Hygrothermal Conditions Research Articles

Interaction of ultrasonic guided waves with interfacial debonding in a stiffened composite plate under variable temperature and operational conditions

Stiffeners play a vital role in strengthening thin panels in a wide range of engineering constructions by reducing additional structural weight. However, these structures are vulnerable to issues such as interlayer delamination or skin-stiffener interfacial debonding due to high stress levels developed from external environmental conditions and operational loadings. In contrast, ultrasonic-guided wave (UGW) techniques exhibit an efficient and precise approach for monitoring discontinuities or damages in composite structures. There is a lack of research on understanding the characteristics of the interaction between UGW and interfacial debonding when in-plane edge loading and environmental factors are simultaneously taken into account. Therefore, this study is motivated by the need to develop a multiphysics numerical model which employs a commercially available finite element software, COMSOL Multiphysics®, to simulate UGW propagation in a stiffened composite plate with debonding at the plate-stiffener interface through a piezoelectric transducer under the combined influence of in-plane edge load and hygrothermal environment. The stiffened plate and piezoelectric patches are modelled with the tetrahedral element, and the bottom surface of the attached stiffener has a through-width 0.1 mm deep groove simulated for debonding. The developed FE model is validated against the results of the conducted experiments and those found in the available literature through the correlation coefficient. Further, the study conducts a comprehensive parametric investigation on stiffened cross-ply (0/90/0) laminated plates, considering variations in debonding size, in-plane load, and hygrothermal load intensity through the excitation of A0 mode. The acquired response is processed to compare the peak amplitude of various modes and energy of the waveform. Additionally, statistical indices such as normalised correlation moment (NCM) and variance of the continuous wavelet transform (CWT) peak are estimated to understand the impact of various parameters on waveform. The results show that the presence of a 90° lamina in the cross-ply laminate generates a low amplitude S0 mode in the scattered response. Moreover, a mode conversion from A0 to S0 mode is observed due to perfect bonding between the plate and the stiffener, providing insights into the bonding state in the panel. Furthermore, it is found that the magnitude of the in-plane loading marginally affects the peak amplitude of various modes in the scattered response. Additionally, when temperature intensity rises, the energy and amplitude of the UGW signals acquired through piezoelectric patches positioned in a direct line with the actuator gradually increase. The NCM value enhances with debonding regardless of exposed hygrothermal condition and reduces with increasing temperature intensity. In addition, the variance of the CWT peak reduces with debonding. The findings of this research are expected to be helpful for the development of efficient algorithms for detecting damages for structural health monitoring of stiffened composite panels.

Moisture effect on tensile and low‐velocity impact tests of flax fabric‐reinforced PLA biocomposite

AbstractThis study investigates the hygrothermal aging effect on the tensile and impact behavior of flax/PLA biocomposites. Specimens underwent up to four weeks of conditioning at 40°C in a climate chamber with water. Analysis covered porosity, moisture diffusibility, and transversal microstructure, enabling assessment of tensile strength, tensile modulus, and impact performance in relation to moisture uptake and fiber orientation. The study of tensile properties revealed that at approximately 12% moisture content, stiffness and yield stress decrease, while strength remains constant. Moisture diffusivity is higher in warp and weft yarn directions than the out‐of‐plane direction. Tensile testing at environmental equilibrium moisture reveals greater stiffness in the weft direction, correlated with lower crimp percentage and yarn angle. The main contribution of the paper is the study of the influence of moisture on the impact behavior, the results show that energy absorption capability of flax/PLA biocomposite increases with moisture content.Highlights Fully biodegradable composite material by heat‐compression molding subjected to hygrothermal aging conditions for up to four weeks. The moisture diffusivity in both the warp and weft yarn directions registered higher values in comparison to the out‐of‐plane direction. Tensile testing at environmental equilibrium moisture revealed that the stiffness in the weft direction presented higher values. At 12% of moisture uptake, the stiffness and yield stress reached their lowest values, while strength remained constant. However, the low‐velocity impact properties of the composites exhibited improvement with moisture.

An Experimental Investigation of the Mechanism of Hygrothermal Aging and Low-Velocity Impact Performance of Resin Matrix Composites.

