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This study focuses on coupled vibrations of rotating thin-walled composite beams subjected to hygrothermal effects. In the existing literature, many studies have been conducted on coupled bending-torsional vibration and resonance in hygrothermal environments. Few studies considered the coupled flapwise-edgewise and resonances of composite thin-walled beams. Considering this, the flapwise-edgewise coupling effects and resonant characteristics of rotating thin-walled composite beams in a hygrothermal environment are studied. The Rayleigh–Ritz method is used to solve the equations of the beam. Results indicate that flapwise-edgewise coupling factors are essential for the vibration analysis of rectangular thin-walled beams. The ply angle and setting angle strongly affect the internal and external resonances. Large ply angles can significantly reduce the chances of primary internal and external resonances occurring when the permitted rotational speed is lower. [ABSTRACT FROM AUTHOR]

The blades of helicopters and wind turbines undergo pitch motion in hygrothermal environments. Few works have studied the effect of this movement on blade frequencies. This paper focuses on the bending characteristics of a pitching thin-walled beam, including the effects of hygrothermal environments, rotor speeds, and composite materials. The harmonic pitch function is introduced into the dynamics model of a rotating beam. The method of multiple scales and the Galerkin method are used to solve these equations. Results indicate that the pitch amplitude A dramatically influences natural frequencies. Conclusions are obtained: (1) The flapping frequency varies periodically with pitch amplitude A, and monotonically changes every 2/π. The pitch amplitude A significantly impacts frequencies, while the pitch frequency ωp and pitch phase C don’t influence frequencies. (2) An increase in temperature or humidity will decrease the flapping frequencies of a pitching beam. (3) The ply angle, coupled with the dynamic pitching angle, dramatically influences the flapping frequencies of the composite beam. The larger the ply angle, the greater the effect of pitch amplitude A on frequencies. [ABSTRACT FROM AUTHOR]

A novelty FG honeycomb-reinforced laminate (FGHRL) with viscoelastic material is constructed for an industrial exoskeleton. The study examines how FGHRL dissipates energy under hygrothermal conditions. Low-velocity impacts on the exoskeleton's load-bearing structure are simulated using a spherical object. Parameters for the FGHRL, made of a two-phase material, are estimated using the Halpin-Tsai model and macroscopically uniform theory. In-plane deformations are determined using Reddy's HSDT, while the impact response is evaluated with Newmark-β and Wilson-θ methods. Energy dissipation properties are calculated using the energy balance model, validated through CUF-based FEM and LS-DYNA explicit solution, and factors influencing energy dissipation are investigated. [ABSTRACT FROM AUTHOR]

We develop a computational framework to model damage and delamination in laminated polymer composite structures incorporating the effects of temperature and moisture content. The framework is founded on a recently developed comprehensive multi-layer thin-shell formulation based on Isogeometric Analysis, which includes continuum damage, plasticity and cohesive-interface models. To incorporate hygrothermal effects in the modeling, we propose a scaling law that is based on the Arrhenius equation and material glass transition temperature that establishes the dependence of the intra- and interlaminar material properties on the temperature and moisture content. We compute several classical test cases using a combination of environmental conditions and demonstrate that the resulting modeling approach shows a good agreement with the experimental data, both in terms of failure loads reached as well as failure modes predicted. [ABSTRACT FROM AUTHOR]

In recent decades, the aviation industry has increasingly adopted composite materials for various aircraft components, due to their high strength-to-weight ratio and durability. To ensure the safety and reliability of these structures, Health and Usage Monitoring Systems (HUMSs) based on fiber optics (FO), particularly Fiber Bragg Grating (FBG) sensors, have been developed. However, both composite materials and optical fibers are susceptible to environmental factors such as moisture, in addition to the well-known effects of mechanical stress and thermal loads. Moisture absorption can lead to the degradation of mechanical properties, posing a risk to the structural integrity of aircraft components. This research aims to quantify and monitor the impact of moisture on composite materials. A new formulation of the Bragg equation is introduced, incorporating mechanical strain, thermal expansion, and hygroscopic swelling to accurately measure Bragg wavelength variations. Experimental validation was performed using both uncoated and polyimide-coated optical fibers subjected to controlled hygrothermal conditions in a climate chamber. The results demonstrate that uncoated fibers are insensitive to humidity, whereas coated fibers exhibit measurable wavelength shifts due to moisture absorption. The proposed model effectively predicts these shifts, with errors consistently below 2.6%. This approach is crucial for improving the performance and reliability of HUMSs in monitoring composite structures, ensuring long-term safety in extreme environmental conditions. [ABSTRACT FROM AUTHOR]

