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Elium® is a novel liquid Methylmethacrylate thermoplastic (TP) matrix that can be cured at room temperature. Elium® resin provides the properties of TP polymer composites and is a competitive alternative to epoxy-based composites. In this work, a quadriaxial non-crimp (NC) glass fabric/Elium® based polymer composites were manufactured using vacuum assisted resin transfer moulding (VaRTM). The static mechanical properties of these composites were evaluated under the tensile and flexural loads. Further, interfacial properties like interlaminar shear strength (ILSS) and fracture toughness (Mode-I and Mode-II) were characterised. To study the influence of ageing on the mechanical properties of glass fabric/Elium® (GF/E) based composites, the test samples were aged in distilled water at a temperature of 60°C for 60 days. The obtained mechanical properties were compared between dry samples (control) and post-water immersed (aged) samples. It was found that water ageing degraded the properties of tensile, flexural strength, ILSS and Mode-II toughness by 30%, 59%, 46%, 37%, respectively. On the contrary, Mode-I fracture toughness of aged samples was 90% higher, indicating significant fibre bridging due to ageing. The microscopy and scanning electron microscopy (SEM) revealed significant failure as the degradation of fibre/matrix interface due to hygrothermal ageing. The overall performance of GF/E composites with infusible TP resin is sensitive to hygrothermal ageing. [ABSTRACT FROM AUTHOR]

In this study, nonlinear aeroelastic instability of a composite sandwich panel subjected to a supersonic airflow and thermal loading is investigated. The sandwich panel is composed of three-phase composites with polymer/Graphene platelet/fiber skins at the top and bottom surfaces and an auxetic honeycombs core layer with a negative Poisson's ratio. The motion equations of the panel within the framework of higher-order shear deformation theory (HSDT) and von Kármán nonlinearity are driven. In addition to Krumhaar's modified supersonic piston, unsteady aerodynamic pressure in the supersonic flow regime is considered. The governing equations of the sandwich panel are derived by implementing Hamilton's principle and solved by the generalized differential quadrature method (GDQM). Validation of the present formulation is assessed by comparing the numerical results with those available in the open literature. Then, the effects of several parameters such as geometric parameters, volume fraction, Mach number, different boundary conditions, yaw angle, and different inclined angles on the nonlinear aeroelastic stability of sandwich panels are examined. Finally, it was found that the ratio of core thickness, inclined angle, and Mach number have significant effects on aerodynamic pressure. [ABSTRACT FROM AUTHOR]

The effect of hygrothermal aging on the tribological and mechanical behaviors of UHMWPE biocomposites reinforced with different weight fractions of mollusc shell (MS) particles was investigated. The developed MS-UHMWPE biocomposites samples were subjected to accelerated hygrothermal aging in Ringer's solution at 80°C. The results showed that hygrothermal aging increases the tribological behavior of these biocomposites. The effect of hygrothermal aging on the wear resistance depends on the reinforcement rate of the MS particles. The mechanical properties of the MS-UHMWPE biocomposite were affected by hygrothermal aging as well as by the weight content of the added MS particles. The modulus of elasticity increased with increasing MS particle content for all tested materials. A scenario of the wear mechanism of aged biocomposites will be proposed. [ABSTRACT FROM AUTHOR]

The moisture content has an important influence on the strength and stability of coal pillar dams in underground reservoirs. In this paper, the uniaxial compression test was performed on coal samples with three different moisture contents, to study the mechanical properties of them under water intrusion condition. The results show that, water has a significant damaging and softening effect on the mechanical properties of the samples. As the moisture content grows of coal samples, the peak strength and elastic modulus fall exponentially while the peak strain increases exponentially. In addition, the energy characteristics of coal samples with different moisture contents under uniaxial compression were analyzed. The releasable elastic strain energy, dissipated energy and total input energy all experience a decline with water content, and the proportion of the releasable strain energy in the total energy drops. The capacities of energy storage and strain energy release of coal samples both fall under influence of moisture, and more energy was transformed. Given this result, the damage variable on the basis of energy dissipation was introduced to explore the influence of moisture contents on the damage variable of coal samples and the damage evolution equations of coal samples with three different moisture contents were calculated from the results of uniaxial loading tests. Finally, the compaction coefficient considering the influence of immersion time was proposed to describe the compaction degree and being used to amend the damage constitutive model obtained, and the fitted results of the models were compared to experimental data. Results show that the values of damage variables increase exponentially with strains, and the modified damage constitutive equation can give a more accurate description of the compressive deformation of coal samples that contain varying contents of moisture. [ABSTRACT FROM AUTHOR]

The vibration analysis of an axially functionally graded material non-prismatic Timoshenko beam under axial thermal variation in humid environment is carried out using the harmonic differential quadrature method. In this modeling, the length and width of the beam remains constant whereas thickness of the beam is linearly varied along the axis of the beam. The material properties are temperature dependent and are assumed to be varied continuously along the axial direction according to power law distribution. Three types of temperature variations are considered in this study, that is, uniform temperature rise, linear temperature rise, and non-linear temperature rise. The temperature of the beam remains constant under uniform temperature rise condition and it is varied linearly and nonlinearly along the length of beam for rest of the conditions. The beam is subjected to uniform moisture concentration to impose humidity. Hamiltonian's approach is used to derive the governing equations of motion. The resultant governing equations are then solved using the harmonic differential quadrature method to obtain the natural frequencies of the axially functionally graded material non-prismatic beam. The results obtained using the harmonic differential quadrature method are compared with results obtained for special cases. The effects of thermal variation, humidity, non-homogeneity parameter, and end conditions on natural frequencies of the non-prismatic beam are reported. [ABSTRACT FROM AUTHOR]

The nonlocal strain gradient theory, when combined with the first-order shear deformation theory, provides many capabilities in size-dependent structures. The aim of the present study is evaluation of the free vibration behavior of two vertically aligned fluid-conveying single-walled boron nitride nanotubes in hygrothermal environments considering slip boundary condition based on Knudsen number. These two adjacent nanotubes are coupled in the context of linear deformation through van der Waals interaction according to Lennard–Jones potential function. Actually, the contribution of the present work, compared with those previously reported, is investigating the simultaneous effect of hygrothermal loading and slip boundary condition on the dynamic behavior of two vertically aligned fluid-conveying single-walled boron nitride nanotubes. As an additional step to achieve a more accurate model of low-scale structures, both hardening and softening effects of materials are taken as important variables in the nonlocal strain gradient approach. To derive the motion equations and associated boundary conditions, Hamilton's variational principle is used. The equations are then solved with the aid of differential quadrature method. Numerical studies are also performed to depict the effects of a number of parameters such as boundary conditions, size scale, aspect ratio, inter-tube distance, and temperature alteration on the variations of dimensionless eigenfrequency and critical flow velocity. [ABSTRACT FROM AUTHOR]

