This paper describes an ultrasonic scanning system for the inspection of stiffened composite panels used in modern aircraft construction based on elastic constant identification. Conventional ultrasonic scanning systems track individual features of the propagating waves (e.g. amplitude, arrival time, etc.) to detect damage by tracking the associated wave scattering. An ability to map the elastic constants of the composite part can greatly enhance the sensitivity of the scan to structural damage that can be detected and quantified by a corresponding reduction in the stiffness components. Elastic constants can be also correlated to residual strength. The proposed scanning system utilizes a “single-input-dual-output“ (SIDO) scheme whereby ultrasonic guided waves are excited by an impact and detected by two air-coupled ultrasonic sensors. The phase velocity dispersion curves of the waves are experimentally measured under a single excitation at each scanning line using a phase-spectrum technique. These curves are then inverted using a Semi-Analytical Finite Element (SAFE) method as the wave propagation forward model and Simulated Annealing as the optimization routine. This approach yields estimates of several elastic effective constants of the composite panel at each scanning line. These constants are then related to impact damage in stiffened stringer-skin composite panels.

The bimodal cell structure with both large and small cells has been reported to have more excellent thermal insulation, vibration damping and mechanical properties. In this work, UHMWPE/PEG porous materials with bimodal cell structure were prepared by a simple one-step decompression micro-foaming technology using only supercritical carbon dioxide (sc-CO2). The effects of PEG content, molecular weight of PEG, saturation temperature, pressure and time on the evolution of cell morphology were investigated. The results show that when the content of PEG is 7%, the UHMWPE/PEG composite porous material exhibits stable bimodal cell morphologies in most process range, and a mechanism for the formation of bimodal cell is proposed. Moreover, UHMWPE/PEG porous materials with bimodal cell structure show higher mechanical properties. When the relative densities are similar, the anti-compression properties of bimodal porous materials are significantly better than those of unimodal porous materials.

This paper investigates the stability performance of a large-scale carbon fiber reinforced composite (CFRP) truss used in the keel structure of a stratospheric airship. An ultimate load-bearing test was performed on an approximately 4 m long curved CFRP thin-walled pipe truss, which was reduced to sliding supports at both ends to simulate force conditions in the normal service phase. A modified initial geometric imperfection (GI) simulation method was employed to conduct a linear and nonlinear finite element (FE) buckling analysis of the structure, considering the composite lay-up of the thin-walled pipe. Parametric analysis studies were also conducted to improve simulation accuracy and lightweight design. Results show that the structure exhibits linear behavior under the ultimate load level, with failure occurring at the span position of the lower chord. The FE model predicts the ultimate load capacity and stiffness of the structure accurately, and the parametric analysis reveals the significance of choosing a reasonable imperfection form and amplitude for the FE analysis, as well as the significant effect of ply orientation on structural performance. This study provides valuable insights into the design and optimization of CFRP trusses used in the keel structure of stratospheric airships.

This paper proposes a 1-3-type piezoelectric composite structure based on a three-layer cascade structure, utilizing flexible silicone rubber to promote the thickness vibration mode of piezoelectric ceramics. The sandwiched ceramic layer is adopted to improve the mechanical stability. Based on the uniform field theory, we build a theoretical model for the cascade composite and analyze how the thickness of the sandwiched layer and the volume fraction of the ceramic component affect the performance parameters, including the thickness electromechanical coupling coefficient, the density, the sound velocity and the characteristic impedance. Finite element analysis is employed to evaluate the material and validate the theoretical model. The three-layer cascade piezoelectric composites are fabricated using the “dice-and-fill” technique. The tested results demonstrate that the cascade piezoelectric composite exhibits a concentrated thickness vibration mode, and its thickness electromechanical coupling coefficient is up to 0.71, which is 15.5% higher than that of traditional 1-3 piezoelectric composites. It has great potential in developing promising ultrasonic transducers with high transmitting-receiving response.

