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Efficient structural element-based models are essential for fast simulations of wave propagation in composite structures for model-based and physics-informed data-driven structural health monitoring. This article introduces the first efficient multiphysics time-domain spectral structural element for wave propagation analysis of beam and panel-type composite structures with piezoelectric transducers (patch or full layers) containing delaminations. A general framework is presented to model multiple delaminations and transducer patches located arbitrarily. The intact host and patch transducer-bonded laminates and sub-laminates between delaminations are modelled separately using an electromechanically coupled efficient layerwise zigzag theory (ZIGT) for kinematics and a piecewise quadratic variation for the electric potential across piezoelectric layers. The high-order spectral element (SE) features a virtual electric node to model equipotential surfaces of piezoelectric transducers apart from the usual physical nodes having mechanical and internal electric degrees of freedom. A hybrid point-least squares continuity approach is employed to maintain continuity at the intersections of delaminated sub-laminates or the patch-bonded laminate with the host laminate. The model's performance in capturing electroelastic waves' interaction with delamination is examined with reference to the conventional finite element (FE) solution based on the ZIGT, continuum-based FE solutions, and the SE solution based on a non-layerwise version of the laminate theory. Finally, the model is used to examine the impact of interfacial location and size of delaminations on wave propagation behaviour. • First efficient multiphysics spectral structural element for smart composite strips with delaminations. • Enforces nonlinear displacement field continuity at delamination and piezo patch intersections. • Employs virtual electric node to satisfy equipotential conditions on piezo surfaces. • Yields faster computation and better accuracy compared to the standard FE counterpart. • It will benefit guided wave-based structural health monitoring of delaminated structures. [ABSTRACT FROM AUTHOR]

• Three damage modes of carbon fiber composite under impact damage are considered. • Physics-based modeling of mixed-mode damage and acoustic emission energy is made. • AE energy-based damage quantification model is established and calibrated. • Accuracy and reliability of the model is validated by independent testing data. In-service composite laminates are susceptible to impact-induced damage, which can substantially reduce its integrity and service life. The damage prediction remains a great challenge due to mixed damage modes and varying damage patterns. This study develops a novel acoustic emission (AE) energy method for predicting damage areas under three typical damage modes. Laboratory testing of composite laminate specimens subject to quasi-static indentation is performed in conjunction with in-situ AE monitoring to acquire AE data. By bridging two sets of energy formulations developed, namely, the one that correlates the damage area and the released strain energy of each damage mode and another that relates the released strain energy to the AE energy, an analytical model for predicting damage areas using AE energy components is derived. Proper signal procedure procedures are established to extract the energy components from AE monitoring data, and numerical and testing data are used to calibrate the model parameters. The effectiveness of the proposed model is further validated by comparing the prediction results of the damage areas with the actual damage areas of specimens tested under different indentation depths. The result indicates that the proposed AE energy method can yield reliable predictions of the damage area under mixed damage modes. [ABSTRACT FROM AUTHOR]

• The impact damage tolerance of quadriaxial laminates was compared with equivalent Double-Double (DD) laminates. • A computer tool was developed to find equivalent DD laminates and assess the homogenization condition. • A validated high-fidelity finite element model was used to study the compression after impact (CAI) of DD laminates. • DD laminates can be used instead of conventional quadriaxial laminates without compromising impact damage tolerance, while benefiting from the design advantages of the DD concept. Tailorability is a key advantage of fiber-reinforced composites over other material systems. While tailoring a single isolated laminate is relatively simple, challenges arise when designing larger integrated components while ensuring compatibility between laminates and avoiding sharp changes in local stiffness. The innovative Double-Double (DD) laminate design method simplifies the optimization and processing of laminates by incorporating 4-ply building blocks consisting of +ϕ, −ϕ, +ψ, and −ψ ply orientations. As a relatively new concept, DD laminate design requires careful assessment to ensure its performance is equivalent to that of conventional designs. The current study compares impact damage tolerance of quadriaxial (QUAD) laminates consisting of 0°, 90°, and ±45° ply orientations with equivalent DD laminates under Low-Velocity Impact (LVI) and Compression After Impact (CAI) loadings. To this end, a validated three-dimensional high-fidelity finite element model capable of capturing fiber breakage, splitting, kinking, as well as matrix cracking and delamination, was used. A computer tool was developed to identify equivalent DD laminates and to find the best stacking sequence for achieving layup homogenization. Three equivalent DD laminates were selected for the [0/45/90/−45] 4s. The first laminate had an equal in-plane stiffness [A] matrix ([67.5/–22.5/22.5/−67.5] 8T), the second laminate had an equal flexural stiffness [D] matrix ([64.5/−17/17/−64.5] 8T), and the third laminate ([65.5/−18.5/18.5/−65.5] 8T) had a similar [D] matrix while keeping the difference between each element of [A] matrices below 10 %. The results indicate that the QUAD laminates can be replaced by equivalent DD without compromising impact damage tolerance while benefiting from the improved design and manufacturing ease of the DD laminate configuration. [ABSTRACT FROM AUTHOR]

• Inspired by the microstructure of the lobster cuticle, a hybrid-helicoidal composite laminate was proposed to enhance its impact resistance. • Combining various helicoidal structures, such as single-helicoidal and double-helicoidal, improved the toughness and strength. • The effects of helicoidal angle growth rate, position, and thickness on low-velocity impact resistance were analyzed. • The damage mechanisms of bio-inspired helicoidal laminates were detailed revealed. The exoskeleton of the Homarus americanus lobster feature a hybrid-helicoidal structure of chitin-protein fibers, with distinct helicoidal configurations in the exocuticle and endocuticle, exhibiting strong impact resistance. Taking inspiration from this biological structure, combined with single-helicoidal and double-helicoidal structures, various helicoidal configurations of composite laminates were designed. Both linear and nonlinear helicoidal angles, including sinusoidal and exponential configurations, were considered. The interlaminar and intralaminar damage mode were adopted to simulate material damage initiation and evolution. The effect of helicoidal angles, position, thickness and angle variations of endocuticle on low-velocity impact resistance was analyzed, revealing the damage mechanisms of bio-inspired laminates. The results show that bio-inspired hybrid helicoidal structures with special features could significantly enhance the impact resistance of composites, with laminates featuring sinusoidal-exponential double helicoidal structures showing superior performance. Sinusoidal configurations, being less prone to penetration, are more suitable for the exocuticle. The introduction of double-helicoidal configurations could enhance the toughness and strength of the structure. This studying deepened an understanding of failure mechanisms of bio-inspired helicoidal composite laminates under low-velocity impact and provide a design strategies for developing high-performance, impact-resistant composite materials. [ABSTRACT FROM AUTHOR]

• The proposed FE modeling methodology can accurately predict the impact response of composite laminates. • Fiber breakage, pull-out, splitting, kinking, crushing, and matrix cracking are predicted using the enhanced LaRC05 criteria. • Delamination and intralaminar matrix cracking interactions are modeled. • The matrix fracture plane and the fiber kink band angle can be found 48 % faster using the SRGSS algorithm. • The detailed sequence of impact damage occurrence is predicted by analyzing the histories of dissipated energies. A finite element model was developed using both continuum and discrete damage modeling techniques to provide detailed predictions for ply-by-ply damage progression in composite laminates during low-velocity impact (LVI) events. A new fiber failure model was incorporated into the LaRC05 failure criteria to predict fiber pull-out and fiber crushing during the fiber damage evolution. In addition, the selective range golden section search (SRGSS) algorithm was implemented to efficiently predict fiber breakage, pull-out, splitting, kinking and crushing, and matrix cracking. The delamination was captured by cohesive element layers embedded between every adjacent composite ply. The interactions of intralaminar matrix cracking and delamination were modeled by deploying cohesive elements within each composite ply. The prediction results were validated by 30 J and 75 J drop-weight tests with different-sized impactors, as well as X-Ray CT inspections on 254 mm by 304.8 mm [0/45/90/-45] 4 s IM7/977–3 laminates. The model predicted the maximum deflection and contact duration with <2 % error, and the peak load, damaged areas, and absorbed energy with <8 % error. The matrix fracture plane and the fiber kink band angle were found with 1° precision 48 % faster via the SRGSS algorithm. The detailed sequences of damage occurrence were predicted by analyzing the energy dissipation histories through various damage modes. Although this modeling methodology was developed for LVI scenarios, it has broad applications for predicting failures in composites. [ABSTRACT FROM AUTHOR]

