Evgeny Twerdowski

Pipe wall damage detection by electromagnetic acoustic transducer generated guided waves in absence of defect signals

Most investigators emphasize the importance of detecting the reflected signal from the defect to determine if the pipe wall has any damage and to predict the damage location. However, often the small signal from the defect is hidden behind the other arriving wave modes and signal noise. To overcome the difficulties associated with the identification of the small defect signal in the time history plots, in this paper the time history is analyzed well after the arrival of the first defect signal, and after different wave modes have propagated multiple times through the pipe. It is shown that the defective pipe can be clearly identified by analyzing these late arriving diffuse ultrasonic signals. Multiple reflections and scattering of the propagating wave modes by the defect and pipe ends do not hamper the defect detection capability; on the contrary, it apparently stabilizes the signal and makes it easier to distinguish the defective pipe from the defect-free pipe. This paper also highlights difficulties associated with the interpretation of the recorded time histories due to mode conversion by the defect. The design of electro-magnetic acoustic transducers used to generate and receive the guided waves in the pipe is briefly described in the paper.

A differential method for the determination of the time-of-flight for ultrasound under pulsed wide band excitation including chirped signals

For applications involving the determination of variations of the time-of-flight in pulsed echo or transit experiments a method has been developed based on Fourier transformation with forced optimized compression of the reference signal to an only bandwidth limited approximation of a Dirac-function. The respective transformation of time shifted response signals allows the effective separation of otherwise overlapping signals and the detection of differences in the time-of-flight for the individual components with high resolution. The developed processing scheme corrects for dispersion and attenuation in the electronics, the transmission lines, and the transducers. The method is presented and applications are demonstrated.

The influence of the radius of the electrodes employed in Coulomb excitation of acoustic waves in piezoelectric materials

Coulomb excitation and detection of ultrasonic waves in piezoelectric crystals by spherical electrical probes is discussed in view of the opening angle of the cone of longitudinal waves coupling to such a probe. The electric field distribution in the piezoelectric crystal under the probe is modeled by means of finite elements in order to determine the effective size of the probe normalized to the sphere radius. The dynamic impedance of the probe is estimated, and it is shown that a probe of a size appropriate to illuminate or detect from the piezoelectric half space has a frequency-independent impedance of about 3 k&OHgr; under idealizing assumptions. Measurements of the directionality of ultrasound emission and detection at a frequency of about 100 MHz are presented for three probes with different tip radii, varying from about 30 &mgr;m to 2.5 mm. As expected, larger probes yield a higher directionality. A relatively large forward contribution is observed even for small spheres.

Acoustic holography of piezoelectric materials by Coulomb excitation

Electric surface excitation of ultrasound in the Coulomb field of scanned electrically conductive spherical local probes and similar detection has been employed for imaging of the transport properties of acoustic waves in piezoelectric materials including singlecrystalline wafers. The employed Coulomb scheme leads to a fully predictable and almost ideal point excitation and detection. In combination with two-channel quadrature transient detection it allows high precision spatially and temporally resolved holographic imaging. Via modeling of the excitation and propagation properties, the effective elastic tensor and the piezoelectric properties of the observed materials can be determined with high resolution from a single measurement. The generation and detection scheme as well as the theoretical background are demonstrated and applications are exemplified.

Recent Advances on Pipe Inspection using Guided Waves Generated by Electromagnetic Acoustic Transducers

For several years guided waves have been used for pipe wall defect detection. Guided waves have become popular for monitoring large structures because of the capability of these waves to propagate long distances along pipes, plates, interfaces and structural boundaries before loosing their strengths. The current technological challenges are to detect small defects in the pipe wall and estimate their dimensions using appropriate guided wave modes and to generate those modes relatively easily for field applications. Electro-Magnetic Acoustic Transducers (EMAT) can generate guided waves in pipes in the field environment. This paper shows how small defects in the pipe wall can be detected and their dimensions can be estimated by appropriate signal processing technique applied to the signals generated and received by the EMAT.

