scrimber through experimental research. The experimental results show that, both loading rate and pre-tension force will impact the mechanical behaviors of bamboo scrimber. Increasing the loading rate leads to an increase in the maximum bending load, bending fracture deflection, total fracture strain, and total dissipated energy. Increasing the pre-tension force will cause an increase in the maximum bending load, effective bending modulus, and total dissipated energy. The fracture positions become more random with the increase in pre-tension force. SEM fracture analysis shows loading rates and pre-tension forces has significant influences on the fracture mechanism. The fracture mode transitions from delamination dominated to fiber/matrix fracture dominated as pre-tension force rises from 0 N to 6000 N. This research offers valuable guidance for the practical implementation of bamboo scrimber beam in engineering applications. The findings provide data support for the design of fiber-reinforced composite and biomimetic bamboo structures subjected to complex stress conditions.

Experimental investigation on Compression-After-Impact (CAI) response of aerospace grade thermoset composites under low-temperature conditions assisted with acoustic emission monitoring

Low-velocity impact (LVI) can provoke catastrophic failure depending on the impact energy, lay-up configuration, and the operating temperature in fiber-reinforced composite structures via damage accumulation during the service. Additionally, extreme operating temperature is an important factor affecting the damage initiation and growth mechanism and understanding the compression-after-impact (CAI) response is a promising way to predict the damage-growth dynamics. For this purpose, this study investigates the LVI response and CAI strength of composite laminates under the −55 °C temperature considering the low-temperature (LT) service conditions of aerospace applications. The experiments are performed on cross-ply ([0/90°]4s) and angle-ply ([±45°]4s) lay-up composites for impact energy levels of 1.4 J and 10 J. After LVI tests, the occurred damages are examined by infrared thermography (IRT). Furthermore, the effect of the impact damage on the failure mechanism under compression loading is elaborated using acoustic emission (AE) inspection. The CAI test results reveal that the LT increases the compressive residual strength. However, this change is directly related to the load and lay-up configuration. The alternation in the damage growth mechanism of angle-ply laminates under LT operating temperature is more significant compared to the cross-ply laminates.

Evaluation of interlaminar shear strength of co-cured fiber-reinforced fabric composite structures by lap joint method

Interlaminar shear strength (ILSS) indicates the resistance to interlaminar delamination of fiber-reinforced composite structures. The short beam shear (SBS) method has been commonly used for ILSS measurement, but unwanted failure modes can appear like a compressive or tensile failure in surface, and diagonal shear failure, causing poor measurement accuracy. The lap joint method has advantage that leading to a clear measurement of the shear strength. This paper proposed the lap joint method for extracting ILSS values of co-cured carbon or glass fiber-reinforced fabric composites (CFRC or GFRC) by minimizing the discrepancy between the experiment and finite element analysis of ILSS test. The lap joint method can compensate for the shortcomings of the SBS method. The calculated ILSS based on the lap joint method (LJ-ILSS) with correction factors showed similar values to the ILSS values by SBS method (SBS-ILSS) done by our work and other works of literature. Therefore, the proposed lap joint method has shown potential as a method to measure ILSS of the co-cured fiber-reinforced fabric composites, but also it can be extended to other types of fiber-reinforced composites.

Hybrid effect of 3D-printed coaxial continuous hybrid fibre-reinforced composites

Fibre-reinforced composite thin-walled structures are often subjected to complex working conditions with multiple loads. By combining various fibres, continuous hybrid fibre-reinforced composites (CHFRCs) can achieve good comprehensive performances and positive hybrid effects, which will improve the ability of composite structures to withstand complicated loads. Coaxial CHFRCs were prepared quickly without the use of a mould by 3D printing. The influence of the hatch spacing on hybrid effect, dynamic and static bending properties and failure mode were investigated by the research of fibre content, hybrid ratio, and interface. The results showed that the 3D-printed coaxial CHFRCs had a positive hybrid effect for the impact strength. A high positive hybrid effect could be attained by modifying the process parameters. At the same time, the composite’s comprehensive performances were enhanced. The impact strength of 3D-printed coaxial CHFRCs was 1.94 times greater than that of the continuous carbon-fibre-reinforced composites, and its flexural strength was 1.75 times greater than that of the continuous Kevlar-fibre-reinforced composites at same process parameter. With their high comprehensive performances and high positive hybrid effect, 3D-printed coaxial CHFRCs have a lot of potential use in thin-walled structures in areas such aerospace.

Stress-based topology optimization for fiber composites with improved stiffness and strength: Integrating anisotropic and isotropic materials

Fiber-reinforced materials offer high stiffness- and strength-to-mass ratios to lightweight composite structures. To design stiff, strong, and lightweight fiber-reinforced composite structures, we propose a multimaterial anisotropic stress-constrained topology optimization framework that simultaneously optimizes geometry, distribution of anisotropic (i.e., orthotropic) and isotropic material phases, and local orientations of fiber reinforcements in the anisotropic phase. To achieve high performance in both stiffness and strength, we discover that both isotropic and anisotropic materials are needed: anisotropic materials are preferred in uniaxial members to increase stiffness, while isotropic materials are crucial at multi-axially stressed joints to enhance strength. We introduce multimaterial interpolation schemes to characterize both the stiffness and strength of composites made up of anisotropic and isotropic materials. The characterization of strength is enabled by a novel load factor-based yield function interpolation that consistently integrates anisotropic Tsai–Wu and isotropic von Mises yield criteria. We optimize stress-sensitive domains considering materials with various levels of stiffness and strength anisotropy as well as multiple load cases. The anisotropic stress constraints in the proposed framework effectively inform geometries to reduce stress concentration in fiber composites. The proposed framework provides a rational design paradigm for composite structures, capitalizing on dissimilar stiffness and strength properties of anisotropic and isotropic materials, to potentially benefit various engineering applications.