Glass fiber reinforced polymer (GFRP) has been criticized for causing environmental problems in the process of fabrication and disposal, but it is still widely used in many small ships, such as fishing boats and leisure boats. The reason is that composite materials have great advantages over metal materials in terms of excellent specific strength characteristics, durability, workability, and economic feasibility.
In general, there have been design rules related to GFRP hull structure design, such as ISO standards and classification society rules widely known for certification of small ships such as leisure boats. According to these design rules, ship design factors such as displacement, speed, subdivision and stiffener lay-out, and design pressure, as well as various material design factors such as fiber and fabric type, weight fraction (Gc) of fiber and resin, and mechanical properties of laminate should be considered in GFRP hull structure design. Fabric combination and mechanical properties in changes with Gc are important variables that determine the stability of the hull structure and are affected by the fabrication quality. However, the effect of such fabrication quality on mechanical properties is not considered in the design rules.
Due to the nature of the composite material, it is impossible to avoid internal defects such as voids during manufacturing. In general, composite materials are used in several fields.
Composite materials used in automobile and aviation fields have a thickness of 2 to 3mm and are manufactured by methods such as infusion, vacuum assisted resin transfer molding process (VARTM), and prepreg, resulting in a relatively small number of internal defects such as voids. However, the composite material used for the hull structure is relatively thick, unlike the composite material used in other fields, and has the characteristic of being mainly produced by the hand lay-up method. For this reason, it is difficult to manufacture the composite hull structure as designed, with a high possibility of defects such as voids.
When manufacturing the GFRP hull structure according to the design, not only the manufacturing method and environmental factors such as temperature and humidity but also the skill of the operator affect the fabrication quality of the composite hull plate. This difference in fabrication quality can have a great effect on the strength performance of the composite hull plate, cause safety problems in the hull structure, and reduce the service life. Therefore, it is necessary to consider these manufacturing uncertainties from the hull structure design phase.
This study aims to propose a design method that considers not only the general design conditions of GFRP composite hull plates but also their manufacturing characteristics. In order to reflect the characteristics of GFRP composite hull plates, E-glass fiber Chopped Strand Mat (CSM), Woven Roving (WR) fabric and polyester resin, which are widely used in manufacturing, were selected to design GFRP hull plates with Gc ranging from 0.30 to 0.70. Composite hull plates were designed in two combinations: a single material group and a combined material group. The composite hull plates were fabricated by the commonly used hand lay-up method.
The tensile strength, flexural strength, and bum-off tests were performed for the fabricated GFRP composite hull plates. Statistical methods were used to analyze the significance of the effects of uncertainty in design conditions such as fabric combination type and Gc as well as fabrication quality on the GFRP hull structure. These analysis results revealed that it was necessary to improve the estimation equation presented in the ISO standard and classification society rules. Furthermore, the GFRP strength estimation equation was derived using multiple linear regression analysis to consider the influence of design conditions and fabrication quality on the mechanical properties of GFRP composite hull plates at the hull structural design stage. Using the derived estimation equation, the GFRP hull structure design equation based on the ISO international standards and classification society rules was improved. A case study was conducted to analyze the effect of the improved GFRP hull structure design equation.