Thin-walled Composite Structures Research Articles

Energy Absorption and Failure Modes of Different Composite Open-Section Crush Elements under Axial Crushing Loading.

In order to study the energy absorption characteristics of the open-section thin-walled composite structures with different cross-sections, axial compression tests were carried out at loading speeds of 0.01 m/s, 0.1 m/s, and 1 m/s. Finite element models were built to predict the crushing response and energy absorption behaviors of these open-section structures. The effects of the cross-section's shape, cross-section aspect ratio, trigger mechanism, and loading speed on the energy absorption characteristics of the composite structures were analyzed. The results show that the average crushing loads of the hat-shaped and Ω-shaped open-section structures are 14.1% and 14.6% higher than those of C-shaped open-section structures, and the specific energy absorption (SEA) values are 14.3% and 14.8% higher than that of C-shaped open-section structures, respectively. For the C-shaped open-section structures, a 45° chamfer trigger is more effective in reducing the initial peak load, while a 15° steeple trigger is more appropriate for the hat-shaped open-section structures. The average crushing loads and SEA of C-shaped, hat-shaped, and Ω-shaped open-section structures are reduced when the loading speed is increased from 0.01 m/s to 1 m/s. The increase in loading speed leads to the splashing of debris and thus reduces the loading area and material utilization of open-section structures, leading to a decrease in energy absorption efficiency.

Stability of Thin-Walled Composite Structures with Closed Sections Under Compression

Stability of Thin-Walled Composite Structures with Closed Sections Under Compression

Natural Fibre and Hybrid Composite Thin-Walled Structures for Automotive Crashworthiness: A Review.

Natural fibres, valued for their low density, cost-effectiveness, high strength-to-weight ratio, and efficient energy absorption, are increasingly emerging as alternatives to synthetic materials in green composites. Although they cannot fully replace synthetic counterparts, like carbon, in structural applications due to their inferior mechanical performance, combining them through hybridization presents a potential solution. This approach promotes a balance between environmental benefits and mechanical efficiency. Recently, the transportation sector has shifted its focus towards delivering lightweight and crashworthy composite structures to improve vehicle performance, address safety concerns, and minimise environmental impact through the use of eco-friendly materials. The crashworthiness of energy absorbers, typically thin-walled structures, is influenced by several factors, including their material and geometric design. This paper presents a comprehensive overview of recent studies focused on the crashworthiness of fibre-reinforced, thin-walled composites under axial crushing. It explores different aspects, such as their materials, cross-sections, stacking sequences, triggering or filling mechanisms, and the effect of loading rate speed. Emphasis is placed on natural-fibre-based materials, including a comparative analysis of synthetic ones and their hybridization. The primary objective is to review the progress of solutions using green composites as energy absorbers in the automotive industry, considering their lightweight design, crashworthiness, and environmental sustainability.

Evaluation of Machinability Indicators in Milling of Thin-Walled Composite Structures

Evaluation of Machinability Indicators in Milling of Thin-Walled Composite Structures

Geometric Accuracy and Energy Absorption Characteristics of 3D Printed Continuous Ramie Fiber Reinforced Thin-Walled Composite Structures

The application of continuous natural fibers as reinforcement in composite thin-walled structures offers a feasible approach to achieve light weight and high strength while remaining environmentally friendly. In addition, additive manufacturing technology provides a favorable process foundation for its realization. In this study, the printability and energy absorption properties of 3D printed continuous fiber reinforced thin-walled structures with different configurations were investigated. The results suggested that a low printing speed and a proper layer thickness would mitigate the printing defects within the structures. The printing geometry accuracy of the structures could be further improved by rounding the sharp corners with appropriate radii. This study successfully fabricated structures with various configurations characterized by high geometric accuracy through printing parameters optimization and path smoothing. Moreover, the compressive property and energy absorption characteristics of the structures under quasi-static axial compression were evaluated and compared. It was found that all studied thin-walled structures exhibited progressive folding deformation patterns during compression. In particular, energy absorption process was achieved through the combined damage modes of plastic deformation, fiber pullout and delamination. Furthermore, the comparison results showed that the hexagonal structure exhibited the best energy absorption performance. The study revealed the structure-mechanical property relationship of 3D printed continuous fiber reinforced composite thin-walled structures through the analysis of multiscale failure characteristics and load response, which is valuable for broadening their applications.

