Characterization of hollow particulate and graded composites using ultrasonic technique

An experimental study has been conducted to characterize hollow particulate composites (syntactic foams) using ultrasonic pulse echo techniques. Materials tested for this study consisted of low viscosity epoxy matrix with embeded soda-lime-borosilicate glass micro-balloons of different volume fractions. Three sizes of microballoons ranging from 30-65 microns were tested. Measurements of longitudinal and shear wave speed and attenuation of ultrasonic wave in syntactic foams were taken. These wave speed values were further utilized to calculate the various moduli of the material. After understanding the behavior of syntactic foams for low volume fractions, functionally graded materials (FGM) with linear variation of increasing volume fraction were manufactured and studied. Further quasi-static compression and low velocity impacts were also performed to better understand the static and absorption behavior of both syntactic foams and FGM materials. It was found that larger microballoon size had higher attenuation values but not necessarily higher wave speeds in syntactic foams. Matrix absorption was the main attenuation parameter. Ultrasonic tests on FGMs suggest higher degree of interaction due to the impedance mismatch between each layer. Lower volume fractions had higher compressive strength than higher volume fractions. This knowledge is important in understanding the bond strength between the particulates and the epoxy matrix. The peak stress in impact loading decreased with increasing volume fraction and was highest for the smallest size microballoon. Peak load of smallest microballoon size FGM was higher than plain syntactic foam of similar density