Local defect resonance and nonlinearity of porosity air bubbles in solids

Acoustics of bubbles is quite developed field mainly due to multiple cavitation-related ultrasonic applications in liquids. New applications, which require detailed studies of ultrasound encounter with bubble in solid materials, have become apparent recently and are concerned with detectability of porosity in advanced solid materials based on layered technology, like composite and additive manufactured structures. To elucidate the transition from liquids to solids the present paper starts from theoretical similarity between both and proceeds to experimental study of the resonance acoustic effects of air bubbles in epoxy resin. The LDR frequencies are shown to be reciprocal to the bubble radius so that the latter can be evaluated if the frequency is known. The bubbles excited at the LDR frequencies and their subharmonics (superharmonic resonance) manifest extraordinary wide higher harmonic spectra that implies a nonlinear means for nondestructive testing of porosity in composites and other materials.