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The mechanical response of three different structural core sandwich panels in out-of-plane compression and shear has been analyzed. Specific core shapes examined are arc-tangent, wavy trapezoidal and hemispherical. Unit cells consisting of representative elements of the core attached to face sheets were selected for analysis. Both face sheets and core were assumed made from paper. Finite element analysis employing large deformation and rotations and orthotropic elastic–plastic behavior was used. The results show that the arc-tangent and trapezoidal cores are prone to collapse by extensive bending and buckling, whereas the hemispherical core behaved more stably in compression and shear. Core sheets with a hemispherical shape were prepared from copy paper sheets in a specially designed forming machine. Sandwich test specimens were prepared from this core and tested in out-of-plane compression, and the load-displacement response was compared to predictions from finite element simulations. The experimental and finite element results were consistent. [ABSTRACT FROM AUTHOR]

Natural fibers are gaining much interest in composite materials because of their potential to replace conventional glass fibers. In particular, unidirectional flax fiber composites have shown excellent mechanical properties with different thermoset matrices. However, few data are available when they are made with a biodegradable thermoplastic matrix. This paper compares the mechanical properties of two types of unidirectional flax composites using polylactic acid as the matrix: one made with layers of aligned flax rovings alone and the other containing an additional paper layer fabricated using paper making techniques. The results show that specific tensile properties of the flax/PLA (252 MPa·cm 3 ·g −1 ) and flax-paper/PLA (217 MPa·cm 3 ·g −1 ) composites are similar to those made using woven glass fabrics impregnated with epoxy (227–278 MPa·cm 3 ·g −1 ). Very high impact strength (600 J/m) was also obtained for UD flax-paper composites compared to unreinforced resin (15 J/m). Together these mechanical properties are promising for industrial applications of this new reinforcement. [ABSTRACT FROM AUTHOR]

This paper presents sound absorbing structure constructed using multiple layers of fibrous papers. Sound absorbing materials converts sound wave into thermal energy due to the porosity. The proposed sound absorbing structure has helical-like shapes in layered format with cyclic winding of the mulberry papers and coconut fibers. The acoustic properties changes depending on the thickness of the air cavity between the thin layers and porosity of the layers themselves. Sound absorption coefficients were measured using two-microphone impedance tube in order to investigate the acoustic property variations with porosity. The measured results were compared to predictions utilizing wave propagations through a slit for verification of the sound absorbing mechanism. A simple method for improving the sound absorption with less material by composite structure is proposed and verified through the experiment. Dynamic stiffness and loss factor of the helical-shaped sound absorbers were measured and compared by analyzing supporting properties to the vibrating beam. Depending on the construction method, the frame dynamic properties showed variation even when the sound absorption was identical. This information is required for optimal design of absorbing layers for specific noise control applications. [ABSTRACT FROM AUTHOR]

Abstract: A finite element procedure, developed in order to account for the effect of high anisotropy at folding of coated papers, is presented. The anisotropic behaviour (with very low stiffness in the thickness direction) is modelled using stiff structural elements (trusses and beams). The numerical results indicate that high elastic anisotropy leads to lower strain levels at folding than reported in previous analyses where this effect was not accounted for. High plastic anisotropy, on the other hand, will contradict this result. [Copyright &y& Elsevier]

Folding of coated paper is examined numerically using the finite element method. Particular emphasis is put on the behaviour of field variables relevant for cracking of the coating layers. In the numerical analysis, the basepaper is modelled as an anisotropic elastic–plastic material (both elastic and plastic anisotropy is accounted for) while the constitutive behaviour of the coating layers are approximated by classical (Mises) elastoplasticity. The numerical results suggest, among other things, that particular forms of plastic anisotropy can substantially reduce the maximum strain levels in the coating. It is also shown that delamination buckling, in the present circumstances, will have a very small influence on the strain levels in the coating layer subjected to high tensile loading. [Copyright &y& Elsevier]

The quality of transformer insulation is related to the safe and stability operation of the power grid, and the life of the transformer oil-paper insulation system mainly depends on the state of paper insulation. With the continuous improvement of the voltage level, the requirements for insulating paper are becoming higher and higher. Based on the molecular dynamics simulation technology, the effects of carbon nanowires with three different structures on the thermodynamic properties of cellulose insulating paper were analyzed from the aspects of mechanical parameters, MSD (mean square displacement), CED (cohesive energy density) and hydrogen bond. The simulation results showed that the carbon nanowires with three different structures have improved the thermodynamic properties of cellulose. Among them, the hard chiral carbon nanowires have the most significant improvement in the thermodynamic properties of cellulose. The elastic modulus, bulk modulus, and shear modulus have been increased by 30%, 25.6%, and 32.9%, respectively. It was found that the thermal stability of cellulose insulation paper modified by three different structure carbon nanowires was improved. The increase of thermal stability was mainly due to the increase of the cohesive energy density and the number of intermolecular hydrogen bonds. [ABSTRACT FROM AUTHOR]

Paper-like electronics with thin multi-layer structures, which can be repeatedly bent and twisted like paper, are promising for diverse innovation application. Employing surface-mounted flexible strain sensors is a general way to monitor the bending characteristics of each layer inside paper-like electronics, however, the deviation of measured strain will reach up to 15% on account of the layer-to-layer interface sliding, neutral axes offset resulted from the extra effect of the flexible strain sensor. Here, we proposed a precise measurement method to evaluate the surface bending strain of paper-like electronics by using flexible sensors. In combination with the effects of the thickness, length, etc, a strain compensation model has been established to compensate the measurement deviation. Meanwhile, a slip failure model has been developed to guide the design of the sensitivity unit within the non-sliding area of the flexible sensor. Validated by finite element analysis and experiment, the bending strain of paper-like electronics (eg., ∼50 μm) can be evaluated much more accurate compared to the frequent access one. The newly-proposed method provides a powerful basis for further research of paper-like electronics. [ABSTRACT FROM AUTHOR]