This paper aimed to evaluate hygrothermal aging effects on polymethyl methacrylate modified with TiO2, SiO2, and Al2O3 nanoparticles in 0.5, 1, and 2% weight fractions. The distribution of nanoparticles was characterized by the scanning electron microscopy (SEM) method. Moisture absorption behavior and mechanical properties of samples in terms of elastic modulus, tensile strength, impact strength, and hardness were investigated. Furthermore, the coefficient of hygrothermal expansion (CHE) for each sample was calculated thanks to experimental data. Finally, by applying the multi-criteria decision making (MCDM) technique, the optimum composition for superior performance was obtained in 0.5 wt% of nanoparticles, more specifically for SiO2. [ABSTRACT FROM AUTHOR]

This work aims to characterize the influence of hygrothermal conditioning on the mechanical and thermal behavior as well as the fractographic aspects of the interface of poly(ether imide) and glass fiber composite joints welded by electrical resistance using 400 mesh of AISI 304 stainless steel. The composites were mechanically characterized by Lap Shear Strength (LSS) and End Notched Flexure (ENF) testing to investigate maximum shear stress and energy from mode II interlaminar fracture toughness. Fractography was performed by SEM, while the influence on glass transition temperature and working temperature were evaluated by Dynamic-Mechanical Analysis and thermogravimetry. In the conditioned samples, the mechanical properties reduced 23% in the LSS test and 28% in the ENF test, while the fractography studies revealed elements of interlaminar and intralaminar fracture in both conditions. Thermal properties did not change significantly to disqualify this composite when applied to welding. [ABSTRACT FROM AUTHOR]

The present article deals with the vibration and damping characteristics of functionally graded carbon nanotubes reinforced hybrid composite skewed shell structure in different hygrothermal conditions. Carbon nanotube reinforced polymer as a matrix phase and carbon fibre as a reinforcing phase are used, and carbon fibre is graded with uniform distribution along the thickness direction for the shell panel according to the power law distribution. The Mori–Tanaka scheme and strength of materials are used to determine the mechanical properties of such functionally graded carbon nanotubes reinforced hybrid composite materials. Finite element modelling has been done by considering an eight-noded shell element with the transverse shear effect according to Mindlin's hypothesis, and an oblique coordinate system is used for the functionally graded carbon nanotubes reinforced hybrid composite skewed shell structures. Damping is incorporated into such carbon nanotube–based hybrid skewed shell structure based on the Rayleigh damping model. A MATLAB-based in-house computer code has been developed for the proposed formulation and verified with published research work before using for the present dynamic analysis of functionally graded carbon nanotubes reinforced hybrid composite skewed shell structure under hygrothermal conditions. The effect of the carbon nanotube, carbon fibre, material distribution as per power law index and hygrothermal conditions on the damping behaviour of such functionally graded carbon nanotubes reinforced hybrid composite skewed shell structures have been studied. Furthermore, parametric studies are carried out for the first resonant frequency, absolute amplitude, settling time and carbon nanotube impact on the vibrational behaviour of different functionally graded carbon nanotubes reinforced hybrid composite skewed shell structures under different hygrothermal conditions. [ABSTRACT FROM AUTHOR]

A new fourth-order in-plane displacement field based refined higher-order zigzag theory is proposed for analysis of laminated sandwich plate (for both skew and non-skew shaped) subjected to hygro-thermal conditions. In order to predict behavior of thick laminated sandwich plates, third-order transverse displacement field is taken. Zig-zag effects are introduced using linear unit Heaviside step function. The theory satisfies zeros transverse normal and shear stress condition at the bottom and top surface of the plate along with continuity condition at interface. The proposed model is free from any kind of C-1 or penalty function requirements. Nine-noded isoparametric finite element having twelve degrees of freedom per node is used during analysis. Since, the present theory incorporates transverse displacement field along with continuity conditions, is able to predict the behavior of thick sandwich plates more efficiently. In literature no results are present for skew laminated composite and sandwich plates therefore, present results for skew plates are entirely new and will serve as benchmark for future studies. [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]

The optimized design of composite adaptive structures puts forward higher requirements and challenges to the actual configuration of the structural section. In this paper, a trapezoidal laminate model of composite materials is established. Based on the classical laminates theory, the bending problem of trapezoidal laminates is solved by using the Kantorovich method and the principle of minimum potential energy. The analytic form of the solution is found to satisfy the Euler equation and displacement boundary conditions. Taking the wing of a jet transport aircraft as an example, the accuracy of the analytical solution is verified by the finite element method. The Differential Evolution algorithm is used to realize the multi-objective optimal design of the bending–twisting coupled trapezoidal laminates, and the hygrothermal stability of laminates is verified by the finite element method. Finally, based on the Monte Carlo method, the robustness analysis of the bending–twisting coupling effect of laminate is realized; meanwhile the feasibility and reliability of the design scheme are verified. The stress and strain functions at each point of the trapezoidal laminate can be further obtained by the analytical solution, making it more convenient to analyze the stresses and calculate the static forces of the trapezoidal laminate and its composite structures, which is of great significance to effectively improve the comprehensive mechanical properties of the cross-section structure of composite materials. [ABSTRACT FROM AUTHOR]

A new simple "four-variable shear deformation" plate model is proposed in this work to demonstrate the hygro-thermal environment effects on dynamic and buckling of functionally graded material "sandwich plates" supported by "Winkler–Pasternak" elastic foundations. Equations of motion are obtained from Hamilton's principle containing the hygro-thermal influences. This model uses only "four variables" and considers trigonometric variation of "transverse shear stress." In addition, these stresses become zero at the upper and lower surfaces of the "sandwich plate." The novelty of this formulation is the addition of the integral term in the field of displacement, which leads to a reduction of the number of unknowns and basic equations. Various kinds of functionally graded material "sandwich plates" are examined in this study. To verify the validity of the model, the calculated results are compared with the existing results. Comparison investigations reveal that the computed results of the proposed formulation are in excellent agreement with those of other "higher shear deformation theories." Parametric investigation is established to demonstrate the effects of temperature rise, moisture condition, elastic foundation coefficients, and power law index on the vibration and buckling of functionally graded material "sandwich plates." [ABSTRACT FROM AUTHOR]

