Hygrothermal Conditions Research Articles

The Species Richness-Environment Relationship for Cherries (Prunus subgenus Cerasus) across the Northern Hemisphere

Understanding large-scale patterns of biodiversity and their drivers remains significant in biogeography. Cherries species (Prunus subgenus Cerasus, Rosaceae) are economically and ecologically important in ecosystems and human agricultural activities. However, the mechanisms underlying the patterns of the species richness–environment relationship in Cerasus remain poorly understood. We collected and filtered worldwide specimen data to map the species richness of Cerasus at the global scale. The map of Cerasus species richness was created using 21,043 reliable recorded specimens. The center of Cerasus diversity was determined using spatial cluster analysis. Stepwise regression analysis was carried out using five groups of 21 environmental variables and an integrated model was included to assess the impact of the overall environment. We calibrated each of the four integrated models and used them to predict the global Cerasus species richness and that of the other continents. Our results revealed that Cerasus species have two centers of diversity (the southwest of China and Honshu Island in Japan) with differing environmental variables influencing the distribution patterns of these two centers. In the southwest of China, hygrothermal conditions are the main driving factor while in Japan, habitat heterogeneity is the main driving factor. The relationship between the abundance of Cerasus and the various groups of factors generally supports both the productivity and the habitat heterogeneity hypotheses. However, these hypotheses do not fully explain the Cerasus species richness pattern, indicating that other factors such as historical environment, topography, and human activities likely played a role in pattern formation. The high level of habitat heterogeneity and better hygrothermal conditions may have played an important role in the establishment of its globally consistent richness–climate relationship. Our results can provide valuable information for the classification and conservation of Cerasus natural resources as well as contribute to furthering our understanding of biogeography at a global scale.

Study of mechanical degradation of freestanding ALD Al2O3 by a hygrothermal environment and a facile protective method for environmentally stable Al2O3: toward highly reliable wearable OLEDs.

Al2O3 deposited via atomic layer deposition (ALD) has been used as an insulating and barrier film for thin-film transistors, organic electronics, and microelectromechanical systems. However, ALD Al2O3 films are easily degraded by hydrolysis under harsh hygrothermal conditions, owing to their poor environmental stability. In this study, the mechanical properties and water-vapor transmission rate (WVTR) of environmentally degraded Al2O3 films were investigated by varying the temperature and relative humidity (RH). The hygrothermal environment led to surface and pinhole-concentrated degradation based on aluminum hydroxide, which caused an increased WVTR and reduced elongation of the films in harsher environments. In particular, the elongation of the degraded Al2O3 films was reduced to 0.3%, which is one-third of that of as-deposited Al2O3, and their WVTR increased on the order of 10-1 g m-2 day-1, which is more than 1000 times that of as-deposited Al2O3. Therefore, we introduced a functional silane-based inorganic-organic hybrid layer (silamer) onto the Al2O3 films to improve their environmental stability. The silamer helped preserve the characteristics of Al2O3 films by forming a strong and continuous aluminate phase of Al-O-Si at their interface in hygrothermal environments. Furthermore, the silamer-capped Al2O3 was shown to be an environmentally stable encapsulation for application in wearable organic devices.

Effect of ambient hygrothermal conditions on wood moisture content in the crawl spaces of wooden houses with insulated and airtight foundations as well as conventional foundations in Hokkaido and Akita

わが国の木造住宅の床下環境は構法の変遷とともに変化しており，現在は床下土壌現しよりも床下防湿コンクリート打ちが主流で，寒冷地を中心に床下換気孔を持たない基礎断熱工法も広まってきている。そこで本研究では，居住者のある住宅を対象に，至近に建つ4棟の木造住宅の床下温湿度を1年間実測する調査，新しい基礎仕様である基礎断熱の木造住宅１棟の床下等の温湿度および木材含水率を17年間の長期にわたり実測する調査の2つを行い，異なる基礎仕様が土台等の床下木材に与える影響を検討した。その結果，基礎断熱の住宅の床下環境はいずれも年間を通して相対湿度40～80％の低湿に保たれており，床下木材が高含水率となる危険性はなかった。一方，床下換気孔を持つ在来型の基礎の場合は夏季に相対湿度が80％を超える高湿度期間が確認され，床下木材が高含水率となる懸念があった。さらに床下防湿コンクリート打ちの住宅は床下土壌現しの住宅よりも高湿度の影響が大きく，これは床下が低温となることに起因することが分かった。