[Display omitted] • The digital twin of a CNT-polymer nanocomposite is developed. • CNT intrinsic resistivity cannot be neglected. • Uniform boundary potentials overestimate effective electrical conductivity. • Control of CNT torsion removes the dependence on the segmentation length. • The critical percolation index lies in the range 1.2–1.7. The paper describes digital twins for an electrically conductive isotropic CNT-polymer nanocomposite, using representative volume element with boundary conditions being periodic geometrically and electrically. During the CNT generation, the torsion and curvature of CNTs are controlled. The percolation threshold is analysed with scaling towards the infinite RVE. The digital twin sensitivity to model parameters is found as follows: (1) restrictions on the curvature/torsion ensure the independence of the homogenised conductivity on the CNTs discretisation; (2) the conductivity of the nanocomposite increases (for the same CNT volume fraction) with the increase of the CNT length and maximum torsion and decrease of the maximum curvature; (3) the percolation threshold, is in the range of 0.3–0.7% for different CNT parameters; it reduces in half with doubling the CNT length; the curvature and torsion have lesser influence. The critical percolation index is 1.2–1.7, which corresponds to the theoretical values for 3D networks. [ABSTRACT FROM AUTHOR]

Folding of coated papers is examined numerically using the finite element method. The analysis is focused on the influence from dynamic effects on the folding process. In particular, the behaviour of field variables relevant for cracking of the coating layers are studied in some detail. The results presented indicate that dynamic effects are of little importance as regards maximum strain levels in the coating but will influence the stress and strain distributions. Accordingly, a quasi-static analysis of the problem will be sufficient in order to describe many of the important features related to cracking. [Copyright &y& Elsevier]

This review paper provides a key information, allowing readers to comprehensively understand the direct tensile behavior and test methods for textile reinforced cementitious composites (TRCCs). The tensile parameters of TRCCs corresponding to bilinear or trilinear tensile stress–strain behavior were summarized and the influencing factors on the tensile parameters were discussed. Various direct tensile test methods for TRCCs were evaluated according to the geometry of specimens, load transmitting mechanism, measurement and loading methods. Conclusive requirements for a standard test method for assessing the tensile stress–strain behavior of TRCCs were proposed. [ABSTRACT FROM AUTHOR]

The dynamic compression behaviors of aramid paper honeycombs, i.e. samples with heights 2.10 mm, 3.10 mm, 5.14 mm, and 10.11 mm, were investigated by the split Hopkinson pressure bar devices (SHPB). The results showed abnormal size effect in the peak stress for samples with height 2.10 mm. The high speed camera was used to capture the deformation process. Two deformation modes, i.e. the in-plane shear wrinkles (characterized as larger deformation and lower strength in the stress-strain curves) and the out-of-plane wall buckling (characterized as smaller deformation and higher strength in the stress-strain curves), were distinguished from the complicated deformation processes. The abnormal size effect was resulted from the dominance of in-plane shear failure for samples 2.10 mm, rather than the out-of-plane wall buckling for other samples. Further investigations were conducted in details on the effects of cell width, wall thickness, and ends constraint condition (i.e. ends bonded with face sheets) to the deformation process based on the experiments, theoretical analyses of buckling, and Finite Element method (FEM). The transition of two deformation modes was analyzed, and the abnormal size effect would disappear for samples with larger wall thickness. It might be helpful for design purpose of honeycombs to some extent. [ABSTRACT FROM AUTHOR]

• The shear properties of aramid paper are characterized for the first time.. • Characterizing shear properties of paper makes the prediction of honeycombs better. • The failure mode map of paper honeycombs subjected to complex loads is obtained. Aramid paper honeycomb cores have been widely used in lightweight structures. However, shear properties of aramid paper can hardly be obtained accurately due to the shear buckling of the paper. Therefore, we make the paper into a laminate to prevent buckling and apply the v-notched beam test method to the laminate to obtain the in-plane shear properties of aramid paper. The shear strength and failure strain of aramid paper are characterized experimentally for the first time. The properties are then used in finite element simulations to study the shear behaviors of paper honeycombs and the good agreement with the experimental results indicates the validity of the shear properties. Furthermore, the failure mode map of the paper honeycomb subjected to combined shear and compressive loads is obtained, which presents the importance of the accurate characterization on the shear properties of the paper. This characterization method can also be applied to other types of paper. [ABSTRACT FROM AUTHOR]

A serious error exists in the above paper. [ABSTRACT FROM AUTHOR]

• Accurate prediction of tape-spring folding behavior with new analytical model. • Proposed model outperforms classical models in predicting peak and steady-state moments. • Stretching and bending strain energy studied and analyzed during folding process. • Three key geometric parameters identified for optimizing tape-spring design. The tape-spring is widely used in aerospace, mechanical, and civil engineering due to its unique mechanical properties and compact size. This paper aims to establish a new analytical model to more accurately predict the folding behavior of the tape-spring, including the geometric configuration, folding moment versus longitudinal curvature curve, peak moment, and propagation moment. This model comprehensively considers the influence of stretching strain energy and bending strain energy on the total strain energy, and determines the folding behavior of the tape-spring through the minimum energy principle. The proposed analytical model was compared with Wuest's model, Yee's model, and Yao's model. Experiments of tape-springs made from two different materials were utilized for validation. The study shows that the proposed model has the highest prediction accuracy compared with the classical models, with the error of approximately 10%. The reason for the significant difference in the prediction results between the proposed model and Yao's model was given, and the relationship between stretching strain energy and bending strain energy during the folding process of the tape-spring was analyzed. The proposed model has broader applicability than classical models and provides new insights and solutions for accurate prediction of the folding behavior of the tape-spring. [ABSTRACT FROM AUTHOR]

Auxetic materials are attracting increasing interest due to their extraordinary or even abnormal mechanical properties. Different from the traditional truss-lattice structures, this paper proposes a series of novel auxetic plate-lattice structures with an excellent auxetic effect. Utilizing a dual-materials glue-free assembly design involving hyperelastic and plastic materials, these plate-lattices exhibit numerous advantages such as disassembly, assembly, replacement, recyclability, reusability, excellent energy absorption, high specific stiffness, and impact resistance performances. Quasi-static/dynamic tests and simulations are conducted to assess mechanical properties, including elastic constants, deformation modes, and mechanical responses. Simultaneously, the effect of the principal geometric parameter, concave angle, on the structural response was analyzed with various impact velocities. Findings suggest that the concave angle influences the structural elastic constants under compressive loading. The stress–strain response exhibits a distinct dual-plateau phenomenon. Taking the A65 (concave angle is 65°) configuration as an example, the stress on the second plateau is approximately 2.7 times that of the first plateau, significantly enhancing the structure's energy absorption capability. Under dynamic impact loadings, different configurations and varying impact velocities affect the structure's energy absorption performance. This paper introduces a series of assemblable plate-lattice structures with an auxetic effect, offering design insights and guidance tailored to the convenient transportation, unconventional structures, and rapid assembly needs of lightweight, impact-resistant mechanical metamaterials. [ABSTRACT FROM AUTHOR]

