Date of Award

8-2012

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Roberto A. Lopez-Anido

Second Committee Member

William G. Davids

Third Committee Member

Eric N. Landis

Abstract

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. Vessels in service also experience changes in temperature of the hull. The polymer foam core material typically used in sandwich construction has shown decreases in strength and stiffness properties when tested at a higher temperature. However, the design of hull structures is usually conducted based on properties obtained from quasi-static tests at standard temperature conditions. Sandwich hull design could be optimized if the effects of strain rate and temperature on properties were taken into account during the design process. First, the 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. Three strain rates were considered ranging from quasi-static to simulated wave slamming conditions. Three temperatures were considered, ranging from low temperature to high temperature. Second, the work presented attempts to predict the effect of strain rate and temperature on the load-deformation performance of a composite sandwich panel structures. The approach is based on implementing a non-linear sandwich beam bending model, which accounts for the foam core material response as a function of strain rate and temperature. Third, the work presented attempts to characterize the effect of high strain rates on the fatigue life of sandwich panels. The approach is based on developing stress range versus number of cycles (S-N) curves for two strain rates using a fully reversed sinusoidal waveform. The foam core experiments at the material level utilized a modified version of the ASTM C273 standard test procedure. Two foam core materials and three densities for each material, which are common in marine construction, were selected for the experimental work. The load-deformation performance of composite sandwich panels was evaluated in a 3-point bending configuration using a modified version of ASTM C393 test procedure. Based on input from the shipbuilding industry, five different types of sandwich panel construction, which simulate the hull structure of a broad range of vessels, were manufactured and evaluated at the selected strain rates and temperatures. 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 composite sandwich beam analysis based on a First Order Shear Deformation Theory (FSDT). The effect of the non-linearity in the shear stress-strain response of the core was incorporated in the modified FSDT analysis. 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 sandwich beam flexure strength increased as much as 49% over the range of strain rates, which is consistent with the trend observed for the foam core. The sandwich beam flexure strength exhibited an inverse relation with temperature, which agrees with the changes in shear strength and stiffness properties of the polymer foam core material with temperature. The S-N curve method allowed the characterization of the fatigue life for different strain rates including an endurance limit. The two fatigue result populations were statistically different but not enough to show that strain rate has an adverse effect on fatigue life.

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