Search

Composite materials are widely used in various applications, but their mechanical and physical properties can be significantly influenced by environmental factors such as temperature, humidity, and UV radiation during service life. This study aims to investigate the accelerated aging behavior of glass/epoxy composites under hygrothermal conditions and compare the results with naturally aged samples to understand the reliability of these materials in harsh environments. NOL GFRP samples were fabricated using the filament winding process and subjected to accelerated aging for varying time periods (ranging from 100 to 1200 hours in 50-hour intervals) under hygrothermal conditions. Additionally, naturally aged samples over several years (1, 1.5, 2, 2.5, and 3 years) were compared. Tensile strength measurements were conducted to assess the mechanical properties of the composites. Machine learning models, including linear regression, polynomial regression, Artificial Neural Network (ANN), random forest regression, and Support Vector Regression (SVR), were utilized to predict natural aging times from accelerated aging data. Hygrothermal aging led to significant matrix deterioration and fiber exposure, resulting in a notable reduction in tensile strength. The study observed a 35.60 % reduction in strength in three-year naturally aged samples and a 37.57 % reduction in 1000-hour accelerated aged samples. Among the machine learning models, the random forest regressor demonstrated the best performance in predicting natural aging times across different accelerated aging periods, while SVR exhibited poorer performance. Polynomial regression and ANN models showed moderate predictive capabilities. This study contributes to a deeper understanding of the aging process of composite materials, providing insights into the reliability and durability of glass/epoxy composites in various applications. By employing machine learning models, the research offers a novel approach to predicting natural aging times based on accelerated aging data, which can be beneficial for optimizing the design and maintenance of composite structures to enhance their long-term performance and minimize the risk of failure in service. • Investigated the impact of hygrothermal conditions on the accelerated aging of glass/epoxy composites over various time intervals. • Compared accelerated aging results with natural aging samples spanning multiple years to assess composite degradation. • Identified significant reductions in tensile strength due to hygrothermal aging, highlighting matrix deterioration and fiber exposure. • Employed five machine learning models to predict natural aging times from accelerated aging data, with random forest regression yielding the most accurate results. • Provided valuable insights into the aging behavior of composite materials through correlation analysis between natural and accelerated aging. [ABSTRACT FROM AUTHOR]

The thermal properties of closed-cell foam insulation display a more complex behaviour than other construction materials due to the properties of the blowing agent captured in their cellular structure. Over time, blowing agent diffuses out from and air into the cellular structure resulting in an increase in thermal conductivity, a process that is temperature dependent. Some blowing agents also condense at temperatures within the in-service range of the insulation, resulting in non-linear temperature dependent relationships. Moreover, diffusion of moisture into the cellular structure increases thermal conductivity. Standards exist to quantify the effect of gas diffusion on thermal conductivity, however only at standard laboratory conditions. In this paper a new test procedure is described that includes calculation methods to determine Temperature Dependent Long-Term Thermal Conductivity (LTTC(T)) functions for closed-cell foam insulation using as a test material, a Medium-Density Spray Polyurethane Foam (MDSPF). Tests results are provided to show the validity of the method and to investigate the effects of both conditioning and mean test temperature on change in thermal conductivity. In addition, testing was conducted to produce a moisture dependent thermal conductivity function. The resulting functions were used in hygrothermal simulations to assess the effect of foam aging, in-service temperature and moisture content on the performance of a typical wall assembly incorporating MDSPF located in four Canadian climate zones. Results show that after 1 year, mean thermal conductivity increased 15%–16% and after 5 years 23%–24%, depending on climate zone. Furthermore, the use of the LTTC(T) function to calculate the wall assembly U -value improved accuracy between 3% and 5%. [ABSTRACT FROM AUTHOR]