Ultraviolet light cured-in-place pipe is a popular trenchless rehabilitation technology used to repair existing buried pipelines without soil excavation. A polymer composite liner is manufactured directly in the field. After curing is complete, the new liner is placed in service to convey flowing water. Long-term water exposure has the potential to degrade the fiber reinforced cured-in-place pipe liners. Like other composites, liner hygrothermal stability could be a major challenge, especially for undercured materials. The objective of this study was to investigate the effect of aggressive environmental conditions such as water, salt solution, and simulated concrete pore solution at 50℃ on cured-in-place pipe liner mechanical and thermo-mechanical performance. Liners were exposed to aqueous solutions ranging from a typical exposure environment to a very aggressive alkaline environment. Cured-in-place pipe liner samples were collected from New York storm sewer installation sites. Specimens were assessed for the degree of curing as determined by differential scanning calorimetry and TGA. Samples were then immersed in aforementioned solutions for investigating the stability of the liners as a function of degree of cure. Both interlaminar shear strength and thermo-mechanical properties significantly differed based on the degree of cure. Cured-in-place pipe liner mechanical and thermo-mechanical properties were significantly impacted by water, salt, and pore solutions exposure. Long-term exposure to salt and alkaline environments may have a detrimental effect on the performance of cured-in-place pipe liners and desires additional scrutiny. [ABSTRACT FROM AUTHOR]

The hygrothermal effect on the short-term creep behavior of extruded thermally modified wood fiber–high-strength styrenic copolymer plastic composites (wood–plastic composites (WPCs)) was investigated on specimens preconditioned for 1 month under water immersion (distilled water (DW) and saltwater (SW)). These specimens were then tested in the same conditions for short-term creep and creep-recovery response using a submersible clamp. The short-term creep tests of WPC specimens (that are immersed in water as a function of different temperatures) have not yet been reported in previous studies. The objective of this study was to determine whether the hygrothermal creep response of WPC material evaluated through water immersion differs from the creep response published in the literature for other environmental exposure conditions. The experiments included measuring 30 min of creep and 30 min of creep recovery on the specimens immersed in SW and DW at two different levels of flexural stresses (9% and 14% of the flexural strength) and three temperature values (25, 35, and 45°C). The average creep strain recovery (%) of the specimens was higher for the specimens immersed in SW during testing than the control specimens. The WPC material is considered to have a potential use in structural applications in environments where the temperature is below 45°C because of the following factors: the low deformation under the short-term sustained loading, the decrease in the deformation rate with respect to the increase in load duration, maintaining the modulus of elasticity over a range of temperatures from 25°C to 45°C under sustained load, and the ability to recover more than 69% of the average creep strain under water immersion when the loading source is removed. The creep strain fractional increment (CSFI) of the WPC in this study under all conditions was 13% which is 86% lower than the CSFI of the WPCs reported in previous studies. [ABSTRACT FROM AUTHOR]

This study aims to produce sandwich-structured composite boards with flame retardance, far-infrared emissivity, and electromagnetic shielding effectiveness using nonwoven, weaving processes, and heat treatment. Needle punching and roller-type hot pressing are used to improve their tensile strength, tensile elongation, puncture strength, and burst strength. The limiting oxygen index is 30 regardless of whether the flame retardance/far-infrared emissivity/electromagnetic shielding effectiveness composite board is stainless steel (SS), SS+Ni–Cu (nickel-coated copper), or SS+Ni–Cu+Cu (copper) composite fabrics. SS+Ni–Cu and SS+Ni–Cu+Cu composite boards both have optimal thermal conductivity at the eighth test hour. SS–B composite board exhibit far-infrared emissivity of 0.81; moreover, they have optimal electromagnetic shielding effectiveness of −41dB at 2450 MHz when they are laminated into three layers at a 90° lamination. [ABSTRACT FROM AUTHOR]

The mechanical response of composite structures may be affected by harsh environments, particularly when the matrix has a major contribution, e.g. with off-axis plies. This study aims at investigating the influence of the winding pattern on the axial compressive behavior of filament wound composite cylinder under hygrothermal conditioning. Carbon fiber-reinforced epoxy cylinders were manufactured via filament winding with 1/1, 3/1, and 5/1 mosaic winding patterns and submitted to distilled and artificial seawater environmental conditioning. Water uptake for each hygrothermal conditioning was periodically monitored. The winding pattern influenced both compressive strength and stiffness, and the environmental conditioning decreased strength up to ≈10%. The winding pattern with three diamonds around the circumference of the cylinders provides the properties in term of compressive strength and stiffness. [ABSTRACT FROM AUTHOR]

The viscoelastic behavior of an extruded wood plastic composite (WPC) made from thermally modified wood under hygrothermal treatment was studied and modeled. Multiple three-point bending creep/recovery tests were carried out using a dynamic mechanical thermal analyzer (DMTA) equipped with a submersible clamp. WPC specimens with a 15-mm span were subjected to two initial applied stresses; 9% and 14% of the flexural strength in 30 min of creep and 30 min of creep recovery under the combined effects of temperature (25°C, 35°C, and 45°C) and water immersion (saltwater (SW) and distilled water). A dry condition WPC control was used to compare the hygrothermal effects with respect to the control conditions. The WPC material in this article exhibited a linear viscoelastic behavior under the effect of temperature, whereas a nonlinear viscoelastic behavior was observed under immersion conditions. A power law model is considered a useful model to describe the creep behavior of WPC specimens with a 15-mm span in the control and the SW conditions and at 45°C. A power law model was used to describe 180-day creep deflection of WPC lumber beams with an 853-mm span subjected to 12 MPa of the flexural strength in four-point bending at 50% relative humidity and at 21°C. The power law model predicts that the WPC lumber will reach a flexural strain in outer fiber of 1% in approximately 150 years. [ABSTRACT FROM AUTHOR]