Resin matrix composites (RCs) have better thermal and chemical stability, so they are widely used in engineering fields. In this study, the aging process and mechanism of two different types of resin-based three-dimensional four-way braided composites (H15 and S15) under different hygrothermal aging conditions were studied. The effect of aging behavior on the mechanical properties of RCs was also studied. Three different aging conditions were studied: Case I, 40 °C Soak; Case II, 70 °C Soak; and Case III, 70 °C-85% relative humidity (RH). It was found that the hygroscopic behavior of RCs in the process of moisture-heat aging conforms to Fick's second law. Higher temperatures and humidity lead to higher water absorption. The equilibrium hygroscopic content of H15 was 1.46% (Case II), and that of S15 was 2.51% (Case II). FT-IR revealed the different hygroscopic mechanisms of H15 and S15 in terms of aging behavior. On the whole, the infiltration behavior of water molecules is mainly exhibited in the process of wet and thermal aging. At the same time, the effect of the aging process on resin matrices was observed using SEM. It was found that the aging process led to the formation of microchannels on the substrate surface of S15, and the formation of these channels was the main reason for the better moisture absorption and lower mechanical strength of S15. At the same time, this study further found that temperature and oxygen content are the core influences on post-aging strength. The LVI experiment also showed that the structural changes and deterioration effects occurring after aging reduced the strength of the studied material.

A thermodynamically consistent physics-informed deep learning material model for short fiber/polymer nanocomposites

This work proposes a physics-informed deep learning (PIDL)-based constitutive model for investigating the viscoelastic–viscoplastic behavior of short fiber-reinforced nanoparticle-filled epoxies under various ambient conditions. The deep-learning model is trained to enforce thermodynamic principles, leading to a thermodynamically consistent constitutive model. To accomplish this, a long short-term memory network is combined with a feed-forward neural network to predict internal variables required for characterizing the internal dissipation of the nanocomposite materials. In addition, another feed-forward neural network is used to indicate the free-energy function, which enables defining the thermodynamic state of the entire system. The PIDL model is initially developed for the three-dimensional case by generating synthetic data from a classical constitutive model. The model is then trained by extracting the data directly from cyclic loading–unloading experimental tests. Numerical examples show that the PIDL model can accurately predict the mechanical behavior of epoxy-based nanocomposites for different volume fractions of fibers and nanoparticles under various hygrothermal conditions.

Optimizing ventilated packaging design for strawberries: Assessing the impact on fruit quality from farm to retailer using physics-based modeling

Amongst fruits and vegetables, strawberries (Fragaria × ananassa) are one of the most perishable products. Consequently, it is challenging to maintain their quality until they reach the consumer. Therefore, keeping optimal hygrothermal conditions around the berries from farm to the consumer is crucial to preserving them and avoiding food loss. Using ventilated packaging design is an established way to address this technological, economic, and environmental problem. Until now, strawberry packaging is often designed using a trial-and-error approach, and a multitude of packaging designs exist. Ideally, the packaging should be designed in such a way that it provides the best preservation of the fruit all the way from farm to retailer. This is challenging since the hygrothermal conditions change significantly along the supply chain. In this study, we use physics-based modeling to evaluate new strawberry packaging designs in regard to cooling performance, risk of condensation, mass loss, and respiration-driven shelf life of the fruit and assess their performance along the whole post-harvest export supply chain from Spain to Switzerland. Twelve packaging designs for strawberries with existing and newly-designed vent hole configurations were investigated. Simulations quantified how the berries physiologically and hygrothermally evolve inside these packages along the whole post-harvest supply chain. Implementing appropriate ventilation in open-and top-sealed trays for strawberries can help to minimize losses along the post-harvest supply chain. As commonly known, strawberries cool significantly faster in better-ventilated trays than in unventilated trays, and condensation-based time of wetness could be reduced by 45% by adding additional ventilation holes in a top-sealed tray. The height of the secondary packaging, the total opening area (TOA), the size, and the placement of the vents have a substantial influence on the performance of the packaging. We recommend a TOA between 5.5% and 7% and a diameter of the vents between 5 and 12 mm. Also, they should be evenly distributed. However, there is always a trade-off in packaging design: a reduced risk of condensation comes along with a higher mass loss of the strawberries. Strawberry packaging models enabled a unique insight into the evolution of the fruits along the supply chain and visualized the spatial distribution of condensation and mass loss inside the package.