Buckling of Compressed Thin-Walled Composite Structures with Closed Sections

Buckling of Compressed Thin-Walled Composite Structures with Closed Sections

Damage analysis of deployable thin-walled composite shell structure during coiling up

Thin-walled coilable composite shells have been successfully used in deployable space payloads due to their high storage efficiency and stiffness-to-weight ratio. However, in real-life applications the deployable composite shells requiring repeatable coiling up and deployment are easy to be damaged, and their damage or failure behaviors due to large deformations has been rarely studied. This paper aims to investigate their damage and failure behaviors during the snap through, equal-sense and opposite-sense coiling up. First, multiscale models are established to compute the mechanical properties of plain-woven fabrics and unidirectional prepregs with consideration of stiffness reduction of the composites. Then, damages of each ply within the thin-walled shell are studied during snap through and coiling up. Simultaneously, effects of hub diameter on the damage of the thin-walled shell were investigated, followed by experimental verification using strain gauges and a scanning electron microscope (SEM). Simulation and experimental results show that the dominated damage of the thin-walled shell is failure of matrix. The SEMs of the edge damaged show failures of both resin and carbon fibers. Tensile damages of matrix can be relieved as the increment of the hub diameter, and no obvious failure is observed at the hub diameter of 50 mm.

Exploring guided wave propagation in composite cylindrical shells with an embedded delamination through refined spectral element method

The elastic wave propagation in a thin-walled composite structure carries information concerning its material properties and structural discontinuities. Towards the damage identification, a refined spectral element method (SEM) is presented to explore the guided wave propagation in composite cylindrical shells with embedded delamination. The governing equations and the natural boundary conditions are derived using the first-order shear deformation theory along with the Hamilton's principle. The spectral shell element is established via the exact strong-form solutions, and the dynamic stiffness matrix is formulated through the force-displacement relations. Transforming the wave differential equations into the frequency domain, the numerical Laplace transform is applied to solve the inverse problems. Following a sub-laminate approach, the delamination is modeled by means of four spectral elements and the interface continuity conditions. In addition, the stiffness reduction method is introduced to approximate the delamination. A comparison with published results in terms of the natural frequencies is made to verify the accuracy of the SEM. The dispersion characteristics of the cylindrical shell are examined including the wave number, phase and group velocities. The interaction of guided waves with the delamination is revealed. The influences of the length, depth, and position of the delamination on the wave responses are demonstrated.

Buckling Analysis of Thin-Walled Composite Structures with Rectangular Cross-Sections under Compressive Load.

The purpose of this research was the analysis of the stability of compressed thin-walled composite columns with closed rectangular cross-sections, subjected to axial load. The test specimens (made of carbon-epoxy composite) were characterized by different lay-ups of the composite material. Experimental tests were carried out using a universal testing machine and other interdisciplinary testing techniques, such as an optical strain measurement system. Simultaneously with the experimental studies, numerical simulations were carried out using the finite element method. In the case of FEA simulations, original numerical models were derived. In the case of both experimental research and FEM simulations, an in-depth investigation of buckling states was carried out. The measurable effect of the research was to determine both the influence of the cross-sectional shape and the lay-up of the composite layers on the stability of the structure. The novelty of the present paper is the use of interdisciplinary research techniques in order to determine the critical state of compressed thin-walled composite structures with closed sections. An additional novelty is the object of study itself-that is, thin-walled composite columns with closed sections.

Multi-Objective Optimization of Thin-Walled Composite Axisymmetric Structures Using Neural Surrogate Models and Genetic Algorithms.

Composite shells find diverse applications across industries due to their high strength-to-weight ratio and tailored properties. Optimizing parameters such as matrix-reinforcement ratio and orientation of the reinforcement is crucial for achieving the desired performance metrics. Stochastic optimization, specifically genetic algorithms, offer solutions, yet their computational intensity hinders widespread use. Surrogate models, employing neural networks, emerge as efficient alternatives by approximating objective functions and bypassing costly computations. This study investigates surrogate models in multi-objective optimization of composite shells. It incorporates deep neural networks to approximate relationships between input parameters and key metrics, enabling exploration of design possibilities. Incorporating mode shape identification enhances accuracy, especially in multi-criteria optimization. Employing network ensembles strengthens reliability by mitigating model weaknesses. Efficiency analysis assesses required computations, managing the trade-off between cost and accuracy. Considering complex input parameters and comparing against the Monte Carlo approach further demonstrates the methodology's efficacy. This work showcases the successful integration of network ensembles employed as surrogate models and mode shape identification, enhancing multi-objective optimization in engineering applications. The approach's efficiency in handling intricate designs and enhancing accuracy has broad implications for optimization methodologies.

