Hygrothermal Environment Research Articles

Aging damage mechanism and mechanical properties degradation of 3D orthogonal woven thermoset composites subjected to cyclic hygrothermal environment

3D orthogonal woven thermoset composites (3DOWCs) can be applied to fabricate the bearing structures of supersonic aircrafts due to their excellent mechanical properties. However, 3DOWCs would be subjected to the cyclic hygrothermal aging caused by the specific service environment of supersonic aircrafts. This paper aims to reveal the cyclic hygrothermal aging mechanism and the consequent degradations of mechanical properties of 3DOWCs. Accelerated hygrothermal cyclical ageing tests were carried out in a simulated service environment of supersonic aircrafts. The aging damage mechanisms were investigated with moisture content monitoring, micromorphology observation and Fourier transform infrared (FTIR) analysis. The evolution of mechanical properties was evaluated by quasi-static mechanical tests (tension, compression and interlaminar shear) and Dynamic mechanical analysis (DMA). The results show that the mechanical properties of 3DOWCs deteriorate significantly with the increasing of hygrothermal aging cycle. Interface debonding, crack propagation, and embrittlement of fiber bundles are the main reasons of the degradation.

Finite element solution of vibrations and buckling of laminated thin plates in hygro-thermal environment based on strain gradient theory

The paper aims to develop a finite element methodology to deal with vibrations and buckling of laminated thin plates subjected to thermal and hygroscopic effects, once a second-order strain gradient theory is included to overcome the limitations of conventional elasticity and to capture nonlocal phenomena. The numerical scheme takes advantage of Hermite approximation for both membrane and bending primary variables, since the strain gradient introduces higher-order derivatives of the nodal displacements. Its versatility is proven by dealing with general lamination schemes and arbitrary boundary conditions. The analyzed configurations cannot be solved analytically.

Fatigue Life Prediction of CFRP-Strengthened RC Beams with Flexural Crack under Hygrothermal Environments

The durability of reinforced concrete (RC) beams strengthened with carbon fiber-reinforced polymer (CFRP) is a worldwide concern in structural engineering. As an important part of the strengthened beam, the performance of the CFRP–concrete interface under hygrothermal environments is a delicate problem. In this paper, the fatigue behavior of CFRP-strengthened RC beams is analyzed by a theoretical model. In the model, CFRP–concrete interface degradation under hygrothermal environments is involved. Since interface debonding and rebar fracture induced by intermediate cracking are two typical failure modes, the damage models of rebar and the CFRP–concrete interface are established. Based on the theoretical model, the failure mode of CFRP-strengthened RC beams can be predicted, and fatigue life can be determined. The results showed that IC debonding is more likely to occur under hygrothermal environments. The accurate prediction of failure modes is essential for fatigue life prediction.

Free Vibration Analysis of a Spinning Composite Laminated Truncated Conical Shell under Hygrothermal Environment

This paper is concerned with free vibration characteristics of a spinning composite laminated truncated conical shell subjected to hygrothermal environment. Hygrothermal strains are introduced into the constitutive law of single-layer material, and fiber orientation lies symmetrically with respect to the midplane of the composite shell. Considering the spin-induced Coriolis and centrifugal forces, as well as initial hoop tension, the governing equations of free vibration of the composite conical shell with hygrothermal effects are derived on the basis of Love’s thin-shell theory and Hamilton’s principle. The solution of the equations is derived using the Galerkin approach. Then, a detailed parametric study on natural frequencies and critical spinning speeds is numerically performed. Results indicate that the Coriolis force induces an asymmetric influence on natural frequencies of forward and backward traveling waves, while the centrifugal force enhances the frequencies of both traveling waves symmetrically. Initial hoop tension plays a major role in the increase of critical spinning angular speed. Temperature, moisture concentration, and design parameters show the significant influence on the free vibration characteristics of the conical shell, and thermal expansion deformation is nonnegligible in the free vibration analysis.

