Cristina Alía

Degradation in Seawater of Structural Adhesives for Hybrid Fibre-Metal Laminated Materials

The adhesives used for applications in marine environments are subject to particular chemical conditions, which are mainly characterised by an elevated chlorine ion content and intermittent wetting/drying cycles, among others. These conditions can limit the use of adhesives due to the degradation processes that they experience. In this work, the chemical degradation of two different polymers, polyurethane and vinylester, was studied in natural seawater under immersion for different periods of time. The diffusion coefficients and concentration profiles of water throughout the thickness of the adhesives were obtained. Microstructural changes in the polymer due to the action of water were observed by SEM, and the chemical degradation of the polymer was monitored with the Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The degradation of the mechanical properties of the adhesive was determined by creep tests with Mixed Cantilever Beam (MCB) specimens at different temperatures. After 180 days of immersion of the specimens, it was concluded that the J-integral value (depending on the strain) implies a loss of stiffness of 51% and a decrease in the failure load of 59% for the adhesive tested.

Influence of post-curing temperature on the structure, properties, and adhesion of vinyl ester adhesive

Vinyl ester (VE) resins are widely used as thermoset adhesives in structural joints and composites, but complete curing under environmental conditions is not produced. The existing literature dealing with the effect of post-curing on the structure, viscoelastic, mechanical, and adhesion properties of VE resin is scarce. Therefore, in this study, VE resin was subjected to different post-curing temperatures (50–140 °C) for one hour, and the changes in structure and properties were assessed. The degree of cross-linking of the VE resin depended on the post-curing temperature and cure started to be completed above 100 °C, a temperature close to the glass transition temperature (115 °C) of the completely cross-linked polymer. Furthermore, gel formation in VE resin was evidenced for post-curing temperature below 100 °C. In order to fully cross-link the VE resin, post-curing at 140 °C for one hour was necessary, and it was evidenced by an increase in the glass transition temperature and in the mechanical properties; an increase in adhesion to cold rolled steel was obtained although the shear strength was lower than in the joint produced with the non-post-cured VE resin.

Mode II fracture energy in the adhesive bonding of dissimilar substrates: carbon fibre composite to aluminium joints

The end-notched flexure (ENF) test calculates the value of mode II fracture energy in adhesive bonding between the substrates of same nature. Traditional methods of calculating fracture energy in the ENF test are not suitable in cases where the thickness of the adhesive is non-negligible compared with adherent thicknesses. To address this issue, a specific methodology for calculating mode II fracture energy has been proposed in this paper. To illustrate the applicability of the proposed method, the fracture energy was calculated by the ENF test for adhesive bonds between aluminium and a composite material, which considered two different types of adhesive (epoxy and polyurethane) and various surface treatments. The proposed calculation model provides higher values of fracture energy than those obtained from the simplified models that consider the adhesive thickness to be zero, supporting the conclusion that the calculation of mode II fracture energy for adhesives with non-negligible thickness relative to their adherents should be based on mathematical models, such as the method proposed in this paper, that incorporate the influence of this thickness.

Fracture energy in structural adhesive joints of composite-aluminum under adverse environments conditions

This work analyzes the degradation of composite-aluminum adhesive joints when they are exposed to the weathering and environmental pollution in Madrid for a long period of time. Two adhesives (epoxy and polyurethane) and several surface treatments for adherends have been considered. End-notched flexure bending tests have been performed to evaluate the loss of mechanical properties (failure stress and fracture energy) of adhesive joints that were exposed to the weathering and environmental pollution. Tests results have shown that the environmental degradation of the adhesive leads to a loss of mechanical properties in the adhesive joints. Considering the relative percentage, the reduction of failure stress in the polyurethane is higher than in the epoxy (31.9% for the polyurethane and 21.1% for the epoxy). Similarly and considering relative percentage, fracture energy reduction is 42.0% for polyurethane and 41.5% for epoxy. Likewise, tests have shown that the loss of mechanical properties does not decrease linearly with the time when the samples have been exposed to the weathering. This reduction occurs during the first few weeks. In summary, tests results have allowed to conclude that adhesive joints with epoxy resist the environmental pollution better than the adhesive joints with polyurethane.

ENF test in the adhesive bonding of aluminium–composite joints and evaluation of its reliability with Weibull distribution

The composite materials of the polymeric matrix reinforced with carbon fibre have an extensive industrial application as they provide light and resistant structures. However, in many products (automobiles, aircraft, etc.), the composite materials must be joined to other components manufactured with aluminium alloys. The use of structural adhesive to bond these materials may be a good alternative if a specific design of the adhesive joint is carried out by maximising its performance and reducing its limitations. In the current work, the end-notched flexure (ENF) fracture test is used to assess the mechanical behaviour of the adhesive joint to facilitate the choice of the best adhesive and surface treatment of the adherends. However, in industrial applications of a great technical requirement (where the safety of staff or property may be at risk), the former experimental results are not enough and must be complemented with methods that can provide additional guarantee for a suitable reliability. For this purpose, a statistic analysis of the obtained experimental data has been carried out by means of the application of a Weibull distribution, in order to propose the adhesive and surface treatment that best combines the mechanical performance and high reliability. At the end, we can conclude that the epoxy adhesive with sand-blasting treatment for the aluminium and the peel ply for the carbon fibre has the greatest reliability (more than 90% for loads until 900 N).