A comprehensive analysis for real-time shape and strain sensing of composite thin-walled structure

This study presents an effective and reliable shape sensing strategy to model the flat cross section deformation of laminated composite thin-walled structure. The proposed strategy is derived by using the customized isogeometric I-beam elements and Murakami zigzag theory (MZT), named IG-MZT(I). Compared to the traditional reconstruction model, the IG-MZT(I) offers numerous advantages for displacement and strain analysis of composite thin-walled structure such as high-order continuity representation, simple mesh refinement, and fewer strain sensors. Initially, the theoretical framework of composite I-beam element based on the MZT deformation theory is presented. Then, the isogeometric analysis is introduced to approximate the displacement field, and the order of B-spline basis functions can be determined by the constitutive equations.Furthermore, an intuitive strain conversion model is established for solving the geometric complexities in strain measurements. Finally, utilizing a least-square variational function within IG-MZT(I) enables the establishment of the shape sensing formula. Several numerical analyses and experiment results demonstrate the outstanding accuracy and reliability of the proposed approach for reconstructing composite I-beams.Overall,the present model is simple and independent of the traditional composite reconstruction models that require embedded sensors,making the IG-MZT(I)sufficiently general to analyse the composite structure in service.

Energy absorption of variable stiffness composite thin-walled tubes on axial impacting

In order to realize the engineering application of automated fiber placement for composite laminates, a method of the variable angle fiber placement was proposed to design the tube structures of variable stiffness composites based on the quadratic Bezier curve. The axial crushing responses were simulated to investigate the energy absorption characteristics of composite tubes. The results showed that the method of the variable angle fiber placement contributed to the improvement of the energy absorption effects. The maximum crushing force efficiency of the variable stiffness composite thin-walled tubes designed by the method was 49.04% which was 106.48% higher than the constant stiffness composite tube. The results could be helpful for the process of automated fiber placement and the design of the energy absorption for composite thin-walled structures.

Failure Mechanism of Tensile CFRP Composite Plates with Variable Hole Diameter

Real thin-walled composite structures such as aircraft or automotive structures are exposed to the development of various types of damage during operation. The effect of circular hole size on the strength of a thin-walled plate made of carbon fibre-reinforced polymer (CFRP) was investigated in this study. The test object was subjected to tensile testing to investigate the strength and cracking mechanism of the composite structure with variable diameter of the central hole. The study was performed using two independent test methods: experimental and numerical. With increasing diameter of the central hole, significant weakening of the composite plate was observed. The study showed qualitative and quantitative agreement between the experimental and numerical results. The results confirmed the agreement of the proposed FEM model with the experimental test. The novelty of this study is the use of the popular XFEM technique to describe the influence of the hole size on the cracking and failure of the composite structure. In addition, the study proposes a new method for determining the experimental and numerical damage and failure loads of a composite plate under tension.

Investigations Into the Stability of Thin-Walled Composite Structures with Top-Hat Cross-Sections

Abstract This paper presents a study of compressed thin-walled composite columns with an open cross-section. The tested specimens with a top-hat cross-section were made of CFRP material. Two arrangements of composite layers [0/-45/45/90]s and [90/0/90/0]s were compared. The paper focuses on the buckling phenomenon and the determination of the critical loads of the structure. It includes both numerical analyses using the finite element method (FEM) and validation on real specimens made using the autoclave technique. A comparison is made between the results obtained by both methods. The critical forces of the real specimens were determined using the P-wc3 approximation method. Both the evaluation of the buckling shape and the values of the critical forces showed a significant correlation between the experimental and numerical tests. This paper also compares the tested lay-ups.

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.

Adaptive modal identification of honeycomb thin-walled composite structures with pit defects under thermal modal testing using variational mode decomposition technique based on digital image correlation

The active imaging blue light high-speed three-dimensional digital image correlation (3D-DIC) system was combined with a transient aerodynamic heating system and random pulse excitation technology to establish a high-frequency thermal vibration optical test system capable of multi-temperature zone testing at 900 °C to accurately obtain the thermal modal parameters of thin-walled structures. The full-field temperature distribution of a novel honeycomb thin-walled composite structure (HTWCS) with pit defects under non-uniform temperature was accurately simulated through reinforcement learning. The first six modal frequencies of honeycomb thin-walled composite structure with pit defects at room temperature increased compared with the non-destructive honeycomb thin-walled composite structure, and the third and fourth modal shapes interchanged. An adaptive modal parameter identification method for full-field three-dimensional vibration measurements under high-temperature environments was developed by combining the successive multivariate variational mode decomposition (VMD) with the modal superposition method. This method can effectively identify close modes. The area ratio-based approach was used to evaluate the damping of the measured response. The finite element model (FEM) of the HTWCS with pit defects at different temperatures was updated using a multi-state step-by-step model updating method and the temperature-dependent material properties were established. The results showed that the introduction of a damping ratio improved the accuracy of the updated FEM of HTWCS with pit defects. The proposed technique is anticipated to provide an effective method for high-frequency thermal vibration optical measurement and modal identification of thin-walled structures under multi-temperature regions.