Flexural behaviour of wood beams strengthened by flax-glass hybrid FRP subjected to hygrothermal and weathering exposures

This paper addressed the flexural behaviour (i.e., maximum load, ductility index, flexural strength and stiffness) of laminated veneer lumber (LVL) beams strengthened by 2-layer flax-glass hybrid fibre reinforced polymer (HFRP) exposed to hygrothermal (50℃ and 95% RH) and weathering (cyclic water spray-ultraviolet radiation) conditions for one month. Effects of environmental exposure and fabric sequences on flexural behaviour of LVL-HFRP beams were studied. LVL-FG and LVL-GF beams (inner plies bonded to LVL were flax and glass fibres, respectively) were tested under four-point bending. Tensile test on HFRP flat coupons was also conducted to provide the tensile parameters of HFRP in LVL-HFRP beams. This study showed that the hygrothermal and weathering exposures reduced the maximum load (by 11.4–32.2%), flexural strength (by 19.1–40.0%) and increased the ductility index (by 18.3–41.7%) of LVL-HFRP beams. The hygrothermal exposure did not affect flexural stiffness. The fabric sequence affected the flexural behaviour of LVL-HFRP beams. LVL-GF beams showed lower maximum load and flexural strength than those of the LVL-FG beams, which was attributed to the HFRP delamination in the LVL-GF beams. A theoretical prediction model for the ultimate bending moments of LVL-HFRP beams was used, and the calculated values showed a good agreement with the experiments.

Modelling and validation of hygrothermal conditions in the air gap behind wood cladding and BIPV in the building envelope

Materials used in the building envelope are exposed to a wide range of varying and harsh conditions over extended periods. Knowledge about these precise conditions allows for improving the design of testing schemes and eventually extending the durability of building materials. In this study, a numerical model in WUFI-Pro Ver. 6.5 is calibrated with field measurements in the ventilated air gap of a Zero Emission Building located in Trondheim, Norway. Measurements were taken from September 01, 2020 until August 31, 2022 and involved recording the surface temperature of the wind barrier (19 locations) and the relative humidity of the air (11 locations) in the middle of the air gap behind wood cladding and building integrated photovoltaics. Several different air change rates in the air gap are investigated in the model. Using a constant air change rate of 100 h−1 showed the overall best performance (R2 = 0.940 for the wind barrier's surface temperatures and R2 = 0.806 for the relative humidity of air in the middle of the air gap). The largest deviations between simulation results and measurements, however, can be attributed to the uncertainty of climate data input. The developed model can be used in future studies that significantly contribute to establishing better testing schemes and test conditions for building materials such as wind barriers and adhesive tapes, and eventually improve the long-term air tightness of buildings.

A sequential finite element model updating routine to identify creep parameters for filament wound composite cylinders in aggressive environments

In this paper, a Finite Element Model Updating (FEMU) procedure is developed to find the best creep parameters for filament-wound cylinders under radial compression in harsh environmental conditions. Three winding angles are considered, each under three different hygrothermal conditions. The two–stage creep model captures i) primary creep through a time–hardening approach whilst ii) secondary creep is captured by Norton’s law. Given the high number of parameters in this two–stage creep model and the complexity of determining them experimentally, the FEMU routine utilises an optimisation scheme that sequentially couples a Genetic Algorithm (GA) with a gradient-based (GB) Levenberq-Marquardt Algorithm (LMA) to find all required creep input parameters to feed the model that best simulates experimental results. This framework finds the global optimum through an initial screening of the optimum area within the design space with GA, clearing the path to allow the GB algorithm to find the global optimum, substantially reducing the chance or even avoiding falling in local minima. The global search is driven by experimental data of cylinders loaded in radial compression under aggressive environments. The numerical results show excellent agreement with experimental results with reasonably low computational efforts.

Effects of recycling and hygrothermal environment on mechanical properties of thermoplastic composites

Recyclability of nine types of composites including natural and mineral fillers in polypropylene and nylon matrix have been studied through injection molding and mechanical grinding. No significant degradation in mechanical properties of natural fiber composites occurred after recycling or water absorption tests. In contrast, glass fiber composites showed decrease in tensile strength and modulus by up to 45% and 40%, respectively after five rounds of recycling mainly due to severe fiber attrition. Glass bubbles breakage occurred after recycling, which diminished its ability to provide lightweighting. Only glass fiber reinforced nylon composites were affected by hygrothermal conditioning, which were characterized as a function of recycling. At saturation point, water absorption for nylon composites was 7.7% by wt. after 45 days of immersion, which significantly affected the mechanical properties. Tensile strength and modulus of glass fiber nylon composites reduced from 88.4 MPa and 5.6 GPa to 36.2 MPa and 1.8 GPa, respectively.

