| This thesis comprises two main sections, namely a critical evaluation of the use of the Ultrasonic pulse echo immersion technique to measure attenuation and an application of the same to particulate composites. The former consisted of testing the main assumptions adopted by the different approaches developed to carry out the attenuation coefficient measurement. The first assumption states that a perfectly bonded interface between water and specimen exists. A second assumption requires consistent reflection coefficients every time the specimen is immersed. Finally, some existing methods assume equal reflection coefficients on either side of any specimen during a particular immersion. Herein, it is experimentally shown that while these conditions hold true for some materials (i.e. Polycarbonate), they are nevertheless violated for others (some hydrophobic materials). The materials that violate all three assumptions are more likely to be those that present hydrophobic surfaces. Due to their hydrophobicity the bond between water and the specimen is very weak and random distributions of air molecules can be trapped and retained over the surfaces during the immersion. In these cases, all current techniques would provide erroneous values for the attenuation coefficient. Therefore, a new method was proposed, tested and validated to measure the attenuation coefficient of these special materials and any others. A new methodology having been derived, it was then applied to glass/epoxy particulate composites where longitudinal wave speeds and attenuation coefficients were measured for several specimens with different solid glass microspheres and different volume fractions. Contrary to expectations, it was observed that the presence of microspheres is not always beneficial, if an increase in the attenuation coefficient is desired, and often adversely affects the behavior of the matrix. |