This work utilizes the Immersed Boundary Conformal Method (IBCM) to analyze linear elastic Kirchhoff–Love and Reissner–Mindlin shell structures within an immersed domain framework. Immersed boundary methods involve embedding complex geometries within a background grid, which allows for great flexibility in modeling intricate shapes and features despite the simplicity of the approach. The IBCM method introduces additional layers conformal to the boundaries, allowing for the strong imposition of Dirichlet boundary conditions and facilitating local refinement. In this study, the construction of boundary layers is combined with high-degree spline-based approximation spaces to further increase efficiency. The Nitsche method, employing non-symmetric average operators, is used to couple the boundary layers with the inner patch, while stabilizing the formulation with minimal penalty parameters. High-order quadrature rules are applied for integration over cut elements and patch interfaces. Numerical experiments demonstrate the efficiency and accuracy of the proposed formulation, highlighting its potential for complex shell structures modeled through Kirchhoff–Love and Reissner–Mindlin theories. These tests include the generation of conformal interfaces, the coupling of Kirchhoff–Love and Reissner–Mindlin theories, and the simulation of a cylindrical shell with a through-the-thickness crack. [ABSTRACT FROM AUTHOR]

As with many tasks in engineering, structural design frequently involves navigating complex and computationally expensive problems. A prime example is the weight optimization of laminated composite materials, which to this day remains a formidable task, due to an exponentially large configuration space and non-linear constraints. The rapidly developing field of quantum computation may offer novel approaches for addressing these intricate problems. However, before applying any quantum algorithm to a given problem, it must be translated into a form that is compatible with the underlying operations on a quantum computer. Our work specifically targets stacking sequence retrieval with lamination parameters, which is typically the second phase in a common bi-level optimization procedure for minimizing the weight of composite structures. To adapt stacking sequence retrieval for quantum computational methods, we map the possible stacking sequences onto a quantum state space. We further derive a linear operator, the Hamiltonian, within this state space that encapsulates the loss function inherent to the stacking sequence retrieval problem. Additionally, we demonstrate the incorporation of manufacturing constraints on stacking sequences as penalty terms in the Hamiltonian. This quantum representation is suitable for a variety of classical and quantum algorithms for finding the ground state of a quantum Hamiltonian. For a practical demonstration, we performed numerical state-vector simulations of two variational quantum algorithms and additionally chose a classical tensor network algorithm, the DMRG algorithm, to numerically validate our approach. For the DMRG algorithm, we derived a matrix product operator representation of the loss function Hamiltonian and the penalty terms. Although this work primarily concentrates on quantum computation, the application of tensor network algorithms presents a novel quantum-inspired approach for stacking sequence retrieval. [ABSTRACT FROM AUTHOR]

Manufacturing processes of composites involve a margin of parameter variability (e.g., geometric, mechanical, loading) which results in an inaccurate prediction of their dynamics when considered with exact assumptions. Real-time calculation of such structures confronts engineers with several challenges (e.g., dimension of finite element model, size of parameter space, uncertainty level, nonlinearity). To guarantee accuracy while saving computing time, a double-process Reduced Order Model (ROM) is proposed. It allows reducing both offline data acquisition and online data interpolation for real-time calculation. The learning phase is gradually becoming one of the most critical part of data-driven models. To overcome this problem, a set of reduced bases are built using the Proper Orthogonal Decomposition (POD) from a set of solutions computed using a regression-based Polynomial Chaos Expansion for a properly chosen Design of Experiments. In the online phase, the POD bases are interpolated on a Grassmann manifold using the Inverse Distance Weighting at a non-sampled set of the uncertain parameters' values. The proposed double-process ROM allows to accurately approximate the nonlinear dynamics of a laminate plate with uncertain thickness and fiber orientation of two layers, with a drastically reduced computing time compared to a Full Order Model solving based on classical statistical data-sampling and postprocessing. • Double-process online and offline reduction of a non-intrusive data-driven model. • Nonlinear dynamics of laminate structures with multiparametric uncertainties. • Polynomial Chaos Expansion with Grassmann interpolation of Proper Orthogonal Modes. • A powerful decision-making tool for real-time simulations. [ABSTRACT FROM AUTHOR]

In this paper, a fatigue-creep damage model that can take into account the interaction of fatigue and creep damage is proposed under high temperature cyclic loading. In the proposed model, the effect of temperature on creep damage, the variation of creep damage under different high temperature cyclic loading conditions, and fatigue-creep interaction damage are considered. In addition, in order to accurately describe the creep behavior of unidirectional laminates with different orientations, the damage mechanism of unidirectional laminates was also analyzed. The creep and fatigue test results at different temperatures showed that the proposed creep rupture time model and the fatigue-creep damage model considering the damage mechanisms can successfully predict the creep and fatigue lives of unidirectional laminates at high temperature, and the prediction results are in good agreement with the experimental data. [Display omitted] [ABSTRACT FROM AUTHOR]

Under static loading, measuring experimentally the mode II and mixed mode fracture toughness of composite materials and adhesive joints is well standardised. However, under dynamic loading, no standard procedure has been defined yet. Therefore, this paper proposes an experimental methodology to measure the mode II and mixed mode interlaminar fracture toughness of composite materials and adhesive joints. The methodology is based on a modified split Hopkinson compression bar. Two different data reduction schemes are explored and compared, one based on measuring the crack length, and another based on measuring the force from the strains in the transmitted bar. The two data reduction methods provided considerably different results. By using the method based on measuring the force, the mode II and mixed mode fracture toughness for both interlaminar and adhesive joints decreased for higher strain rates, while the opposite was found with the other approach. The method based on the crack length measurement was deemed to be unreliable due to the difficulties in measuring it. • Experimental study of the mode II and mixed mode fracture toughness under dynamic loading. • Interlaminar fracture toughness of composite materials and adhesive joints is explored. • Two different data reduction methods are employed and compared. • Significant strain rate is found, but results are dependent on the data reduction method. • Advantages and limitations of the data reduction methods are discussed. [ABSTRACT FROM AUTHOR]

• Neither ageing severity nor layups have effects on mode I fatigue delamination. • Ageing severity and layups have negligible influence on material degradation and fatigue delamination mechanisms. • This study provides extra information for ISO mode I fatigue delamination test standard development of composite laminates. Ageing is known to have significantly detrimental effect on mode I fatigue delamination growth (FDG) in unidirectional (UD) composite laminates. However, composite structures are usually designed with multidirectional (MD) layups, which raises the question that is it enough to only conducted fatigue delamination experiments on specimens with a UD layup. The aim of this study is therefore to explore mode I FDG in MD composite laminates with 45//45 interface after different ageing, i.e. at 70 °C 85 % relative humidity (RH) and immersion in 70 °C water bath. Fatigue delamination experiments were conducted at stress ratios R = 0.1 and 0.5. The fatigue data, interpreted via Paris-type fatigue laws, demonstrated that: (1) the change of ageing severity has no influence on mode I FDG in MD composite laminates; (2) FDG remains the same in composite laminates after different ageing, regardless of layups. In all cases, the same master resistance curves can be obtained to determine the intrinsic mode I fatigue delamination resistance of UD and MD composite laminates after different ageing. The physical reasons for these findings were discussed based on the moisture content analysis, Fourier transform infrared (FTIR) analysis, dynamic mechanical thermal analysis (DMTA), and fractographic examinations. It was found that material degradation and delamination mechanisms remain the same for UD and MD layups, as well as for 85 %RH and water bath conditioning. [ABSTRACT FROM AUTHOR]