The inverse finite element method (iFEM) has been used to achieve the shape sensing of small displacements based on linear elastic theory. However, with the development of smart structure technology, the formulation is not suitable for the anisotropic composite structures with large deformation in practical engineering. Therefore, a nonlinear iFEM algorithm is proposed to monitor the linear and nonlinear deformation of the anisotropic composite beams. The formulation not only involves the effect of orientation of composite fibers on strain distribution into the shape sensing model, but also accounts for the effect of shear deformation without any requirement of shear correction factor. Initially, the third-order shear deformation theory (TSDT) is reviewed along with deriving the nonlinear strain field based on von-Karman strain theory. Considering the problem that the couple term of the shear strain and bending strain is unmeasurable, the relationship between shear and bending displacements is established according to the derived constitutive equations. Then, the proposed nonlinear iFEM method reconstructs the deformed structural shape, where isogeometric analysis (IGA) approach is used to construct the displacement functions and the experimental section strains are calculated from the discretized surface strains. Finally, several examples are solved to verify the proposed methodology. Numerical results demonstrate that the nonlinear iFEM algorithm can improve the reconstruction accuracy by 4% with respect to the linear iFEM method for beam structures. Hence, the proposed approach can be used as a viable tool to predict nonlinear deformation of composite structure.

The current study focuses on the bond behavior of CFRP sheets-to-steel double-lap shear (DLS) joints with different steel surface treatments. A total of 18 DLS joints are tested in tension. The effects of six different steel surface treatments on the mechanical response of the DLS joints are investigated. The steel surface treatments include power tool polishing with four different sandpapers in grit sizes, grit blasting and pre-coating silane coupling agent. The surface topography and roughness of the steel substrates with different treatments are evaluated from test results, and their relationship with mechanical behavior and failure modes is established qualitatively. The geometric characteristics of the surface are strongly correlated with the average ultimate loads of the DLS joints. Based on the measured load-slip curves, the bond-slip (B-S) relationship is predicted, which removes the effect of the actual deformation of the steel substrates on the slip. Regression analyses of quantifying the B-S relationship are conducted to establish the simplified B-S model applied to the CFRP sheets-to-steel bonded interface, and the effects of different steel surface treatments are considered in this model. The accuracy of the simplified B-S model is verified through comparison with the experimental results.

In this study, analytical solutions are derived for three-dimensional, transversely isotropic and multilayered isosceles-right triangular plates under simply supported edge conditions. For any homogeneous layer, we construct the general solution in terms of a simple formalism that resembles the Stroh formalism and the dual-variable and position method. For multilayered isosceles-right triangular plates, we derive the analytical solution by the new propagating method in terms of the symmetry theory. Numerical results clearly show the influence of load distribution, stacking sequence, and mechanical and geometric parameters on the elastic fields. The proposed solutions can also serve as benchmarks for other numerical methods such as the finite element method.

A hierarchical multi-scale numerical model is developed to predict the effect of cross-sectional geometric properties on the mechanical response of three-dimensional (3D) braided composites. In this model, the effective properties are transferred from the fiber bundle scale to the mesoscale, and the finite element and analytical methods are used to predict the macroscopic mechanical properties. Damage evolution was evaluated, based on a continuous damage mechanics approach. A user-defined material subroutine (UMAT), in a nonlinear finite element analysis, has been developed to implement the proposed model and further determine the response and subsequent damage evolution in 3D braided composites subjected to quasi-static tension. The elastic properties of the composites are predicted using a hybrid multi-scale model. The agreement between the numerical simulations and the experimental results is good. It has been shown that the cross-sectional geometry of the fiber bundle properties has a significant influence on the tensile behavior of the composite, which is captured by the proposed multi-scale damage model. This research offers a theoretical analysis for selecting the optimized fiber bundle geometry for using in FE predictions of 3D braided composites.

In this paper, a plug-in – FibrePlug – developed within the ABAQUSTM Simple GUI (RSG) and Fox GUI development environments is introduced. In contrast to traditional meso-scale modeling, FibrePlug provides an advanced user-friendly environment for simultaneously modeling fibers at micro- and meso-scales. With FibrePlug, the yarn level fabrics may be replaced by a specific user-defined number of representative bundles also known as virtual fibers. Further customizations, including the modification of the number of virtual fibers, are available within the GUI. In addition to the FibrePlug’s microscale modeling capabilities, readers are also introduced with the beam and solid element discretization features within the plug-in and their implications to computational efficiencies. The feature of the plug-in’s hybrid multi-scale model generation is also demonstrated. Furthermore, the accuracy of a uniform beam-element model generated with FibrePlug is validated by simple tensile tests performed with the 3D DIC equipment. Despite noticeable deviations in mechanical properties with respect to traverse direction, these properties are satisfactorily predicted along the loading direction.