Assessing the deterioration risk of polychrome clay sculptures based on the hygrothermal environment: A case study of Baosheng temple, China

The polychrome clay sculptures of the Buddha at Baosheng Temple in Suzhou, China are important and famous cultural heritage with a history of more than 1000 years. Currently, they are tarnished by cracking, peeling, and fading. To evaluate the current conservation environment and deterioration risk to the sculptures, the hygrothermal environment of Luohan Hall and the temperature and moisture content of the surfaces of the Buddhas were analyzed by field environmental monitoring and a heat and moisture transfer model. The results show that the air temperatures near the sculptures have different fluctuation patterns in different orientations and that the relative humidity of the air near the sculptures fluctuates sharply, with the daily fluctuation exceeding set limits most of the time. The fluctuations of the air temperatures and relative humidities in Luohan Hall are mainly due to air exchange with the outdoors. The surface layer of the sculpture is in a dynamic state of absorption and evaporation depending on the humidity of the surrounding air, and evaporation from the sculptures’ surfaces is more significant in autumn and winter. The air tightness and insulation performance of Luohan Hall should be improved, and dehumidification during periods of high humidity might be needed to reduce the evaporation of the surface layer of the sculpture.

Historic Churches and Their Hygrothermal Environment: A Review of Criteria Related to Building Fabric, Artefacts, Artwork and Occupants

Regarded as important cultural heritage, historic churches have been utilised over hundreds of years for worship and community benefit. Simple on/off space heating systems are installed in many churches to increase human comfort. However, the conservation of the important historic artefacts and artwork contained within may not have been fully considered. This review attempts to appraise the standards in place for artefacts and artwork. A consensus of 15–25 °C and 40–65% relative humidity is established as safe from the standards reviewed. Consideration is given to the environment within the church to understand if such exacting conditions can be met. The review finds that the conservation and preservation of artefacts and buildings are aligned goals, although striving to meet specified target ranges for artefact types is not fully compatible with historic churches. The stability of the internal environment is clearly an important factor in conservation and benefits human comfort expectations. Churches may contain microclimates throughout the building, complicating the use of target ranges for artefacts, artwork and comfort. The findings of this study can assist historic churches in managing the change, alteration or installation of heating systems.

Wavelet solution for hygrothermomechanical bending of initially defected plate undergoing large deformation on nonlinear elastic foundation

An efficient wavelet approach has been developed for geometrically nonlinear bending analysis of largely deformed plate with arbitrary initial curvature on nonlinear Winkler–Pasternak foundation loaded with sinusoidally hygrothermal stresses. Translational and rotational constraints have been analyzed and equivalently transformed into the Cauchy-type boundary conditions of Airy stress function, which are efficiently approached by interpolating Coiflet-type wavelet. A newly hygrothermomechanical bending model of plate subjected to the adiabatically impermeable edges or with boundaries of sinusoidal temperature and humidity has been firstly formulated. The highly coupled and nonlinear governing equations have been decomposed into linear differential terms by homotopy transformation and successfully solved by the Coiflet-type wavelet Galerkin method. The wavelet strategy has been validated superior in dealing with extremely nonlinear bending and high-precision reconstitution of initial deflection in hygrothermal environment, while the obtained solutions are in excellent agreement with published results. Effect of initial deflection, hygrothermal load and elastic foundation has been investigated, which implies the hygrothermal effect on plate in linear bending is prominent but gradually negligible in large deformation, while the nonlinear effects of elastic foundation become significant by the increase of deflection but not obvious at smally deformed state.

Application of the Higher-Order Hamilton Approach to the Nonlinear Free Vibrations Analysis of Porous FG Nano-Beams in a Hygrothermal Environment Based on a Local/Nonlocal Stress Gradient Model of Elasticity.