Frequency Analysis of Asymmetric Circular Organic Solar Cells Embedded in an Elastic Medium under Hygrothermal Conditions

This research represents the first theoretical investigation about the vibration behavior of circular organic solar cells. Therefore, the vibration response of asymmetric circular organic solar cells that represent a perfect renewable energy source is demonstrated. For this purpose, the differential quadrature method (DQM) is employed. The organic solar cell is modeled as a laminated plate consisting of five layers of Al, P3HT:PCBM, PEDOT:PSS, ITO, and Glass. This cell is rested on a Winkler–Pasternak elastic foundation and assumed to be exposed to various types of hygrothermal loadings. There are three different kinds of temperature and moisture variations that are taken into account: uniform, linear, and nonlinear distribution throughout the cell’s thickness. The displacement field is presented based on a new inverse hyperbolic shear deformation theory considering only two unknowns. The motion equations including hygrothermal effect and plate–foundation interaction are established within the framework of Hamilton’s principle. The DQM is utilized to solve these equations. In order to ensure the accuracy of the proposed theory, the present results are compared with those reported by other higher-order theories. A comprehensive parametric illustration is conducted on the impacts of different parameters involving the geometrical configuration, elastic foundation parameters, temperature, and moisture concentration on the deduced eigenfrequency of the circular organic solar cells.

Molecular dynamics supported thermal-moisture aging effects on properties of silicone rubber

Silicone rubber materials are widely used as protective coatings in multiple fields. In this work, accelerated artificial aging tests of silicone rubber under different thermal and hygrothermal conditions were performed to better understand how temperature and humidity affected aging behavior and mechanism. It was noticed that due to the presence of oxygen molecules, the main chain of silicone rubber broke and produced free radicals, while water molecules brought hydrolysis reactions and produced silicone hydroxyl structures. Furthermore, computer aided molecular simulation was carried out to better understand the diffusion of small particles in silicone rubber materials, as well as to study how they affect the chain structure, providing support for the mechanism of aging. Finally, two lifespan prediction models were proposed respectively to determine the usage life of silicone rubber under certain temperature and humidity conditions, which provides guidance value for the use of silicone rubber.

Response of Moisture and Temperature Diffusivity on an Orthotropic Hygro-thermo-piezo-elastic Medium

This research explores the complex interaction between piezoelectric waves and heat-moisture diffusion within a semi-infinite piezoelectric material under hygro-thermal conditions. By employing a two-dimensional Cartesian framework, novel governing equations for a thermo-piezoelectrically orthotropic medium influenced by moisture effects are developed. Accurate representations for key parameters are obtained by utilizing normal mode analysis. The investigation examines the influence of critical factors like moisture content, diffusivity, and temperature diffusivity on the spatial distribution of various physical fields. Additionally, a particular scenario of significance is highlighted. These results have the potential to improve sensor, actuator, and energy-harvesting device performance and dependability.

The effects of temperature and hygrothermal aging on the properties of CF/PA6 composite and its compression failure mechanisms

AbstractCarbon fiber reinforced polyamide composites have outstanding properties, but their applications are significantly affected by the service environment. This work evaluates the effects of temperature (25, 80, 140, and 200°C) and hygrothermal conditions on the mechanical properties of CF/PA6 composite containing delamination damage, and studies its damage behaviors and failure mechanisms. After hygrothermal aging, recrystallization of PA6 improves the heat resistance of CF/PA6 composites, but water absorption reduces the interfacial strength of composites, resulting in the decrease of modulus and compressive strength (24% and 42%, respectively). With the increase in temperature, the compressive strength of CF/PA6 composites gradually decreases (from 132 MPa at 25°C to 24 MPa at 200°C), and the failure mode transitions from fiber fractures, interlayer shear fractures, delamination to kinking. In addition, the compressive failure process of CF/PA6 composites is divided into three stages: buckling, delamination bulging and kinking failure.Highlights Hygrothermal aging reduces the fiber/resin interface performance. Fiber fracture, interlayer shear and delamination convert to kinking failure. The failure process is divided into buckling, delamination and kinking.

Numerical investigation on mechanical behavior of 3D orthogonal woven composites in hygrothermal environments

3D orthogonal woven composites (3DOWCs) are increasingly utilized in various engineering applications due to their superior mechanical properties. However, their mechanical properties and damage mechanisms could be significantly influenced by the environmental factors such as temperature and moisture. This study aims to investigate the mechanical behavior of 3DOWCs under hygrothermal conditions through meso-scale finite element (FE) simulations. The proposed FE model is validated using existing experimental results under standard temperature and humidity conditions, and subsequently applied to predict the performance in hygrothermal environments. The effects of temperature, humidity, and loading conditions on the mechanical properties of 3DOWCs are analyzed and the corresponding failure mechanisms are revealed. This study offers significant insights into the hygrothermal behavior of 3DOWCs, thereby facilitating their optimal design and implementation in various engineering applications.