In this work, our new quadrature schemes based on the Richardson extrapolation (RE) and centroid or midpoint rule are further applied to the quadrilateral Mindlin plate element for static and free vibration analysis. In the process of generating element matrices, to apply RE and midpoint quadrature rule, there are two approximations are particuarly considered. As the first approximation, element matrices are calculated at centroid of whole plate element. For stabilizing function or second approximation, each quadrilateral element is divided into four sub-quadrilaterals, either centroid of the each sub-quadrilateral plate for Element midpoint method (EM-plate method) or midpoint of the element edges are used for Element edge method (EE-plate method) to compute the all the element matrices. Then, both the approximations are added with the RE based weighting functions. Generally, midpoint based quadrature avoids the shear-locking phenomenon in the plate elements and the RE enhances accuracy and rate of convergence of the final solution without hourglass or zero-energy issues. The numerical examples validated that the RE based EM-plate method and EE-plate method are free of shear-locking, ability to pass the patch test, better rate of convergence with less number of sampling points. Also, the new schemes confirms the three important properties such as (i) can produce high accurate solutions with better rate of convergence for problems for irregular geometries in the static analysis; (ii) can produce high accurate solutions without hourglass issues in free vibration analysis; and (iii) can generate the accurate values of high frequencies of plate elements over irregular meshes.

Impact damage of carbon fiber reinforced polymers (CFRP) composites is a common form of damage in composite structures. In this work, a model integrating the structure model of composites, failure model and contact model with friction based on ordinary state-based peridynamics (PD) theory is developed to simulate multiple impacts of CFRP. The simulation results of single impact are in good agreement with the existing experimental and finite element simulation results, validating the proposed PD model. Then the effects of impact energy, stacking sequences and impact angles on the CFRP damage behaviors under multiple impacts are thoroughly investigated. Multiple high energy impacts may cause a non-linear increase in the amount of damage. The peanut-shaped stress distributions induced by impact along different directions are typical of the stress responses, which usually correspond to different fiber directions. Moreover, the laminates with stacking sequences of different fiber directions have a better impact damage resistance, as they can hinder the propagation of stress waves across the layers. In addition, as the impact angle increases, the stress and damage become more pronounced. The insights gained shed light on the repeated impact damage mechanisms of CFRP.

The induced ovalization or out-of-plane deformation in the circular and transition airfoils of modern MegaWatt (MW) size wind turbine rotor blades is caused by the combined effect of the applied far-field bending loads and the local structural curvature. In general, the phenomenon of the ovalization or out-of-plane deformation of the circular elastic cross sections under the applied bending load or the internal/external pressure is termed the Brazier effect [1], [2]. Considering the poor mechanical performance in the out-of-plane direction of the typically used laminated glass composites for wind turbine rotor blade construction, the current research work aims to develop a thoroughly validated computational methodology to understand the ovalization (out-of-plane deformation) induced failure behavior in circular cross-sections located between the root and transition region of a rotor blade, using a substructure level testing and modeling under representative load conditions. To this end, detailed manufacturing, testing, and a computational modeling campaign is devised and executed at different length scales starting from coupon to cylinder (substructure) level. The required elastic, strength, and fracture properties for the cylinder progressive damage analysis are evaluated using coupon-level experiments following various ASTM standards.The developed numerical methodology is thoroughly validated using the substructure level experimental load–displacement curves, strain, and damage profiles. Detailed experimental and numerical studies reveal that the induced ovalization leads to a non-linear relation between the applied crushing force and the local radius of curvature until the unstable collapse of the cylinder. Based on the thorough local stress and damage analysis, it is evident that along with the dominant interlaminar shear stress (ILSS), the presence of the interlaminar tensile (ILT) stress component is the key to the delamination induced collapse of the cylinder.