• The first-ever systematic experimental program on the transverse tension tests of PGFRP profiles using non-standard short coupons was presented. • Size effects on transverse tensile properties in terms of coupon width and gauge length were examined. • Finite element models were developed to provide a better understanding of the failure mechanisms of the transverse short coupons under tension. • Recommended short coupon configuration for transverse tension tests of PGFRP profiles was eventually proposed. Pultruded glass fiber-reinforced polymer (PGFRP) profiles, manufactured via a pultrusion process, are increasingly used in civil engineering. However, due to the anisotropic nature, their transverse material properties are weaker than the longitudinal ones, necessitating robust material characterization methods for transverse properties. Commercially available PGFRP profiles often have insufficiently large cross-sections for standard-sized transverse tensile tests, prompting the need to investigate the effect of coupon size on transverse tensile properties and propose a feasible shorter coupon size. This paper therefore aims to examine the coupon-size effects and propose suitable short coupon geometries for transverse tensile tests. Standard material characterization tests were conducted on channel-section PGFRP profiles, followed by transverse tensile tests on six types of non-standard short coupons. The effects of coupon width and gauge length on transverse tensile properties were evaluated and compared across six aspects, with potential failure mechanisms further explored using finite element analysis (FEA). The paper concludes by proposing a feasible short coupon configuration for transverse tensile tests. [ABSTRACT FROM AUTHOR]

Space debris is a major threat to the safety of spacecraft in orbit, and advanced protective materials with lightweight and high impact resistance are effective passive protection solutions. In order to investigate the hypervelocity impact protection performance of aluminum matrix ceramic ball composite materials plate, this paper has carried out the hypervelocity impact test of aluminum matrix ceramic ball composite materials plate, and verified the accuracy of the numerical model based on the FE-SPH adaptive method, and then carried out a comparative study on the hypervelocity impact protection performance of aluminum plate and aluminum matrix ceramic ball composite materials plate with the same area density. The results show that: compared with the aluminum plate conditions, the composite materials plate has a better ability to broken the projectile. Compared with the aluminum plate, the kinetic energy absorbed by the composite materials plate is increased by 37.04%, and the kinetic energy of the generated debris cloud decreased by 69.57%. The proportion of the internal energy dissipation of the composite materials impact system reaches up to 72.03%. The secondary pollution generated by the composite materials plate is less and the hazardous debris have less threat to the rear wall. The research in this paper can provide reference for the selection of hypervelocity impact protection materials. [ABSTRACT FROM AUTHOR]

The aim of this paper is the multiscale investigation of composite pressure vessel structures wound with varying fibre tension. The paper explores the potential of "programming" the fibre tension force during the winding process of a high pressure hydrogen storage vessel. Two series of vessels were wound with two different tensions: Two series of vessels wound with forces of 3 N and 80 N respectively were experimentally investigated. Other technological factors, such as the type and weight of carbon fibre used, were kept constant throughout the study. Quasi-static burst tests were used to measure the elastic deformation of the vessel surface. Microscopic studies were also carried out to analyse the structure of the composite at different length scales. The fibre content, the geometric parameters of the voids and the distribution of the three main components of the composite – carbon fibre, technological defects and resin – were also determined. Representative RVE elementary cells were generated for both materials using statistical modelling. The composite was then homogenised and elasticity matrices were obtained for the fibre tensile forces in both materials. [ABSTRACT FROM AUTHOR]

This paper presents an extensive experimental performance study of bonded joints (BJs), only bolted joints (OBJs) and hybrid bonded-bolted joints (HJs). For each joint type, three test configurations are considered: namely, short, medium and long overlap lengths. In each case, the adherends comprise quasi-isotropic twill woven CFRP. For each joint type/overlap length combination, three specimens are tested for statistical representation. HJs demonstrate 1.4, 1.5 and 1.5 times higher failure load and 1.5, 1.6 and 1.9 times higher stiffness than OBJs, for short, medium and long overlap, respectively. In all test cases, HJs outperform BJs except for short overlaps where BJs outperform both HJs and OBJs. OBJs perform poorly in terms of failure load and Hooke's stiffness. Nevertheless, due to bearing deformation at bolt hole locations, OBJs experience higher failure displacements than BJs and HJs leading to a desirable energy absorption mechanism compared to HJs and BJs. This is thanks to the bearing failure mode of the joint despite much lower failure load in OBJs. It was found that increasing the overlap length generally benefits BJs. However, for medium length overlap specifically, HJs show better performance than BJs. Stress–strain behaviours show a linear behaviour for all test groups with significant joint rotation for OBJs compared to BJs and HJs. Failure mechanism studies presented in the paper show that BJs fail in cohesive failure mode for all test groups. OBJs fail in bearing mode, which is followed by net tension failure. OBJs experience matrix cracking and delamination at bolt hole locations. On the contrary, HJs experience considerably less bearing failure at bolt holes due to the load bearing contribution from the adhesive. [ABSTRACT FROM AUTHOR]