Hygrothermal conditions for the aging of red wine from experimental data

Despite the importance of the aging process, there is great disparity in the hygrothermal conditions in which to carry it out. The aim of this work is to propose contrasted data, based on successful cases and millions of experimental data, which can be taken as a reference for the climate control of aging rooms. For this purpose, the hygrothermal environment of 19 red wine aging rooms with multiple construction designs was monitored and statistically characterized. The results showed a great variability in the hygrothermal environment. Frequency histograms and psychometric diagrams revealed the true hygrothermal environment of red wine aging rooms. There were large differences in stability and uniformity depending on the construction design (up to four times). The indoor hygrothermal environment was not kept within the comfort ranges mentioned by a wide list of authors (2–65% of time out of range). A contrasted psychometric diagram establishing monthly comfort intervals and other useful data (stability, uniformity …) were proposed, based on the hygrothermal environment of wineries with quality wines. The results should be a valuable tool for the wine sector, in order to control the evolution of aging, set the programming of air conditioning equipment and simulate the behavior of new wineries.

Numerical Study of the Hygrothermal Effects on Low Velocity Impact Induced Indentation and Its Rebound in Composite Laminate

Impact indentation is believed to be an effective indication of low-velocity impact (LVI) damage for polymer matrix composites. However, it has been discovered that an indentation can partially rebound over time. Impact indentation and its rebound behavior over a period of time are significantly affected by hygrothermal conditions, especially moisture absorption. Therefore, a good understanding of the moisture-dependent impact indentation and its rebound behavior is helpful for impact damage assessment for composites. In this paper, moisture effects are considered for both the intra-laminar transverse property model and the interlaminar interface model in the simulation of impact indentation. Then, in these two models, viscosities are introduced to represent the indentation rebound over time. In order to validate the proposed models, LVI experiments with different impact energies were conducted on dry and hygrothermal conditioned carbon fiber/epoxy matrix composite laminates. For the specimens, the initial depths of impact dents and their rebounds over time were measured. The specimens of hygrothermal conditions were found with deeper dents compared with dry ones under the same impact energy; and their rebounds were also more significant. These phenomena were explained by the fact that moisture softens epoxy in composite and meanwhile elevates its viscosity. This indentation and its rebound phenomenon were simulated in ABAQUS by considering the moisture effects and viscoelasticity with user-defined material subroutines. These experiments were simulated using the proposed models, and the numerical predictions conformed well with the experimental observations.

The effect of hygrothermal aging on the adhesion performance of nanomaterial reinforced aluminium adhesive joints

Adhesive joints are subjected to various environmental factors such as humidity and temperature, depending on their use in industrial applications. It is important to examine and improve the performance of adhesive joints formed with polymeric adhesives, which are known to be sensitive to humidity and temperature parameters, under these conditions. For this reason, in this study, FPL-etched AA7075-T6 aluminium adherends bonded with nonreinforced and BNNP (boron nitride nanopowder), ANP (alumina nanopowder) and MWCNT (multiwalled carbon nanotubes) reinforced epoxy adhesive as a single lap adhesive joint. Thereafter, performance of the adhesive joints that exposed hygrothermal aging conditions of 1, 2, 4 and 6 weeks at 100% relative humidity (RH) and 60 °C were investigated. Glass transition temperature (Tg) values of nonreinforced and reinforced epoxy adhesives before and after hygrothermal aging were determined by DSC (Differential Scanning Calorimeter) analyses. Depending on the nanomaterial types, the tendency of the adhesive joints to water uptake was determined and compared with nonreinforced adhesive joints. When the shear strength findings were investigated, it was defined that the adhesive joints reached almost saturation after 2 weeks of hygrothermal aging. In addition, the most effective nanomaterial type against hygrothermal aging was determined as MWCNT. After 2 weeks hygrothermal aged, the shear strength decreases of MWCNT reinforced adhesive joints were 21.77% and 16.97% less than BNNP and ANP reinforced adhesive joints, respectively. Although the adhesive deteriorates chemically after hygrothermal aging, its adhesion performance improved with the physical benefits of nanomaterials. However, ANP, which is cheaper in terms of price/shear strength performance in long-term aging times, showed similar results with MWCNT. In addition, fracture mechanisms were examined in more detail with SEM (Scanning Electron Microscope) analyses before and after aging, and the effects of hygrothermal aging on the adhesive joints were determined.

