Strain Rate and Temperature Effects of Polymer Foam Core Material in Marine Composite Sandwich Panels

Composite sandwich panels are often used in high-speed vessel construction because of lightweight and durability properties. In service, high-speed vessels often experience wave slamming events that induce dynamic forces resulting in high strain rates in the hull.  The polymer foam core material typically used in sandwich construction has shown increases in strength and stiffness properties when evaluated at a higher strain rate. However, the design of hull structures is usually conducted based on properties obtained from quasi-static tests at standard temperature conditions. First, the research work presented in this Thesis attempts to quantify the effect of high strain rates and high and low temperatures on polymer foam shear properties at the material level.  Second, this Thesis presents a model to predict the effect of strain rate and temperature on the load-deformation performance of a composite sandwich panel structures. Finally, the Thesis presents an approach to characterize the effect of high strain rates on the fatigue life of sandwich panels.

The foam core experiments at the material level utilized a modified version of the standard direct shear test procedure.  The load-deformation performance of composite sandwich panels was evaluated in a 3-point bending configuration using a modified version of the sandwich beam bending test procedure. The experimental stress-strain response of the polymer foam was synthesized numerically for each core material type, density, strain rate and temperature.  The resulting stress-strain functions were incorporated into a beam analysis model.  The effect of the non-linearity in the shear stress-strain response of the foam core was considered.  This analysis was correlated with the experimental load-deformation response of composite sandwich panels.

The polymer foam core material shear strength and stiffness increased by as much as 45% and 16%, respectively, over the range of strain rates.  The polymer foam core material shear strength and stiffness exhibited an inverse relation with temperature.

The fatigue experiments on composite sandwich beams developed stress level versus number of cycles (S-N) curves for two strain rates using a fully reversed sinusoidal waveform. The S-N curve method allowed the characterization of the fatigue life for different strain rates including an fatigue threshold.