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.

Linear vs nonlinear ultrasonic testing of kissing bonds in adhesive joints.

Kissing bonds in adhesive joints are precursors to damage and failure in materials and components used in safety-critical industries. They are zero-volume, low-contrast contact defects widely regarded as "invisible" in conventional ultrasonic testing. In this study, the recognition of the kissing bonds is examined in automotive industry-relevant aluminum lap-joints with standard bonding procedures using epoxy- and silicone-based adhesives. The protocol to simulate kissing bonds comprised customary surface contaminants PTFE oil and PTFE spray. Preliminary destructive tests revealed brittle fracture of the bonds with typical single-peak stress-strain curves indicating ultimate strength reduction due to adding contaminants. The curves are analyzed by using nonlinear stress-strain relation with the higher-order terms containing the higher-order nonlinearity parameters. It is shown that the lower-strength bonds manifest a high nonlinearity while the high-strength contacts are candidates for a low nonlinearity. Based on that, the nonlinear approach is set side by side with linear ultrasonic testing for experimental locating of the kissing bonds fabricated in the adhesive lap-joints. The sensitivity of the linear ultrasound is shown to be adequate to detect only a substantial bonding force reduction caused by the irregular interface defects in adhesives, while a minor contact softening due to kissing bonds remains undistinguishable. On the contrary, the probing of the kissing bonds vibration with nonlinear laser vibrometry reveals dramatic growth of the higher harmonic amplitudes and thus validates highly-sensitive detectabilty of these troublesome defects.

Ultrasonic frequency mixing via local defect resonance for defect imaging in composites.

A new approach to nonlinear frequency mixing based on local damage resonance is proposed, analysed and tested experimentally for flexural waves in composites. The method is free from stringent requirements on the mode types and frequencies for interacting waves. The resonance of damage enhances strongly its higher-order nonlinear response and boosts the efficiency of generation for numerous-order combination frequencies. The damage resonance combined with its strong nonlinearity also provides locality of nonlinear interaction even for continuous wave operation. The combination frequencies generated locally in the damaged area are the footprints of damage and used for its detection, location and visualization. A single C-scan yields a number of images of the defect corresponding to various nonlinearly generated frequencies. Various versions of the resonant frequency mixing are considered and applied to nonlinear imaging of defects in composite materials.

Thermosonic Chladni figures for defect-selective imaging.

Thermosonic patterns produced by resonant vibrations of simulated and realistic defects are experimentally observed and compared with conventional vibration Chladni figures. The patterns are interpreted on the basis of hysteretic damping model that accounts for in-plane polarization component of vibrations. The analysis and simulation results show that thermosonic Chladni figures are the patterns of dissipation of vibration energy determined by a square of the in-plane strain developed in the resonant vibrations. The difference in nodal patterns between the two types of Chladni figures is due to additional extension-compression of the material mainly in near boundary area. The contribution of various order resonance modes in single-frequency imaging is illustrated and their superposition is shown to produce a full-scale thermosonic image of a resonant defect in a wideband excitation mode.

Air-coupled laser vibrometry: analysis and applications.

Acousto-optic interaction between a narrow laser beam and acoustic waves in air is analyzed theoretically. The photoelastic relation in air is used to derive the phase modulation of laser light in air-coupled reflection vibrometry induced by angular spatial spectral components comprising the acoustic beam. Maximum interaction was found for the zero spatial acoustic component propagating normal to the laser beam. The angular dependence of the imaging efficiency is determined for the axial and nonaxial acoustic components with the regard for the laser beam steering in the scanning mode. The sensitivity of air-coupled vibrometry is compared with conventional "Doppler" reflection vibrometry. Applications of the methodology for visualization of linear and nonlinear air-coupled fields are demonstrated.

Instability, chaos, and "memory" in acoustic-wave-crack interaction.

A new class of nonlinear acoustic phenomena has been observed for acoustic wave interactions with cracked defects in solids. Parametric modulation of crack stiffness results in fractional acoustic subharmonics, wave instability, and generation of chaotic noiselike acoustic excitations. Acoustic-wave impact on a crack is shown to exhibit amplitude hysteresis and storage for parametric and nonlinear acoustic effects. The measured storage time amounts to several hours and is believed to be due to a long-term relaxation of thermally induced microstrain within a crack area.