Wave Propagation Analysis of Functionally Graded Graphene-Reinforced Piezoelectric Sandwich Nanoplates via Nonlocal Strain Gradient Theory

This article elaborates on the dispersion of waves in piezoelectric sandwich nanoplates resting on a viscoelastic foundation. The nanoplate comprises a functionally graded (FG) graphene-reinforced composite core layer with two piezoelectric surface layers. By combining the Halpin–Tsai model and related mixture rules, the properties of the composite material have been obtained. The Euler–Lagrange equation is obtained using the third-order shear deformation theory (TSDT) and Hamilton’s principle. Subsequently, based on the nonlocal strain gradient theory (NSGT), the equation of motion is presented. Finally, the effects of scale parameters, hygrothermal conditions, graphene distribution, and viscoelastic foundation on the propagation characteristics are numerically studied. The results reveal that the scale effect is more evident when the wave number is larger. Furthermore, critical damping increases with a rise in the wavenumber and Winkler modulus.

A longitudinal study on the effects of hygro-thermal conditions and indoor air pollutants on building-related symptoms in office buildings.

We conducted a longitudinal epidemiological study for over 1year in Tokyo and Osaka, Japan, beginning June 2015, to examine the association between indoor environmental factors and building-related symptoms (BRSs) in office workers. Data were obtained from 483 subjects (225 females and 258 males) in 24 office rooms in 11 office buildings. Environmental monitoring was conducted for hygro-thermal conditions and carbon dioxide and sampling was performed for indoor air pollutants. Questionnaires were concurrently administered to collect information on participants' perceptions of their comfort and health and the conditions of the work environments. Multivariable analyses revealed that upper respiratory symptoms were significantly correlated with a decrease in both relative [odds ratio (OR): 0.77; 95% confidence intervals (CI): 0.62-0.95; p=0.014] and absolute humidity (OR: 0.89; 95% CI: 0.81-0.97; p=0.008). Statistically, significant evidence was found that average relative humidity of <38% (OR: 2.68; 95% CI: 1.36-5.27; p=0.004) showed the most significant association with increased risk of upper respiratory symptoms. Air concentrations of carbon dioxide showed no significant correlation with BRSs at mean concentrations <1000 ppm in most buildings surveyed. Most indoor air pollutant concentrations were relatively low or lower than the values set by indoor air quality guidelines and the values of thresholds for sensory irritation. Air concentrations of indoor air pollutants showed no significant correlation with BRSs. Our data emphasize the importance of appropriate humidity control during low humidity in winter.

Influence of circular cutouts and elastic foundation on vibration characteristics of porous sandwich FGM plates under hygro-thermal environments

The present article investigates the effect of cutouts, porosity inclusion, and hygro-thermal environment on the vibrational characteristics of sandwich functionally graded material (FGM) plates resting on Pasternak elastic foundation. The governing equations for the sandwich FGM plates have been developed using the finite element method (FEM) with the conjunction of non-polynomial-based higher-order shear deformation theory (HSDT). The trigonometric shear-strain shape function has been used for the first time for sandwich FGM plates. The effective material properties with porosity inclusions have been computed using modified power–law distribution. The present results have been acquired by employing a C0 continuity isoparametric finite element (FE), using 4-noded elements with 7-DOF's per node. The accuracy of the current solution has been confirmed by convergence and validation studies. The influence of circular cutout, elastic foundations, hygro-thermal conditions, and geometric and boundary conditions on the vibrational frequency of porous sandwich FGM plates have been discussed in detail.

A characteristic curve including hygrothermal effects for predicting the failure of composite multi-bolt joints

A characteristic curve that considers hygrothermal effects was developed based on the traditional characteristic curve. A nondimensional temperature was introduced to represent the severity of seven hygrothermal conditions experienced by primary aircraft structures. The tensile characteristic lengths and compressive characteristic lengths under the seven conditions were determined using a numerical strategy. Expressions between the characteristic lengths and the nondimensional temperature were established by fitting the data with exponential functions. Consequently, an equation of the characteristic curve formulated as a function of the nondimensional temperature and the angle, was established. The developed characteristic curve can be used to predict the failure of composite multi-bolt joints under any hygrothermal environment.