• Both quasi-static and dynamic properties of 3DP-CFRPs are reported. • 3DP continuous fibre-reinforced composites has enhanced the impact resistance. • 3DP-CFRPs behaves like that of laminated CFRPs. • The bio-inspired 3DP-CFRPs performed best compared with the control groups. This paper explores the mechanical properties of 3D-printed continuous carbon fibre (CCF)-reinforced helicoidal laminates inspired by the laminar structures found in crustacean exoskeletons. For comparison, conventional cross-ply layups, both with and without CCF reinforcement, were also examined. The laminate specimens were manufactured layer by layer through a 3D printer using short carbon fibre-reinforced nylon and CCFs. To evaluate their mechanical properties, both quasi-static and dynamic tests were conducted using a universal testing machine and an instrumented drop-weight impact facility. Experimental data were collected to analyse the progression of mechanical failure and energy absorption in these 3D-printed composites. Additionally, micro-computed tomography (μ-CT) scans were used to analyse internal damage, such as fibre fracture and delamination, in the tested samples. The results revealed distinct dynamic failure mechanisms in the 3D-printed composites compared to their quasi-static behaviour, indicating potential real-world applications. The inclusion of CCFs in 3D-printed composites significantly enhanced their mechanical performance. While the cross-ply laminates outperformed the bio-inspired helicoidal laminates under quasi-static bending loads, the helicoidal laminates exhibited greater stiffness and superior energy absorption during low-velocity, out-of-plane impacts, surpassing their cross-ply counterparts. [Display omitted] [ABSTRACT FROM AUTHOR]

This study proposes a thermo-elastic coupled nonlinear finite element (FE) model for laminated composite beams with active SMA layers based on a higher-order layerwise theory and Brinson's model of SMA's phase transformation constitutive relations. The developed FE Model is further discretized by using the Newton-Raphson time integration scheme to update SMA's phase transformation progress step by step. This new model can be used for thermal-mechanical behavior analysis of hybrid SMA-composite laminated beams, which especially enables accurate prediction for static response of moderately thick to thick beams, large deformation analysis, through-thickness variations of interlaminar stresses and strains. The accuracy and effectiveness of the proposed approach are verified by several numerical examples, and the results are compared with those obtained from corresponding alternative solutions and experimental tests. Using the developed FEM, the effects of temperature, geometrical nonlinearity, pre-strain of SMA, and stacking sequence of composite laminates on the static response of hybrid SMA-composite beams are studied. • A novel coupled finite element model is developed for thermal-mechanical analysis of SMA hybrid laminated composite beams. • The proposed nonlinear method naturally accounts for both material nonlinearity of SMA and geometrical nonlinearity. • The Newton-Raphson method is used to update SMA's phase transformation progress at each iteration step. • The employment of higher-order layerwise theory enables more accurate predictions of stresses at layer level. [ABSTRACT FROM AUTHOR]

Tow steering technologies, such as automated fiber placement, enable the fabrication of composite laminates with curvilinear fiber, tow, or tape paths. Designers may therefore tailor tow orientations locally according to the expected local stress state within a structure, such that strong and stiff orientations of the tow are (for example) optimized to provide maximal mechanical benefit. Tow path optimization can be an effective tool in automating this design process, yet has a tendency to create complex designs that may be challenging to manufacture. In the context of tow steering, these complexities can manifest in defects such as tow wrinkling, gaps, overlaps. In this work, we implement manufacturing constraints within the tow path optimization formulation to restrict the minimum tow turning radius and the maximum density of gaps between and overlaps of tows. This is achieved by bounding the local value of the curl and divergence of the vector field associated with the tow orientations. The resulting local constraints are effectively enforced in the optimization framework through the Augmented Lagrangian method. The resulting optimization methodology is demonstrated by designing 2D and 3D structures with optimized tow orientation paths that maximize stiffness (minimize compliance) considering various levels of manufacturing restrictions. The optimized tow paths are shown to be structurally efficient and to respect imposed manufacturing constraints. As expected, the more geometrical complexity that can be achieved by the feedstock tow and placement technology, the higher the stiffness of the resulting optimized design. [ABSTRACT FROM AUTHOR]

• The failure modes of the designed discrete interleaved laminates are studied by LVI and CAI tests. • An integrated numerical model is developed for discrete interleaved laminates. • The control mechanism of discrete interleaving on the delamination propagation is revealed. • The relationship between residual compressive strength and failure mode is studied based on numerical results. The poor delamination resistance of laminated composites is always regarded as a potential threat to the service safety of the primary load-bearing structure. However, delamination is inevitable during low-velocity impact (LVI) events, as a result, the residual compressive strength of laminates is significantly reduced. Discrete interleaving is a novel strategy to enhance the damage tolerance of these composites, yet the damage mechanism of laminates modified by this method is more complicated. In this paper, we investigated the delamination failure mechanism of quasi-isotropic laminates through thermal deply experiment, and proposed a discrete interleaving scheme based on its damage characteristics. Accordingly, a series of LVI and compression-after-impact (CAI) tests were conducted to validate the design philosophy. Besides, the damage constitutive relation and evolution model applicable to discrete interleaved laminates were explored in detail, a numerical model embedded strain-rate-dependent progressive damage criterion and modified cohesive zone model was established to further study the damage mechanism. Experimental and numerical results exhibit excellent alignment. The results demonstrate that the CAI strength is enhanced by 16.48 % according to the proposed discrete interleaving method. The main reason is attributed to the employed method can manipulate the delamination propagation during the low-velocity impact loading, and then maneuver the compressive failure mode of laminates to achieve a favorable benign failure (implying higher CAI strength). [ABSTRACT FROM AUTHOR]

RFID (Radio Frequency Identification) is commonly used to monitor goods along the supply chain. The feasibility of the application of a RFID tag to monitor CFRP components was demonstrated in a previous work by some of the authors. This work is aimed at evaluating how the integration of a RFID tag inside a CFRP laminate affects the quasi-static strength and the failure mode. Finite element modelling is used to design a specimen with an embedded tag, that may be representative of the tensile state of stress in a larger laminate. Tensile testing is performed both up to failure and by interrupting tests at different load levels in order to inspect the specimen by C-scan. Acoustic Emission (AE) and Digital Image Correlation (DIC) are used as additional non-destructive monitoring techniques. Some interrupted tests were selected to extract micrographic sections that illustrate damage development in the laminate. The extensive destructive and non-destructive characterization allowed to quantify the effect of embedding a RFID tag in the laminate in terms of strength decrease and failure mode. [Display omitted] [ABSTRACT FROM AUTHOR]

• A novel vibration-based damage detection method is proposed by using 2D continuous wavelet transform to provide a baseline-free damage detection with tunable resolution. • The detecting resolution of damage identification can be arbitrarily manipulated by actively selecting suitable wavelet parameters, and the relationship between key parameters and detectable damage size has been investigated quantitatively. • Numerical and experimental results exhibit that this method has high sensitivity to damage and can achieve a multi-resolution detection for identifying multi-size damage in composite laminates. Composite laminates are increasingly applied in advanced engineering structures, but they are prone to be damaged in service. However, multiple defects with different sizes challenge the current damage detection methods to identify all of them without a baseline. In this paper, a baseline-free damage identification method based on vibration is developed, and a novel damage index with tunable resolution of detection is proposed based on two-dimensional continuous wavelet transform by manipulating wavelet parameters. The relationship between the smallest detectable damage size and wavelet parameters is explored quantitatively, and the effectivity and reliability of proposed method are numerically and experimentally verified to directly detect damage in composite laminates without baselines. By a comparison with other vibration-based damage detection methods, results show that the proposed method can precisely locate and visualize multi-damage with determinate resolution of detection, and can realize multi-resolution of damage detection for identifying various sizes of defects. [ABSTRACT FROM AUTHOR]