Nonlinear transverse free vibrations of porous functionally-graded (FG) Bernoulli–Euler nanobeams in hygrothermal environments through the local/nonlocal stress gradient theory of elasticity were studied. By using the Galerkin method, the governing equations were reduced to a nonlinear ordinary differential equation. The closed form analytical solution of the nonlinear natural flexural frequency was then established using the higher-order Hamiltonian approach to nonlinear oscillators. A numerical investigation was developed to analyze the influence of different parameters both on the thermo-elastic material properties and the structural response, such as material gradient index, porosity volume fraction, nonlocal parameter, gradient length parameter, mixture parameter, and the amplitude of the nonlinear oscillator on the nonlinear flexural vibrations of metal–ceramic FG porous Bernoulli–Euler nano-beams.

Effect of marine environment on the mechanical properties degradation and long-term creep failure of CFRP

The carbon fiber reinforced plastics (CFRP) has broad application prospect in the marine field, but its significate long-term performance and failure mechanism are still unclear. In this paper, the mechanical property degradation and creep failure mechanism of CFRP in marine environment were investigated by the molecular dynamics (MD) method. Based on the CFRP molecular model established in dry, pure water and salt immersion environment, the effect of service conditions on it in marine environment was explored. The results show that the marine environment can lead to a significant decrease in the mechanical properties of CFRP only in the vertical fiber direction, and the degradation extent increases with the increase of temperature. The effects of pure water immersion and salt immersion environment on its mechanical properties are basically the same. High stress level, hygrothermal environment and high temperature salt immersion environment will accelerate the creep failure of CFRP. Later, the creep failure mechanism of CFRP is analyzed from microstructure and microscopic energy. Microstructure analysis shows that water molecules could accelerate the movement of epoxy fragments and lead to CFRP failure. Microscopic energy analysis shows that the conversion process of strain energy and the decline of interface performance are the important reasons for creep failure. The finds of this paper introduce a new method to study the long-term performance of CFRP and can provide reference for the engineering application of CFRP in marine environment.

Analysis of the annual hygrothermal environment in the Maijishan Grottoes by field measurements and numerical simulations

Fluctuation in the hygrothermal environment can facilitate the deterioration of cultural relics in the Maijishan Grottoes. The purpose of this research is to develop a hygrothermal simulation model of grottoes to analyze the changes in the hygrothermal environment under different influencing factors. First, the temperature and relative humidity (RH) in Cave 101 of the Maijishan Grottoes were monitored throughout the year. The results indicated that the hygrothermal environment in Cave 101 was in an annual cycle from low temperature and RH to high temperature and RH. Then, a hygrothermal simulation model of the cave was established via the coupled heat and moisture transfer method. In addition, the accuracy of the model was verified by error analysis. Finally, the influences of different factors on the hygrothermal environment in the cave were evaluated by the model. The corridor and door can effectively weaken the influence of climate change on the hygrothermal environment in the cave. With increasing size and decreasing air change per hour, the annual amplitude and diurnal change in temperature and RH in the cave decrease. However, the environment in the cave remains moist in summer. The ventilation dehumidification design could be adopted as a regulation measure. This research provides a quantitative research method for the regulation of the hygrothermal environment and promote the development of preventive environmental protection technology in the Maijishan Grottoes.

Ultralight and hydrophobic MXene/chitosan-derived hybrid carbon aerogel with hierarchical pore structure for durable electromagnetic interference shielding and thermal insulation