Experimental characterization of desiccation cracking mechanisms in clayey soils

Desiccation cracking in clayey soils is a complex phenomenon investigated by researchers over many decades. Understanding and mitigating this issue will have a significant influence on the performance of raw earth construction materials and geotechnical engineering. In the current study, the digital image correlation technique was used to provide a thorough insight about the shrinkage anisotropy and the cracking mechanisms during drying. A series of free shrinkage tests were performed on an initially saturated slurries in order to assess the effects of the intrinsic characteristics of the material, the surface tension, the sample thickness and the geometric boundary conditions on the crack initiation and propagation in clays. The results show that increasing sample thickness prevents cracks formation in a non-swelling clay and delays it in a swelling clay. A vertical uplift was observed in the swelling clay due to a concave bending of the material during the desiccation shrinkage. An important anisotropy in the vertical shrinkage was observed compared to the horizontal shrinkage. The geometric boundary conditions indirectly influence the internal drying stress that becomes anisotropic. The intrinsic characteristics have a greater effect on the shrinkage behavior of the material than the hygrothermal and geometric boundary conditions.

Effects of Moisture Diffusion on a System-in-Package Module by Moisture-Thermal-Mechanical-Coupled Finite Element Modeling.

Epoxy molding compounds (EMCs) are commonly used in electronic products for chip encapsulation, but the moisture absorption of EMC can induce significant reliability challenges. In this study, the effects of hygrothermal conditions and structure parameters on moisture diffusion and the consequent influences (such as moisture content on die surfaces and stress distribution) on a system-in-package module have been systematically investigated by moisture–thermal–mechanical-coupled modeling. Hygroscopic tests were carried out on a new commercial EMC at 60 °C/60% RH and 85 °C/85% RH, followed by evaluations of diffusion coefficients by Fick’s law. It was found that the moisture diffusion coefficients and saturation concentrations at 85 °C/85% RH were higher than those at 60 °C/60% RH. From the modeling, it was found that the consequent maximum out-of-plane deformation and stress of the module at 85 °C/85% RH were both higher than those at 60 °C/60% RH. Influences of thicknesses of EMC and PCB on the moisture diffusion behavior have also been studied for design optimization. It was found that the maximum moisture concentration on die surfaces and resultant stress increased notably with thinner PCB, whereas the effects of EMC thickness were limited. This can be attributed to the comparison between the thicknesses of EMC and PCB and the shortest existing diffusion path within the module. These findings can provide helpful insights to the design optimization of electronic modules for hygrothermal conditions.

Hygrothermal conditions for the biological aging of sherry wine

The present work analyzed the hygrothermal environment of several prestigious warehouses where sherry wines of “Fino” and “Manzanilla” type are made. Through descriptive statistics, frequency histograms and psychrometric diagrams of temperature and relative humidity variations have been obtained. In addition, other factors such as the stability and uniformity of the interior environment have been quantified. The results showed significant temperature variations throughout the year (limit values of 5 °C and 31 °C), with a more stable relative humidity (between 60% and 90% for most of the year). Indoor conditions varied much more rapidly than in wineries for the aging of red wine, as a result of constant ventilation. Therefore, the annual average stability was 2.0 ± 0.8 °C per day and 0.14 ± 0.12 °C per hour. Vertical uniformity decreased in spring and summer, with stratification values close to 0.5 °C/m and 3% r.h./m. The monitored indoor environment is outside the comfort ranges described in the literature for extended periods of time (in the most extreme case, 96% of the time outside the recommended interval). For that reason, a reference psychrometric comfort diagram was proposed. Industrial relevanceFor the first time, the hygrothermal behavior of several wineries producing high quality wines (very high scores in the oenological guides) was extensively analyzed. The results of the study, which include monthly comfort intervals, should be a very useful tool for the sherry wine industry, in order to control and ensure optimal development of the flor yeast film. It should also be taken into account for the correct design of new wineries or the rehabilitation of abandoned warehouses.

