Doctor of Philosophy (PhD)
Adhesively bonded sandwich structures comprising of particulate composites as core and graphite epoxy skins as stiffeners are widely used for various applications in the marine and aerospace industry. The core material and the stiffener are held together by an adhesive bond. Particulate composites are made from a mixture of a polymer resin and hollow or solid particles. Hollow particulate composites are known as syntactic foams. Particulate composites possess attractive mechanical and physical properties such as high compressive strength etc, making them attractive materials for use in structural applications. Characterization of the adhesive bondline and core material in sandwich structures is important for ensuring structural stability and reliability. Nondestructive evaluation [NDE] techniques such as ultrasound are used for better evaluation of these sandwich structured materials. The present study addresses the problems of detection of disbonds, bond surface characteristics and porosity in the adhesive panels along with characterization of particulate composites separately using NDE. The importance of the attenuation coefficient in computing the longitudinal velocities of the ultrasonic wave in particulate composite samples is also discussed. Five sets of adhesively bonded carbon epoxy composite specimens with varying bond surface preparation, twenty four different types of hollow syntactic foams and six different types of solid particulate composites, are fabricated. The adhesively bonded panels are made by including known defects in the bond layer of the samples. The particulate composites (syntactic foams and solid particulates) are fabricated by varying the volume fraction of each of the four types of microballoons and solid particle from 10% to 60%. Pulse echo UI method is selected for use in the present work. The results of this research provides a better understanding of adhesive joints and particulate composites and thus help in characterizing structures composed of these constituents. One of the major findings in this research is the discovery of a nondestructive method to determine the dynamic modulus of particulate composites. In addition, a constitutive model explaining the effect of particle size, porosity, radius ratio on the ultrasonic attenuation coefficient in particulate composites is developed.
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Mylavarapu, Phani Surya Kiran, "Characterization of advanced composites- a nondestructive approach" (2007). LSU Doctoral Dissertations. 1878.