This paper introduces quasi-periodic homogenization schemes for quasi-periodic media, those without strict periodicity, but that can be mapped to a parent periodic medium. Quasi-periodic homogenization relies on the conceptual idea of mapping a non-periodic domain to a reference periodic one through a point mapping of material points within the domain of an identified repetitive unit cell. The theoretical background of quasi-periodic homogenization introduced in the first part of this paper relies on expressing the microscopic position of micropoints within a physical unit cell as a sum of the macroscopic position (the center of area of the unit cell) and the relative position of micropoints with respect to the center of area. This decomposition parameterized by the small-scale parameter entails a corresponding additive decomposition of the tangent map defining the geometrical transformation of the periodic UC into the quasi-periodic one in terms of an additive decomposition into macroscopic and microscopic contributions. The quasi-periodic homogenized effective moduli are then determined, starting from the average of the microscopic energy, those being expressed in terms of the periodic moduli and a perturbation term, both expressed in a volumetric format as surface integrals over the reference unit cell domain in a 2D context. In the second part of this contribution, a surface formulation of the quasi-periodic moduli is derived, based on the notion of shape derivative of the total potential energy stored within the unit cell. This approach relies on introducing a shape velocity field at the boundary of the periodic unit cell to change its design, driven by the normal projection of Eshelby stress onto both the internal and external boundaries of the unit cell. This second scheme offers comparatively to the first one a simpler way to compute the quasi-periodic moduli as it only requires the evaluation of the mechanical fields on the unit cell boundaries. Application of the proposed homogenization schemes are done for inclusion-based composites, underlining the importance of strain gradient energetic contribution in the situation of a micrograding of the unit cell geometry. [ABSTRACT FROM AUTHOR]

Abstract: Natural continuous bast fibres (hemp, flax, etc.) are gaining popularity in composite materials because they can advantageously replace glass fibres. It is found that intrinsic properties of unidirectional (UD) composites are almost equivalent to those of unidirectional glass fibre composites. Unfortunately it is difficult to get repeatable results because of the inherent variability in the properties of bast fibres compared to glass fibres. Their quality is largely affected by the weather conditions, the extraction location along the plant and the techniques used to extract them (retting, bleaching, etc.). In this work, unidirectional hemp/paper/epoxy and flax/paper/epoxy composites are manufactured by adding one or two sheets of paper at the surface of a unidirectional layer of hemp or flax fibres before molding. The composites are tested under tensile loads and the results compared with those obtained for UD composites made without paper. The results show a significant increase in strength and modulus repeatability when the paper layer is present. A significant increase in strength and modulus was also obtained compared to the base epoxy properties. The resin permeability of the flax–paper reinforcement has also been evaluated and compared to that of a standard glass fabric. Globally, the results suggest that adding thin sheets of paper at the surface of reinforcement layers stabilizes and increases the composite strength but slightly reduces the Young’s modulus. However the permeability of the flax–paper reinforcement was much lower than that of the glass fabric. Modifications to the paper architecture will be required to improve this aspect. [Copyright &y& Elsevier]

A major disadvantage of polymer-based composite materials is their poor performance when exposed to relatively high heat flux. At temperatures typically above 300–400 °C, the organic matrix decomposes releasing heat, smoke, and toxic volatiles, jeopardising the composite mechanical performances. The heat flux resistance of composites can be improved by integrating protective coatings on their surface. In the present work, graphene nanoplatelet (GNP) films are employed as thermal barrier coatings (TBCs) to mitigate the thermal degradation of Carbon Fibres Reinforced Plastic (CFRP). The films were bonded onto the heat-exposed surface of CFRP laminates. Laser spot heating tests, with different laser power densities (25–150 kW/m2), were adopted to assess the effectiveness of the coating. The effect of the thickness of the protective layer has been tested too, by changing the number of layers (from 1 to 2 layers). From the results, a significant reduction in the temperatures and damages were observed. Moreover, the damage extension and residual mechanical performances of exposed samples have been assessed as a function of the power density and the protection level. A significant improvement of the post-heat flexural moduli and a significant reduction of the damaged areas have been obtained in laminates protected with high-loaded graphene nanoplatelet films. [ABSTRACT FROM AUTHOR]

Digital image and volume correlation (DIC and DVC) are increasingly being used to measure the deformation and damage behaviour of composite materials. The heterogeneous nature of the composite microstructure results in complex local behaviour, which cannot be captured using traditional experimental characterisation techniques. DIC and DVC offer significant advantages in monitoring this behaviour for fibre-reinforced composite materials. However, as acceptance of these techniques is growing, careful consideration of testing setup and approach is nevertheless crucial for achieving accurate results. This paper reviews digital image and volume correlation methods for the characterisation of composite materials and reports current research applications. The review discusses essential considerations and provides recommendations to aid researchers in achieving quality deformation measurement and damage identification in composites. Finally, an outlook on possible future applications is presented as a motivator for further studies. • Digital image & volume correlation use with fibre-reinforced composites is explored. • Considerations for deformation, damage and defect characterisation are discussed. • Results of innovative recent studies are reported at micro-, meso- and macroscales. • The potential use of experimental deformation in high-fidelity models is evaluated. • Perspectives on existing advantages, limitations and future directions are presented. [Display omitted] [ABSTRACT FROM AUTHOR]

This communication is to correct the sensitivity analysis formula of frequencies for composite structures in the paper [1]. It is further pointed out that even though the sensitivity analysis formula of frequencies has been corrected, the first derivatives of frequencies with respect to design variables cannot be achieved by direct differentiating the equations of eigenvalues and eigenvectors because the first derivatives of vibration modes with respect to design variables are still unknown. The readers are referred to the analytical solution of the sensitivity analysis of frequencies and modes for composite structures in the literature [2], which is derived using linear space theory. [ABSTRACT FROM AUTHOR]

Since carbon fiber woven composite panels contain complex fiber bundle structures, the aim of this paper is to investigate the factors influencing the macro-equivalent properties and topological flexibility of composite panels resulting from variations in fiber bundle configuration. Firstly, the structural topology optimization design of carbon fiber composite plates was achieved using a multi-scale modeling approach with the variable density method. Nonlinear relationships between carbon fiber bundle configuration parameters and macroscopic equivalent properties, as well as macroscale topological flexibility, were determined using deep learning methods, respectively. The maximum error in the predicted values of the deep learning network model does not exceed 0.5% when compared with the results of the finite element calculations. In addition, utilizing deep learning networks to predict macro-equivalent properties of composites can significantly decrease computational time. The impact of varying the number of filaments in fiber bundles and the spacing of the fiber bundles on the flexibility of the topology was determined using a deep learning network. The optimal topological flexibility, along with its corresponding fiber bundle spacing and number of fiber bundle filaments, are globally searched within the design domain using an adaptive genetic algorithm. The study in this paper can provide a reference for designing fiber bundle configurations of composites for practical applications. [ABSTRACT FROM AUTHOR]