The performance of fiber reinforced polymer externally bonded to concrete is greatly influenced by the environmental conditions to which it is exposed during service. Temperature and humidity are the two common environmental factors that alter the bond behavior of externally bonded fiber reinforced polymer. This paper reviews the experimental and computational approaches used to evaluate the hygrothermal effects—that is, the effect of temperature and humidity—on the durability of the fiber reinforced polymer–concrete bond, as well as on the bond's performance under loading conditions. Some experimental testing conducted in the laboratory and in situ are critically reviewed and presented. Implemented approaches for improving bond performance under hygrothermal conditions and their modeling techniques are also presented. The paper concludes by discussing the review's salient issues. The ongoing wide application of externally bonded fiber reinforced polymer creates opportunities for new research on improving and predicting the bond strength of fiber reinforced polymer concrete under hygrothermal conditions. [ABSTRACT FROM AUTHOR]

In this study, non-linear low-velocity impact response of a simply supported sandwich plate with CNTRC face sheets subjected to the impactors with different geometrical shapes is investigated. It has been assumed that the sandwich plate is made up of two face sheets reinforced with CNTs graded along their thickness as X profile and a homogeneous core. In CNT-reinforced layers, a micromechanical model has been used to obtain the effective material properties and the analysis is performed in the framework of the Reddy's higher order shear deformation theory with regard to thermal effects. An analytical model is proposed to capture the response performance of the three-layer sandwich plates under different thermal environments. Through the proposed analytical study, in order to characterize the contact force between the sandwich plate and the impactors, the modified Hertz contact law is utilized. Rayleigh-Ritz method is applied to the Hamilton principle in order to find the set of equations of motion for the impactor as well as the CNTRC sandwich plate. Afterwards, the solution in the time domain is obtained based on Newmark's numerical time integration scheme. After validating the proposed approach, in order to examine the influences of various involved parameters, different parametric studies are conducted. It has been demonstrated that the variation of the initial kinetic energy as one of the parameters under study has a significant effect on the central displacement, contact force, and indentation in both conical and cylindrical impactors and the change in the radius of the cylinder has an insignificant effect on the central displacement. As well, in the case of equal masses, the cylindrical impactor causes more amount of indentation with respect to conical. [ABSTRACT FROM AUTHOR]

Mechanical and tribological behaviors of polyamide 66 reinforced by various weight fractions of short glass fiber were investigated. All studied materials were subjected to accelerated hygrothermal aging tests. Three-point bending flexural tests were carried out to determine the mechanical behavior. Reciprocating tribotester was employed to determine friction and wear behaviors. Tribological tests were carried out, without lubrication and under ambient conditions. Ball on flat contact configuration was adopted for friction tests. Wear tests were carried out against an abrasive counterface. Results showed that flexural strength and elastic modulus increased when increasing glass fiber rate for all tested materials. In the contrary, the coefficient of friction and the wear resistance decreased. Results showed that the hygrothermal aging decreased the flexural features and increased the coefficient of friction. The effect of hygrothermal aging on the wear resistance depends on the reinforcement rate. [ABSTRACT FROM AUTHOR]

The present work deals with the hygrothermal aging of the bolt joints prepared from glass fiber reinforced plastics. To investigate the effect of nanoclay on joint performance, nanoclay content was varied from 0 to 5 wt%, with laminates prepared from 3 wt% of nanoclay content demonstrating the best mechanical properties. Nanoclay acts as a mechanical interlock between the fiber and the epoxy and thus improves the interfacial bonding. A good dispersion of nanoclay also improves moisture barrier properties which in turn reduces the degradation of the composite material hygrothermal conditions. Bolt joints were prepared from woven glass fiber reinforced laminates incorporating 3 wt% of nanoclay content. To design the bolt joint, ASTM D5961 was used and the geometric parameters, i.e. edge distance to hole diameter (E/D) ratio and width to hole diameter (W/D) ratio were fixed to 5 and 6, respectively. Three different temperatures, i.e. 25℃, 50℃ and 75℃ were considered for the aging to three different duration of exposure, i.e. 1, 2 and 3 weeks. The effect of different levels of bolt torque, i.e. 0, 2 and 4 Nm were considered for the failure analysis of the joint. A full factorial design of experiment was conducted on important control factors, i.e. water temperature, exposure time, bolt torque and material variation. It was found that the hygrothermal conditions degraded the material with temperature as the most contributing factor. [ABSTRACT FROM AUTHOR]

In this research, nonlinear free and forced vibration behaviors of the multiscale doubly curved composite shells have been investigated. By using Reddy's third-order shear deformation theory (TSDT) and von-Karman type nonlinearity the strains and stresses are obtained. Three-phase composites shells with polymer/carbon nanotube/fiber and polymer/graphene platelet/fiber and shape-memory alloy (SMA)/matrix according to the Halpin–Tsai model have been analyzed. The governing equations of composite shell have been derived by implementing Hamilton's principle, and according to the multiple scales perturbation method have been solved. The obtained results are validated through the comparison with the available results. Finally, the effects of different parameters such as volume fraction of SMA, temperature rise, various distributions pattern, aspect, and curvature ratio considered on nonlinear frequency have been studied. The results indicate that the changes of the fundamental vibrational mode shape have a significant influence on the nonlinear frequency of the composite shell. [ABSTRACT FROM AUTHOR]

This paper is devoted to the buckling problem of the composite cylindrical shell subjected to hydrostatic pressure. Both analytical and numerical methods are applied to investigate the buckling behavior. Based on the study of analytical formulas, it is found that the composite cylindrical shells with the same length-to-diameter ratio, diameter-to-thickness ratio, and type of layup have the same buckling pressure. Thus, a scale model experiment method is then proposed, which uses the scale model to replace the full-scale model in pressure test experiment to reduce the manufacturing cost of the test specimen. The feasibility of this method is verified by numerical calculation. The influences of ply orientation angle and length of shell on buckling shape and critical buckling pressure have been investigated numerically and demonstrated by several examples. Based on the study of the influence of shell length on critical buckling pressure, a modified finite element model, which can overcome the conservatism of optimization result due to the stress concentration caused by boundary conditions, is combined with the genetic algorithm to optimize the laminations for mass reduction. [ABSTRACT FROM AUTHOR]

