Impedance, acoustic layered structures

A resonance in the layered stracture occurs when echoes between two boundaries travel back and forth due to differences in acoustic impedances at the boundaries. For multi-layer structures a number of resonances can be observed depending on their geometry and condition. For each particular defect-free structure and given transducer we obtain a characteristic resonance pattern, an ultrasonic signature, which can be used as a reference. 

The new instrument introduced for inspection of multi-layer structures from polymeric and composite metals and materials in air-space industry and this is acoustic flaw detector AD-64M. The principle of its operation based on impedance and free vibration methods with further spectral processing of the obtained signal. 

Bias-induced reverse piezoelectric response Broadband dielectric spectroscopy (BDS) Dielectric permittivity spectrum Dielectric resonance spectroscopy Elastic modulus Ferroelectrets Electrical breakdown Acoustic method Characterization Dynamic coefficient Interferometric method Pressure and frequency dependence of piezoelectric coefficient Profilometer Quasistatic piezoelectric coefficient Stress-strain curves Thermal stability of piezoelectricity Ferroelectric hysteresis Impedance spectroscopy Laser-induced pressure pulse Layer-structure model of ferroelectret Low-field dielectric spectroscopy Nonlinear dielectric spectroscopy Piezoelectrically generated pressure step technique (PPS) Pyroelectric current spectrum Pyroelectric microscopy Pyroelectricity Quasistatic method Scale transform method Scanning pyroelectric microscopy (SPEM) Thermal step teehnique Thermal wave technique Thermal-pulse method Weibull distribution... 

When selective layers are deposited, the whole structure must be treated as a multiple resonator in which the reflection and/or refraction of the acoustic energy occurs at each interface. For example, when a polymer film is deposited on top of the gold electrode of the QCM, it is the polymer-Au interface with which we are concerned. When the mass loading of multiple structures becomes too high, the effect of the impedance mismatches becomes significant and the crystal ceases to oscillate. Even approximate treatment of the multiple resonator is difficult because densities, as well as thicknesses and shear moduli, of the individual layers must be known. 

Figure 32 shows the schematic structure of the tested CFRP sample. The CFRP sample had a multilayered structure of CFRP cloth. The carbon fibers in one layer of cloth were aligned unidirectionally and the adjacent layers of cloth were set cross-directionally. The thickness of CFRP cloth was 70 pm, and the examined CFRP sample was composed of 72 layers of doth (about 5 mm thickness in all). In order to model the separation of the CFRP layers (defect), a Teflon sheet (80 pm in thickness) was sandwiched between the 54th and the 55th layers of CFRP cloth, at a level 3.8 mm below the CFRP top surface. The sound velocity and the acoustic impedance of the CFRP sample are about 3 X 10 m/s and 5 x 10 Pa s/m, respectively. Water was used as the coupling medium because of its low ultrasonic attenuation characteristics, and the CFRP... 

The technique involves evaluating reflection (echo) and transmission characteristics of an ultrasound radiation, when crossing layers of different density, which present different acoustic impedance. Illumination intensity of different zones using echography is proportional to the density of the crossed structures, thus, the technique detects the presence... 

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