Study on the preparation of an epoxy-based electromagnetic wave-absorbing adhesive film and its hygrothermal aging

With the wide application of structural adhesives in the aerospace field, the structural adhesive films with only bonding property have not met the current needs. The structural-functional integrated adhesive films have become the current study hotspot. In this paper, an epoxy-based electromagnetic wave-absorbing adhesive film was prepared by adding the magnetite/SWCNTs as the absorbing agent to the epoxy adhesive. And then, it was subjected to hygrothermal aging for 1500h at 70°C and 90% RH. The results show that the absorbing filler was uniformly dispersed in the epoxy resin matrix. The reflection loss (RL) peak of the magnetite/SWCNTs/epoxy adhesive film reached −27.5 dB at the frequency of 8.75 GHz when the absorbing agent content was 20 wt%. The effective bandwidth was 3 GHz with 25 wt% absorbent content. Moreover, after 1000h of hygrothermal aging, the lap shear strength of the magnetite/SWCNTs/epoxy adhesive film decreased by 19.6%, indicating that it can perform excellent service under hygrothermal conditions.

Probing Interfacial Aging of Model Adhesion Joints under a Hygrothermal Environment at a Molecular Level.

Generally, for adhesive joints, the polar water molecules in humid environments can have a critical effect on the interfacial structures and structural evolution adjacent to the solid substrates. Regarding this, it is still a big challenge to detect and understand the interfacial hygrothermal aging process at the molecular level in real time and in situ. In this study, to trace the interfacial hygrothermal aging process of a classical epoxy formula containing diglycidyl ether of biphenyl A (DGEBA) and 2,2'-(ethylenedioxy) diethylamine (EDDA) with sapphire and fused silica in a typical hygrothermal environment (85 °C and 85% RH), sum frequency generation (SFG) vibrational spectroscopy was used to probe the molecular-level interfacial structural change over the time. The structural evolution dynamics at the buried epoxy/sapphire and epoxy/silica interfaces upon hygrothermal aging were revealed directly in situ. The interfacial delamination during hygrothermal aging was also elucidated from the molecular level. Upon hygrothermal aging, the interfacial CH signals, such as the ones from methyl, methylene, and phenyl groups, decreased significantly and the water OH signals increased substantially, indicating the water molecules had diffused into the interfaces and destroyed the original interactions between the epoxy formula and the substrates. Further analysis indicates that when the integrated signals in the CH range declined to their minimum and leveled off, the interfacial delamination happened. The tensile experiment proved the validity of these spectroscopic experimental results. Our study provides first-hand and molecular-level evidence on a direct correlation between the diffusion of the surrounding water molecules into the interface and the evolution/destruction of the interfacial structures during hygrothermal aging. More importantly, it is proved, SFG can be developed into a powerful tool to noninvasively reveal the local interfacial delamination in real time and in situ under extreme hygrothermal conditions, complemented by the mechanic test.

Modeling study on the mechanical performance of CFRP/Al single-lap rivet joints under hygrothermal environmental conditions

Carbon Fiber Reinforced Polymers (CFRP) and aluminum alloys, owing to their exceptional mechanical properties and lightweight attributes, are extensively used in aircraft manufacturing. However, prolonged exposure to a hygrothermal environment can compromise the mechanical integrity of composite joint structures. In this paper, accelerated aging experiments were designed to test the mechanical properties of CFRP after hygrothermal aging, a novel mechanical property prediction model tailored for the hygrothermal coupled environment is presented. The model accurately predicts the modulus and strength of CFRP after hydrothermal aging. A three-dimensional finite element model for the CFRP interference riveted structure, considering a tri-coupled state of moisture, temperature, and force was established by the application of subroutine and field superposition. The precision of this finite element model has been affirmed through accelerated aging tests. By integrating finite element analysis with experimental methods, this research delves into the failure modes and mechanisms of CFRP and its joints under hygrothermal conditions. It was discerned that the hygrothermal environment undermines the bond between fibers and the matrix, resulting in pronounced interlaminar delamination and shear failure in CFRP. The failure forms gradually change from “flaky” at lower levels of aging to “filamentary” at higher levels of aging. For CFRP riveted structures, the hygrothermal conditions influence their loadbearing capability and shift the primary positions of failure.

Antifungal Susceptibility Assessment of Innovative and Non-Conventional Lime Mortars Incorporating Almond-Shell Powder Bio-Waste Subjected to Particle-Dispersion Technique.