The origin of fiber print-through is commonly based on the influence of temperature during cure and post cure thermal treatments on polymeric matrix systems due to chemical and thermal shrinkage. Furthermore, it is proven that especially polymeric matrices are hygroscopic and therefore, have a disposition to soak up water in surroundings with a high level of humidity. Currently, to characterize influences like these on coated surfaces of carbon fiber reinforced plastics short and long wave values are applied which are actually developed to describe waviness of coating film structures of conventional materials like aluminum or steel. A fitted value regarding the phenomenon of fiber print-through is missing today to consider various influences on the surface topography. This investigation delivers a novel approach with a specific value to determine averaged altitudes caused by fiber print-through. For this purpose, the used confocal chromatic sensor by Fries Research and Technologies creates 3D images of surface topography and fiber print-through. This elaboration allows to correlate the change of mass of carbon fiber reinforced plastics due to thermal and hygrothermal post curing treatments with the characteristic of fiber print-through. Mass increase of 0.25% due to hygrothermal influence leads to a rise of print-through effect of about 210% and a decrease of mass of 0.5% due to thermal influence rises the characteristic of print through of about 75%. [ABSTRACT FROM AUTHOR]

For the application of continuous fibre-reinforced thermoplastics in car body structures it has to be ensured that they can pass through the automotive paint shop. There are substantial chemical, hygroscopic and thermal impacts throughout this process that can affect the shape of such components. The aim of this study is to model the deformation behaviour of continuous fibre-reinforced thermoplastics under hygrothermal loading. Relaxation tests were performed to assess temperature-dependent and viscoelastic behaviour. Input parameters for temperature and moisture distribution were determined experimentally and computed using finite difference method. A straightforward approach based on classical laminate theory is presented that comprises anisotropic viscoelasticity as well as hygroscopic, thermal and polymer-chemical effects. The introduced phenomenological model was successfully used to describe the deformation of five different laminate layups both dry and fully saturated with moisture. [ABSTRACT FROM AUTHOR]

In this article, free vibration of functionally graded (FG) viscoelastic nanobeams embedded in viscoelastic foundation exposed to hygro-thermal loading is investigated based on nonlocal strain gradient elasticity theory and a higher order refined beam theory which captures shear deformation influences without the need for any shear correction factor. Also, the exact position of neutral axis is determined. The visco-Pasternak foundation is consists of parallel springs and dashpots as well as a shear layer. Temperature-dependent material properties of FGM beam are graded across the thickness based on the power-law form. Hamilton's principle is used to obtain nonlocal governing equations of embedded strain gradient viscoelastic nanobeam which are solved analytically for various boundary conditions. The results are validated with those available in the literature. The impacts of visco-Pasternak foundation parameters, structural damping coefficient, hygrothermal loading, nonlocal stress parameter, nonlocal gradient parameter, power-law exponent, mode number, boundary conditions and slenderness ratio on the damping frequency of nanoscale viscoelastic FG beams are evaluated. [ABSTRACT FROM AUTHOR]

The boundary layer hygrothermal stresses in the thick sandwich cylinder with laminated face are investigated. Uniform and through the thickness steady-state distribution for temperature and moisture content can be considered in the analysis. A displacement based layer-wise formulation is presented for analysis of thick sandwich composite cylinders subjected to hygrothermal loading conditions. Considering a general displacement field and employing a displacement based layer-wise theory, the governing equations of thick laminated sandwich cylinder are obtained. The displacement based formulation is derived for thick sandwich cylinder, which is subjected to non-uniform hygrothermal loading conditions. The faces of the sandwich cylinder are made of laminated composite with general layer stacking. The governing equations of the system include a set of coupled differential equations on the displacement components of the numerical surfaces. A semi-analytical solution is developed and the governing equations are solved for free edge boundary conditions. The accuracy of the numerical results is validated by the results of the finite element simulation and good agreements are seen between the predicted results. The free edge interlaminar stresses distributions are presented for thin and thick sandwich composite cylinders for uniform and non-uniform loading conditions. It is concluded that the presented layer-wise formulation is efficient and accurate method for analysis of thermal and hygroscopic stresses in thick and thin sandwich cylinders with general layer stacking. [ABSTRACT FROM AUTHOR]

Premature local failure of web/flange junctions of the majority of unstiffened commercially produced pultruded fiber-reinforced polymer composites profiles is inescapable due to lack of fiber continuity at the resin-dominated junction zones. This paper focuses on evaluating both short- and long-term behavior of such junctions when subjected to axial tensile stresses. In this study, the axial mechanical behavior of web/flange junctions of pultruded bridge deck components before and after hygrothermal aging effect is investigated. In the experimental phase, axial tensile tests were performed on six different types of pultruded web–flange junctions to assess axial characteristics of each web/flange junction type. In order to evaluate the hygrothermal behavior of such junctions, axial tensile tests were conducted on web/flange junction specimens that were exposed to both fresh water and artificial seawater environments at temperatures of 40℃, 60℃, and 80℃. Several controlling parameters affecting both strength and stiffness of pultruded fiber-reinforced polymer junctions were identified and discussed that include: (i) Junction's web and flange thicknesses, (ii) fillet radius, and (iii) variation of hygrothermal exposure environments. Results of this study indicated that the axial tensile capacity of a pultruded profiles is increased as the thickness of both web and flange increases. Also, it was found that the larger fillet radius of the pultruded fiber-reinforced polymer profile may contribute to a larger axial tensile capacity of pultruded profiles. However, due to variable size and geometry of the resin-rich junction zones and the common manufacturing defects in the form of folded fabric and other fibers misalignment effects, it is difficult to generalize the preceding results. Experimental results also showed that the degradation of web/flange junctions axial tensile strength increases when junctions are exposed to higher temperatures. Also, it was found that the axial tensile characteristics for junctions specimens exposed to freshwater environment are slightly different than those that were exposed to artificial seawater environments. [ABSTRACT FROM AUTHOR]

