Composite Structures最新文献

Lattice structures with tunable expansion properties have been investigated in multidisciplinary fields to control the temperature effects of structures or materials. The expected thermal adaptivity can be achieved by optimizing the structural geometry. A novel method for the form-finding of thermal-adaptive pin-bar assemblies is developed in this paper by considering the control of structural temperature effects as the minimization of the potential energy of the system. Based on the stationarity condition of the potential energy with respect to the nodal coordinates, the compatibility relationship between the thermal elongations of members and the target nodal displacements is proven to be the sufficient and necessary condition for structural thermal adaptivity. The solvability of the compatibility equation is determined by the rank equality between the compatibility matrix and its augmented form, which can be measured by the number of nonzero eigenvalues of its Gramian matrix. The analytical relationship between the eigenvalues of the Gramian matrix and the nodal coordinates is established using the matrix perturbation theory. A numerical strategy based on Newton’s method is proposed in which the eigenvalues are gradually modified by adjusting the nodal coordinates until the rank equality is satisfied. To address the existence of multiple solutions with structural thermal adaptivity, structural symmetry and periodicity constraints are introduced to narrow the solution space. The thermal-adaptive configurations of three illustrative pin-bar assemblies are analyzed using the proposed form-finding method, and the expected thermal deformations are verified for the obtained configurations using the finite element software ABAQUS. Comparing the results obtained by the proposed method with those obtained by nonlinear programming and the genetic algorithm validates the advantages of the proposed method in terms of computational time, optimality of the obtained configuration and applicability to complex structural geometries.

Quartz fiber-reinforced polymer (QFRP) is a vital non-polar material used in aviation wave-transparent structural components. Automatic characterization of delamination defects in QFRP is critical to aviation structural component safety. Terahertz time-domain spectroscopy (THz-TDS) is one of the new non-destructive testing (NDT) methods with highly accurate characterization of internal defects in non-polar material. Hence, attempts to extract features of THz time-domain signals for automatic characterization have been made by using deep learning algorithms. In this work, a Transformer-based neural network to classify the THz time-domain signals collected from a QFRP curved structure for automatic characterization of pre-embedded delamination defects has been reported. A THz-TDS system combined with a collaborative robot for collecting the THz signals from QFRP curved structure has been built. An automatic characterization method framework is developed. Results show that the precision rates of Transformer-based neural network for $1_{st}$ delamination to $5_{th}$ delamination are 1.0, 1.0, 1.0, 0.985, 1.0, and $F_1$ score of it is 0.982. During the process of testing, delamination defects inside the QFRP curved structure were visualized using pixels with different colors. Results indicate that the Transformer-based neural network can characterize all pre-embedded delamination defects while minimizing false identification of non-defective areas, performing outstanding generalization.

The study focuses on the free vibration analysis of beams composed of functionally graded porous materials and characterized by a variable cross-section along their length. A broad range of beams is examined encompassing various tapered configurations, porosity profiles, and porosity content. The equations of motion are derived using Hamilton’s principle within the framework of Timoshenko beam theory. These equations are solved semi-analytically using the differential transform method, which has been adapted to incorporate various boundary conditions such as clamped–clamped, clamped–free, clamped–pinned, and pinned–pinned constraints within a general formulation of the beam eigenvalue problem. To validate the proposed solution technique, computed natural frequencies are compared with existing literature results for tapered inhomogeneous beams and uniform porous beams. Notably, new results are obtained for tapered porous beams. In this regard, a comprehensive parametric study explores the influence of various factors on the natural frequencies and mode shapes of functionally graded porous beams with variable cross-sections. These factors include the type of porosity profiles, a range of porosity parameters, cross-section taper ratios, and specific boundary conditions. The findings deepen our understanding of the modal characteristics of functionally graded porous beams, providing valuable guidance for engineering design and structural optimization in relevant applications. Additionally, they may serve as benchmarks for other researchers.

Automated Fiber Placement (AFP) is a key additive manufacturing process for the production of complex composite structures. This study focuses on the in-situ AFP process for thermoplastic composites with Hot Gas Torch (HGT), which includes heating, consolidation, and solidification steps. Temperature control is critical to achieving high quality parts as it affects bond quality, crystallization, and solidification. However, previous studies have oversimplified convective heat transfer by assuming a constant coefficient, resulting in discrepancies between simulations and experiments. This paper introduces a novel distribution function to model the convective heat transfer coefficient, thereby improving temperature predictions. An optimization loop is used to determine the parameters of the function, which ensures agreement with experimental data. The proposed approach accurately predicts the temperature distribution, which is validated against unseen experimental results. By incorporating the distribution of the convective heat transfer coefficient, this study improves the understanding of heat transfer mechanisms in AFP for thermoplastic composites, leading to improved manufacturing processes and part quality.

This study investigates the vibrational performance of a new generation of hybrid composites based on a short fibre core for structural dynamic applications. The dynamic characterisation was conducted through dynamic mechanical analysis (DMA) and the free vibration of a cantilever beam at different lengths. The short fibre composite exhibited a superior specific damping capacity, producing a maximum damping of 0.42. The values extracted from DMA indicated no visco-plastic contribution from the PEEK resin system, implying that the internal microarchitecture drives the material damping, with friction at the fibre/matrix interface being the primary dissipation mechanism. The hybrid laminate presented an improvement in damping performance of 39% due to the addition of the short fibre core compared to a baseline quasi-isotropic laminate of similar flexural stiffness. These findings show hybridisation’s advantages in designing structural components with improved damping performance and reduced cost for a variety of dynamic applications in industries such as automotive.