AbstractThis study investigates the influence of moisture and Thermo-hygroscopic influence on bio-inspired helicoidally laminated composite plates (BHLC) for strengthening damaged reinforced concrete (RC) beams. Borrowing inspiration from nature’s design principles, BHLC plates represent a novel approach to rehabilitation. An analytical approach based on deformation compatibility is employed, assuming constant shear and normal stresses across the adhesive layer within the framework of linear elasticity. BHLC plates are implemented to restrict crack propagation within the RC beam. The analytical methodology considers equilibrium and deformation compatibility across all components: the concrete beam, BHLC plate, and adhesive layer. A parametric study explores the sensitivity of interfacial stresses to parameters such as laminate and adhesive stiffness, plate thickness, and helicoidal layup configuration. The results reveal a significant influence of these factors on the magnitude of maximum interfacial shear and normal stresses. This study establishes a foundation for future analyses of damaged RC beams strengthened with BHLC plates, potentially leading to advancements in rehabilitation methodologies and improved crack propagation modeling in RC beams. [ABSTRACT FROM AUTHOR]

For naval applications, composite sandwich structures are of significant interest, and are often manufactured using thin (2 to 4 mm) composite facings made from carbon or glass fiber reinforcement, attached to a thick (25 to 50 mm) section of PVC cellular foam or balsa wood-based core materials using a suitable polymeric resin. In the present study, we focus on the hygrothermal effect on the fiber-dominated compression properties of carbon fiber reinforced vinyl ester resin based polymeric composite (CF/VE), used as "skin" for a polymeric composite sandwich material. Hygrothermal conditioning is achieved by saturating samples in simulated seawater at 40°C. Compression properties are evaluated for coupons extracted along warp and fill undergoing- no conditioning, conditioning till saturation (up to 6 months), and long-term conditioning (2 years). Sea-water saturation yields in up to 12% drop in compression strength with a further 3–4% drop resulting from long-term conditioning. No statistically significant modulus degradation is noticed due to short or long-term hygrothermal exposure. The failure mechanism of the warp extracted coupon, which fails in a splitting failure mode originating due to the delamination between the 0/90 interface, or the fill extracted coupon, which fails due to the instability caused by tow micro-buckling, remains unchanged due to combined exposure (short or long-term) of seawater and temperature. The loss in strength is attributed to the degradation of the fiber-matrix interface, which is validated via conducting single fiber push-in tests with a nominal diameter of 7 micron for conditioned and unconditioned coupons. [ABSTRACT FROM AUTHOR]

This analysis investigated the impact wave response and propagation on a composite sandwich shell when subjected to a low-velocity external shock, considering hygrothermal effects. The sandwich shell was crafted using face layers composed of functional gradient metal–ceramic matrix material and a core layer reinforced with negative Poisson's honeycomb. The honeycomb layer consisted of a combination of viscoelastic polymer material and elastic material. The equivalent parameters for the functional gradient material in the face layers were determined using the Mori–Tanaka and Voigt models, and the parameters for the negative Poisson's ratio honeycomb reinforcement core layer were obtained through Gibson's unit cell model. Parameters relevant to a low-velocity impact were derived using a modified Hertz contact law. The internal deformations, strains, and stress of the composite sandwich shell were described based on the higher-order shear deformation theory. The dynamic equilibrium equations were established using Hamilton's principle, and the Galerkin method along with the Newmark direct integration scheme was employed to calculate the shell's response to impact. The validity of the analysis was confirmed through a comparison with published literature. This investigation showed that a multilayer negative Poisson's ratio viscoelastic polymer material honeycomb-cored structure can dissipate impact wave energy swiftly and suppress shock effectively. [ABSTRACT FROM AUTHOR]

The effects of hot-water aging on the quasi-static and dynamic compression properties of unidirectional E-glass/epoxy laminates were investigated. E-glass/epoxy specimens were aged in water at 60°C for 4900 h and then aged and unaged specimens were tested in compression at a rate of 1.3·10–3 s–1 and by a split Hopkinson pressure bar apparatus at varying strain rates. Their diffusion behavior was successfully described by the two-stage model whose parameters were found by the nonlinear regression method. The strain-rate-sensitivity of aged and unaged E-glass-reinforced epoxy specimens in the longitudinal direction was studied. Their dynamic and static compression properties were compared for specimens with the same dimensions. Empirical models were proposed to predict dynamic properties as functions of strain rate. SEM micrographs showed a low degradation of the resin matrix and fiber-matrix interface at hot-water aging for a time up to 4900 h. [ABSTRACT FROM AUTHOR]

