ULS analysis of FRP composite sandwich plates - Sandwich PULS – development of a semi-analytical method

The use of fibre-reinforced polymers (FRP) composite sandwich structures is desirable because of their advantages compared to using steel structures. Many of the advantages are due to reduced weight, for example improved stability, lower fuel consumption, higher top speed, and increase in payload. Other advantages such as lower maintenance and long fatigue life contribute to an overall lower lifetime cost. A sandwich plate is a three-layer geometry consisting of two thin faces separated by a thick core. The purpose of this thesis is to develop Sandwich PULS; a semi-analytical calculation tool for predicting the ultimate limit state (ULS) of FRP composite sandwich plates. This was done by extending and improving DNV GL’s semi-analytical non-expert calculation tool Composite PULS, which is used for quick estimation of the ULS for unstiffened composite plates. ULS was evaluated in terms of the first buckling load and the first ply failure (FPF). The Sandwich PULS code was developed by implementing formulations for sandwich plate theory. First-order shear deformation theory (FSDT) was implemented in Sandwich PULS to include the transverse shear deformations that are highly important for sandwich plates with poor shear stiffness of the core. The Sandwich PULS code was evaluated against non-linear finite element analyses (FEA). It was concluded that Sandwich PULS shows good agreement with FEA predicted critical buckling loads. For all inspected plates, Sandwich PULS shows improved results compared to Composite PULS. Differences between Sandwich PULS and FEA are caused by the difference in evaluating shear stiffness. It has been shown that neglecting shear stiffness of faces results in good agreement between Sandwich PULS and FEA, while use of conventional shear correction factors proved to be unfavourable for sandwich plates. It was found that Sandwich PULS is limited in terms of slenderness. Sandwich plates with soft core should not have slenderness below 20 to assure an accurate solution. FPF loads according to Hashin-Rotem failure criteria are on the conservative side, but differences compared to FEA are up to 40% for sandwich plates. It was concluded that a more extensive investigation of failure initiation load prediction should be performed for better understanding