A favorable environment for fungi colonization in building materials' surfaces can emerge when certain hygrothermal conditions occur. Thus, reducing fungal growth susceptibility is of major interest. Furthermore, if the integration of bio-wastes is performed in parallel with the development of innovative materials for this purpose, a more sustainable and environmentally friendly material can be obtained. In this study, the fungal susceptibility of lime mortars incorporating almond-shell powder (ASP) microparticles (2 and 4%, wt.-wt. in relation to the binder content) was evaluated. The particle-dispersion technique was employed to prepare the bio-waste introduced in the mixtures. The fungal susceptibility of ASP samples was compared with nanotitania (n-TiO2) with recognized antifungal properties. Mechanical strength, water absorption, and wettability tests were also performed for a better characterization of the composites. Although the addition of 2% ASP led to mechanical properties reduction, an increase in the compressive and flexural strength resulted for 4% of the ASP content. Difficulties in fungal growth were observed for the samples incorporating ASP. No fungal development was detected in the mortar with 2% of ASP, which may be correlated with an increase in the surface hydrophobic behavior. Furthermore, mortars with ASP revealed a reduction in water absorption by capillarity ability, especially with 4% content, suggesting changes in the microstructure and pore characteristics. The results also demonstrated that an improvement in the physical and mechanical properties of the lime mortars can be achieved when ASP microparticles are previously subjected to dispersion techniques.

Hygrothermal progressive damage in open-hole compression of composite laminates with aligned carbon nanotube interlaminar reinforcement studied by X-ray micro-computed tomography

Hierarchical nanoengineered composite laminates were created with through-thickness-aligned carbon nanotube (A-CNT) reinforcement of unidirectional carbon (micro) fiber/epoxy laminae in their failure-prone interlaminar regions, forming a multiscale “nanostitched” architecture. Motivated by previous mechanical enhancements for similar nanostitched laminates in ambient environments, hygrothermal effects on the 3D strengthening and toughening mechanisms of nanostitched laminates are investigated here for the first time, focusing on standard open-hole compression (OHC) testing of quasi-isotropic laminates. We report that the hygrothermal conditions of −55 °C/DRY and 100 °C/DRY correspond to ultimate strength increases of 2.6% and 5.9%, respectively, for nanostitched laminates over the baseline; and, room temperature/50% relative humidity (RT/50% RH) and 100 °C/wet conditions exhibit no change in ultimate strength, suggesting correlation of positive nanostitch effects with increased polymer brittleness. Subsequently, damage progression in step-wise interrupted OHC loading for selected hygrothermal conditions (RT/50% RH and 100 °C/DRY) was performed in coordination with high-resolution ex situ X-ray micro-computed tomography for various load steps up to 98% of failure load. No clear differences due to nanostitch in the damage progression are found up to 98% of ultimate strength, suggesting that the reinforcement effect is associated with interlaminar strength at ultimate load, and not pre-ultimate damage. This study contributes first insights from high-resolution experimental mapping of nanostitched composite progressive damage subjected to hygrothermal conditions that may guide and inform mechanical enhancement approaches and improved progressive damage models. Future work is suggested to expand the environmental conditions considered, as well as visualize progressive damage nearer to failure to identify nanostitch effects on complex multimodal failure.

Influences of hygrothermal conditions and structure parameters on moisture diffusion behavior in a system‐in‐package module by moisture-thermal-mechanical-coupled finite element modeling

Purpose This paper aims to investigate the moisture diffusion behavior in a system-in-package module systematically by moisture-thermalmechanical-coupled finite element modeling with different structure parameters under increasingly harsh environment. Design/methodology/approach A finite element model for a system-in-package module was built with moisture-thermal-mechanical-coupled effects to study the subsequences of hygrothermal conditions. Findings It was found in this paper that the moisture diffusion path was mainly dominated by hygrothermal conditions, though structure parameters can affect the moisture distribution. At lower temperatures (30°C~85°C), the direction of moisture diffusion was from the periphery to the center of the module, which was commonly found in simulations and literatures. However, at relatively higher temperatures (125°C~220°C), the diffusion was from printed circuit board (PCB) to EMC due to the concentration gradient from PCB to EMC across the EMC/PCB interface. It was also found that there exists a critical thickness for EMC and PCB during the moisture diffusion. When the thickness of EMC or PCB increased to a certain value, the diffusion of moisture reached a stable state, and the concentration on the die surface in the packaging module hardly changed. A quantified correlation between the moisture diffusion coefficient and the critical thickness was then proposed for structure parameter optimization in the design of system-in-package module. Originality/value The different moisture diffusion behaviors at low and high temperatures have seldom been reported before. This work can facilitate the understanding of moisture diffusion within a package and offer some methods about minimizing its effect by design optimization.