The present work addresses the critical buckling of circular cylindrical nano-shells containing static/dynamic nanofluids under the influence of different thermal fields that can also lead to appear the effect of thermal moisture so-called hygrothermal forces fields. To this end, the classical Sanders theory of cylindrical plates and shells is generalized by utilizing the non-classical nonlocal elasticity theory to derive the modified dynamic equations governing the nanofluid-nanostructure interaction (nano-FSI) problem. Then, the dimensionless obtained equations are analytically solved using the energy method. Herein, the applied nonlinear heat and humidity fields are considered as three types of longitudinal, circumferential, and simultaneously longitudinal-circumferential forces fields. The mentioned cases are examined separately for both high- and room-temperatures modes. The results show a significant effect of nanofluid passing through the nanostructure and its velocity on the critical buckling strain of the nano-systems, especially at high temperatures.

• Dynamic instability problem is investigated in hygrothermal effect. • Delamination is considered in piezoelectric effect. • Total energy equation is obtained. To use effectively the shell in high-performance applications, analyzing the effect of hygrothermal effects on a piezoelectric delamination event is necessary. A laminated piezoelectric cylindrical shell with delamination is first established under hygrothermal effects, followed by an internal force analysis and an energy analysis of the shell. Then, a parametric vibration equation of the Mathieu for the shell is obtained by using the energy variation of the Rayleigh-Ritz method; later on, the harmonic balance method is employed to solve this equation. Finally, the effects of delamination parameters, geometric parameters, hygrothermal effects, and piezoelectric materials on the shell under uniformly distributed axial load are discussed. [ABSTRACT FROM AUTHOR]

Recurrent neural network (RNN)-based accelerated prediction was achieved for the long-term time-dependent behavior of viscoelastic composite laminated Mindlin plates subjected to arbitrary mechanical and hygrothermal loading. Time-integrated constitutive stress-strain relation was simplified via Laplace transform to a linear system to reduce the computational storage. A fast converging smooth finite element method named cell-based smoothed discrete shear gap was employed to enhance the data generation procedure for straining RNNs with a sparse mesh. This technique is applicable under varying hygrothermal conditions for real engineering structure problems with fluctuating temperature and moisture. Hence, accurate RNN-based long-term deformation prediction for laminated structures was realized using the history of environmental temperature and moisture condition. [ABSTRACT FROM AUTHOR]

In the present study, the viscoelastic analysis is investigated for composite laminated plates using a smoothed finite element method called cell/element based smoothed discrete shear gap method. Moreover, the hygrothermal effects is considered on the viscoelastic responses of composite laminated plates. The first-order shear deformation theory is employed due to its simplicity and accuracy. With the help of the convolution theorem in Laplace transformation, the complex stress-strain relationship in integral form is simplified to linear in transformed domain. Therefore, all computing procedures are performed in the transformed domain and then, using inverse techniques (Fast Fourier Transform) to converted back to the real-time domain. The study provides an effective computational tool to analyze the viscoelastic response of laminated composite taking into account the influence of the time and hygrothermal effects. [ABSTRACT FROM AUTHOR]

This parametric study investigates the RC Beams Rehabilitated with angle-ply composite laminate plate [θn/90m]s. This work is based on a simple theoretical model to estimate the interfacial stresses developed between the concrete beam and the composite with taking into account the hygrothermal effect. Fibre orientation angle, effects of number of 90° layers and effects of plate thickness and length on the distributions of interfacial stress in the concrete beams reinforced with composite plates have also been studied. [ABSTRACT FROM AUTHOR]

Abstract In the present study, the dynamic response of functionally gradient (FG) honeycomb reinforced and viscoelastic material (VEM) embedded composite laminate plates with low-speed impact in the hygrothermal environment is investigated. The equivalent effective mechanical properties of the two-phase macroscopic constructed layers are determined according to Hill's generalized self-consistent model. The Reddy's high-order shear deformation theory (HSDT) is employed to evaluate the in-plane displacement variables. By combining the extended Hamilton's principle with the finite element method and adopting the Rayleigh's proportional damping, the global mass, damping and stiffness matrices are determined first. Then, the central displacement of the FG honeycomb reinforced laminate plate is obtained with the aid of the Newmark's method. Next, several key factors affecting the central displacement in the whole impact duration are analyzed and compared, such as the hygrothermal effects, the radius of the impactor and the functionally gradient factor. Moreover, the finite element method (FEM) is applied to obtain the mode parameters and to verify the convergence of our analysis. Finally, the LS-DYNA explicit solution is employed to validate the present results. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to study the hygrothermal effect on fatigue behavior of quasi-isotropic carbon/epoxy plain weave aerospace laminates containing artificial flaw under axial tension–tension loading. Dry and wet specimens were tested at tensile load-controlled cyclic loading with a stress ratio R = 0.1 and a load frequency of 7 Hz at room temperature (RT) and at 82℃ under different stress levels. Allowable stiffness change as a failure criterion was used to determine the delamination propagation onset threshold under cyclic tensile loading at each environmental condition. The delamination propagation onset was verified using the ultrasonic imaging (C-Scan) technique. The experimental results show that (1) fatigue life of CFRP specimens was more individually affected by moisture than by temperature and (2) combined moisture and temperature cause a drastic decrease in fatigue life. Finally, an investigation of the effect of hygrothermal conditions on stiffness degradation and damage of composite laminates subjected to tensile fatigue loading has been also carried. On the basis of the residual stiffness degradation, a damage variable was presented and phenomenological damage models were proposed by employing fatigue modulus and secant modulus concepts as measure of material damage. [ABSTRACT FROM AUTHOR]