Non-destructive test technique for monitoring delamination failure under complex load and environments is still not mature until now. The purpose of this paper is to study mixed-mode delamination failure properties of carbon fiber/epoxy composite laminates under hygrothermal environment using acoustic emission. Different water-saturated composite specimens with initial intralaminar and interlaminar defects are tested. Two loading modes including single-leg and over-leg three-point bending are applied under hygrothermal environment. By analyzing the responses of acoustic emission parameters including amplitude and energy, the effects of the hygrothermal environment, layup pattern and initial defect on the delamination behaviors of composite specimens are studied. Besides, different failure modes are observed through scanning electron microscope. Quantitative acoustic emission results show hygrothermal environment and load mode affect the delamination properties of composites remarkably. [ABSTRACT FROM AUTHOR]

Fibre metal laminates are high strength materials made up of alternating layers of metal alloy and composite materials. Commercially available fibre metal laminates include CARALL, ARALL and GLARE. Other than the commercially available fibre metal laminates, number of researches have been performed with the alternate metal alloys such as stainless steel, Ti, Mg alloys and polymer matrices based on polypropylene, polyimide, polyester, etc. In this study, various factors affecting the mechanical properties of fibre metal laminates have been addressed along with the different types of failure modes observed by various researchers in their experiments. Hygrothermal conditioning which involves the subjection of fibre metal laminates to moisture and temperature (80℃ and 90% humidity as per ASTM standard 5229), their behaviour and influence on mechanical properties has also been reported in this study based on the results from the various researchers. [ABSTRACT FROM AUTHOR]

Three interface joint models for calculating energy release rate are extended to include hygrothermal effect with consideration of uniform or nonuniform changes of temperature or moisture through layer thickness in a cracked bilayer beam system using both J-integral and correction methods. The method of J-integral leads to a general expression of total energy release rate in terms of the crack-tip moments and forces, while the correction method gives explicit formulas of total energy release rate with inclusion of a separate energy release rate term subjected to only mechanical loads. Several bilayer beam fracture specimens subjected to transverse load or equivalent bending moment and thermal load are considered to validate accuracy of two methods in connection with three interface joint models, and the results show that the flexible joint model produces the best predictions for all the specimens. A parameter study is conducted to evaluate influences of thickness ratio, modulus ratio of upper and lower layers, change of temperature, and temperature gradient on the energy release rate. All the factors show a pronounced influence on the energy release rate, and a special attention is paid to evaluate effect of temperature change and temperature gradient on the energy release rate of bilayer beams. The moisture change and its gradient have the same effect and formulations on the energy release rate of bilayer beams as those of temperature, and the moisture portion of derivations and its effect are thus not given in detail. [ABSTRACT FROM AUTHOR]

Bamboo fibers reinforced with non-woven fabric polypropylene composites were prepared via compression molding. Three types of bamboo fibers were used in the research: untreated, treated with a sodium hydroxide solution and treated with a sodium hydroxide solution followed by three-aminopropyltriethoxysilane. The effects of the preformed structures and chemical treatments on the composite performances were investigated. The results showed that the composite attained optimum mechanical properties at an alkali concentration of 5% and at a silane coupling reagent concentration of 3%. The preformed structure composed of materials with bamboo fiber/polypropylene mass ratios of 50/50 and 70/30 exhibited ideal mechanical properties. The moisture absorption behavior of the composites with different bamboo fiber contents exhibited good agreement with Fick’s law. Scanning electron microscopy showed that the composites prepared using alkali-silane-treated fibers demonstrated better bonding between the fibers and matrix and had fewer voids in the composites. We also confirmed that the modifications of the bamboo fibers improved the hygrothermal properties of the composites because of the improved interfacial adhesion between the fiber and matrix. [ABSTRACT FROM AUTHOR]

Bamboo fibers reinforced with non-woven fabric polypropylene composites were prepared via compression molding. Three types of bamboo fibers were used in the research: untreated, treated with a sodium hydroxide solution and treated with a sodium hydroxide solution followed by three-aminopropyltriethoxysilane. The effects of the preformed structures and chemical treatments on the composite performances were investigated. The results showed that the composite attained optimum mechanical properties at an alkali concentration of 5% and at a silane coupling reagent concentration of 3%. The preformed structure composed of materials with bamboo fiber/polypropylene mass ratios of 50/50 and 70/30 exhibited ideal mechanical properties. The moisture absorption behavior of the composites with different bamboo fiber contents exhibited good agreement with Fick’s law. Scanning electron microscopy showed that the composites prepared using alkali-silane-treated fibers demonstrated better bonding between the fibers and matrix and had fewer voids in the composites. We also confirmed that the modifications of the bamboo fibers improved the hygrothermal properties of the composites because of the improved interfacial adhesion between the fiber and matrix. [ABSTRACT FROM AUTHOR]

Analytical and numerical results on transverse shear stresses of angle-ply composite plates subjected to hygrothermal loadings are scarce in literature. Thus, a higher order global-local model a priori satisfying the continuity condition of transverse shear stresses is proposed for hygrothermal analysis of angel-ply composite plates. Based on the higher order global-local model, a triangular element is developed by using the refined element approach. Comparing the results from available three-dimensional elasticity solution, the proposed model can produce promising transverse shear stresses directly from constitutive equations without any smoothing technique. By analyzing hygrothermal response of composite plates with general configurations, the effects of material properties, lamination angles, boundary conditions, transverse normal strain, and hygrothermal expansion coefficients on the hygrothermal response have also been studied. Differing from previous work, the corrected hygrothermal expansion coefficients are used for hygrothermal analysis of composite plates. [ABSTRACT FROM AUTHOR]

Exposure to hygrothermal environments affects mechanical properties of laminated composites through two principle mechanisms, namely, moisture absorption and the structural deterioration due to it. Previous studies have indicated that moisture plays a positive role in carbon fiber reinforced polymer composites by alleviating their damage under impact. In this study, carbon fiber reinforced polymer composites were exposed to long-term hygrothermal ageing to investigate the possibility of the initiation of structural defects due to it and their influences on the various behaviors of the laminated composites. A major type of observable structural defects was identified and the related initiation mechanism was proposed. It was found for the first time that the structural defects formed during hygrothermal ageing in turn changed the moisture uptake profile of the laminated composites. The effects of structural defects on the impact resistance of carbon fiber reinforced polymer composites were exclusively evaluated after removing the absorbed moisture. Tensile testing on vacuum-oven-dried laminates was conducted to assist the understanding of the mechanical strength of the carbon fiber reinforced polymer composites exposed to the hygrothermal environments. [ABSTRACT FROM AUTHOR]