Effects of tree density variations on outdoor heritage conservation: Numerical study of an ancient brick city wall with four orientations

Trees possess the potential to safeguard outdoor cultural heritage through the regulation of microclimates in their surroundings. The Leaf Area Index (LAI), a collective measure of the foliar portion within vegetation structure, reflects the regulatory ability of trees. In this study, numerical simulations were performed to compare the hygrothermal conditions of the Nanjing City Wall facing four orientations under the influence of evergreen trees with four LAI values, while also assessing the potential risks of salt weathering and algal growth. The results were as follows: 1) under the protective canopy of trees with identical LAI, the west and east walls experienced the highest temperature fluctuations, followed by the south and north sides. Wall surface temperature fluctuation was negatively correlated with LAI; 2) except for LAI = 1.5, the north wall had the least water content fluctuation under equivalent LAI levels. Water content fluctuation generally exhibited a decreasing trend with rising LAI; 3) at the same LAI level, the south wall presented the highest risk of salt weathering (including efflorescence and subflorescence), whereas the north wall faced the lowest risk. The risk of salt weathering decreased with an increase in LAI; 4) When shielded by trees with identical LAI, the north wall exhibited the greatest risk of algal growth, while the east wall faced least the risk. Higher LAI values were associated with an increased risk of algal growth on the wall surface. These results could provide a theoretical basis for large outdoor immovable heritage conservation and plant management around heritage sites.

Energy absorption and dissipation characteristic of FG honeycomb reinforced laminate embedded with viscoelastic material in hygrothermal conditions under low-velocity impact

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

Durability of hybrid flax fibre-reinforced epoxy composites with graphene in hygrothermal environment

Natural fibre composites are highly sensitive to the hygrothermal environment (humidity and elevated in-service temperature). Enhancing the long-term behaviour of such composites can be achieved through additive manufacturing using nanomaterials as a constituent. Thus, this study investigated the mechanical properties of hybrid flax fibre-reinforced epoxy composites with 0%, 0.5%, 1% and 1.5% of graphene nanoparticles after exposure to a relative humidity of 98% for 1000, 2000, and 3000 hours and at temperature of 20°C, 40°C, and 60°C. The degradation behaviour of hybrid natural fibre composites was then evaluated by flexural and inter-laminar shear tests. Hygrothermal conditioning simulation of these hybrid composites was then performed using Arrhenius model based on accelerated aging data. The results of this study showed that graphene nanoparticles played a significant role in the reduction of moisture absorption and in the improvement of mechanical properties after hygrothermal conditioning. Compared to the specimens without graphene nanoparticles, the flexural and interlaminar shear strength in hybrid composites with 0.5%, 1.0%, and 1.5% graphene increased by 77.7%, 72.0%, and 77.1%, respectively, and up to 75.5%, 70.6%, and 73.5%, respectively after exposure for 3000 hours at 40°C. However, the degradation of the hybrid composites increased with the increase of conditioning temperature and exposure duration due to the moisture diffusion into the flax fibres and resin plasticisation. Nevertheless, the 0.5% graphene nanoparticles were found optimal in retaining the mechanical properties of aged hybrid composites due to their better distribution within the matrix. Accelerated test results showed that the hybrid composites can retain at least 57% and 49% of its flexural and interlaminar shear strength, respectively, after 100 years in service in hygrothermal environment at a temperature of 30°C representing the average annual temperature in Australia.

Timber-framed exterior walls insulated with wood shavings: A field study in a nordic climate

Wood processing residues provide a possibility to produce thermal insulation material with low environmental impact. This paper deals with the hygrothermal performance of timber-framed exterior walls with wood shavings as insulation material. The presented study consisted of field measurements of five exterior wall structures with two different types of wood shavings insulation and three reference walls. The study was performed in cold Nordic climate, where hygrothermal conditions at outer parts of a wall structure are critical. Untreated wood shavings and wood shavings coated with pulverized clay were compared as insulation materials. Different wind barriers as well as a water vapour barrier and sheathing acting as vapour retarder were also used. Results indicated no suitable conditions for mould growth inside any wall. The thermal resistance of wind barriers as well as the clay-coating of wood shavings had positive effect on the hygrothermal performance of the test walls. Wood shavings proved to be suitable insulation materials for well-insulated structures from the hygrothermal point of view, when the overall design, construction and maintenance are carried out in a proper way.