In this study, single fiber (∼7-μm diameter) push-out tests are conducted to evaluate hygrothermal effects on the interfacial shear strength (IFSS) of carbon fiber/vinyl ester (CF/VE) composites. Hygrothermal conditioning is achieved by saturating samples in simulated seawater at 40 °C for two years. An investigation has been conducted on the preparation, validity, and interpretation of the push-out test results. First, the authors present a polishing methodology that results in thin films of CF/VE composites in the thickness range of 15–120 μm and produces an average 41.2% drop in IFSS due to long-term hygrothermal exposure. Using scanning electron microscopy (SEM), we show that during the push-out tests, the failure initiates locally at the zone of minimum bond strength at the bottom (away from the indenter), then propagates along the length of the interface. The influence of radial tensile stresses originating due to bending is found to be negligible. Using the SEM imaging of the pushed-out fibers, we validate the failure of the interface to be the primary source of failure. The associated results are found to depend on the thickness of the interface. We then reevaluate the results using the Weibull distribution, knowing that the failure mechanism is analogous to the weakest link theory. The results show a 25.5% drop in the IFSS of the CFVE composite, measured at an infinitesimal scale due to long-term to hygrothermal conditioning at 40 °C. A significant drop in IFSS was observed after reheating above glass transition temperature (T g) and cooling. [Display omitted] [ABSTRACT FROM AUTHOR]

Hygrothermal effects on free vibration of woven fiber glass/epoxy laminated composite cylindrical shallow shells are investigated both numerically and experimentally. In the present finite element analysis a composite doubly curved shell model based on first order shear deformation theory (FSDT) is used for free vibration of cylindrical shell panels in hygrothermal environment. B&K FFT analyzer is used to determine the natural frequencies of vibration experimentally by conducting non-destructive testing. The effects of curvature ratios, lamination sequences and boundary conditions on natural frequencies of vibration under hygrothermal loading are investigated. The frequency of vibration decreases with increase of temperature and moisture. [ABSTRACT FROM AUTHOR]

Analytical and numerical results on hygrothermal behaviors of angle-ply composite plates resting on elastic foundation are scarce in literature. To fill this gap, a higher-order global-local model (HGLMA) is proposed for hygrothermal analysis of angle-ply composite plates resting on elastic foundation. Based on the model HGLMA, a triangular plate element is formulated by using the refined element approach, in which a new finite element formulation including effects of elastic foundation has been developed. By analyzing hygrothermal response of composite plates with general configurations, the effects of elastic foundation parameters, lamination angles, boundary conditions, transverse normal strain, and hygrothermal expansion coefficients on the hygrothermal response have all been studied. Numerical results showed that elastic foundation has a significant impact on the behaviors of composite plates and models neglecting transverse normal strain fail to simulate the effects of the elastic foundation parameters on displacements and stresses of symmetric composite plates. [ABSTRACT FROM AUTHOR]

The interlaminar stresses are confined to a region near the free edge. Therefore, the laminate stacking sequence and the free edge finishing are some of the factors that affect the strength of the laminate and limit its life. The use of molded edges eliminates the need for trimming and machining the laminates edges thus improving productivity. However, this fabrication technique may have a detrimental effect on the laminate strength for certain stacking sequences. This effect in the presence of moisture has not been characterized. This work presents the results of a comparative study of the resistance to delamination of laminates with machined edges and molded edges. Additionally, two environmental conditions were considered: dry laminates and laminates saturated with moisture. The tensile strength of the laminates were measured and micrographs were used to analyze the microstructure of the laminates near the free edges. It is concluded that the mechanical properties of advanced composites depend on the environmental conditions and the fabrication techniques used to produce the laminates. Therefore, it is necessary to account for these factors when experimentally determining the design allowables.