• The study introduced physically meaningful parameters to quantify the viscoelastic hysteresis phenomenon and the evolution of cumulative cyclic strain in composites under fatigue loading. • The hysteresis loops of composites under stress-controlled fatigue loading have a constant hysteresis phase that is unaffected by fatigue damage or accumulated plastic strain. • The hysteresis loops and cyclic hysteresis energy models of composites were developed in order to describe deformation behavior and energy dissipation under tensile-tensile fatigue loads. • The models were validated against a series of tensile-tensile fatigue tests of four typical composite laminates, including CFRP [0] 8 , [90] 16 , [(0/±45/90) 2 ] S and GFRP [(0/90) 2 ] S. Fatigue constitutive curve is the presentation of mechanical response and damage mechanism of composites. The hysteresis phenomenon in composites is essentially attributed to the viscoelastic properties of the polymer matrix, causing the strain response to lag behind the stress excitation. Combined with the fatigue damage modes of composites, the contributions of elastic, plastic and viscoelastic strains to the cumulative cyclic strain are analyzed to determine the evolution law of the hysteresis loops with cycling. The hysteresis loops and cyclic hysteresis energy models of composites under tensile-tensile fatigue loads are proposed, and the models are verified by the experimental results of four kinds of laminates under several fatigue loads. [ABSTRACT FROM AUTHOR]

• A damage localization method based on wave front shape for composite laminates is proposed. • The a priori knowledge of the direction-dependent acoustic velocity profile is not needed. • The localization stability of the two wave front shape models is discussed. • The feasibility of the proposed method is verified by numerical simulations and experiments. Composite laminates are widely used in various fields, but their structures are prone to cracks and damage. Due to the difference in angles of the instantaneous direction of the wave front propagation and the direction of the energy flow in an anisotropic material, the use of Lamb waves for damage localization in composite laminates is a challenging task. Establishing the wave front shape equation can overcome the difficulty of damage localization caused by anisotropy, but this usually requires a priori knowledge of the acoustic velocity distribution of the laminates, which is not convenient for efficient damage localization. In this paper, a damage localization method based on wave front shapes for composite laminates without any knowledge of the velocity profile is presented. Numerical simulation and experimental results show that the proposed method works. This method shows good damage localization accuracy and has broad application prospects in non-destructive testing for plate structures with strong anisotropy. [ABSTRACT FROM AUTHOR]

• A new optimization scheme by NRBTO model is proposed for composite laminates with interval and convex uncertainties. • Convex-theoretical method is explored to determine response bounds of multi-layer anisotropic structures. • Reliebility-oriented design sensitivity is deduced to drive large-scale variable updating issue. With the rapid development of intelligent manufacturing and smart design, the optimal design conception for composite laminates is arising more attentions in both academic and engineering fields. Note that most of the current researches focus on the deterministic laminate design whereas the uncertainty-oriented topological design scheme for heterogeneous composites are rarely considered. In view of this, this study will present a new layout optimization process which combines the hybrid uncertainty quantification analysis and non-probabilistic reliability-based topology optimization (NRBTO). Owing to the sample limitation of multi-source uncertainties in material properties and external loads, the non-statistical convex-theoretical set theory is first explored to determine displacement response bounds of multi-layer structures under anisotropic constitutive relationship. As the safety judgment, a novel reliability index corresponding to the local stiffness features is defined via interval and convex mixed uncertainties, and its explicit expression is further deduced by the set-interference principle. To ensure the robustness and convergence of the large-scale iteration in NRBTO framework, the adjoint equations are also analyzed for the design sensitivity updating and the gradient algorithm is involved as well. Several typical optimization cases are presented to demonstrate the validity and effectiveness of the developed methodology. [ABSTRACT FROM AUTHOR]

The current challenges facing data-driven impact localisation methods primarily involve accurately localising impacts occurring outside the training impact coverage area, particularly for anisotropic composite structures. This study addresses these challenges by integrating the physical dispersion relations of impact-induced waves into the localisation process, thereby enhancing overall accuracy and reducing reliance on extensive training impact data. The dominant impact-induced flexural waves, derived from first-order shear deformation theory and classical laminate theory, exhibit significant dispersion. The phase velocity of these waves is governed by a dispersion equation dependent on structural stiffness, wave frequency, and direction. Solving this dispersion equation for a given wave frequency and known structural stiffness yields a wave velocity profile (WVP). Unlike conventional data-driven methods, which approximate the WVP with a black box and relate the time difference of arrival to the impact location, this study explicitly formulates and optimises the WVP with respect to structural stiffness and wave frequency based on the training impacts. Leveraging the partial derivatives of the dispersion equation, optimisation is performed using fast gradient-based techniques such as the Newton's method and the trust-region reflective algorithm. The optimised WVP is then employed for impact localisation using the triangulation method. Experimental validation of the proposed hybrid physics-based and data-driven impact localisation method involves conducting drop-mass impact testing on a composite laminate flat panel and a stringer-stiffened panel. The localisation results confirm the efficiency and accuracy of the proposed hybrid method with minimal training impact data, demonstrating its capability to accurately localise impacts even for stiffened panels, including those occurring outside the coverage area of the training impacts. [Display omitted] • A novel hybrid physics-based and data-driven impact localisation method. • Dispersion relations explicitly parameterise impact-induced wave velocity profile. • Optimising wave velocity profile solely involves fast gradient-based algorithms. • Accurately localise impacts occurring outside the training coverage area. • Impact localisation with minimal training impacts, even for stiffened panels. [ABSTRACT FROM AUTHOR]

• Investigate modal properties of edge waves in composite laminates. • Explore the sensitivity of edge waves in detecting edge delamination. • Provide examples of edge waves detecting delamination through experiments. Detecting near-edge damage in composite structural elements using guided wave-based techniques can be challenging, primarily due to the complexity of wave analysis arising from material anisotropy. Furthermore, scattered waves containing damage information can be contaminated by waves reflected from the edges, which makes detecting near-edge damage difficult to implement. In the literature, studies showed that in elastic materials, the edge of a structure can serve as a waveguide, enabling the existence of typical edge modes with concentrated energy at the edge. However, studies regarding edge waves in composite structures have received limited attention. This paper aims to explore the potential of detecting edge delamination damage in composite laminates using edge waves. The modal properties of edge waves in [(0/90) 2 ] s composite laminates are investigated using the Semi-Analytical Finite Element (SAFE) method. Additionally, dispersion curves for quasi-isotropic composite laminates are calculated. Following this, numerical and experimental studies were conducted to investigate the sensitivity of edge waves in detecting edge delamination in the [(0/90) 2 ] s composite laminates. The outcomes of this study offer physical insights into the modal properties of edge waves and confirm their effectiveness in detecting damage near the edges. [ABSTRACT FROM AUTHOR]

• A novel damage model for matrix crack damage under fatigue loading is proposed based on damage mechanics at macro level. • The damage model takes into account the 'edge effect' of matrix cracks in composite laminate under fatigue loading. • The model is applicable to laminated composites under fatigue loading with different stress levels. • Experimental validation demonstrated the effectiveness and predictive capability of the model. A damage model for transverse matrix cracks in composite laminates under fatigue loading was presented in this paper. The model was prosed based on the theory of continuum damage mechanics (CDM) at macro level and could capture the 'edge effect' of matrix cracks in composite laminate under fatigue loading. To obtain the constitutive equation of UD lamina containing transverse matrix cracks, a polynomial Gibbs free energy function was derived, and a macro damage parameter was introduced to characterize the influence of matrix cracks on multidirectional stiffness properties of UD composite. To predict the initiation and evolution of matrix cracks, a previously deduced initial crack initiation life function and a Paris-like equation in terms of damage driving force were employed, respectively. Prediction results of two cross-ply laminates under fatigue loading with different stress levels were compared to the experimental results and prediction results obtained from previously proposed model. The comparison results indicate the presented model is more effective and reliable in predicting the initiation and evolution of matrix cracks and the corresponding stiffness degradation in composite laminates under fatigue loading. [ABSTRACT FROM AUTHOR]