Facilitator of moisture accumulation in building envelopes and its influences on condensation and mould growth

The hygric performance of building envelopes determines the degradation progress. Multiple factors, including the climate parameters (e.g., solar radiation and wind-driven rain) and building structure (e.g., thermal bridges), significantly affect the hygrothermal conditions in building envelopes. Inappropriate thermal insulation measures may exacerbate the humidity within the building envelope and further induce condensation and mould growth, which threatens the building structural safety and indoor air quality. However, the key facilitators of moisture accumulation in building envelopes are yet to be fully determined. Herein, a well-validated coupled heat and moisture transfer (HAMT) model was adopted to investigate moisture accumulation in building envelopes. The target building was located in a Chinese city (Hangzhou) where the climate is characterized as a hot and humid summer and a cold winter. The results demonstrate that the moisture content as well as condensation and mould risk is closely related to the building wall orientation. High heat flux at the exterior surface can reduce the overall moisture content of the building envelope. Conversely, it could facilitate moisture accumulation when the building envelope is exteriorly isolated by a layer of thermal insulation material (e.g., expanded polystyrene). Subsequently, the insulation layer could increase condensation risk. Three prone periods for condensation as well as spore germination and mould growth in a year were detected, indicating that the conditions that are tendentious for the above risks are the same. The adoption of an external insulation layer could prevent mould from growing inward while increasing the condensation risk.

Mechanical and failure analysis of thick composites under hygrothermal conditions by a novel coupled hygro-thermo-mechanical multiscale algorithm

A novel coupled hygro-thermo-mechanical multiscale-APFEA (HTM-multiscale-APFEA) algorithm is developed to precisely predict the mechanical and failure behavior of thick fiber-reinforced laminated composites under hygrothermal conditions. The Fourier and Fick's equations are simultaneously solved by the proposed fully-coupled temperature-moisture concentration (FCTM) algorithm to account for their mutual effects on each other. Two subroutines, one responsible for the homogenization, failure analysis, and also linking the macro and micro models of the composite material, and another for establishing the constitutive equations and failure analysis of the cohesive/adhesive material, are integrated into a user-defined material code extended for also defining the materials failure behavior. The effects of temperature and moisture conditions on the stiffness, strength, coefficients of hygrothermal expansions (CHEs), and fracture toughness of matrix and cohesive/adhesive materials are considered. The results of the HTM-multiscale-APFEA algorithm are verified against the results of experimental data, analytical high-order theories, and reliable numerical analysis. Furthermore, the failure analysis of composite laminate under hygrothermal conditions coupled with mechanical loading (bending) is carried out by the HTM-multiscale-APFEA algorithm, and compared with experimental data. The results have shown that the transverse tensile strength, shear strength, and transverse Young's modulus of the composite material degrade respectively about 72%, 13%, and 40% in the most severe hygrothermal conditions at the material boundaries; while the normal stiffness and normal strength of the cohesive/adhesive material decline by about 73% and 47%, respectively, at outer layers. In addition, the mixed-mode fracture toughness of the adhesive material elevates from 0.55 ×103N/m at inner layers to 2.14 ×103N/m at outer layers of adhesive material. The proposed algorithm is highly flexible and can easily be adapted to model different types of laminated composites.

Effect of hygrothermal condition on single-lab shear behavior of induction-welded CF/PEKK thermoplastic composites

This study analyzed the effects of hygrothermal conditions on the single-lap shear strength of a carbon fiber/poly-ether-ketone-ketone (CF/PEKK) thermoplastic composite material fabricated by induction welding. Specimens were exposed to an 85 °C/85% environment using a temperature and humidity chamber to identify the effect of moisture on single-lap shear strength, while their moisture saturation was assessed through weight measurement. Single-lap shear strength tests were performed on the dried and saturated specimens at 25 °C and 100 °C to 180 °C at 20 °C intervals. At 160 °C, the strength of the CF/PEKK thermoplastic composites rapidly declined to 76% (dried specimens) and 78% (moist specimens). The fracture surfaces and failure modes were analyzed using scanning electron microscopy images, which confirmed an increase in the degraded areas and naked fibers at higher testing temperatures. In addition, it was found that exposure to a moist environment changes the failure mode from fiber bundles and fiber/matrix failure to naked fiber and matrix failure owing to the reduction in interfacial adhesion properties. The findings confirmed that hygrothermal conditions directly affect the degradation of the CF/PEKK thermoplastic composites and that a rapid reduction in the single-lap shear strength occurs above Tg.