Ultimate strength study of composite plates

Abstract

Highly energy efficient ships are valuable assets in the maritime transport industry; however, energy efficiency often comes at a higher price. In this work “energy efficient ships” are considered to be ships designed with lightweight structures, which lead to lower fuel-consumption or larger payload capacity. The material of the lightweight structures addressed in this thesis is fibre-reinforced plastic, a material known for its high strength to weight ratio. The material is also known to be expensive and traditional design principles with over-conservative estimates need to be reviewed in order to limit the costs. The purpose of this thesis is to contribute to the development of methods for calculating the ultimate limit state (ULS) of fibre- reinforced plastic panels with less conservative estimates compared to traditional methods used in the industry. This contribution was achieved by extending the formulation of an existing computer software known as Panel Ultimate Limit State (PULS), which is used for estimating the ULS capacity for steel and aluminium panels, to include unstiffened single-skin composite panels. The ULS was defined as the ultimate load-carrying capacity. Two traditional lay-ups used in the maritime industry were analysed for different breadth/thickness-ratios under compressive load conditions. The results were compared with a finite element analysis and another semi-analytical model. Through comparisons of the developed software against a finite element analysis, it is found that composite PULS provides reasonable conservative ULS results for plates with high breadth/thickness-ratios. Composite PULS has its limitations when it comes to estimating the ULS of plates with low breadth/thickness-ratios. It was experienced that the balance between accuracy and computational time is hard to establish. Using composite PULS in order to estimate the ULS capacity is through this thesis shown to give a less conservative ULS compared to traditional design criteria. Composite PULS is based on Hashin-Rotem’s failure criteria, and estimated the ULS to be 73-94% of the ULS calculated with a finite element analysis.