Failure analysis in non-crimp fabric (NCF) based fibre reinforced composite materials is challenging due to the complexity associated with multiaxial fabric architectures and the interaction of different failure modes. In this study an in-situ 3-point bend test under scanning electron microscopy (SEM) has been performed to analyse the failure behaviour of a NCF basalt epoxy composite manufactured by vacuum assisted resin transfer moulding (VaRTM) technique. The failure mechanisms observed during the in-situ 3-point bend test are compared to those from standard (ex-situ) flexure and interlaminar shear strength (ILSS) tests on the same material. Finite element analysis of each test configuration is also conducted to analyse the in-ply stress distributions and to predict the damage initiating stresses. The study reveals that fibre/matrix debonding leading to matrix cracking in the 90° sub-ply is the damage initiating failure mode, with final failure occurring due to fibre buckling in neighbouring 0° sub-plies. The finite element analysis reveals that matrix cracking in the 90° sub-ply is controlled by shear stresses ranging between 35-45 MPa.

Bio-inspired helicoidal carbon fiber reinforced polymer composite (CFRPC) laminates are one of the novel composite structures that have potential applications in many engineering fields. The optimization design of helicoidal layups with linear, nonlinear Recursive, Fibonacci, Exponential and their combination-form arrangements are carried out using ABAQUS and ASA algorithm on Isight platform. Two cases of the compressive postbuckling under uniaxial compression and the thermal postbuckling under uniform temperature rise of helicoidal CFRPC laminated plates stacked with optimal layups are performed and the secondary buckling and mode jumping in the postbuckling region are examined for the first time. Numerical investigations are carried out for 16-ply symmetrical helicoidal laminated plates with various width-to-thickness ratios under different boundary conditions and are compared with quasi-isotropic counterparts. The results show that the bio-inspired helicoidal CFRPC laminated plates can enhance the buckling load and the buckling temperature as well as the compressive and the thermal postbuckling strength. In some cases, the secondary buckling and mode jumping may occur for rectangular helicoidal laminated plate in the postbuckling region.

Strain-hardening cementitious composites (SHCC) has become increasingly prevalent in structural design. Compared to ordinary concrete, SHCC exhibits ultra-high tensile ductility, significant strain-hardening behavior, very fine multi-point cracking, high energy dissipation, and good durability. One robust model for simulating reinforced concrete behavior is the so-called Lattice Discrete Particle Model (LDPM), and specifically LDPM-F which includes the effect of fiber reinforcement. The present cycling constitutive law implemented in LDPM and the cycling fiber-bridging law in LDPM-F though cannot accurately capture the residual tensile plastic strain, loading–unloading path, and energy dissipation of SHCC during cyclic tension. To solve these concerns, the cycling tension–compression constitutive law and the nonlinear cycling fiber-bridging law were reformulated. Further, the new model was used to simulate the cycling tensile behavior of plain concrete, SHCC, and Fiber-Reinforced Polymer (FRP) grid reinforced SHCC (FRP-SHCC). Simulation results show that the multi-point cracking, crack widths, and ultra-high ductility properties are correctly captured by LDPM-F. In addition, LDPM-F with the modified cycling constitutive model can effectively simulate the cycling tension–compression behavior, accurately reproducing the stress–strain relationship, residual plastic strain, and overall dissipated energy. Finally, the simulated failure modes agree well with the actual fracture planes of these materials under cyclic tension.

Hybrid carbon/glass fibers composites can possess pseudo-ductility of progressive failure through material design to achieve the function of load early warning. The printing design and forming thermal environment of parts have important influence on the pseudo-ductile behavior of hybrid fiber composites. In this paper, the hybrid continuous carbon/glass fibers reinforced nylon composites with progressive failure pseudo-ductility were prepared by additive manufacturing, and the effects of printing layer thickness, carbon/glass fibers layer ratio and forming thermal environment on the tensile properties and pseudo-ductility of hybrid fiber composites were investigated and characterized. The results indicate that the printing layer thickness has no significant impact on the tensile strength of hybrid fiber composites, the pseudo-ductility is obvious when the layer thickness of [G5C2G5] is 0.075 mm and 0.125 mm. With the same material ratio, reducing the layers or setting the printing platform temperature (100 °C) can decrease defects generation to achieve the better pseudo-ductility. However, the pseudo-ductility immensely weakens using the annealing treatment of 120 °C for 3 h. The pseudo-ductility of the [G5C2G5]0.075, [G5C2G5], [G3C1G3] and [G4C1G4] samples have been discovered successfully.