In this paper, we study the buckling characteristics of curvilinear fibre composite laminates exposed to hygrothermal environment. The formulation is based on the transverse shear deformation theory and it accounts for the lamina material properties at elevated moisture concentrations and thermal gradients. A 4-noded enriched shear flexible quadrilateral plate element is employed for the spatial discretization. The effect of a centrally located cut-out, modelled within the framework of the extended finite element method, is also studied. A detailed parametric investigation by varying the curvilinear fibre angles at the centre and at the edge of the laminate, the plate geometry, the geometry of the cut-out, the moisture concentration, the thermal gradient and the boundary conditions on the buckling characteristics is numerically studied. [ABSTRACT FROM AUTHOR]

This paper presents a layup optimisation algorithm for composite marine propellers, including the hygrothermal effects, using Non-Uniform Rational B-Splines (NURBS) based FEM coupled with real-coded Genetic Algorithm (GA). The use of Iso-Geometric Analysis (IGA) enables accurate representation of complex marine propeller blades, coupled with GA for both continuous and mixed integer ply angle optimisation. The optimisation scheme was further investigated multi-objective, multi-material and multiple layer thickness optimisation scenarios. Furthermore, the IGA-FEM solver was constructed based on constitutive equations that take into account the hygrothermal effects that must be addressed to enable the optimisation of non-symmetric layups. Such non-symmetric layup patterns are proposed to be used to create extension-twist coupling to gain further control in layup optimisation. The optimisation technique is presented here as a method to widen the efficiency envelope of marine propellers. However, the same approach can potentially be adopted to cater many other practical applications such as composite wind turbine blades, aircraft propellers and other general composite ply angle optimisation scenarios. The paper discusses the proposed framework for optimisation of marine propeller, numerical tools used in the process and results under different conditions. [ABSTRACT FROM AUTHOR]

The durability of the composites has been assessed in terms of the shear-lag parameter, n through the use of Raman spectroscopy. The decrease of the shear-lag parameter indicates damage due to hygrothermal ageing. The stability of the specimens subjected to the wet and dry conditions has been compared by the determined n. The n values for the wet composites have been found to decrease faster than those of the dry ones. The composites exposed to water vapour and liquid water environments were also investigated in terms of the moisture content, the mid-fibre strain and the debond length. There was no interfacial debonding for the specimen exposed to water vapour whilst the ones aged in the liquid water experienced such failure.

Abstract: The effect of moisture concentration and the thermal gradient on the free flexural vibration and buckling of laminated composite plates are investigated. The effect of a centrally located cutout on the global response is also studied. The analysis is carried out within the framework of the extended finite element method. A Heaviside function is used to capture the jump in the displacement and an enriched shear flexible 4-noded quadrilateral element is used for the spatial discretization. The formulation takes into account the transverse shear deformation and accounts for the lamina material properties at elevated moisture concentrations and temperature. The influence of the plate geometry, the geometry of the cutout, the moisture concentration, the thermal gradient and the boundary conditions on the free flexural vibration is numerically studied. [Copyright &y& Elsevier]

This paper deals with buckling of analysis multilaminated cylindrical shell panels subjected to axial and hygrothermal loadings. The geometrical non-linear analysis is carried out using the Finite Element Method based on a single layer first shear deformation theory. A nine-nodal isoparametric element with 5 degrees of freedom per node is considered. The effects of different number of layers, lamination angles, length to width ratios and hygrothermal effects are studied.

Abstract: The present work develops an optimization procedure for a geometric design of a composite material stiffened panel with conventional stacking sequence using static analysis and hygrothermal effects. The procedure is based on a global approach strategy, composed by two steps: first, the response of the panel is obtained by a neural network system using the results of finite element analyses and, in a second step, a multi-objective optimization problem is solved using a genetic algorithm. The neural network implemented in the first step uses a sub-problem approach which allows to consider different temperature ranges. The compression load and relative humidity of the air are assumed to be constants throughout the considered temperature range. The mass, the hygrothermal expansion and the stresses between the skin and the stiffeners are defined as the optimality criteria. The presented optimization procedure is shown to yield the optimal structure design without compromising the computational efficiency. [Copyright &y& Elsevier]

In the present work, the wave propagation behavior of sandwich plates with graphite particles filled viscoelastic material core under the hygrothermal environments is investigated. A four-variable integral higher-order shear deformation theory (HSDT) and the Hamilton's principle are applied to derive the equations of motion and the corresponding wave dispersion relations. The accuracy and effectiveness of the present model are verified by comparison with previous research. The effective properties of the particulate composite are accurately determined by a differential scheme with consideration of the particle interactions. Results imply that increased effective moduli are obtained through incorporating the graphite particles, or decreasing the temperature or increasing the moisture. Influences of the volume fraction of graphite particle, the core-to-face layer thickness ratio, temperature and moisture on the wave dispersion responses are quantitatively analyzed and qualitatively assessed. It has been found that the hygrothermal environments play a significant role in the wave propagation characteristics. [ABSTRACT FROM AUTHOR]