• The standard-sized 100 mm × 150 mm compression after impact (CAI) coupons are inadequate for high-energy impact scenarios. • Larger 254 mm × 304.8 mm specimens with the proposed support plates can be used to assess a wider range of impact scenarios. • The high-fidelity FE model can accurately predict the compression after impact (CAI) behavior of composite laminates. • Fiber breakage, pull-out, splitting, kinking, crushing, as well as matrix cracking and delamination are predicted. • The detailed damage occurrence sequences during CAI tests are predicted by analyzing the histories of dissipated energies. This study focuses on developing and validating a high-fidelity finite element model for predicting damage evolution and residual strength in fiber-reinforced composite panels. Impact and compression after impact (CAI) tests were conducted at both barely visible impact damage (BVID) and clearly visible impact damage (CVID) levels. The ASTM D7137 standard 100 mm × 150 mm CAI coupons were inadequate to cover the range of experimental studies required for model validation. Therefore, larger 254 mm × 304.8 mm laminates were investigated under two CAI testing conditions: one a scaled-up version of ASTM standard coupon, and the other with additional anti-buckling support plates to reduce unsupported areas to 127 mm × 177.8 mm. The model captured inter- and intra-laminar failure modes, including fiber breakage, splitting, kinking, pull-out, and crushing as well as matrix cracking, delamination, and their interactions. This was achieved by cohesive zone modeling technique and enhancement of the LaRC05 failure criteria through modeling the fiber damage evolution and utilizing an efficient search algorithm to determine the matrix fracture plane and fiber kink band angle. This study underscores the efficacy of the high-fidelity modeling approach in accurately predicting both impact damage and CAI strength in typical aircraft impact damage scenarios. Additionally, it provides insights into complex CAI failure mechanisms and energy release associated with various damage modes and highlights the effect of global buckling on the failure behavior and compressive strength of composite laminates. Furthermore, it shows that the proposed fixture with support plates is suitable for testing a broader range of impact scenarios without experiencing global buckling. [ABSTRACT FROM AUTHOR]

In this work, we present a mixed-mode phase field approach to model delamination progression and its migration. The proposed model includes the volumetric and deviatoric effects that occur during the mixed-mode delamination and migration. The volumetric–deviatoric split is combined with a power law criterion of delamination to capture the mixed-mode effects. An anisotropic tensor is used in the crack surface density function for capturing the anisotropic fracture. Two history parameters are introduced to ensure the irreversibility of the damage field throughout the evolution process of delamination and to consider the maximum value of the strain energy. A staggered approach is implemented for solving the equilibrium and evolution equations. The algorithm enables to obtain stable numerical solutions with faster convergence. Several examples of mode I and mode II as per standard tests have been incorporated to demonstrate the working of the proposed method. The numerical examples are validated by comparison with experimental results from the literature. It is observed that the proposed model can predict crack paths and angles closer to the experimental results. Examples of multiple delamination and delamination migration are also studied to understand the kinking in the delamination scenario. It is observed that the delamination migration is governed by the ratio of notch length a to the distance of the load from the edge L , and by the stress state in front of the crack tip. In the case of laminates with multiple interphases, the event of delamination impinging and kinking at interphase depends on the fracture toughness ratio of the bulk and the interphase. The example of the single-edge notched composite lamina and open-hole tension test with different fiber orientations has been validated with the literature and experiments. [ABSTRACT FROM AUTHOR]

• Dynamic damage of CF/BMI impacted by titanium alloy flyer at high speed are studied by experiment and numerical simulation, and the real model of CF/BMI were established. • CF/BMI were modeled by TexGen, and VUMAT subroutine interface in Fortran environment, the Hashin failure criterion were implanted into the numerical simulation. • Time history curve of strain, macroscopic damage process and microscopic damage morphology were obtained. In this paper based on the turbofan engine fan casing, the rectangular cross-section TC4 flyer high-speed impact carbon fiber/bismaleimide (CF/BMI) composite laminate was used to simulate the impact of failed blade fragments on the casing. Based on mechanical properties tests and equivalent mechanics theory, the mechanical properties parameters of laminates were obtained. The mesoscopic CF/BMI laminates model was built by TexGen. The Hashin failure criterion was implanted into the VUMAT subroutine. ABAQUS/Explicit was used to simulate the normal penetration and oblique penetration of ballistic impact experiments to obtain the strain time history curves, macroscopic damage process and mesoscopic damage morphology. The reliability of the numerical simulation method was verified by ballistic impact test. On this basis, the mechanical response and impact damage characteristics of flyer penetrating CF/BMI laminates under different impact velocities (260 m/s, 300 m/s), incident angles (45°, 60°, 90°) and flyer flight attitudes were studied. The damage characteristics of CF/BMI laminates under high-speed impact of flyer were obtained by combining the dynamic response analysis of two components of carbon fiber layer and bismaleimide resin in the laminates. [ABSTRACT FROM AUTHOR]

Polymer composite laminates have established themselves as essential materials across a wide type of industrial fields because of their specific mechanical properties such as high strength and low weight. Among the main issues they face is susceptibility to delamination damage. This comprehensive review paper investigates various damage mechanisms and associated phenomena that obvious during delamination within polymer composite laminates. Delamination can primarily arise in Mode I, Mode II and mixed Mode I &II loading scenarios. Notably, the damage features can vary significantly between these conditions. This paper aims to characterize and identify delamination-dominated damage features by conducting a comprehensive examination of the parameters that influence these features, all based on an extensive literature review and utilizing fractography analysis. The findings of this review illustrate the valuable insights that can be obtained from delamination fracture surfaces through the utilization of fractography images and the examination of damage features. For instance, it is possible to recognize details such as determining of global crack growth direction, calculating the rate of fatigue crack growth, and anticipating of strain energy released rate. This deeper understanding aids in pinpointing the key factors contributing to delamination damage. It could offer valuable insights for designing composites resistant to delamination. Additionally, it may assist in determining the underlying causes of catastrophic failures in tragic events. • Fractographic Analysis of Damage Mechanisms Dominated by Delamination in Composite Laminates. • Characterizing and identifying damage features associated with delamination. • Fractography analysis in composite laminates provides valuable insights for both the repair and design of composite structures. • The global crack growth direction has been determined using cusps pattern in mode II loading condition. • The fatigue crack grow rate (da/dN) can be calculated using striation damage features.. [ABSTRACT FROM AUTHOR]

Over the last decades, the intense need for more robust and lightweight structures, together with the dramatic improvement of computational power, had, as a result, the introduction of simulations in the traditional product development. As a simulation, it is considered any computer process that imitates a real system by generating similar responses over time. Simulations allow the designers to create virtual prototypes that can speed up the design phase and, thus, the product development time in total. This design paradigm shift is called simulation-based design (SBD) and includes several simulations and optimization techniques. The most notable of these techniques are; computer-aided design (CAD), finite element analysis (FEA), topology optimization (TO), and parametric optimization (PO). A combined SBD methodology, including these techniques, is presented here. This methodology is a two-stage optimization process. During the first stage, traditional compliance TO using the SIMP approach was conducted, while at the second, a PO with an evolutionary algorithm was applied. The presented methodology is focused on the optimization of composite laminates. In particular, an angle-ply laminated beam made by carbon fiber reinforced polymer (FRP) was used as a case study and optimized both for its topology and fibers' direction. The results of this research are presented and tested using a commercial example. The suggested methodology resulted in a lighter and more robust design solution. These design solutions can be constructed either by conventional manufacturing processes (CMP) or by additive manufacturing (AM). Designers looking for interesting and lightweight composited structures can exploit the results found in this paper. The implemented process can easily be modified in order to cover any possible optimization of FRP products. [ABSTRACT FROM AUTHOR]