Lightweight, robust and durable high-performance electromagnetic interference (EMI) shielding materials with excellent thermal insulation properties are highly desired for modern integrated electronic systems used in military and aerospace applications. Although MXenes have exhibited high shielding performance, it remains a great challenge to build high-performance EMI shielding MXene-based aerogels with minimal MXene content and high durability. Here, hierarchically porous hybrid carbon aerogels have been constructed by assembling chitosan (CS) hydrogels and MXenes via freeze-drying followed by a suitable heat treatment strategy. Due to the special MXene–TiO2–C heterostructures and high conductivity resulting from heating at 800 °C under an Ar atmosphere, an ultralight hybrid carbon aerogel (11.9 mg cm−3) with 0.1322 vol% MXene exhibits a high EMI shielding effectiveness (EMI SE) of ∼61.4 dB and a specific SE of 5155.46 dB cm3 g−1 at a thickness of ∼3 mm. Furthermore, the hybrid carbon aerogel also provides significant thermal insulation (∼25.88 mW m−1·K−1 at 25 °C and ∼62.65 mW m−1·K−1 at 800 °C under Ar) as well as fire resistance. Superior EMI shielding and thermal insulation performance can be retained even after storage in a hygrothermal environment for 30 days, which guarantees long-term application of the hybrid carbon aerogel in humid or high-temperature environments.

Study of the Influence of the Type of Aging on the Behavior of Delamination of Adhesive Joints in Carbon-Fiber-Reinforced Epoxy Composites.

This study analyzes the behavior under the static delamination and mode-I fracture stress of adhesive joints made on the same composite material with an epoxy matrix and unidirectional carbon fiber reinforcement and two types of adhesives, one epoxy and the other acrylic. Standard DCB tests (for mode-I fracture) were used to quantify the influence on the interlaminar fracture toughness of the type of adhesive used. Both materials were subjected to two different degradation processes, one hygrothermal and the other in a salt-fog chamber. After aging, the mode-I fracture has been evaluated for both materials. From the experimental results obtained, it can be deduced for the epoxy adhesive that exposure to the hygrothermal environment used moderately modifies its behavior against delamination, while its exposure to the saline environment produces a significant loss of its resistance to delamination. For the acrylic adhesive, the hygrothermal exposure causes an improvement in its delamination behavior for all the exposure periods considered, while the saline environment slightly modifies its behavior. There is, therefore, a clear influence of the type of aging on the fracture behavior of both adhesives.

Mechanical performance evolution and life prediction of prestressed CFRP plate exposed to hygrothermal and freeze-thaw environments

The long-term prestressed loss of anchorage system and mechanical property evolution play the significant role for the steel/concrete structures strengthened with prestressed CFRP plates. In the present study, a wedge-extrusion bond anchorage system was applied to provide the reliable anchorage load-bearing capacity for CFRP plate. A new prestressed tension device was put forward to realize the coupling exposure of elevated temperature (20 °C, 60 °C and −20 to 30 °C), distilled water and sustained loading (20%, 40% and 60%). The mechanical and thermal properties of CFRP plate were conducted to obtain the long-term evolution exposed to the above environments. The mechanical analysis and tensile tests of anchorage system showed the tensile failure modes under the static and cyclic loads were the CFRP burst, and there was no debonding occurring in the anchor. The higher anchorage capacity was attributed to the uniform stress distribution of CFRP plate parallel to the greatest dimension of cross section in the anchor. The new prestressed tension device can realize the maximum prestressed level (∼70%) with an allowable prestressed loss (∼300 με). Meanwhile, the prestressed loss of CFRP plate exposed at the maximum temperature (60 °C) and longest time (90 days) was found to be ∼300 με. Immersed at higher temperature and prestressed level brought about an additional degradation rate of tensile strength (5%∼10%). The minimum strength retention of CFRP plate was found at the freeze-thaw exposure for 90 days. The long-term life prediction showed the residual tensile strength retentions of CFRP plates were more than 50% for the maximum prestressed level (60%) for the service life of 30 years in civil engineering structures.

