# Sewater Effect on Fatigue Behaviour of Notched Carbon/Epoxy Laminates
## 3 Results
For [PERSON] and [PERSON] [41], moisture absorption promotes changes in the thermophysical, mechanical and chemical characteristics of the matrix by plasticisation and hydrolysis. Regarding the fibres, the amount of moisture absorbed is completely different from that absorbed by the matrix. Hence, due to different volumetric expansions, stress and deformation fields located in the composite arise [39]. Furthermore, debonding of the fibre/matrix interface also occurs due to possible chemical attacks and mechanochemical effects, such as osmotic pressure. Therefore, all these damage mechanisms caused by moisture affect the load-bearing capacity of the composite and, consequently, lead to the lower fatigue strength observed. From the data collected throughout the fatigue tests, it was also found that the maximum and minimum axial displacements varied due to the decrease in stiffness (E) caused by fatigue damage [42; 43; 44; 45]. Figure 7 plots E/E\({}_{0}\) versus N/N\({}_{t}\), for different tests, where E\({}_{0}\) is the initial value of E, N is the current number of cycles and N\({}_{t}\) is the number of cycles to failure. These curves, which illustrate the evolution of residual stiffness, are, according to the literature, a suitable parameter for fatigue damage assessment. They reflect the degradation that occurs during the fatigue lifetime [42; 43; 44; 45]. It is possible to note that after an initial value of E there is a significant drop in the stiffness modulus during the first 5-30% of the fatigue life, followed by a second dominant stage where the stiffness modulus slowly decreases until near the final failure. Finally, a third stage occurs during the last 5% of fatigue life, where the stiffness modulus suddenly drops. According to the open literature, this is a consequence of stress release, temperature rise and internal damage development [44; 45; 46; 47]. Regardless of the pre-damage introduced during the drilling process, and despite all the care taken in this procedure, the initial application of the fatigue load introduced matrix cracking and its propagation, or even the propagation of pre-existing cracks, promoting, in this case, the decrease in stiffness modulus observed during the first stage [46]. However, independent of the stress amplitude (Figure 7a), this decrease was higher for wet samples (independent of the immersion time and seawater type) than dry samples due to the damage introduced by the seawater. As reported above, some debonding of the fibre/matrix interface and matrix plasticization (matrix becomes more pliable) were expected. These facilitate the appearance of new matrix cracks and the propagation of pre-existing cracks/defects. In the second stage, the propagation of pre  
Figure 7: E/E\({}_{0}\) versus N/N\({}_{t}\) for tests: (**a**) with different stress amplitudes using samples not immersed and (**b**) with different immersion times and types of seawater (natural and artificial seawater). existing cracks and cracks in the matrix contributed to a higher debonding fibre/matrix interface, mainly due to a triaxial stress state developed inside the composite, which became more complex due to the presence of seawater. In this case, the moisture absorbed by the samples created a hydrostatic pressure at the crack tips, which accelerated crack propagation and damage of the matrix [47]. Therefore, while no significant differences were observed for wet samples (independently of the immersion time and seawater type), the dry samples evidenced different second stages. For example, higher stress amplitudes were responsible by lower second stages and longer first stages (see curve for 308 MPa), proving that higher fatigue loads are responsible for more extensive initial damage over time, but their propagation is faster. Furthermore, considering the typical dispersion of these curves, the stress amplitudes of 261 MPa and 277 MPa were not sufficiently different to promote significant differences at the level of the second stage's extension.