In this paper, the static and fatigue behaviors under compression of Multi-Directional (MD) Carbon Fiber Reinforced Polymer (CFRP) laminates with impact damage were investigated. After low-velocity impact performed on three 24-ply Quasi-Isotropic Quasi-Homogeneous (QIQH) laminates having different stacking sequences, the influence of the distribution of interfaces between two plies on the compression after impact and post-impact compression fatigue properties was evaluated. The damages on the surfaces and internal delamination were accessed via visual and optical observation and ultrasonic C-scan inspection. The thermographic approach, which has been developed and validated on numerous non-impacted composite materials, was applied for the rapid determination of the fatigue limit together with the S-N curves of post-impact fatigue specimens. Experimental results showed that with 5 J impact energy, the three stacking sequences gave similar impact results. The residual strengths measured by compression after impact tests were also similar to each other, so did the global damage modes. The determined fatigue limits of impacted specimens of different stacking sequences were nearly identical and the predicted S-N curves were also close to each other despite the exact crack propagation and orientation were not the same, under both compressive static and cyclic loads.

High packaging efficient lightweight jointless deployable folding structures are nowadays fabricated from combination of rigid and thin flexible elastic hinges using fiber reinforced polymers composites (FRPCs) for various applications such as space, robotics, flexible electronics, etc. It is of high importance to optimize these structures in the design phase using computer aided analysis tools for which the structural-level elastic constants are challenging to find experimentally. In this work, a generic X-ray computed tomography (micro-CT) driven framework based on voxel modeling approach was developed to virtually characterize the smallest building block of a typical large-scale foldable structure that consists of a rigid region and elastic hinge needed for folding operation. The voxel-based description of a real representative volume element (RVE) was generated using the images obtained from the micro-CT. The homogenized mechanical properties for each of the material systems was then obtained by multi-parameter component segmentation and subsequent directional averaging of the corresponding voxels. Three-dimensional void analyses were also shown using the micro-CT based voxel modeling. The accuracy of the non-destructive framework was demonstrated for four materials systems, based on different combinations of the matrix and fabric architectures in comparison with the mechanical tests. The homogenized mechanical properties obtained using the present framework based on the real RVE showed good agreement with the experimental results providing a foundation for a structural-level computer aided analysis.

The stress-shielding (SS) effects following orthopedic implantation can lead to implant loosening. Functionally graded materials (FGMs) and coatings (FGCs) are special heterogeneous composites with an altered gradient from one surface to the other. Using such materials in the implant structure with the ability to mimic the bone's characteristics may lead to proper consequences. This study reviewed papers to argue for two groups of studies: 1) research focusing on fabrication methods of FGM prostheses and research focusing on the design and optimization of knee and hip arthroplasty components. Manufacturing techniques with advantages and disadvantages have been classified based on solid, liquid, and gas-based processing. Moreover, 3D printing is used to fabricate arthroplasty components from FGMs. Despite the production of cement or cementless FGMs for orthopedic applications, the optimal design of implants is essential for preparing long-span products. This research highlighted FGMs and their applications in the manufacturing, design, and optimization of hip and knee arthroplasty components. While several researchers have focused on the design of implants via finite element analysis to reduce SS, further scientific studies are required in the future to prove their clinical applications. Furthermore, we suggest using 2D and 3D FGMs in the design of TKA and THA.

Spatial thin-walled laminated composite beams under large deformations with torsional warping effects are studied. A novel two-dimensional cross-section model for composite beams with shear and torsion-warping deformations is proposed wherein the three-dimensional warping and coupling effects caused by inhomogeneous anisotropic materials are considered. A weak form quadrature element formulation is established for nonlinear analysis of geometrically exact composite beams with torsional warping and shear deformations. By combining the two-dimensional cross-section model and the nonlinear one-dimensional beam model, the three-dimensional stress and strain fields can be obtained. Numerical examples are provided to investigate the torsional warping effects and verify the effectiveness of the present formulation. Results of the present beam formulation which are achieved at a significantly lower computational cost are in excellent agreement with those of shell and solid models of commercial codes.