This paper presents an asymptotic method-based analysis of composite laminates having interfacial imperfections. In general, imperfect interfaces are simply modeled by introducing a linear, spring-layer model, which empirically assumes that the displacement jumps that occur at the weakened interface are proportional to the transverse shear stresses of interface positions. In this study, we propose a composite plate model derived by using asymptotic expansion that does not make any assumptions, other than the scaling of coordinate systems. Within the framework of the asymptotic analysis, the spring-layer model is introduced to describe the effect of weakened interfaces, which is realized by the separation of domains in the through-the-thickness direction, and the integration of piecewise continuous warping functions. As a result, we newly define a spring parameter that is exactly the same as the stiffness of a spring. Therefore, a set of spring elements are added to the through-the-thickness modeling of the microscopic analysis, and the plate equations derived in the macroscopic problem are the same as those of the perfectly bonded laminates. As a consequence, we also derive the proposed plate model with the mathematical rigor that the previous asymptotic models contain. We provide some numerical results verifying that the proposed method shows good agreement with the elasticity and 3D FEM solutions. [ABSTRACT FROM AUTHOR]

[Display omitted] • The dynamic model of the flexible composite laminates structure with MFC is established. • The performance index of the controllability grammian matrix eigenvalue is used to establish an optimization model. • The influence of actuator layout on control performance and natural frequency of matrix structure is considered. • The correctness of the basic model is verified by experiments, and the optimization model is solved and analyzed. The configuration and optimization of actuators are essential to improve the control performance in active vibration control. In this paper, flexible composite laminates with multiple macro fiber composites (MFC) actuators are used as an example to study the position optimization of the MFC actuators on the structural surface. The dynamics model of the above structure is established using classical laminate theory and Hamilton's principle, which can be used as the basic model in the subsequent optimization analysis. For the establishment of the optimization model, firstly, the performance index of the controllability grammian matrix eigenvalue is adopted as the optimization criterion to establish a single objective optimization model; secondly, a multi-objective optimization model is established based on the reciprocal of the controllability grammian matrix eigenvalue and the weighted value of the natural frequency changes of each order with or without MFC actuator on the structure surface; finally, based on the idea of the penalty function, a geometric constraint is proposed to prevent position interference between each actuator. A linear quadratic regulator (LQR) is designed for active vibration control of flexible composite laminates. Experimental systems for modal testing and active control response testing are built. By comparing the solution results of the basic model with the experimental test results, it is found that the maximum error of the first four natural frequencies is 7.9%, and the vibration response error of the structure with or without active control is kept at about 15%, which fully proves the correctness of the basic model. Finally, the two optimization models are solved and analyzed to verify the rationality of the optimization model and the necessity of considering the influence of the actuator position on the natural frequency of flexible matrix structure. [ABSTRACT FROM AUTHOR]

• Neural network model and decision tree regression model can predict the impact resistance of laminates well. • A highly precise prediction model can be generated by training tiny sample sets. • The impact velocity directly determines the impact resistance of the laminates. • The combination of FE simulation and machine learning is a practical approach to performance prediction. Composite laminates subjected to dynamic impacts are usually investigated by experimental or numerical techniques. Numerical simulations, as an excellent complementary tool to experiments, are capable of reproducing microscopic results that cannot be observed in experiments, but require time-consuming calculations. Therefore, this work demonstrates the capability and efficiency of neural networks and decision tree models for the real-time prediction of projectile penetration of aramid/carbon hybrid fiber laminates impacted at variable angles for different initial velocities. To obtain accurate prediction models, a combination of experimental and finite element methods has been adopted and the experimentally validated finite element models have been used to provide data for training the prediction models. Consequently, the prediction model is able to accurately predict the residual velocity after projectile penetration of unknown hybrid laminates. The research demonstrates that using a large dataset generated by finite element analysis can help the prediction model to give more accurate predictions. The decision tree model outperforms the neural network model in known datasets, but the neural network model has better generalization capabilities of handling unknown feature inputs and giving accurate results. [ABSTRACT FROM AUTHOR]

Monitoring material degradation and in-service damage in fibrous composites at an early stage is challenging, yet critical for many industrial applications. Second harmonic Lamb waves, which result from the interaction of fundamental waves with material microstructural defects, have great potential for Structural Health Monitoring (SHM) applications. However, understanding of the second harmonic generation in composite laminates remains largely insufficient, which hinders practical SHM applications. This issue is investigated in this study to highlight the quasi-cumulative second harmonic B 2 mode Lamb wave in the low frequency range. Theoretical analyses are conducted to ascertain how anisotropy affects the quasi-cumulative effect, followed and validated by numerical simulations and analyses considering different wave propagation angles. The characteristics of the second harmonic B 2 mode Lamb waves are further explored in terms of the cumulative feature, robustness to wave beam divergence, and excitability. Experiments are finally carried out to monitor material degradation during the thermal aging of a carbon fiber reinforced panel. The results confirm the omnidirectional cumulative effect of quasi-phase-matched second harmonic B 2 mode Lamb waves. It is revealed that the 0° propagating Lamb waves exhibit superior cumulative effect, high robustness to wave beam divergence, and good excitability, rendering them an ideal tool for SHM applications. Furthermore, the high sensitivity of the second harmonic B 2 mode waves to material degradation is demonstrated, confirming their promise for material degradation monitoring applications in composite structures. • The influence of the anisotropy of composite materials on the second harmonic generation is theoretically analyzed. • The cumulative feature, robustness to wave beam divergence, and excitability of the second harmonic B 2 mode Lamb waves are numerically explored. • The second harmonic B 2 mode Lamb waves are experimentally applied to monitor material degradation during the thermal aging of a carbon fiber reinforced panel. [ABSTRACT FROM AUTHOR]

Based on identification of the two distinguishing delamination mechanisms within the two delamination zones associated with Mode I fracture toughness testing of composite laminates using the well-known ASTM standard double cantilever specimen (DCB), a new mechanism based cohesive zone model (MB-CZM) is proposed in this work. Overcoming the limitations with the widely used superposed cohesive zone models, the proposed MB-CZM develops two traction-separation relations to individually represent the two distinctive delamination mechanisms. One for the quasi-brittle linear elastic behavior of composite material and another for the nonlinear characteristics of fiber bridging which is commonly simplified with tri-linear to multi-linear approximation in the previous cohesive zone models (CZMs). Energy decomposition is carried out based on different damage and toughening mechanisms associated with delamination initiation and propagation. The proposed new MB-CZM is implemented in the finite element analysis via two UMAT subroutines and used in the numerical simulations. The good agreement of the simulation results with the experimental results provides the verification and demonstration of the capabilities of the proposed MB-CZM. [ABSTRACT FROM AUTHOR]

In this research, we developed a new 2D triangular lattice model with axial and angular springs to simulate fracture problems in composites. The inclusion of angular springs in the model formulation resolves the constraint related to the choice of the shear modulus, which is present in other lattice models that only consider axial springs. Moreover, it accounts for progressive damage in composites by effectively integrating a linear softening constitutive law. This model is based on equating the strain energy stored in a representative hexagonal cell to its corresponding continuum solid. Four numerical examples demonstrate the ability of this method to predict elastic constants and dynamic and quasi-static fracture problems in unidirectional composites. The results will be compared with the numerical and empirical results documented by the other researchers. • Novel 2D triangular lattice model for anisotropic fracture simulations. • No calibration needed; applicable to general geometries and boundary conditions. • Axial and angle springs based on stiffness matrix in the model's interactions. • Hashin criterion used for progressive damage evolution. • Accurate predictions for elastic modulus, crack paths, failure load. [ABSTRACT FROM AUTHOR]