• The cohesive bond model demonstrates good agreement with experimental results. • Significant dynamic enhancement occurs at the concrete-CFRP strip interface during blast loading. • Blast wave propagation induces interface damage between concrete and CFRP strip. • A dynamic increase factor (DIF) is necessary for modelling CFRP-concrete interface behaviour under blast loading. The paper is structured into two main sections. In the first part, the focus is on the finite element (FE) analysis of bond slip in a single bond shear test between concrete and carbon fiber reinforced polymer (CFRP) strips under static loads. To model the test set-up, a plastic damage material model and an elastic material model are used for the concrete prism and the unidirectional CFRP strip respectively. Three approaches, including a perfect bond model, a cohesive bond model, and contact algorithms based on recent bond slip models, are employed to simulate the bond interface. The numerical model's validity is confirmed through comparison with experimental results from the literature. The paper predicts the debonding failure mode, the strain evolution along the bond length, and the delamination loads of the CFRP strip. The cohesive bond model exhibits good agreement between numerical and experimental data. In the second part of the paper, the developed FE model is tested under blast loading and compared to the experimental results of the blast tests conducted by the authors. The experimental and numerical findings highlight a significant dynamic enhancement effect on bond-slip properties due to the propagation of the blast wave within the concrete and the high loading rate. [ABSTRACT FROM AUTHOR]

Accurately predicting Cure-Induced Distortion (CID) is paramount to ensuring manufacturing precision. Existing prediction models heavily rely on material parameters associated with the CID, which presents challenges for newly developed composites due to the time-consuming, expensive, and limited accuracy of parameter characterization processes. With recognizing the shared curing mechanism across various composites, this paper introduces a transfer learning-based approach that leverages a small dataset of practical CID data from the newly developed composite and historical knowledge from existing composites to achieve rapid and accurate CID predictions. This approach was validated for two typical CID prediction cases by implementing a transfer learning framework, and various sampling methods were explored to assess the stability and repeatability of prediction results. The results indicate that the proposed method can predict the CID within a 5% margin of error. This paper offers a promising approach for predicting CID in the design and manufacturing processes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Honeycomb panels with discrete arranged block reinforcements were numerically designed. • The elastic moduli of the new honeycomb panels are higher than those of heavier panels with slatted. • Tables with new cellular panels are characterized by a stiffness of 85 % of the stiffness of heavier tables with slats. Furniture manufacturers indicate they know the benefits of lightweight honeycomb panels but still expect their higher mechanical properties. This work presents a numerical and experimental approach to designing new tables manufactured of paper honeycomb panels with discreetly arranged reinforcing blocks in the core. The main goal was to determine whether the discrete arrangement of the blocks affects the strength and amount of energy absorbed by the newly designed tables and stiffness of honeycomb panels. Optimization reduced the weight of the panels by up to 30 %. At the same time, their linear elastic modulus was increased from 1579 MPa to 1771 MPa. After ten load cycles, tables with discrete reinforcements have a stiffness up 86.85–87.20 % of the stiffness of the reference table. After destructive tests up 89.96–91.16 %. Ultimately, the test results demonstrate the effectiveness of replacing wide slats with discreetly placed reinforcing blocks in the core structure. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of compaction and curing states on the mechanical properties and void content of AFP-manufactured laminates. For the mechanical characterization, tensile tests, 4-point-bending tests and interlaminar shear strength (ILSS) tests were applied. The internal structure was evaluated via micro CT. For the surface structure evaluation, an optical profilometer was used. The void content was determined via segmentation of the micro CT images. The paper is subdivided into the description of the applied materials and methods, the results and their discussion and a conclusion. The study showcases the potential of AFP-manufactured aerospace composites to attain high-quality results even without the need for an autoclave, making them well-suited for unmanned applications. [ABSTRACT FROM AUTHOR]

• A highly efficient and exact search algorithm of inter-fiber fracture criterion is proposed. • A progressive damage model considering fracture toughness is promoted. • Experimental and simulation data proved the accuracy of the proposed model. This paper presents a quick search algorithm of the fracture angle of inter-fiber fracture (IFF) criterion and a progressive damage model considering the dynamic fracture toughness effect. The fracture angle determination of the IFF criterion is necessary and time-consuming, which has limited its practicality. Therefore, to accurately calculate stress exposure factors under various combinations of stress states, a highly efficient algorithm is needed. In this paper, a novel algorithm using stress tractions on the fracture plane and the golden section search (GSS) is promoted. Through a series of simulation tests, this novel algorithm has been proven to be exact and efficient compared with other two common algorithms, which saves an average of 63.2 % and 28.1 % of the time in 1 million calculations and 2 h 2 min 4 sec and 39 min 2 sec in simulations. Additionally, fracture toughness, which controls material damage evolution process, also has strain-rate effect like elastic modulus and strength. Hence, a new progressive damage model considering the dynamic fracture toughness effect is proposed. The results of dynamic compression and ballistic impact tests and simulations demonstrate the fidelity of the novel damage model. [ABSTRACT FROM AUTHOR]

A unique strategy of low-frequency wideband vibration control by harnessing the concept of feedback control between the piezoelectric bimorph sub-cells which are acting as sensors and actuators is conceptualized in this paper. In the unit cell, voltages from the sensing bimorph are fed back to actuate another bimorph through operational amplifier-based gain circuits. The prime novelty of the paper is the non-trivial formulation of the spectral element of the non-reciprocal piezoelectric beam (NRPB), having electro-mechanically coupled sensing and actuating bimorph in the unit cell, derived from the exact solution of the governing differential equations. The analytical formulation employing the spectral element method enables the elementary assembling of the NRPB units to simulate different boundary conditions and various configurations arising due to the alteration of the position of the sensing and actuating bimorph. The direct frequency responses for various configurations are obtained for a cantilevered and a simply-supported beam with varying feedback gain. The feedback gain induced damping to the system, hence sharp peaks and drops are flattened. Most promisingly, the amplitude of the displacement response for the entire frequency range can be brought down below its corresponding static deformation with the application of positive gain for certain configurations. Hence, the proposed system could potentially be used for application in the field of vibration and acoustics. • Emergence of superior vibration control by non-reciprocal piezoelectric beam (NRPB). • Realization of the constant gain sensing and actuating piezo bimorph via an electrical circuit. • Spectral element formulation for the complete NRPB, including actuating cell. • Ultra-wide bandgap starting from very low frequency. [ABSTRACT FROM AUTHOR]