Abstract: The hygrothermal effects on mechanical, thermal, and electrical properties of highly conductive graphite-based epoxy composites were investigated. The highly conductive graphite-based epoxy composites were found to be suitable for applications as bipolar plates in proton exchange membrane (PEM) fuel cells. The hygrothermal aging experiments were designed to simulate the service conditions in PEM fuel cells. Specifically, the composite specimens were immersed in boiling water, aqueous sulphuric acid solution, and aqueous solution of hydrogen peroxide. The water uptake, changes in surface appearance and dimensions, glass transition behavior and thermal stability, and electrical and mechanical properties were evaluated. The water uptake at short time increased linearly with the square root of time as in linear Fickian diffusion. The presence of graphite significantly reduced both the rate and extent of water uptake. No discernible changes in specimen dimensions, surface appearance, and morphology of the composites were observed. The electrical conductivity and mechanical properties remained almost unchanged. The wet specimens showed slight reduction of glass transition temperature (T g) due to plasticization of epoxy networks by absorbed water, while the re-dried specimens showed small increase of T g. The composites maintained high electrical conductivity of about 300–500Scm−1 and good mechanical properties and showed thermal stability up to 350°C. [Copyright &y& Elsevier]

The wide range of composite material applications results in an almost inevitable contact with liquids and vapors, either organic or aqueous, which can affect both the immediate and the long-term performance of the material. In this study, the influence of the hygrothermal effects on the compression strength of carbon unidirectional tape/epoxy 8552 composites ([0/0]s) has been investigated. The moisture absorption was monitored placing the samples for a long time (until saturation) in a humidity-controlled chamber (80°C and 95% relative humidity (RH)) and other samples in a salt spray chamber (36°C and 95% RH). The compression tests were carried out in two temperatures: the room temperature (22°C) and the high temperature (82°C). It was observed that the samples tested at room temperature and submitted to the salt spray chamber presented a decrease of 8% on the compression strength when compared with the non-conditioned specimens. On the other hand, the samples tested at high temperature and submitted to the humidity-controlled chamber presented a decrease of 25.7% on the compression strength, when compared with non-conditioned samples. However, the specimens submitted to the salt spray chamber presented a decrease of 18.5% on the compression strength when compared with non-conditioned specimens at high temperature. These results showed that the combined effect of the humidity with the high temperature test, decrease the compression strength of the composite materials. [ABSTRACT FROM AUTHOR]

Abstract: Carbon-filled epoxy composites are developed for potential application as bipolar plates in proton exchange membrane (PEM) fuel cells. These composites are prepared by solution intercalation mixing, followed by compression molding and curing. Electrical conductivity, thermal and mechanical properties, and hygrothermal characteristics are determined as function of carbon-filler content. Expanded graphite and carbon black are used as synergistic combination to obtain desired in-plane and through-plane conductivities. These composites show high glass transition temperatures (T g ∼180°C), high thermal degradation temperatures (T 2 ∼415°C), in-plane conductivity of 200–500Scm−1 with 50wt% carbon fillers, in addition to offering high values of flexural modulus, flexural strength, and impact strength, respectively 2×104 MPa, 72MPa, and 173Jm−1. The presence of carbon fillers helps reduce water uptake from 4 to 5wt% for unfilled epoxy resins to 1–2wt%. In addition, morphology, electrical, mechanical, and thermal properties remain unchanged on exposure to boiling water and acid reflux. This data indicate that the composites developed in this work meet many attributes of bipolar plates for use in PEM fuel cells. [Copyright &y& Elsevier]

Abstract: Fiber metal laminates are the frontline materials for aeronautical and space structures. These composites consists of layers of 2024-T3-aluminum alloy and composite prepreg layers. When the composite layer is a carbon fiber prepreg, the fiber metal laminate, named Carall, offers significant improvements over current available materials for aircraft structures. While weight reduction and improved damage tolerance characteristics were the prime drivers to develop this new family of materials, it turns out that they have additional benefits, which become more and more important for today''s designers, such as cost reduction and improved safety. The degradation of composites is due to environmental effects mainly on the chemical and/or physical properties of the polymer matrix leading to loss of adhesion of fiber/resin interface. Also, the reduction of fiber strength and stiffness are expected due to environmental degradation. Changes in interface/interphase properties leads to more pronounced changes in shear properties than any other mechanical properties. In this work, the influence of moisture in shear properties of carbon fiber/epoxy composites and Carall have been investigated by using interlaminar shear (ILSS) and Iosipescu tests. It was observed that hygrothermal conditioning reduces the Iosipescu shear strength of CF/E and Carall composites due to the moisture absorption in these materials. [Copyright &y& Elsevier]