This study evaluated the durability of three innovative multilayer insulation systems incorporating thermal mortars with EPS aggregates and silica aerogel granules after hygrothermal accelerated aging and one year of natural aging at an urban site in Portugal. The loss of performance was assessed after the accelerated aging and every three months of natural aging using non-destructive testing. Chemical-morphological analyses were also carried out prior to and after accelerated and natural aging. Results obtained after accelerated and natural aging were compared, thus contributing towards a deeper understanding of possible synergistic effects of several degradation agents and mechanisms on the long-term durability of multilayer insulation systems. The Coffin-Manson equation showed that the accelerated aging procedure (∼13 days of heat/rain cycles and 5 days of heat/cold cycles) adopted herein corresponds to approximately 11 years of natural aging in typical urban conditions. The results show a significant increase in capillary water absorption and drying capacity after aging. Extensive surface microcracking was observed after accelerated aging and after 3 months of natural aging, especially in the systems facing North. Traces of biological growth were detected on both the artificially and naturally aged systems, whereas aesthetic alterations were more pronounced in North-oriented specimens after 3 months of exposure, with significantly lower surface gloss and a darker tone. On the other hand, color change cannot be detected in the artificially aged systems ( Δ E l a b * < 2), due to the lack of degradation agents such as pollutants or UV radiation that were not included in the accelerated aging procedure. • The durability of multilayer insulation systems was assessed after accelerated and natural aging. • The accelerated aging procedure corresponds to ∼11 years of natural aging in typical urban conditions. • A significant increase in capillary water absorption was registered after aging. • Traces of biological growth were detected after accelerated and natural aging. • Degradation agents identified in long-term field exposure should be considered in optimized accelerated aging procedures. [ABSTRACT FROM AUTHOR]

Durability of FRP composites and their bond to concrete is essential to structural integrity of an FRP repair. Epoxy adhesives are used to form FRP composites, and as a bonding medium between the FRP and concrete substrate. Susceptibility of epoxy to the negative effects of water and high temperature affects the longevity of FRP repairs in hygrothermal environmental conditions. The presented work investigates the effects of such environments on the curing kinetics of epoxy. Competing effects of plasticization and post-cure during accelerated conditioning are discussed; recommendations targeting researchers, practitioners, and manufacturers are made based on the research findings. [ABSTRACT FROM AUTHOR]

This paper addresses the results of accelerated hygrothermal (coupled temperature and moisture) tests on fiber-reinforced polymer (FRP) strengthened clay bricks, aimed at investigating bond degradation mechanisms. The exposures are selected to simulate different environmental conditions and the bond degradation is periodically investigated by visual inspection and by conventional single-lap shear bond tests. The changes in the properties of material constituents were also monitored; the results are presented and critically discussed. A decay model is adopted for simulating the observed degradation in the specimens. The model, once validated, is used for long-term performance prediction of FRP-masonry systems and the results are compared with the environmental reduction factors proposed by available design guidelines. [ABSTRACT FROM AUTHOR]

Abstract: Despite their high performances, composites with polymer matrix are very sensible to the increase in temperature and moisture concentration. During long years of services, both phenomena cause a critical transient hygrothermal transverse stresses, particularly at first-ply; i.e. at two edges of the composite plates. Therefore, significant degradation of hygrothermal characteristics and ultimate strengths of materials are occurred. To get an explicit relation between the durability and the damage probability of the composite, quadratic failure criterion in stress space is used. This criterion enables us to find a direct relation between transient hygrothermal stresses produced by the increase in temperature and moisture concentration and the ultimate strengths. It is necessary to calculate the strength ratio R from initial to saturation time for each condition imposed of temperature and moisture concentration. The strength ratio gives a point of view on the damage probability of the composite plates, where the rupture occurs if R =1. In order to limit the consequences of simultaneous effects of temperature and moisture concentration, unidirectional hybrid composites in graphite epoxy was proposed. To reach this aim, hygrothermal transverse stresses are calculated through the thickness of unidirectional hybrid plate. Finally, the strength ratio was evaluated along of the plate with a gradual increase in temperature and moisture concentration. [Copyright &y& Elsevier]

In this article, both numerical and experimental investigations were carried out on the durability of hemp concrete. For this, an accelerated aging process was performed using cycles of immersion, freezing and drying. Then, an experimental campaign was enabled to determine heat and mass transfer properties, as well as the microstructure for both aged and reference materials. Observations using a digital microscope showed the appearance of cracks at the interfaces and an increase of the porosity of about 6%. These microstructural modifications imply a non-negligible evolution of heat and mass transfer properties. Thus, a numerical model for the prediction of heat and mass transfer was developed. The prediction of physical phenomena was computed using both aged and reference material properties. It highlights the aging effects on the behaviour of the hemp concrete. The numerical simulation results showed significant discrepancies between the predicted relative humidity values for the two configurations (aged and reference) of about 18% and a maximum phase shift of 40 min, due to the amplification of the mass transfer kinetics after aging. Nevertheless, few deviations in temperature values were found. Thus, after aging, sensible heat fluxes were overestimated compared to the reference case, unlike latent heat fluxes, where an underestimation was shown. [ABSTRACT FROM AUTHOR]