Bistable laminates have attracted considerable interest among researchers as prospective candidates for delivering exceptional performance within the domain of morphing structures. These laminates exhibit a distinctive characteristic of featuring two stable states, sustaining their configuration until the application of a critical force that triggers a transition to their second stable position. Before discussing the practical applications involving bistable composite laminates, this paper presents analytical modeling for predicting the room-temperature shape and simulating the snap-through phenomena. The main aspects of the room-temperature shape simulation are discussed, and various snap-through triggering methods are shown. At this point, the paper focuses on a comparative analysis of numerical predictions in contrast to experimental data aimed at discerning the reliability of existing models. Furthermore, this paper not only elucidates existing challenges but also delineates potential avenues for future research. As the investigation into bistable composites progresses from comprehending intrinsic phenomena to practical, real-world applications, this study outlines both the present and potential future research concerning bistable composite laminates. [ABSTRACT FROM AUTHOR]

• This paper provides insight into the conventional methods for predicting critical buckling loads of pultruded FRP columns. • A comprehensive test database of 365 for global flexural buckling of pultruded FRP struts is proposed. • Based on the machine learning algorithm and the database proposed in this paper, the critical load of pultruded FRP struts is predicted. Pultruded fiber-reinforced composites (FRP) are widely used in structural engineering due to their excellent properties along the fiber direction. Its global buckling performance has received increasing attention from academia and engineering. This paper reviews the prediction methods for the critical buckling load of pultruded FRP struts, including the Euler formula critical load prediction method, the shear-corrected critical load prediction method, the Southwell plot method, and the critical load prediction method based on the strength failure mode. Summarizes the basic principles of each method, discusses the internal relationship and difference of each method, and analyzes the critical factors of each method with experimental data. In addition, a unified generalized Ayton-Perry theoretical model is proposed through induction and summary. Finally, based on the experimental database, a Machine learning model for predicting the buckling critical load is established. The results show that the machine learning algorithm can predict the buckling critical load with high prediction accuracy. [ABSTRACT FROM AUTHOR]

Applying the hybrid flux finite element method(HF-FEM) and hybrid thermal stress finite element method (HTS-FEM) is not prevalent in analyzing heat conduction and thermal stresses in particle-reinforced composite materials. Most of the research on this technique is confined to two-dimensional problems. This paper proposes the HF-FEM and HTS-FEM, based on 3D polyhedron-octree, to calculate the steady-state heat conduction and thermal stress of particle-reinforced composites. In addition, this paper presents a method for partitioning spherical particle-reinforced materials into polyhedron-octree elements, each of which contains only one type of material. Each element is divided into multiple Delaunay tetrahedrons, and hammer integration is used to perform the integrals. Additionally, this paper presents a technique to construct the heat flux function for polyhedral elements, and the effectiveness of the above methods is demonstrated by comparing it to several numerical examples of traditional finite element methods. [ABSTRACT FROM AUTHOR]

Anisotropic stiffness properties of carbon fiber-reinforced polymer composites (CFRPCs) can be designed by arranging the fiber path. In this paper, we propose a novel method to optimize the fiber path using the genetic algorithms (GA) and simulated annealing algorithms (SAA) combined with the quasi-uniform cubic B-spline curves. The optimization goal is to obtain the largest reaction force of an open-hole plate under uniaxial tensile loading. After the finite element method (FEM) and experimental verifications, the optimized variable-stiffness specimen owes the higher strength and stiffness, compared with the unidirectional one. Besides, the stress concentration can be reduced. One of the key points of this paper is to use the GA and SAA to control the slight data points to optimize the B-spline curve. By reducing the number of optimization variables, the computational efficiency is improved greatly. Moreover, by increasing the interval variables of the B-spline curve, we further improve the accuracy of optimization results. The range of values for the control points can be easily adjusted according to the manufacturing and design requirements. We also compare the optimization results of GA and SAA. Both of them can get the similar optimization results, and the SAA has a better convergence speed and accuracy. The proposed optimization method can be extended for three-dimensional preform and structure design in the future. [ABSTRACT FROM AUTHOR]

This paper proposes a concurrent topology optimization method for optimizing the structures that are periodically filled with multiple microstructures excited by dynamic response in the time domain. At the macroscale, different microstructures are considered as different materials. To generate the various distribution of different microstructures, a multi-material interpolation approach based on the Solid Isotropic Material with Penalization (SIMP) is integrated. At the microscale, the energy-based homogenization method (EBHM) is employed to determine the macroscopic effective properties of the microstructures. All macroscale elements with identical material are represented by a distinct microstructure. For the proportional damping model, the HHT- α is implemented as a time integration technique to obtain the dynamic response of multi-scale and assumed multi-material structures. The objective function of the topology optimization problem is to minimize the mean dynamic compliance. Combined differentiate-then-discretize method with the adjoint variable method, the sensitivity analysis applies the gradient-based Zhang-Paulino-Ramos Jr. (ZPR) algorithm or Method of Moving Asymptotes (MMA) to update the design variables under multiple constraints in the time and space-discretized system. The proposed approach is numerically performed through 2D and 3D examples to demonstrate its effectiveness. [ABSTRACT FROM AUTHOR]