In this study, the effects of hygrothermal aging on the mechanical properties and impact behavior of glass-epoxy composites were investigated experimentally. The vacuum-assisted resin infusion molding was used to manufacture composites. In manufacturing of composites, unidirectional E-glass fabrics with areal density of 300 g/m2 and a resin system (Araldite LY 564/Aradur 3487 BD) were used. In order to determine mechanical properties such as modulus of elasticity, longitudinal and transverse strengths, shear strength, and Poisson’s ratio, the composite samples were prepared according to the ASTM standards. Then, the samples were conditioned at a constant temperature (95℃) and a constant humidity (70%) for different periods of times, ranging from 0 h to 1200 h. The conditioning temperature was chosen due to the glass transition temperature (Tg) of the intact composites which is determined as 78℃. It is noted that the most affected mechanical property via material degradation is modulus of elasticity in transverse direction (E2) while the least affected one is shear modulus (G12). The perforation threshold of the composite materials also decreases with increase of conditioning time. [ABSTRACT FROM AUTHOR]

In the present study, the influence of temperature and moisture concentration on the bending characteristics of delaminated rotating composite conical shells has been investigated. The shells with initial twist are considered for moderate rotational speeds wherein Coriolis effect is assumed to be absent. The effects of rotary inertia and transverse shear deformation have been incorporated in the present finite element formulation along with the multi-point constraint algorithm for modeling the delamination. Graphical results have been presented depicting the contours of deflection and relative moment along the planform of the conical shell resulting from centrifugal and hygrothermal forces taking into account the influence of twist angle, stacking sequence, rotational speed, and size of delamination present inside the laminate. [ABSTRACT FROM AUTHOR]

In this article, the geometrical nonlinear free vibration behaviour of shear deformable laminated composite single/doubly curved shell panel in hygrothermal environment is investigated. A general nonlinear mathematical model of laminated composite doubly curved shallow shell panel has been developed based on the higher order shear deformation theory and Green-Lagrange geometric nonlinear kinematics. All the nonlinear higher order terms have been included in the present mathematical model to achieve a realistic case. In addition to that, the composite material properties are considered as temperature and moisture dependent through hygro-thermo-elastic constitutive relations. The governing equation of laminated composite curved panel is obtained using Hamilton’s principle and discretised using nonlinear finite element steps. The desired responses are computed numerically using a direct iterative method. The convergence behaviour of present numerical model has been checked and validated by comparing the responses with those available published results. The effect of different geometrical and/or material parameters on nonlinear vibration behaviour of laminated composite doubly curved shell panel under hygrothermal environment is computed and discussed in detail. [ABSTRACT FROM AUTHOR]

CCF300/5228A aero composite stiffened panels achieved absorbing saturation in hygrothermal condition (70℃ distilled water), whose absorbing process followed the Fick law. Many debonding and gaps of matrix to fibers could be observed after hygrothermal exposure by scanning electron microscope (SEM). Compressive experiments were conducted on unaged and aged composite stiffened panels. Failure modes of the two types of specimens are quite similar with several major ways, including the debonding and fracture of stiffeners together with the splitting and ripping of panel. Buckling patterns for both types of specimens are similar and characterized by buckling of panels between stiffeners and buckling of the middle two stiffeners. However, the bending direction of buckling mid-panel for aged specimens is opposite to that of the unaged at same position panel, which shows that hygrothermal condition influences the bending direction of buckling mid-panels seriously. There still exists a post-buckling process for aged specimens under axial compression, whose average buckling load and failure load decrease by 3.1% and 22.2% respectively, comparing to that of unaged specimens. [ABSTRACT FROM AUTHOR]

Low-velocity impact testing was conducted on carbon/epoxy composite laminates exposed to hygrothermal environments. Besides normal laminates, staggered lay-up structures were made from unidirectional prepreg materials. The specimens were immersed in water at 80℃ for different durations before impact testing. Experimental data showed that moisture played a positive role by improving the impact resistance of composite laminates. Laminates with higher moisture level could behave elastically up to higher strain levels and retain their resistance to the striking impactor to larger deflections. After absorbing moisture, more of the impact energy was dissipated by the specimen through elastic deformation and less damage was induced to the laminate. Little degradation in the fiber/matrix interface was observed from SEM graphs. It is postulated that absorbed moisture improved the ductility of the epoxy resin by promoting chain segmental mobility of the polymer molecules, which eventually lead to the better impact response of laminates with higher moisture content. [ABSTRACT FROM PUBLISHER]

Since flax is the most promising plant for the reinforcement of polymer-based composites in structural applications, we have chosen to investigate its hygrothermal characteristics which can be useful for the understanding of the behaviour of other plant fibres. The flax fibres were exposed to different hygrothermal conditions: in an oven at various controlled temperatures (–40 to 140℃) and measured relative humidity, in a climate chamber at 50% relative humidity for define temperatures between 25℃ and 85℃, or different determined aging conditions. The correlation of these hygrothermal conditions to the evolution of the mechanical properties gives evidence of the prominent influence of water over temperature on the microstructural changes of flax fibres. The mechanical parameters drastically decrease in usually prescribed hygrothermal aging conditions for organic matrix composite materials, the strength being particularly sensitive to the presence of water. These evolutions were correlated to the fibre microstructure modifications induced by water absorption as revealed by electron microscopy analyses. These findings could be useful for understanding the behaviour of polymer matrix biocomposites in severe hygrothermal conditions. [ABSTRACT FROM PUBLISHER]

This paper investigated the effect of porosities, environmental factors and impact energies on the impact resistance properties of Carbon Fiber Reinforced Polymer/Plastic (CFRP) laminates. Impact tests on the CFRP laminates with three porosity levels were subjected to five energy levels from 3 J to 15 J at the room temperature. The damage area was assessed with combined effects of different impact energies and porosity levels. The damage area was evaluated by visual inspection and ultrasonic C-scan measurement. The thermal deply technique was used to evaluate the damage evolution behavior. Three-point bending tests were conducted on the non-impacted and impacted specimens that were exposed to the room temperature, the hygrothermal and the drying environments. The curves of the impact resistance of the tested laminates mutate at the impact energy of 9 J. The thermal deply technique reveals the failure mechanism of the impact damage mutation of the tested laminates when the impact energy exceeds the threshold 9 J. [ABSTRACT FROM AUTHOR]

