The use of high-performance composite materials is a reality in a wide variety of industrial sectors such as the aerospace or automobile industry, in which there is great interest in designing and manufacturing lighter components that allow the reduction of fuel consumption, and consequently, reduce operating costs and greenhouse gas emissions, without interfering with its resilience. Therefore, composite materials are increasingly being used for manufacturing, leaving aside the traditionally used metallic materials, also due to their properties of high mechanical resistance, corrosion resistance, fatigue, impact and thermal stability, but the downside to this type of material is its low delamination resistance. In addition to the composite material itself, alternative joining techniques have become a topic of relevance in the industry, replacing conventional screws and rivets. The study of adhesive joints in fibre-reinforced composites has increased significantly.
When the material is subjected to tests to study the phenomenon of adhesion in relation to the phenomenon of delamination in composite materials, it is necessary to take into account the physical, chemical and mechanical properties [1–6] of the material to be adhered and in particular, an adequate treatment of the surfaces that will serve as a connecting surface, which can lead to a significant improvement to the adhesion, modifying the initial properties of said surfaces [7–10], especially in fibre-reinforced polymers due to the low surface tension and wettability they present. Furthermore, it must be taken into account that these types of joints cause an increase in the plastic dissipation energy at the fracture of the adhesive joint [11, 12].
Another important parameter associated to the behaviour of the adhesive joints which is being studied on different materials, including composite materials, is the influence of the type and speed of application of the applied load [13–17].
In addition, there are lines of work associated to the behaviour of adhesive joints against initiation and growth processes of delamination, in which the properties of the joint, the thickness of the adhesive  and concerning different types of adhesives used are studied [19, 20]. Approached with different test methodologies: through pure shear tests [21, 22], or through fracture mechanics, under stress in mode II  and mixed mode (combination between mode I and mode II) [24–26].
On the other hand, the fatigue behaviour of adhesive joints in mixed materials [27, 28] with the incorporation of reinforced nanofibers : silica nanoparticles  and rubber microparticles [31 are being studied under different experimental methodologies, also under different fracture modes [32–34]. A major experimental effort is being made to modify the properties of the adhesive bond, by means of laser surface pre-treatments , the incorporation of additional elements to the bond that collaborate in improving the adhesion and they act as a reinforcement of the union against delamination phenomena, the incorporated particles range from carbon nanotubes (CNT) [36, 37] to graphene nanoplates (GNP) .
Another line of work in this field analyzes under different fibre orientations of the composite material , also the behaviour of adhesive joints in composite materials subjected to different degradation processes such as humidity and its effects on the delamination process under pure fracture modes , exposure to a saline environment [41, 42], freezing and thawing , water absorption in joints with hybrid composite materials , the effects of temperature [45–47] and the combination of the effects of humidity and temperature [48–50].
It is worth noting the important contribution that has been made and is being made to provide a tool for the calculation and verification of structures subjected to different loading conditions and manufactured with technologies associated to composite materials joined by adhesives, the knowledge necessary for the numerical simulation of their behaviour and thus be able to develop interesting methodologies and try to provide a numerical solution to this type of joint [51–55].
The aim of this work is to evaluate the behaviour of adhesive joints against the fatigue delamination phenomenon when they are subjected to different periods of exposure to a saline environment, for which an epoxy matrix composite material with unidirectional carbon fibre reinforcement and an epoxy-based adhesive was selected as a substrate. For the characterisation of the strength of the joint against delamination, the ERR reached by the joint under mode I fracture stress has been taken as a study parameter. Analysing the influence that the applied degradation process, saline environment, has on the adhesive bond in relation to the periods of exposure to which it has been subjected and the influence on its fatigue life, both in its initiation and fatigue crack growth phases.