• The layer-gradient periodic pipe provides a feasible solution for bandgap tuning in limited space. • The layer-gradient periodic pipes can reduce the frequencies of higher-order bandgaps. • The coupling between layers makes the bandgaps of layer-gradient periodic pipes more complex. This paper presents a vibration control strategy that installs layer-gradient periodic pipes on general pipelines to suppress the transmission of bending vibration. Bandgap tuning can be achieved without changing the structural size. Initially, a composite cell is constructed based on the bandgap characteristics of ordinary periodic pipes, and a layer-gradient periodic pipe is innovatively designed. Secondly, the bending vibration model of the fluid-filled pipeline under external axial force is established using Hamilton's principle. Combining Bloch's theorem, the transfer matrix method is applied to derive the transfer formula for bending waves. The band structure of the layer-gradient periodic pipe is obtained by solving the eigenvalue equation. Subsequently, the frequency tuning advantages of the layer-gradient periodic pipes are demonstrated through a comparison with the conventional periodic pipes. The effectiveness of the proposed vibration control strategy is verified by finite element simulation. Finally, the effects of liquid filling, layer thickness ratio, number of layers, component ratio, and different material combinations on the bandgap characteristics are discussed. The results indicate that the layer-gradient periodic pipe can reduce the frequency of high-order bandgaps to a certain extent. This study can provide insights into the structural design, bandgap tuning, and engineering applications of periodic pipes in limited spaces. [ABSTRACT FROM AUTHOR]

In this paper, the experimental, statistical and computational universal analysis methods applicable to most fiber-reinforced friction materials (FMs) are presented for four natural wollastonite fibers (WFs) with different length-to-diameter (L/D) ratios. Machine learning (ML) methods for friction coefficient prediction of friction materials are compared and screened. Numerical statistics and calculations are used in conjunction with the use of finite element (FE) models to determine the performance metrics and friction wear mechanisms of FMs. The experimental and computational results show that the artificial neural network (ANN) model performs the best in COF prediction with an accuracy of 0.961. The friction fluctuation, friction stability and integrated friction performance of the WF-containing samples are better than those of the blank samples. For the first time, the FE model was used to explain the swirling phenomenon on the wear surface of FMs. This study provides new ways and ideas for the design, experimentation, data processing and application of friction composites. [ABSTRACT FROM AUTHOR]

This paper examines the impact of polyurethane foam (PUF) filling on the dynamic response of honeycomb sandwich beams (HSBs) through the blast testing. The HSBs are composed of steel face sheets and an aluminium alloy honeycomb core filled with PUF. A comparative analysis of damage modes is conducted between conventional hexagonal and auxetic HSBs, considering both foam-filled and unfilled configurations. The results reveal that the HSBs with foam filling demonstrate significantly enhanced blast performance compared to their counterparts without foam filling. This superiority is evident in both global deformation and local damage characteristics. Furthermore, a well-validated numerical model is developed to examine the damage mechanisms of PUF-filled HSBs under blast loading. The investigation reveals that appropriate PUF filling methods can effectively enhance bending resistance while minimizing the decline in the energy absorption efficiency. Finally, a multiobjective optimization was conducted using the back-propagation (BP) neural network and the SPEA2 algorithm to identify the optimal position for PUF. By utilizing a reduced foam filling approach as opposed to completely filling the structure, the two optimized configurations show a decrease in deformation of 5.38 % with an equivalent specific energy absorption (SEA) or an increase in the SEA by 5.26 % with an equivalent deformation. [ABSTRACT FROM AUTHOR]

• Two novel auxetic structures with different arrangements of a new enhanced trigram frame unit-cell were proposed. • The mechanical performance of the designed structures was evaluated through theoretical analysis, finite element simulation, and experimental tests. • Single and multi-objective optimization methods were implemented to optimize the geometry parameters of the selected auxetic structure. • The exceptional mechanical characteristics of the present proposed structures, surpassing previous designs in terms of stiffness and specific stiffness, highlight their tremendous potential for load-bearing applications. Superior stiffness is deemed essential for structures intended for several applications. The stiffness enhancement can be achieved by adopting structures that deform based on the stretching-dominated mechanism instead of the bending-dominated mechanism. This paper proposes a novel auxetic unit cell design and two distinct lattice structures dominated by stretching when subjected to quasi-static compressive and tensile loadings. The structures are manufactured via the fused deposition modeling (FDM) 3D printing method with the use of polylactic acid (PLA) filament as the patent material. The investigation also includes theoretical analyses and finite element (FE) simulations, all yielding consistent results. Response Surface Methodology (RSM) is also employed to understand the influence of three geometric parameters. Besides, single-objective optimization is conducted to maximize the stiffness through geometry design, and multi-objective approaches are implemented to improve the stiffness while simultaneously reducing relative density. Finally, different unit cells, which deform based on the stretching-dominated mechanism, are selected to represent their respective classifications, enabling a comparison of their potential stiffness capacity with the designed structures in this study. Remarkably, it is highlighted that the proposed unit cells and structures outperform others in terms of specific stiffness, making them a promising choice for use in structural engineering. [ABSTRACT FROM AUTHOR]

In this paper, three state-of-the-art deep learning uncertainty quantification (UQ) methods – Flipout probabilistic convolutional neural network (CNN), deep ensemble probabilistic CNN, and Bayesian probabilistic CNN – based on the Visual Geometry Group 13 architecture are proposed. They are compared with a traditional Bayesian inference approach for localizing delamination damage in composite. The law of conditional covariance is used to separate and quantify the predictive variance of the three networks into aleatoric and epistemic uncertainty. The network models' performance is enhanced through hyperparameter optimization using Hyperband and warm-up optimization algorithms. The performance of the three networks in measuring the uncertainty is assessed on an out-of-distribution (OOD) dataset and validated on an in-distribution (ID) dataset for localization of composite delamination damage. Results indicate high accuracy in predicting damage locations for all methods on the ID dataset. On the OOD dataset, the Flipout and deep ensemble network have better performance, stably measuring aleatoric uncertainty in both trained and untrained areas, while the Bayesian network's aleatoric uncertainty shows a discernible change across both areas. All three networks effectively measure epistemic uncertainty. Overall in both ID and OOD datasets, the Flipout network provides an optimal balance among training efficiency, UQ effectiveness and accuracy in predicting damage locations. [ABSTRACT FROM AUTHOR]