Cohesive zone modelling is a common approach to capture delamination in composite laminate structures. Recent experimental advancements now enable the direct measurement of Mode I traction-separation responses (TSRs) from a single specimen using the composite rigid double cantilever beam (cRDCB), overcoming a major obstacle in using cohesive zone modelling to model delamination. However, TSRs measured experimentally with the cRDCB specimen capture damage response as well as the stiffness contribution of the adjacent laminae, which can introduce significant artificial compliance into numerical models when modelling delamination separately from intralaminar behaviour. A two-stage analysis procedure utilizing a crack tip compensation function is presented to enhance the TSRs measured with the cRDCB specimen to accurately model Mode I delamination. The analysis procedure is demonstrated to improve the accuracy of delamination prediction within the statistical variation of published experimental data. Furthermore, the transferability of TSRs measured with cRDCB specimens is explored using available experimental DCB data. It is shown that the onset of damage and early damage behaviours measured with the cRDCB specimen appear to be transferable between geometries, whilst large-scale damage mechanics remain geometry dependent. [ABSTRACT FROM AUTHOR]

In this paper, full-field damage forecasting of a laminated composite structure under different low velocity impact (LVI) conditions is realized through the proposed surrogate model, named VQ-SM. First, an efficient surrogate modelling method is proposed based on the advanced Vector Quantised-Variational AutoEncoder (VQ-VAE) proposed by DeepMind. Second, numerical simulation based on the progressive damage model of composite laminates is performed to obtain the training dataset. After training, the performance of VQ-SM is evaluated compared to the surrogate model without a representation learning process. The results show that VQ-SM has better performance with high-precise and good robustness, trained on the small dataset. Finally, the impact damage field of composite laminates is analyzed based on the surrogate model. The proposed surrogate modelling method provides not only the full-field damage forecast model for composite structures, but also an efficient method of improving the performance of the "generative" surrogate model. [ABSTRACT FROM AUTHOR]

An ordinary state-based peridynamic (PD) model for predicting out-of-plane mechanical behaviour of composite laminates has been proposed, in which three types of PD bonds are used to describe the reinforcement properties. The non-local strain energy density and peridynamic formulations are obtained based on the principle of virtual work by using Total Lagrange formulation, and the force density is reformulated by interpolation technique. Since the Mindlin plate is regarded as the research object of this PD model, the transverse shear deformation of laminates is considered. Also, the dilatation term is included in the force density vector, and flaws in Poisson's ratio of materials can be overcome. In the proposed PD, the critical strain energy density rather than the critical curvature is adopted as the failure criterion. The capability of the developed PD model was demonstrated by the bending examples of composite laminates with different fiber orientations, and damage analysis was further conducted to demonstrate the strong capability of proposed PD model in replicating the damage process of laminates. In addition, a single-layer material point model (SLMPM) can be implemented in the proposed PD algorithm, and the computational efficiency of numerical models will be greatly improved. [ABSTRACT FROM AUTHOR]

Fatigue delamination growth (FDG) is an important failure in composite structures during their long-term operations. Hygrothermal aging can have significant effects on interlaminar resistance. It is therefore really necessary to explore FDG behavior in composite laminates with hygrothermal aging. Dynamic mechanical thermal analysis (DMTA), mode I FDG experiments and fractographic examinations were conducted to fully investigate hygrothermal aging effects and the corresponding mechanisms on FDG behavior. The DMTA results indicated that environmental aging can induce obvious T g decrease. Mode I experimental fatigue data interpreted via different Paris-type correlations demonstrated that: Bridging has obvious retardation effects on FDG behavior via the Paris interpretations; The modified Paris relation can well characterize the intrinsic FDG behavior around the crack front; The use of the two-parameter Paris-type relation can appropriately account for R -ratio effects, contributing to a master resistance curve in determining mode I FDG behavior. According to these interpretations, it can be concluded that hygrothermal aging can have adverse effects on mode I FDG behavior. SEM examinations demonstrated that moisture absorption can cause fibre/matrix debonding and resin matrix pores/voids in the composite. However, no obvious difference in damage mechanisms was identified in mode I fatigue delamination for composite with/without environmental conditioning. Both fibre/matrix debonding and matrix brittle fracture were identified on fatigue fracture surfaces. Accordingly, it was concluded that fibre/matrix interface and matrix degradation induced by water absorption were the main reasons for a faster mode I fatigue crack growth in environmental aged composite. [ABSTRACT FROM AUTHOR]

When optimizing the fiber orientation of multilayer composite laminates, with the increasing number of layers, the number of design variables increases sharply, resulting in a large amount of computational cost. To address this challenge, this paper proposes a novel discrete fiber angle optimization method based on the Archimedean spiral function and applies it to a collaborative design framework of topology and fiber orientation. The proposed method uses the normal distribution function as the angle selection function in every layer. To prevent convergence issues in optimizing the fiber angle, a new candidate angle weighting formula is proposed. Further, a discrete fiber angle parameterization method based on the Archimedean spiral function for the thickness direction of laminates is presented, which requires only one variable to represent the fiber angle of any two layers in the element. To circumvent the issue of local optima and expand the design space, a collaborative optimization strategy is employed to improve the discrete fiber angle optimization results. Finally, the numerical examples indicate that in comparison to conventional approaches, the correlation of the proposed method with initial values is significantly reduced ( σ 2 = 0. 00188). Under identical initial conditions, this method can improve the structural stiffness by over 20% at maximum. • A novel parameterization method of discrete fiber angle in laminates based on the Archimedean spiral function is proposed. • A new candidate angle weighting formula suitable for discrete fiber angle optimization is proposed. • The collaborative optimization strategy is used to simultaneously optimize the topology and fiber orientation of variable stiffness composite laminates. [ABSTRACT FROM AUTHOR]

Abstract Over the past decades, a vast number of theories for numerical modeling of laminated composite plates and shells has been developed by various researchers and for diverse reasons. Three-dimensional elasticity theory, equivalent single-layer theories, zig-zag theories and layerwise theories are notable examples. In general, computing 3D elasticity solutions require huge computational time, the ESL theories cannot furnish satisfying results for thick laminates or laminates with distinct properties between layers, and the zig-zag theories cannot directly obtain the transverse stress fields from the constitutive model. The layerwise theory treats each layer individually and C z 0 continuity is satisfied from the beginning; therefore, it yields results comparable to 3D elasticity solutions. These attributes and advantages have driven the prosperity of layerwise theories for analysis of composite laminates and structures. The main aim of this review is to provide the recent development of layerwise theories, their numerical implementation, and application in the analysis of composite laminated structures. The main conclusions and possible future research trends are presented. We expect this review will provide a clear picture of layerwise theory for modeling of composite laminated structures and serve as a useful resource and guide to researchers who intend to extend their work into these research areas. [ABSTRACT FROM AUTHOR]

Abstract Fiber reinforced composite laminates have been increasingly replacing conventional materials in various manufacturing sectors due to their extremely superior mechanical properties. Usually, mechanical drilling is an important final manufacturing process for composite laminates, whereas drilling of high-strength composite laminates is very challenging and difficult. As the most undesirable damage and challenging failure mode, drilling-induced delamination for fiber reinforced composite laminates is a hot research area of immerse engineering importance. A review on the path towards delamination-free drilling for composite laminates can significantly help researchers improve currently-available cost-effective drilling process and develop high performance drilling process. This review paper summarizes an up-to-date progress in drilling-induced delamination for composite laminates reported in the literature. It covers delamination formation mechanism, delamination quantification methodologies and measurement technologies, delamination suppression strategies (including tool design optimization, drilling conditions optimization and high performance drilling methods). This general review of drilling-induced delamination for composite laminates can be referenced as not only a summary of the current results from literature survey but also future work possibilities, giving the researchers the opportunity to deepen specific aspects and explore new aspects for reaching delamination-free drilling for composite laminates. [ABSTRACT FROM AUTHOR]