Multi-resolution non-contact laser-sensing damage detection technique for composite laminates

Numerous ultrasonic (UT) modalities have been developed to perform inspections of engineered structures. However, increased precision often comes at the cost of longer inspection times. As an example, guided waves propagating in plate-like structures allow fast inspection of large areas due to their long propagation distances. Damage characterization accuracy is, however, limited in this case due to relatively large wavelengths. On the other hand, local inspection with high-frequency bulk waves provides much more accurate damage characterization thanks to shorter wavelengths. The drawback of local inspection is the long scanning time necessary for large components. Here, we discuss a two-step laser-interferometer-based non-contact approach for the inspection of carbon-fiber-reinforced structures. First, a full-field screening using guided waves is performed to determine the possible damage locations. The identified hot spots are further evaluated in the second step with a high-frequency bulk wave system. A piezoceramic exciter is used for the guided wave setup, while a pulsed laser excitation source is used in the bulk wave setup. The proposed approach minimizes sample preparation time and is completely non-intrusive. The feasibility of the proposed framework is demonstrated in a complex shape, thin-walled composite structure with multiple defects.

Experimental Investigation on the Failure Behavior of Single-Stiffened Composite Panels under Compressive Load

Whilst the thin-walled composite structures have been widely used to build the primary components in aircraft, Low-Velocity Impact (LVI) can cause severe intra-laminar and inter-laminar damage in the composite material, reducing the mechanical performance of these components. This paper presents an experimental study on the compressive failure behavior of a Single Hat-Stiffener Panel (SHSP) with two typical flaws, i.e., impact-induced damage and skin/stiffener debonding. LVI tests were performed to introduce the damage around the radius edge of the stiffener and the center of the flat skin panel, respectively, while the skin/stiffener debonding was introduced to the SHSP by artificially embedding a PTFE film at the skin/stiffener interface during autoclave curing. The impact energy to cause Barely Visible Impact Damage (BVID) was determined to be 22 J for the thin SHSP configuration. Whilst the considerable damage area was detected by the ultrasonic scan for the thick SHSP at an energy level of 35 J, the impact-induced indentation was not visually detectable (<0.35 mm), indicating a high impact resistance of this thick SHSP. Compression After Impact (CAI) results showed a significant reduction (59.1%) in the ultimate compressive strength of the SHSP when the impact location was on the stiffener compared to the pristine specimen. However, this reduction is less significant (34.7%) when the damage resulted from the same impact energy in the flat skin panel. In this study, the presence of skin/stiffener debonding does not adversely influence the compressive strength of the thick SHSP, while this debonding flaw appears to cause a minor reduction in the compressive strength of the thin SHSP.

Geometrically nonlinear analysis of functionally graded composite shells using MITC4 and MITC9 elements

In this paper, the geometrically nonlinear behaviors of functionally graded composite shells under large displacements and large rotations are analyzed. Instead of constructing conventional layer-wised models, equivalent single-layer composite shell elements based on general displacement fields are developed to model the inhomogeneity of composite materials. The mixed interpolation of tensorial components technique is used to eliminate membrane locking and shear locking phenomena. The assumed covariant strain fields are initially constructed before being used to evaluate the second Piola–Kirchhoff stress and construct stiffness matrices in local Cartesian coordinates. The standard full Newton–Raphson method is utilized to solve a system of nonlinear equations. Geometrically nonlinear analyses are performed on different thin-walled composite structures including thin cantilever beams, slit annular plates, pinched cylindrical shells, and hemispherical shells. Obtained results demonstrate good convergence characteristics and modeling capability of the developed quadrilateral composite shell elements in analyzing thin-walled composite structures.

INFLUENCE OF LAYER ARRANGEMENT ON THE STABILITY AND FAILURE OF THIN-WALLED COMPOSITE STRUCTURES

The purpose of this article is to study the effect of the layup n stability and load carrying capacity of a thin-walled composite structure with an open section - angle. The research part carried out by means of the finite element method, using the commercial software Abaqus®, presents the analysis of the limit states of compressed composite profiles, made of carbon-epoxy (CFRP) material, with a variable layer configuration.