Comparison of operational limit estimations for FRP laminates – effects of material modelling and stochastic behaviour

Licentiate thesis, 2012

Transport companies need vehicles with higher efficiency to remain competitive and comply with environmental regulations. Lightweight construction is one way to increase the efficiency of a vehicle. In some vehicles, lightweight construction can reduce the fuel consumption, increase the cargo capacity and reduce the environmental footprint. Fibre-Reinforced Plastics (FRPs) are convenient materials for lightweight construction due to their high stiffness and strength to weight ratio; however, FRPs are expensive materials compared to steel and sometimes to aluminium. Therefore, a FRP structure should have the least amount of material as possible so as to increase the benefits of lightweight construction and to be as inexpensive as possible; FRP structures should also be beneficial enough to motivate the added acquisition cost. Operational limits determine the magnitude of the load that a material can carry safely, thus, in a sense, operational limits determine how much material it is required for a certain component or part. FRPs are heterogeneous anisotropic materials that exhibit several modes of material degradation and failure. The calculation of their operational limits is generally simplified, which may lead to under predicted operational limits and consequently, to unnecessarily heavy and expensive FRP structures. The work presented in this thesis intends to contribute to the body of knowledge regarding the estimation of operational limits for FRPs, and to determine if more accurate methods for estimating the operational limits of FRPs can motivate higher material utilization compared to current design rules. Both objectives aim at rendering lighter, cheaper and safer FRP structures. This investigation compares the operational limits of tension-loaded FRP laminates calculated through several methods. The methods are intended to be generally applicable to FRP structures; however, in this work, the methods are applied to structures that follow marine structure design rules and guidelines. The comparison evaluates the effects that material modelling and stochastic behaviour have on the estimation of operational limits.