Abstract Benzoxazine and phthalonitrile-based resins play an important role in fields of advanced materials because of their excellent properties. In this work, a kind of matrix resin with various ratios of phthalonitrile and benzoxazine were designed and prepared via co-reaction between aniline and 3-aminophenoxy phthalonitrile in the presence of biphenol and formaldehyde. Curing behaviors and dynamic rheology of matrix resins were investigated with DSC and DRA. Results indicated that obvious double curing stages can be assigned to ring-opening polymerization of oxazine rings and ring-forming polymerization of nitrile groups. Possible polymerization mechanisms were discussed in this work. Thermal stability of cured resins was evaluated by TGA testing and an integral decomposition temperature (IPDT). Then, fiber-reinforced composite laminates were prepared and their mechanical properties were investigated and confirmed with SEM images of fracture surfaces. Results demonstrated that this work proposed an improved matrix resin system with outstanding thermal stability and mechanical properties that broadened the foundation and ideas for subsequent researches. Graphical abstract Image 1 Highlights • A novel benzoxazine composites were prepared with copolymerization of oxazine and nitrile groups. • Improved thermal stability and mechanical properties were obtained due to the enchantment of nitrile groups. • The thermal stability of the fiber-reinforced laminates were discussed with IPDT. • The possible copolymerization processes between oxazine and nitrile groups were proposed. • The enhanced mechanical properties of the composites were backed up with the interfacial adhesion effects. [ABSTRACT FROM AUTHOR]

• Drilling-induced delamination was measured under different drilling parameters. • Quasi-static three point bending tests were performed to investigate damages. • Acoustic emission method was used to monitor specimens' loading and damage mechanisms. • Cohesive Zone Modeling and Continuum Damage Mechanics approaches were conducted. • The results of FE and AE methods showed good consistency with the experimental data. In this paper, a very promising procedure to evaluate damage initiation and propagation of high speed drilled composite laminates base on Acoustic Emission (AE) and Finite Element (FE) techniques is proposed. First, the extent of delamination after high speed drilling was measured in unidirectional and woven composite specimens under different feed rate and cutting speed parameters. Three point bending tests were then performed to investigate the effect of drilling-induced delamination on static strength and the damage mechanisms in the drilled specimens. FE simulation including Cohesive Zone Modeling (CZM) and an improved Continuum Damage Mechanics (CDM) approaches was conducted to predict damage initiation and propagation of each distinct damage. Besides, AE was also used to notice the damage emergence and investigate these damage mechanisms. The results showed that the effect of drilling parameters on static strength is insignificant, however the main accomplishment of this work is illustrating a good consistency between experimental procedure, exhaustive FEM and AE techniques. FEM and AE can be used as efficient ways to predict damage initiation and failure of composite structures and also to evaluate damage mechanisms in drilled composite materials with two kinds of lay-up. Consequently, it is feasible to understand the effect of lay-up on occurring different damage mechanisms. [ABSTRACT FROM AUTHOR]

• Simultaneous excitation of multiple transmitters is efficient for data acquisition. • Sparse reconstruction imaging for Lamb wave simultaneous excitation is proposed. • The proposed method is applied for high-resolution damage imaging in CFRP. Most Lamb wave methods utilize transducer array for damage detection and localization. Simultaneous excitation of multiple transmitters is a strategy for efficient data acquisition, in which matched filtering is generally applied for signal separation from different sources. However, the signal-to-noise ratio (SNR) and subsequent damage imaging performance is limited due to transmitter interference. To obtain both efficient data acquisition and high imaging performance, sparse reconstruction imaging for Lamb wave simultaneous excitation system is proposed in this paper that exploits the sparsity of structural damage. The key of success is to construct accurate damage-reflection Lamb wave model based on the linear characteristic of simultaneous excitation system. In addition, the model performance is further improved by excitation waveform design for each transmitter. The effectiveness of the proposed method is experimentally demonstrated with array imaging of simulated delamination in composite laminates. [ABSTRACT FROM AUTHOR]

Abstract The present article includes the modal analysis of laminated composite structures under thermal effect. Experiments are conducted to study the influence of temperature change on natural frequencies of glass fibre reinforced polymer (GFRP) composites. GFRP laminated plates are prepared in the laboratory using vacuum bagging by resin infusion. The specimens are thermally conditioned to obtain temperature variation ranging from −10 °C to 120 °C. A Green–Lagrange nonlinear finite element (FE) model based on third order shear deformation theory (TSDT) has been developed for the analysis. The FE solutions in terms of natural frequencies are compared with experimental data and an extensive parametric study is carried out. The free vibration modal analysis of a hollow stiffened laminated panel is also carried out to identify the effect of temperature change with proper temperature distribution within the stiffened structures using 1D heat conduction model and the solutions may be used as benchmark. [ABSTRACT FROM AUTHOR]

• Besides dispersion, comprehensive amplitude modulation can also change the appearance of Lamb wave signals. • A dictionary reconstruction strategy is established with considerations of phase and amplitude information. • The performance of sparse imaging method relies on the denoising parameters and numbers of sources. Composite structure is increasingly used in civil and aerospace applications due to its high mechanical performance. Lamb wave based sparse reconstruction imaging for damage localization is promising for structural health monitoring (SHM) and nondestructive evaluation (NDE) by using few measurements. However, this dictionary based method requires accurate atoms to represent Lamb wave propagating features in structure very well. Besides dispersion, signal changes caused by amplitude modulation should be considered for waveform distortion when constructing the dictionary for sparse imaging method. In this paper, a non-contact laser is used for Lamb wave excitation which exhibits a strong amplitude modulation in low frequency. Additionally, the strong attenuation resulting from material damping would also presents a distance-dependent amplitude modulation. To reconstruct an amplitude model of Lamb wave, the decomposition method of system response and attenuation is proposed. Then, the influence of amplitude modulation on signal representation is analyzed, which shows the restriction of dictionary without considering amplitude modulation. On this basis, the amplitude considered dictionary is built together with the phase considered dictionary for sparse imaging in terms of damage detection. Furthermore, according to Lamb wave reflection model, the solution for sparse reconstruction imaging is given. Finally, the performance of sparse imaging method is discussed by experimental investigation with different parameters. The results show the efficiency of the proposed method with improved imaging performance and give comparisons for better parameter choice. [ABSTRACT FROM AUTHOR]

Abstract The triangulation method has been an effective tool for estimating the source locations of various acoustic emissions (AE). However, it is hard to guarantee high accuracy in composite structures due to their anisotropy. Also, the conventional methods basically require the precise arrival time data of AE signals. Because most of commercial fiber optic sensing systems applicable to real structures have limited measurement performance, the arrival times cannot be clearly identified. In this paper, the magnitudes of fiber optic sensor signals were used for estimating the distances between each sensor and impact location. In order to obtain higher correlation between the magnitude and distance, the signal near the roughly estimated arrival time was used for calculating the magnitude. Then, through the neural network training, the accuracy of estimating the distances from the signal magnitudes could be enhanced. Finally, the triangulation method was applied for localizing the impact sources. As a result, our suggested triangulation method showed the acceptable localization results about the non-trained impact points. Because the input data for this method could be reliably obtained from the commercial fiber optic sensing system, it can be useful for constructing a simple impact monitoring system for the real composite structures. [ABSTRACT FROM AUTHOR]