Effects of nonlinear hygro-thermo-mechanical loading on the bending response of nanobeams using nonlocal strain gradient theory

This article analyzes the bending response of functionally graded nanobeams under hygro-thermo-mechanical loadings. The nonlocal strain gradient theory (NSGT) is implemented to consider both hardening and softening impacts on the system response. The nanobeam is constructed of functionally graded materials (FGMs), whose characteristics change smoothly across the thickness direction according to a power-law distribution function. The system is subjected to hygro-thermal environments according to the sinusoidally distributed loading. By employing Hamilton's principle, the static equations of the system are acquired. Static responses of the nanobeam are obtained with the help of the Galerkin technique. The effects of material distribution and different boundary conditions on the system deflection are examined. The impacts of several system parameters, such as the nonlocal parameter, length scale parameter, temperature gradient, and moisture variation, on the static responses of the functionally graded nanoscale beams are determined.

Long term accelerated influence on thermo-mechanical properties of glass/carbon fiber reinforced interpenetrating polymer network hybrid composites

The prime importance of this work is that to compare the influence of hygrothermal analysis on the physical properties of different variant of interpenetrating polymer network (IPN) blend reinforced with E-Glass/Carbon and combination of both (hybrid) fibers. In this study, combinations of epoxy (EP)/polyurethane (PU), vinyl ester (VER)/polyurethane (PU), and epoxy (EP)/vinyl ester (VER) have been taken as the matrix material (IPN) to reinforce the glass, carbon, and combination of both fibers. Moreover, prepared specimens are subjected with boiling water immersion test by maintaining the temperature of 45°C, 55°C, and 65°C in order to thoroughly understand the influence of moisture absorption and temperature in their physical attribution as per ASTM standards. Besides, to better understand the thermal stability and compatibility, thermal-gravimetric (TGA) analysis and burn-off test were conducted as well. During this study, it was found that, combination of VER/PU possesses the high moisture absorption resistance amongst all variants (0.725% for 45°C, 0.854% for 55°C, 1.234 for 65°C). Similarly, epoxy/vinyl ester reinforced glass fiber IPN laminate (EVG) has shown notable TGA value as 418.6°C, as well burn-off test also shown that hybrid IPN composites have better wettability (less void presence) than all other laminates (EPGC-0.9%, VPGC-0.89%, EVGC-0.92%). Further, losses of physical strengths have been noticed on all specimens upon subjection on hygrothermal environment irrespective of IPN blend and fiber constituents.

Effect of simulated hygrothermal environment on the flexural and interlaminar shear strength of particulate-filled epoxy-coated GFRP composites

The application of glass fiber-reinforced polymer (GFRP) composites in civil engineering and construction has gained significant interest in the last decade. Such applications require information on the long-term performance and durability of fiber composite materials because these materials are directly exposed to aggressive environments. This comparative study evaluated the effect of a simulated hygrothermal environment on the durability of GFRP composite laminates with and without a particulate-filled epoxy coating. The three-point bending test was conducted to characterize the degradation of interlaminar shear strength and flexural properties of GFRP composites before and after conditioning at combined high moisture and temperature for 0, 1000, 2000, or 3000 h. Dynamic mechanical analysis was also conducted to characterize the changes in the thermomechanical properties of the composite laminates. The results show that hygrothermal conditioning had a negative impact on the flexural and interlaminar shear strength of uncoated GFRP laminates, but this could be efficiently minimized with the use of a particulate-filled polymer-epoxy coating.

Experimental and Numerical Investigation on Fatigue Properties of Carbon Fiber Cross-Ply Laminates in Hygrothermal Environments.

The fatigue properties of composite materials are degraded seriously in hygrothermal environments, so taking into account their influence is very important when evaluating the fatigue life of composite structures. Tensile fatigue experiments of carbon fiber reinforced resin composite cross-ply laminates were conducted in room temperature/dry (RTD), cool temperature/dry (CTD) and elevated temperature/wet (ETW) conditions. The S-N curves and fatigue failure modes of the cross-ply laminates were obtained in three conditions. On this basis, a finite element model was established to discuss the influence of temperature and moisture content on the fatigue properties, as well as a method for determining environmental factors of fatigue life of cross-ply laminates was established. The results show that the saturation moisture absorption and temperature have a significant influence on the tensile fatigue properties of cross-ply laminates. The high-cycle fatigue property is weakened significantly by the saturation moisture absorption and high temperature, but the low-cycle fatigue properties were strengthened in cool temperature conditions. The delamination failure mode in ETW is the most severe, presenting with an obvious necking phenomenon. The influence of temperature has a greater effect than that of moisture content, but moisture absorption would play its affect obviously when temperature exceeds 40 °C.

