PSI - Issue 42

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

795

3

Fig 1. Fiber layout configurations

2.2. Experimental research At the end of the submersion period, tensile tests (uniaxial tensile tests performed on a Zwick 400 kN universal testing machine with a macro extensometer; 2 mm/min crosshead displacement rate) have been performed on each of the coupons in order to asses the changes of ultimate tensile strength caused by the marine environment. The results of this phase of the experimental research has been presented by the authors of this paper in previous publications (Vizentin et al., 2021; Vizentin and Vukelic, 2022). 2.3. Regression analysis The data collected during the tensile test has been used for regression analysis in order to obtain a predictive failure model that can be used to asses the life cycle duration for glass/polyester composites exposed to the sea. The Classification Societies do prescribe procedure for obtaining S-N curves for composite materials. On the other hand, numerous authors have reported results in modelling S-N curves for composites (Burhan and Kim, 2018; Vázquez et al., 1998; Yasar et al., 2014) and prediction of the fatigue life of composite structures (Djeghader and Redjel, 2017; Kim and Huang, 2021; Silvera et al., 2011). A failure model for groups of similar glass/polyester composites, as the ones used in this research, can be defined in the form:

   

 

( ) N

log

(1)

u

= 

 



max

where N is the number of fatigue life cycles,  u is the ultimate tensile strength (UTS) value,  max is the maximal tensile stress occurring in the loaded structure,  and  are experimental data fitting coefficients that need to be obtained for a specific composite material (Silvera et al., 2011). Material data obtained by other researchers for similar material type and configuration is acceptable according to Classification Societies rules and can be used for design of composite marine structures. A resulting mathematical strength degradation prediction model that showed the best correlation to the experimental data can be then formulated as:

B A  = t e

(2)

where  is the tensile strength value, t is the time of exposure to the marine environment and A and B are experimental data fitting coefficients. The coefficient A represents the average ultimate tensile stress of the non-submerged