PSI - Issue 42

Goran Vizentin et al. / Procedia Structural Integrity 42 (2022) 793–798 Vizentin/ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Fiber reinforced polymer (FRP) composites possibilities of engineering applications are constantly expanding, whether as an exclusive option for construction elements material (Grbović et al., 2022; Kim et al., 2021; Vukelic and Vizentin, 2021) or as a combination with traditional materials, such as steel, concrete (Kožar et al., 2019; Lukács et al., 2021) or rock (Andreazza et al., 2020). In the last couple of decades, significant effort was made to combine the experimental and scientific knowledge obtained so far in these field of research to enable prediction models that can be safely used to achieve sustainable and safe design of engineering structures (Davies and Rajapakse, 2018, 2014; Martin, 2008; Takacs et al., 2020). The marine industry sector is possibly the one currently experiencing the greatest increase of number of practical applications or composite materials, following the aeronautics and automotive industry sectors. Structural applications of any material no matter the industry sector require extensive and well based failure and fatigue life prediction knowledge in order to enable a safe and reliable design. Composites are nowadays being used in more complex structures which makes the necessary prerequisites for mechanical and environmental resilience ever more stringent. Parameters such as limit stress states, durability and life span, failure modes, fracture toughness, fire resistance, and environment influence parameters are crucial for an efficient, sustainable, and secure design process for structures in this demanding industry sector (Kastratović et al., 2021; Khosravani et al., 2022; Kim et al., 2014; Sousa et al., 2020; Tomasz et al., 2022; Vizentin et al., 2021). Perhaps the most important parameter influencing the mechanical properties of composite materials in marine applications is the absorption of seawater (Vizentin and Vukelic, 2019). Available research on this matter is based on immersing test samples, called coupons, in laboratory conditions using accelerated procedures (Morla et al., 2021) to simulate 20+ years of expected lifespan of typical marine structures (Bond, 2005; Eftekhari and Fatemi, 2016). The ageing of composites is usually carried out in climatic chambers in laboratory conditions to reduce the time of the test (Cysne Barbosa et al., 2017; Davies, 2020; de Souza Rios et al., 2016; Panaitescu et al., 2019). It is important to emphasize here that for composite failure data collection methods are still experimentally based (Gljušćić et al., 2022) . The marine sector regulatory bodies rules acknowledge that material properties can “change gradually with time and long exposure times” and that significant changes can occur after one year but neglect to consider the real marine environment influences. The previous phases of the research presented here have indicated to the importance of the real marine environment as the surroundings for the experiments as opposed to the more commonly chosen artificial/laboratory environment conditions opted for by a vast number of researchers (Vizentin et al., 2021; Vizentin and Vukelic, 2020). Considering this a need for a modification in the approach to the modelling of S-N curves for composite materials prescribed by the Classification societies, considering the material properties degradation due to the effects of water absorption, fouling marine organisms and prolonged exposure to the marine environment arises. An experimental approach to obtain modified S-N curves for glass fiber and polyester resin composites is described here. The dominant choice of composite materials in the civil sector of the marine vessels industries is glass fibre reinforced plastics (GRP), both for commercial and leisure vessels hulls (Rubino et al., 2020), resulting in a more cost effective product. The objective of the research presented here is to encompass the real marine environment effects on fatigue parameters of composites, including the effects of moving seawater (waves, tidal changes), weather influences and the impact of the marine organisms that live attached to any and all typical marine structures. 2. Materials and methods 2.1. Materials Epoxy/glass and polyester/glass composite coupons, dimensioned 250×25×3 mm and 250×25×5 mm respectively, with a50±5% fibre reinforced volume have been submerged for a total period of 2 years. Three distinct fibre layout configurations of 8 layers of continuous glass fibres prepreg, namely unidirectional UD0°, cross ply (0/90)s and (0/45/90)s have been used, as shown in Figure 1.