Enhancement of aeroelastic performance of a smart delaminated composite plate under hygrothermal environment

In the present article, the effect of delamination on flutter performance of a smart laminated composite wing-like plate structure under hygrothermal environment has been investigated. A FE model for a laminated composite plate with centrally located mid-plane square delamination along with an AFC actuator at the top of the laminate is developed in MATLAB. Subsequently, the aerodynamic model is developed in NASTRAN and coupled with structural modal parameters through the surface spline technique using the Direct Matrix Abstraction Programming (DMAP). The effect of delamination and its size on aeroelastic performance has been analyzed under adverse moisture level and temperature conditions. It has been seen that the aeroelastic performance of the plate gets considerably deteriorated due to the presence of delamination under adverse hygrothermal circumstances. Voltage is applied to the AFC actuator and it is observed that there is a surge in the stiffness of structure along with the escalation of aeroelastic performances.

Experimental study of quasi-static and dynamic tensile behavior of epoxy resin under cyclic hygrothermal aging

Fiber-reinforced polymer (FRP) structures used in aviation are typically exposed to cyclic hygrothermal environments in which they may be subjected to impact loads that threaten flight safety. Although the effects of hygrothermal aging on the mechanical behavior of FRPs have been extensively explored, few authors have discussed the impact-induced behavior of composites after long-term hygrothermal aging, and hardly any study has examined the effect of the strain rate of neat epoxy resin after hygrothermal aging. This study examines the quasi-static and dynamic tensile properties of TDE86 epoxy resin under cyclic hygrothermal aging for the first time. The resin specimens were aged for 0, 1, 7, 14, 28, 56, 70, and 140 days, representing specimens that were unaged, and had been in service for one month, six months, one year, two years, four years, five years, and 10 years respectively. The strain rate at which they were tested ranged from 7.5 × 10−6 s−1 to 1000 s−1. This yielded the hygroscopic law, the process of evolution of crack-induced damage, and the viscoelastic behavior of the specimens, including the hygrothermal effect and the effect of the strain rate of the epoxy resin. This information can be used to predict the hygrothermal behavior of FRPs based on the micromechanics method.

Research on mechanical properties and durability of flax/glass fiber bio-hybrid FRP composites laminates

In recent years, natural fiber as a green and renewable resources have attracted much attention to be used as reinforcement in composites. The aim of this work is to assess mechanical properties and durability in harsh environments of epoxy-based flax and glass bio-hybrid fiber reinforced plastics (bio-HFRP) composites. With the same fiber mixing ratio, the flexural and dynamic mechanical properties of the bio-HFRP composites were studied by changing the stacking order. The pure flax FRP and bio-HFRP composites were exposed to hygrothermal at 20 °C, 40 °C, and 60 °C for 28 days, respectively. The water absorption and diffusion capacity, the bending and shear properties were analyzed. Compared with the pure flax FRP, the flexural strength of G2F2s group sample was increased by 2.2 times. When exposed to the hygrothermal environment, flexural strength loss rate of the G2F2s group sample is only reduced by 4.35%, 9.00% and 11.51%, and its flexural modulus decreases 15.48%, 15.90% and 23.89%. It shown that the addition of glass fiber effectively improves the water absorption of flax fiber composites. Hybridization of flax fibers with glass fibers was a practical approach to enhance FRP laminates in hygrothermal environmental conditions after synthesizing the stacking sequence, mechanical properties and aging resistance.