Abstract: Epoxy resin-based composites reinforced with three-dimensional braided carbon fabric composites (C3D/EP) were prepared by a vacuum assisted resin transfer molding (VARTM) process. In this study, the moisture absorption behavior of the C3D/EP composites with different interface conditions was investigated. The role of interface condition was evaluated by comparing weight gain, diffusion parameters, and reductions of mechanical properties of the two 3D composites with different interface conditions, as well as by SEM observation. The weight change and diffusion parameters were determined by immersion tests and the mechanical properties degradation was investigated by examining flexural and shear properties during hygrothermal aging. It was found that moisture absorption behavior and losses of mechanical properties were interface dependent. The composites of higher interface strength showed lower moisture absorption rate (k) and diffusion coefficient (D) values at 37°C and 80°C, and a lower equilibrium moisture content (M ∞) at 37°C, but a higher M ∞ at 80°C. A mechanism was proposed to elucidate interface dependence of hygrothermal aging. In addition, a comparison to the unidirectional composites was made to assess the influence of fiber structure. [Copyright &y& Elsevier]

Based on the strong bonding between carbon nanotubes (CNTs) and the polymer matrix, the influence of hygrothermal environments on the initial pull-out force applied on the CNTs in CNTs-reinforced composite systems in investigated by means of thermoelasticity and conventional fiber pull-out models. According to the known literature, the material properties of the composite systems are affected by the variation of temperature and moisture, and the coefficient of thermal expansion of the CNTs is considered as transverse isotropic and is a nonlinear function of the temperature variation. The coefficient of thermal expansion of the polymer matrix is isotropic and is a linear function of the temperature variation. In a fully debonded region, an extended Coulomb's friction law, in which the friction coefficient depends on the pull-out rate, is used to determine the initial frictional pull-out force applied on the CNTs and the interfacial shear stress under various hygrothermal environments. In example calculations, the effects of the temperature variation of CNTs-reinforced composite systems, the moisture concentration variation in the matrix, the pull-out rate imposed on the CNTs end, the chiral vectors and the number of layers of the CNTs on the initial frictional pull-out force applied on the CNTs end in CNTs-reinforced composite systems are described and discussed. [ABSTRACT FROM AUTHOR]

An analytical procedure is developed to design and predict the behavior of fiber reinforced composite pressure vessels. The classical lamination theory and generalized plane strain model is used in the formulation of the elasticity problem. Internal pressure, axial force and body force due to rotation in addition to temperature and moisture variation throughout the body are considered. Some 3D failure theories are applied to obtain the optimum values for the winding angle, burst pressure, maximum axial force and the maximum angular speed of the pressure vessel. These parameters are also investigated considering hygrothermal effects. [Copyright &y& Elsevier]

Residual stresses in composite laminates depend on the thermoelastic properties of the material and processing temperatures. The distribution of these stresses in the various laminates is a function of the stacking sequence and ply orientation. This paper presents a theoretical investigation of the effects of hygrothermal residual stresses on the optimum design of laminated composites. Angle-ply laminates, cross-ply laminates and quasi-isotropic laminates have been considered and these laminates are subjected to different mechanical, thermal and hygroscopic loading conditions. Optimum laminate configurations have been determined by using non-linear optimization algorithms offered by the program OPTICOM. Residual stresses and strains in different layers of laminates are calculated using classical laminate theory and laminate strength ratios are determined by using the Tsai-Wu failure criterion. Hygrothermal effects on minimum thickness and in-plane strength of the laminates are then demonstrated. Analysis of the results indicates that the prediction and measurement of residual stresses are important in relation to production, design and performance of composite structures. [ABSTRACT FROM AUTHOR]

• Hygrothermal coupling conduction model is established. • Geometric nonlinear of shell is investigated in hygrothermal effect. • Navier solution with hygrothermal effect is proposed. During the service life of polymer matrix composites, various loading conditions often occur. To use such materials effectively in high-performance applications, their behaviors under hygrothermal effects should be clearly understood. Firstly, a laminated spherical shell panel is established under hygrothermal effects, followed by geometric properties' analysis and internal forces analysis of the shell. Then, the constitutive relations related to the hygrothermal effects are considered in the laminated spherical shell panel, and equations of motion with geometric nonlinearity are derived by employing Hamilton's principle; thereafter, solution of equations of motion is obtained using the Navier solution technique. Finally, influences of the hygrothermal effects on nondimensionalized center deflections, nondimensionalized fundamental natural frequency, and vibration-deflection amplitude for the shell are discussed. [ABSTRACT FROM AUTHOR]