Noninvasive monitoring of mesenchymal stem cells by 1.2 GHz acoustic microscopy

Cell‐based therapies can benefit from noninvasive and marker‐free monitoring techniques for living cells. For this purpose a phase‐sensitive scanning acoustic microscope operating at a frequency of 1.2 GHz was combined with a commercial confocal laser scanning microscope. The system is equipped with a live‐support system for the long‐term observation of living cells. Confocal acoustic imaging with phase and magnitude contrast and confocal laser scanning microscopy can be performed simultaneously. Both techniques are used in reflection from opposing sides of the object. Time‐lapsed acoustic microscope images of ovine mesenchymal stem cells are presented. For this purpose, a pseudo‐3D representation is generated by encoding the unwrapped phase in the height and the magnitude in the brightness. In the case of highly reflective substrates and sufficiently low reflection from the interface between the cells and the surrounding fluid the echo from the top of the cells can be neglected and the phase contrast ima...

Scanning acoustic defocused transmission microscopy with vector contrast combined with holography for weak bond imaging

Surface focused acoustic transmission microscopy is employed for projection (tomographic) imaging of bonded materials including wafers. Short pulse excitation with apodized focusing transducers operated in transmission and two channel quadrature transient detection are employed for multiple contrast imaging. The achievable contrast schemes are based on mode selection for longitudinal, transverse, mode converted, and scattered modes. The identification of the involved modes including conversion schemes is experimentally accessible by time-gating of the recorded signal and by observation of spatially selected holograms. Perfect bonding, disbonding, and weak bonding can be studied and characterized by the developed mode selective imaging scheme. The characteristic features of weak bonding phenomena are demonstrated and characterized.

Comparative evaluation of ultrasonic lenses and electric point contacts for acoustic flux imaging in piezoelectric single crystals

Conducting micro-spheres approximating point probes have been employed to piezoelectrically excite and detect ultrasonic wave packages in anisotropic single crystals. Imaging based on the detection of magnitude and phase is performed in transmission. The experimental data can be used for the determination of the elastic constants of the material. Here we compare this approach with imaging using conventional ultrasonic lenses and water as a coupling fluid. The large bandwidth and the absence of internal lens echoes in the Coulomb excitation and detection scheme permit unperturbed monitoring of multiple echoes in plane-parallel samples and the detailed investigation of mode conversion processes of longitudinal and transverse waves at the surfaces of the crystal. Due to differences in the coupling between the probes and the ultrasound in the sample, excitation of ultrasound by an acoustic lens or an electrical point contact, respectively, result in noticeably different phonon focusing patterns. This is illustrated for lithium niobate single crystals.

Critical cone channelling in directly bonded wafers

Directly bonded semiconductor wafers have been investigated using ultrasonic transmission tomography and imaging of the phonon focusing patterns at ultrasonic frequencies. Beside of total disbonds, several bonded wafers contained defects that are fully transparent to normally incident waves of longitudinal polarization, and are fully opaque to those of transverse polarization. These defects, which are due to slip boundary conditions at the wafer‐wafer interface, generate an additional acoustic mode by mode conversion at the interface. The additional mode is clearly observable in the experimental phonon focusing patterns and is indicative of the critical cone channelling phenomenon, which is caused by the generation of pseudo‐surface and head waves at weakly bonded solid‐solid interfaces. The effect is also expected to be present in the case of weakly‐bonded isotropic materials. Since the strong pseudo‐surface wave resonance exists only in the case of boundary conditions that allow for the relative displac...

Ultrasonic monitoring for directional solidification experiments onboard of the International Space Station

Ultrasonic monitoring schemes for the detection of the solid-liquid interface during directional solidification have been developed including electronic equipment for the Material Science Laboratory (MSL) of the International Space Station (ISS). Special signal and data processing suitable for automatic monitoring, on board signal averaging, and operation under a limited data transfer condition is discussed. The achievable resolution in the micrometer regime as well as post experimental processing and evaluation for high resolution monitoring are presented and exemplified for typical applications.