A method for fatigue damage quantification in composite materials, based on experimental stiffness degradation data for composite laminates subjected to cyclic load is proposed. Discrete damage mechanics theory is used to calculate crack density vs. number of cycles from elastic moduli-reduction data obtained during fatigue experiments. The calculated crack density simplifies fatigue testing by diminishing the need for counting cracks during testing. Accurate results are achieved and reported. The defect-nucleation rate, which controls the fatigue damage rate, is also obtained from processing the modulus-reduction data. It is observed that the defect-nucleation rate has a small scatter and is independent of applied load magnitude. Furthermore, the onset of delamination observed in the experiments correlates very well with the onset of deviation between the predicted and experimental curves of elastic moduli-reduction versus accumulated crack density. An additional parameter, the defect-nucleation threshold, is here proposed to further characterize the fatigue performance of the composite material under stress-controlled fatigue loading, in contrast to thermal fatigue results from the literature. Furthermore, the difference in damage nucleation rate between strain-controlled and stress-controlled was observed and discussed.

The realization of the dynamic snap-through behaviors provides the new design ideas in the fields of the morphing aircraft and piezoelectric energy harvesting. This paper studies 1:2 internal resonance, nonlinear vibrations and chaotic snap-through phenomena of the bistable asymmetric composite laminated (BSCL) cantilever shell with the lower-frequency and higher-frequency primary resonances for the first time. The transverse foundation excitation subjects to the fixed end of the bistable cantilever shell. The perturbation analysis of two-degrees-of-freedom nonlinear ordinary differential equations is carried out by using the first-order approximate multiple scale method. The analytical results of the frequency-amplitude and force-amplitude response curves are obtained under the small foundation excitation. The obtained results reveal that the BSCL cantilever shell exhibits the double-jumping characteristics when 1:2 internal and primary resonances occur. There is a continuous energy exchange back and forth between two modes of the bistable laminated cantilever shell. As the foundation excitation increases, the BSCL cantilever shell exhibits saturation phenomenon. Numerical simulations are finished to further investigate the effects of the large excitation on the chaotic, quasi-periodic and snap-through vibrations for the BSCL cantilever shell. The vibration experiment is carried out to investigate the internal resonance and dynamic snap-though motions of the BSCL cantilever shell. Using the snap-though behaviors of the BSCL cantilever shell, we obtain the morphing structure of the aircraft wing.

The issues of electromagnetic (EM) emission and its secondary contamination have prompted significant concern among individuals. Hence, the development of efficient absorbing electromagnetic interference (EMI) shielding materials are urgent. Herein, silver-coated and foamed temperature-sensitive microspheres (Ag@FTSM) were synthesized by electroless silver plating. Subsequently, the Ag@FTSM/Fe₃O₄/waterborn polyurethane (WPU) composite aerogels with a porous and asymmetric gradient structure were prepared via freeze-drying process based on the principle of density difference. The introduction of the porous structure can reduce the material density and filler addition, augment the loss interface, and prolong the conduction path resulting in the enhancement of the EM wave absorption property. Through the loss mechanism of “absorption-reflection-reabsorption” and the longer loss path of the aerogels, the EMI shielding efficiency (SE) of the composites reached 48.7 dB with excellent absorption efficiency (46.5 dB) and high absorption coefficient (0.60) when the electromagnetic waves hit the magnetic layer.

This study proposes the application of prestressed near-surface-mounted (NSM) carbon-fiber-reinforced polymer (CFRP) technique in the field of the shear-strengthening of bridges for the first time. Fourteen reinforced concrete (RC) beams shear strengthened with prestressed NSM CFRP were tested under static load. The effect of the CFRP prestressing level, spacing, angle, and end-anchorage measures on the shear-strengthening behavior was evaluated. The experimental results demonstrate that the ultimate shear capacity of prestressed NSM CFRP shear-strengthened beams increased by 65–127% when compared to that of the reference beams, and the width of shear cracks was effectively suppressed. The failure mode of prestressed NSM CFRP shear-strengthened beams without end-anchorage measures was web concrete cover separation, which can be suppressed using a CFRP U-jacket and through-beam screw. Increasing the CFRP prestressing level and percentage enhanced the ultimate shear capacity and cracking resistance of the strengthened beams. However, an excessively high CFRP percentage and prestress level combination resulted in large shear crack angles and decreased the shear contribution of CFRP and concrete. Finally, an analytical model based on the modified compression field theory (MCFT) was proposed to predict the flexural-shear load response of strengthened beams, which was in agreement with the experimental results.

The energy absorption characteristics of the triply periodic minimal surfaces (TPMS) structure may vary significantly due to the anisotropy under multi-directional loading conditions. To address this issue effectively, an isotropic design strategy based on a precise elastic modulus compensation mechanism for different TPMS lattices is proposed. This strategy involves combining a TPMS lattice with a high elastic modulus in the axial direction with another TPMS lattice featuring a low elastic modulus in the same direction, leveraging the complementary effects of elastic modulus to achieve isotropy. The relationship between the relative density and the elastic modulus of six types of TPMS lattices is analyzed through homogenization simulation and finite element analysis. Mathematical expressions are then fitted using the Gibson-Ashby model. Additionally, a Kriging model is employed to establish the relationship between the Zener anisotropy values of hybrid TPMS structures and the relative density of their component lattices. This enables the precise complementary effect of elastic modulus in different TPMS lattice structures, providing a widely applicable selection rule for achieving isotropy. Using the Primitive-Diamond hybrid lattice as an example, the Zener anisotropy index after hybridization is reduced by 65.2 % and 31.37 % compared to single Primitive and Diamond lattices, respectively.