The tensile test of a multidirectional laminate of strip geometry is analysed assuming that the clamping system imposes elastic restrictions to the rotations at both ends. The problem has five redundant unknowns that are determined by applying Engesser’s second theorem. Thus, the stress field can be obtained including clamping effects. The displacement field is determined by applying Engesser’s first theorem in conjunction with the unit load method. Hygrothermal effects are also included in the analysis. A method for determining the end compliances based on longitudinal strain and out-of-plane displacement measurements is proposed. Experiments are simulated by numerical results corresponding to AS4/3501-6 carbon/epoxy composite. Finally, two examples concerning multidirectional laminates with general coupling are analysed. [ABSTRACT FROM AUTHOR]

The hygrothermal effect on the flexural, interfacial and glass transition behaviors of 977-2A carbon/epoxy composites cured in an autoclave and by using Quickstep were evaluated in the previous paper. This article concerns with the hygrothermal effect on the mode I and mode II delamination resistance of the samples cured in an autoclave and by Quickstep. It was observed that the mean initiation and propagation GC values (i.e. GIC and GIIC) of both autoclave and Quickstep 60 cured specimens were apparently reduced after hygrothermal conditioning; however, the data obtained from both autoclave and Quickstep 60 cured specimens in dry and wet states were not statistically significant. The scanning electron micrograph confirmed the enhancement of matrix ductility; however, no effect of this matrix ductility was observed in the GIC and GIIC data. [ABSTRACT FROM PUBLISHER]

The effect of moisture and temperature on the strength of composite single-lap bonded joints were extensively investigated. Secondary bonded-joint specimens were immersed in tap water and 3.5% salt water at 71°C for different durations up to a maximum of 300 days. A total of 160 specimens were tested under six different moisture contents and two different temperature environments. Test results showed that while the strengths of the joints with 2.1% or more tap water moisture content decreased compared with those of dry joints, the strengths of pre-saturated joints with up to 1.0% moisture content increased at both room and elevated temperature. However, the strengths of saturated joints at 2.5% moisture content decreased by 34% (room temperature) and 44% (71°C) compared with those of the dry joints. In the joints with 1.0% or lower moisture content, the higher temperature environments resulted in slightly increased strengths, whereas the elevated temperature degraded the strengths in the joints with 2.1% or greater moisture content. The major failure modes of the joints were delamination and intralaminar failure of the composite adherend; the joints with high moisture content tested under elevated temperature showed cohesive failure of adhesive. The strengths and failure modes of the joints that were immersed in salt water did not show a remarkable difference from the joints that absorbed tap water. [ABSTRACT FROM AUTHOR]

This article presents the hygrothermal effects on the free vibration of laminated composite plates resting on a two-parameter Winkler and Pasternak elastic foundations with random system properties using micromechanical model. System properties such as material properties, hygroscopic expansion coefficients, thermal expansion coefficients and foundation stiffness parameters are modeled as independent basic random variables which are affected by the variation in temperature and moisture based on a micromechanical model of a laminate for accurate prediction of system behavior. A C0 finite element method based on higher-order shear deformation theory has been used for deriving the standard eigenvalue problem. A Taylor series-based mean-centered first-order perturbation technique is used to find mean and standard deviation of the natural frequency subjected to uniform moisture concentration and temperature rise, plate aspect ratio, total number of plies, fiber orientations and elastic foundation parameters with different boundary support under hygrothermal environmental conditions. Typical numerical results have been validated with those available in the literature and independent Monte Carlo simulation. [ABSTRACT FROM AUTHOR]

Refined two-dimensional models are proposed for the static hygrothermoleastic analysis of multilayered composite and sandwich shells. These shell models are developed in the framework of the Carrera's unified formulation (in a general and unified manner) by considering both equivalent single layer and layerwise multilayer description. The principle of virtual displacements contains elastic, thermal and hygroscopic strains. The governing equations allow mechanical, thermal and hygroscopic loads to be applied and they are solved in a closed-form solution. Thermal and hygroscopic loads are defined by means of appropriate temperature and moisture content profiles through the thickness of the shell, such profiles can ‘a priori’ be assumed or they can be calculated by solving the Fourier heat conduction equation and the Fick's moisture diffusion law. Such equations are solved in steady-state conditions and in curvilinear coordinates for the shell geometries. The presence of loads due to hygroscopic and thermal effects (in addition to the mechanical load) modifies the bending response of multilayered shells. Comparisons between classical and refined models, and between assumed and calculated temperature and moisture content profiles are proposed in the cases of composite and sandwich shells. The use of refined models combined with calculated temperature and moisture content profiles through the thickness is mandatory for a correct elasto-thermo-hygroscopic analysis of multilayered structures. [ABSTRACT FROM AUTHOR]

In this article, the effect of hygrothermal aging on the painted surface finish of unidirectional and fabric carbon fibre composite laminates, with and without surfacing film was investigated. The results highlighted the importance of ensuring that the composite surface directly beneath the paint layer is made from a uniform material with a consistent thickness in order to minimise surface defects from occurring during aging. The surfacing film was found to minimise the print through development on the painted unidirectional and twill composite surfaces. However, the surfacing film layer was found to intermingle with the carbon fibre plies during cure, which resulted in an uneven film thickness that caused increased levels of orange peel. The twill laminate painted surface produced high levels of print through and surface waviness that was caused by the large resin rich regions located within the tow intersections at the surface which enlarged due to thermal expansion and swelling of the matrix with hygrothermal aging. It was also noted that the small resin rich regions between the individual carbon fibres on the unidirectional composite surface were sufficiently large to print through the painted surface. [ABSTRACT FROM AUTHOR]

This article focuses on the calculation of transient and cyclical hygrothermal fields in laminated composite plates. Water diffusion is simulated by employing Fick’s law, while thermal fields are assumed to be uniform: temperature and water concentration are coupled through the coefficient of hygroscopic diffusivity, which follows a classical Arrhenius law. Relative humidity and temperature at the plate boundary vary in a transient/cyclic way, reproducing real complex environmental conditions. Analytical solutions of the cyclical hygrothermal fields in laminated plates can be employed as benchmark for numerical calculations and for evaluating the induced hygrothermal stress, therefore, for structural, design and optimisation purposes. [ABSTRACT FROM PUBLISHER]