Compared to traditional textile composites, the coated fabric used in fabric membrane structures is much more flexible. Its macroscopic mechanical behavior is highly nonlinear, anisotropic, and shows stress ratio dependence during the loading process. The main reason is that the yarns within the coated fabric are not completely solidified, and there is a significant crimp interchange process in the yarns under load. This paper proposes a novel mesoscopic finite element (FE) model to predict the nonlinear orthotropic mechanical behavior of PTFE-coated fabric using the virtual fiber method (VFM). By comparing with experimental data, it is shown that the virtual fiber defined by the truss element can visually and effectively simulate the mechanical behavior of the internal fibers and reflect the crimp interchange process between two-way yarns. Combined with PBCs, it can effectively predict the uniaxial and biaxial tensile behaviors of the PTFE-coated fabric. Moreover, a new multi-scale model suitable for structural scale analysis of fabric membrane structures has been attempted by combining the direct FE2 (D-FE2) method with the proposed mesoscopic FE model. Local deformation mechanisms and macro response of fabric membrane structures can be observed simultaneously from these analyses, which provide a better understanding of the mechanical behavior of the fabric membrane structures. [ABSTRACT FROM AUTHOR]

Impact direction is one of the critical factors affecting the waterdrop erosion behavior of materials. In this paper, rain erosion damage behavior of a typical carbon fiber reinforced polymer (CFRP) laminate is investigated as a function of the impingement angle (0° as normal impact and 15°, 30°, 45° as oblique impact deviated from the normal) and the relative fiber orientation (parallel PA and perpendicular PE to the fiber direction of the surface layer) under the waterjet velocity of 320 m/s and the impact frequency of 50 Hz via a pulsating waterjet erosion test rig. After continuous impact of multiple waterjets, the typical damage of CFRP laminates is mainly composed of the central erosion crater and the upheaval of the surrounding surface layer. The onset and evolution of damage are mainly driven by the shear action of lateral jetting and stress concentration of hydraulic penetration exerted at the surface irregularities. There is no significant difference in the damage modes between the normal and oblique impacts, except for the asymmetric distributions of matrix cracking in PA-orientation and delamination in PE-orientation caused by the angle inclination. With the impingement angle increasing, the volume loss and erosion depth decrease while the incubation period increases mainly due to the decreases of water hammer pressure and contact area. The PA-orientation can cause lower incubation period and larger erosion mass loss than PE-orientation before the surface ply is penetrated, with the underlying mechanism related to the distinct tensile properties of CFRP laminates along longitudinal and transversal directions and the asymmetric properties of fiber–matrix interface. According to the fitting results of erosion curves, it is possible to describe the incubation period and the erosion rate of CFRP laminates under oblique impact using a two-branch equation, and Springer model can give reasonable prediction for the average incubation period and PA-orientation erosion rate of a certain oblique impact occasion with known 0° impact erosion results. [ABSTRACT FROM AUTHOR]

The nonlinear stochastic behavior of soft-core sandwich panels and the evolution of uncertainty along the nonlinear path are investigated. The paper focuses on the way the geometrically nonlinear and unstable behavior is affected by uncertainty and considers, for the first time, the evolution of uncertainty along the unstable structural response. The structural modeling relies on the EHSAPT and its integration into the geometrically nonlinear stochastic regime. A tailored nonlinear FE model that is based on a variational formulation and an arc-length solution scheme are developed. The stochastic analysis adopts the Perturbation-based SFEM and its generalization for the evolution of uncertainty along the process axis. A numerical study demonstrates the stochastic methodology and explores the impact of uncertainty on the nonlinear structural behavior. The evolution of the stochastic stress resultants at the displaced end is studied along the two axes of evolution. The differences between the two representations and their physical and engineering significance are highlighted revealing significant changes to the uncertainty evolution pattern with the shift from global buckling to localized wrinkling instabilities. The representation of the stochastic results along the process axis also quantifies the level of uncertainty near critical and singular points where conventional analyses diverge. [ABSTRACT FROM AUTHOR]

Macro fiber composites (MFCs) are widely used in various fields due to their excellent piezoelectric performance. This paper presents a novel method that combines analytical and finite element analysis to accurately model MFCs. The method iteratively calculates the nonlinear poling field generated by interdigital electrodes (IDEs), leading to reliable estimates of the local orientation and properties of the piezoceramic. The local poling state is then incorporated into a 3D representative volume element (RVE) model to determine the strain response of the device under realistic loading conditions. The influence of the poling voltages, piezoceramic fiber thickness, and IDE geometries on the poling and actuating performance of the device is studied. Furthermore, considering the complexity of the modeling the entire MFC device, a simplified method is further presented that is much more efficient and maintains the same level of accuracy. [ABSTRACT FROM AUTHOR]

Bending-active elastica beam is a structural configuration that is based on the elastic deformation of an initially straight beam. This deformation occurs when horizontal displacements are applied to a sliding support, causing the beam to bend into an arched shape. Previous research has focused on the stability of small-scale bending-active structures, thus not considering material strength, which is crucial in real-scale applications and structural design. This paper investigates the short-term structural behavior of bending-active elastica beams using pultruded glass fiber-polymer composite profiles, as used in real-scale structures. A series of short-term bending and service load application experiments were performed considering different bending degrees and loading scenarios. These results were used to validate finite element modeling. They demonstrated that steeper bent beams experience material failure, while shallower ones exhibit snap-through buckling. Material failure initiates on the tensile side of the beams, with cracks initiating at locations of maximum curvature. Higher bending degrees and symmetric loading result in higher maximum loads than lower bending degrees and asymmetric loading. A strain-based failure criterion, which can serve for structural design, is derived. [ABSTRACT FROM AUTHOR]

This paper presents the Green's Functions Stiffness Method (GFSM) for solving linear elastic static problems in arbitrary axially non-uniform Timoshenko beams and frames subjected to general external loads and bending moments. The GFSM is a mesh reduction method that seamlessly integrates elements from the Stiffness Method (SM), Finite Element Method (FEM), and Green's Functions (GFs), resulting in a highly versatile methodology for structural analysis. It incorporates fundamental concepts such as stiffness matrices, shape functions, and fixed-end forces, in line with SM and FEM frameworks. Leveraging the capabilities of GFs, the method facilitates the derivation of closed-form solutions, addressing a gap in existing methods for analyzing non-uniform reticular structures which are typically limited to simple cases like single-span beams with specific axial variations and loading scenarios. The effectiveness of the GFSM is demonstrated through three practical examples, showcasing its applicability in analyzing non-uniform beams and plane frames, thereby broadening the scope of closed-form solutions for axially non-uniform Timoshenko structures. [ABSTRACT FROM AUTHOR]