PSI - Issue 47

Francesco Ascione et al. / Procedia Structural Integrity 47 (2023) 826–841 Author name / Structural Integrity Procedia 00 (2019) 000–000

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� Equivalent crack length P Force applied at midspan in ENF test δ Displacement at midspan in ENF test �� Fracture energy in mode II � measured initial compliance in ENF test �� SLJ compliance in ENF test A Continuous absorption D Continuous desorption � ultimate adhesive shear stress � Sliding at the limit elastic stage of the adhesive � Sliding at the limit softening stage of the adhesive 1. Introducion

In recent years, the use of adhesives in the construction sector has increased, especially relating to the use of composite materials, which are considered a valid and alternative solution to traditional ones, with the aim of realizing more durable structures with lower maintenance requirements [Bakis et al 2002, Green et al. 1994]. However, a wider use of bonded joints is pending on obtaining a better understanding of their long-term behaviour. Within this aim, several studies were recently carried out on the influence of different parameters on the strength and stiffness of a bonded joints, which included: temperature [Michels et al (a) 2016] and hygro-thermal and outdoor ageing as well as moisture [Bowditch et al. 1996]. The effect of these parameters should be taken into account, considering that they may limit the applicability of structural adhesives [Abbey et al. 2010]. The environmental temperature may exceed the glass transition temperature (T g ) of the adhesive formulation entailing relevant changes in its properties, determining, for example, a transition from a hard to a rubbery behaviour, thus compromising its specific application [Michels et al (b) 2015]. Furthermore, due to the different environmental parameters experienced by the assembled structures during their use, which include the temperature values, the structural adhesive can be naturally subjected to a delay or increase in the curing degree. Adverse or positive changes in strength and stiffness can be manifested. The speed and extent of the changes depend on the magnitude and duration of the temperatures experienced by the adhesive [Sousa et al. 2018]. To design and verify the bonded connections, it is crucial to know the fracture energies in mode I and mode II taking into account the durability and relative interface laws as requested by current standards [Report EUR 27666 EN]. Few data about adhesive fracture energies are available in current literature making the use of bonded joints, especially in the field of civil engineering, a challenge rather than a reality [Barra et al. 2017]. Within this literature framework, some authors of this paper started in the recent past the experimental study of the hygro-thermal durability of epoxy resins used for civil engineering applications [Ascione et al. 2021]. The authors focused on the influence of two types of conditioning (immersion in tap water and sea water for a period of fifteen months at the temperature of 30°C) on the mode II fracture energy of two commercial epoxy resins cured at room temperature (about 23±2°C). Near to the fracture energy value, it is crucial to know the relative interface bonding law (in terms of the shape and limit values of both the elastic and softening stages) in order to design and verify any kind of adhesive connection. Furthermore, it is necessary to know how such an interface law could be modified by taking into account the degradation phenomenon due to immersion in water. Within this scopes, this paper aims to implement a Finite Element Model (FEM), by using the commercial Abaqus code, capable of perfectly describing on one hand the End Notch Failure (ENF) test used to evaluate fracture energy in mode II as requested by current standards [Report EUR 27666 EN], and on the other, to evaluate the stiffness of the elastic and softening stages in case of ageing. 2. Brief summary of the experimental program The experimental investigation was related to the study of the hygrothermal durability of the fracture energy in