• Hygrothermal coupling conduction model is adopted. • Nonlinear vibration behavior has been understood in the hygrothermal effects. • Analytical expression of nonlinear vibration has been obtained. Based on the nonlinear static stability analysis, this paper establishes the governing equations of the nonlinear dynamic stability of a spherical shell panel of carbon fiber-reinforced polymer (CFRP) under the hygrothermal effects; the Galerkin method is used to solve the governing equations. Then, for the free vibration of CFRP spherical shell panel, the analytical expression of the ratio of nonlinear vibration frequency to natural frequency is obtained by employing the multiscale method; for forced vibration, the analytical expression of the ratio of excitation frequency to natural frequency is obtained by considering the harmonic excitation load. Finally, the effects of geometric parameters, boundary conditions, loads, and the hygrothermal effects on the amplitude-frequency response curves of the CFRP spherical shell panel are discussed. [ABSTRACT FROM AUTHOR]

Hygrothermal effects and periodic loading often occur in carbon nanotube-reinforced composite (CNTRC) during their service life. For the effective use of such materials for high-performance applications, their nonlinear vibration behavior under hygrothermal effects should be clearly understood. Firstly, this paper establishes a nonlinear vibration mechanical model of the CNTRC plate with different distributions of carbon nanotubes (CNTs). Then, ordinary differential equations of nonlinear vibration are given by using the Galerkin method and solved by using the harmonic balance method; additionally, a primary resonance and stability of the CNTRC plate under transverse harmonic loads are analyzed by employing the multi-scale method. Finally, for a fixed shear deformation function, effects of geometric size, volume fraction, CNT distribution, and damping coefficient on the nonlinear vibration behaviors of the CNTRC plate are discussed. [ABSTRACT FROM AUTHOR]

A rotating composite thin-walled beam structure usually works in environment with strong hygrothermal characteristics. Temperature variation and moisture absorption can influence physical properties of composite, and furthermore change mechanical behavior of the structure. Hygrothermal influences depend on composite, layup scheme, geometry and nonlinearity of the beam. However, a comprehensive analytical model including these factors is lacking. This work aims to establish such a model for rotating composite thin-walled beams subjected to hygrothermal effects so that relevant problems can be solved analytically, which can aid numerical simulation and experimental test. Many important factors including hygrothermal effects, nonlinearity, mode coupling, arbitrary layup schemes, rotation, external forces, transverse shear, non-uniformity of cross section, constrained torsion, warping, setting angle and pretwist angle are considered. Governing equations of the coupled vibration are established by using the generalized Hamiltonian principle. Reduction verification, simplification principles and reduced models are discussed. The solution of the model is illustrated and some examples are given. Finally, the influences of temperature, moisture absorption and ply angle on hygrothermal internal forces of constrained thin-walled beams with CUS and CAS configurations are analyzed. [ABSTRACT FROM AUTHOR]

Vibration of a simply supported rectangular composite laminated plate with four actuating magnetostrictive layers is analyzed in the current study. The studied plate is supported by the two-parameter elastic (Pasternak's) foundations and subjected to a hygrothermal environment. Hamilton's principle and five theories are utilized to derive the kinematic equations considering the transverse shear strain with/without the transverse normal strain. The effects due to modes, lamination schemes, elastic foundations parameters, the magnitude of the feedback coefficient, position of the magnetostrictive layers, temperature rise, the degree of moisture concentration on the damping coefficients, damped natural frequencies, maximum deflection, vibration time and damping ratio, are investigated. Some conclusions about the effect of the temperature and moisture concentrations on the vibration behavior of the plate are formulated. The outcomes refer to that increasing the intelligent actuating layers in the advanced plates improves the vibration damping process, but also the position of these layers plays an effective role in reduction the time interval of damping and improving the vibration damping characteristics. Also, hygrothermal conditions reduce the ability of smart components to suppress the vibration of structures. • Vibration of a composite laminated plate with four actuating magnetostrictive layers is analyzed. • The plate is supported by two-parameter elastic foundations and subjected to hygrothermal environment. • The effects due to many parameters are investigated. • The outcomes show how to improve the vibration damping process. • The hygrothermal conditions reduce the ability of smart components to suppress the vibration of structures. [ABSTRACT FROM AUTHOR]