Issue 45

C. Bellini et alii, Frattura ed Integrità Strutturale, 45(2018) 173-182; DOI: 10.3221/IGF-ESIS.45.15 174 adherends not only causes the onset of corrosive phenomena on the metallic adherend, but can also lead to forms of damage in composite adherend like bubbles and swellings, as noted by Tucker et al. [3]. Moreover, adherend in composite material possess carbon fibres that are electrochemically nobler than most metals, so it is able to behave like a cathode. Consequently, it is possible the formation of a more aggressive environment localized at the fibre-matrix interface. Both in steam and in liquid form, water is one of the most aggressive environments for bonded joints. Being the most widespread substance in the entire globe, the study of its influence on the degradation of the joint is fundamental in every type of application. This peculiarity generally derives from the presence of polar groups in the adhesives, which make the bonding intrinsically hydrophilic. Bowditch [4] has observed that the addition of silicon in the adhesive can decrease the amount of mechanical degradation due to immersion in water for a certain period: the adhesive becomes more stable in water. He also states that increasing the test temperature to accelerate itself can be misleading because it is easy to analyze different damage mechanisms in this way. In fact, often the aggressiveness of the test environment is increased (for example by raising the temperature, increasing the humidity or simply varying the chemical composition), in order to reduce the test time and often neglecting the true phenomena that would occur in the real case. Furthermore, degradation can be exacerbated by the simultaneous presence of a load. Armstrong [5,6] has studied the durability in distilled water of bonded joints realized with aluminum adherends combined with various types of epoxy adhesives. Specifically, he observed that the degradation is directly proportional to the diffusibility and the solubility of the water in the adhesive. Gravimetric analyses are generally used to measure the water absorbed by the joints. He noted that the saturation of water in epoxy adhesives is shown by an increase in weight that can vary from 0.5% to 13%. The temperature is able to accelerate the diffusion process. Wylde and Spelt [7] have analyzed the effects of temperature on water diffusion in epoxy adhesives, observing that the saturation condition is a function of temperature as long as it is below the Tg. Obviously the different nature of the adhesives does not make it possible to generalize the observations made only on specific tests. Wilken et al. [8] have experimentally observed that the presence of an interface between adherend and adhesive amplifies the diffusive phenomenon. Specifically, the diffusion that experimentally observed at the interface of the bonded joints appeared to be about 6 times greater than that observed in the bulk of the adhesive. The molecular structure of an epoxy resin or epoxide-based materials may undergo physical changes caused by exposure at temperatures below the Tg for very long periods. These changes are manifested as the reduction of the free volume and the variation of molecular configuration, phenomena able to influence the reliability of components made with this type of resin. Odegard [9] describes these physical changes occurring in the molecular structure of the epoxy, investigating how they affect the properties of bulk. Degradation of the epoxy resin in water can also lead to irreversible damage, not negligible in case of structural components. In their work, Zhou et al. [10] have observed that a composite laminate immersed in water for a sufficiently long time may show a partial dissolution of the matrix, observable only by gravimetric analysis. The diffusion of water in the degraded laminate leads to a behavior of the laminate that cannot be described by Fick’s law. According to Zhou, this occurs only if the ratio between the temperature and the glass transition temperature exceed the nominal value of 0,25. Still in the same work, Zhou et al. have noted that the expansion due to the absorption of water is much more limited in the direction of the fibres respect the others. Zanni et al. [11] have analyzed the diffusion of water in epoxy adhesives. They noted that the diffusivity is related to the temperature according to Arrhenius law and that the elastic modulus of the adhesive decreases as the diffusion phenomenon progresses. Furthermore, they hypothesize the existence of a phenomenon called capillary diffusion: the presence of surface tension relative to the adhesive-adherend interface leads to an increase in the driving force of water penetration into the joint. Epoxy adhesives are one of the most commonly used adhesive types for bonding composite materials. More recently, McConnell et al. [12] carried out dielectric studies on the effects of freezing and hydrolytic ageing on bonded joints in composite material realized with this type of adhesive. Using gravimetric analysis, they observed that the composite adherends have a water diffusion coefficient that is one order of magnitude higher respect the adhesive and that the freezing causes an increase in voids and micro-cracks in the adhesive itself. Furthermore, the presence of water in the adhesive affects the mechanical resistance of the same in an inversely proportional manner. The research presented in this work is part of an experimental campaign on composite-composite single lap joints. In the previous work of the authors [13], the influence of the surface treatment of the adherends on the mechanical strength of the joint was analyzed. The need to achieve reliable bonding in the long term has led the scientific community to investigate the effect of humidity or temperature on bonded joints, but the results obtained are not always clear if these two factors act simultaneously. The study of the combined effect of hydrothermal ageing and operating temperature on the strength of the bonded joint is a topic that still needs further investigation. The objective of this second experimental phase is essentially to evaluate how the bonded joint, made of CFRP and epoxy adhesives, reacts to ageing induced by hydrothermal stress. To obtain a uniform and repeatable bonding surface, all the adherends have been realized with peel ply treatment.