F. Severin

Acoustic Microscopy of Internal Structure of Resistance Spot Welds

Acoustic microscopy, although relatively new, has many advantages within the industrial quality control process. Its high degree of sensitivity, resolution, and reliability make it ideal for use in resistance spot weld analysis, aiding in visualization of small-scale nugget failures, as well as other defects, at various depths. Acoustic microscopy makes it possible to inspect fine detail of internal structures, providing reliable inspection and characterization of weld joints. Besides weld size measurements, this technique is able to provide high resolution, three-dimensional images of the weld nuggets, revealing possible imperfections within its microstructure that may affect joint quality. The high degree of accuracy allows one to consider the results of acoustic microscopy an authoritative measure of weld size, particularly in the case of high strength steels, dual phase steel, USIBOR steel, etc. Indeed, this technique is effective even when both conventional ultrasound and hammer and chisel methods are not. In this paper, the potential of scanning acoustic microscopy as a means to provide qualitative and quantitative information about the internal microstructure of the resistance spot welds is demonstrated. Thus, acoustic microscopy is shown to be a unique and effective laboratory instrument for the evaluation and calibration of weld quality.

Acoustic imaging of thick biological tissue

Up to now, biomedical imaging with ultrasound for observing a cellular tissue structure has been limited to very thinly sliced tissue at very high ultrasonic frequencies, i.e., 1 GHz. In this paper, we present the results of a systematic study to use a 150 to 200 MHz frequency range for thickly sliced biological tissue. A mechanical scanning reflection acoustic microscope (SAM) was used for obtaining horizontal cross-sectional images (C-scans) showing cellular structures. In the study, sectioned specimens of human breast cancer and tissues from the small intestine were prepared and examined. Some accessories for biomedical application were integrated into our SAM (Sonix HS-1000 and Olympus UH-3), which operated in pulse-wave and tone-burst wave modes, respectively. We found that the frequency 100 to 200 MHz provides optimal balance between resolution and penetration depth for examining the thickly sliced specimens. The images obtained with the lens focused at different depths revealed cellular structures whose morphology was very similar to that seen in the thinly sectioned specimens with optical and scanning acoustic microscopy. The SAM operation in the pulse-echo mode permits the imaging of tissue structure at the surface, and it also opens up the potential for attenuation imaging representing reflection from the substrate behind the thick specimen. We present such images of breast cancer proving the methods applicability to overall tumor detection. SAM with a high-frequency tone-burst ultrasonic wave reveals details of tissue structure, and both methods may serve as additional diagnostic tools in a hospital environment.

High Resolution Ultrasonic Method for 3D Fingerprint Representation in Biometrics

Biometrics is an important field which studies different possible ways of personal identification. Among a number of existing biometric techniques fingerprint recognition stands alone – because very large database of fingerprints has already been acquired. Also, fingerprints are an important evidence that can be collected at a crime scene. Therefore, of all automated biometric techniques, especially in the field of law enforcement, fingerprint identification seems to be the most promising. Ultrasonic method of fingerprint imaging was originally introduced over a decade as the mapping of the reflection coefficient at the interface between the finger and a covering plate and has shown very good reliability and free from imperfections of previous two methods. This work introduces a newer development of the ultrasonic fingerprint imaging, focusing on the imaging of the internal structures of fingerprints (including sweat pores) with raw acoustic resolution of about 500 dpi (0.05 mm) using a scanning acoustic microscope to obtain images and acoustic data in the form of 3D data array. C-scans from different depths inside the fingerprint area of fingers of several volunteers were obtained and showed good contrast of ridges-and-valleys patterns and practically exact correspondence to the standard ink-and-paper prints of the same areas. Important feature reveled on the acoustic images was the clear appearance of the sweat pores, which could provide additional means of identification.

Locating an acoustic point source scattered by a skull phantom via time reversal matched filtering

This paper examines the utilization of the time reversal matched filtering method to resolve the location of an acoustic point source beneath a skull phantom (variable thickness layer), without the removal of this layer. This acoustical process is examined experimentally in a water tank immersion system containing an acoustic source, a custom-made skull phantom, and a receiving transducer in a pitch-catch arrangement. The phantom is designed to approximately model the acoustic properties of an average human skull bone (minus the diploe layer), while the variable thickness of the phantom introduces a variable time delay to the acoustic wave, relative to its entry point on the phantom. This variable delay is measured and corrected for, and a matched filtering time reversed process is used to determine the location of the point source. The results of the experiment are examined for various positions of the acoustic source behind the phantom and compared to the reference cases with no phantom present. The average distance between these two cases is found to be 4.36 mm, and within the expected deviation in results due to not accounting for the effects of refraction.

Adaptive beamforming for ultrasonic phased array focusing through layered structures

Successful realization of ultrasonic imaging through a multilayered composite barrier is hampered by scattering, attenuation, and multiple reflections of acoustic waves at and inside the barrier. These effects tend to distort the beam pattern produced by conventional phased arrays, defocusing the ultrasonic field transmitted through the barrier and causing image quality degradation and resolution loss. To compensate for the refraction and multiple reflection effects, we developed an adaptive beamforming algorithm for small-aperture linear phased arrays. After assessing the barriers local geometry, the method calculates a new timing distribution to refocus the distorted beam at its original location. The procedure is in fact a construction of a matched filter that automatically adapts the transmission pattern of the phased array to the local geometry of the barrier and cancel its distorting effect In this work, the adaptive beamforming algorithms, in transmission mode, for the barriers in the form of a flat homogeneous layer, a layer with a smooth, randomly curved back surface and a two-layered combination of the above have been developed and experimentally verified on custom-engineered samples with prescribed acoustical properties. The algorithms were implemented on ULA-OP, an ultrasound advanced open-platform (University of Florence), controlling 64 active elements on a 128-elements phased array. Experimental measurements of original, distorted and corrected beam profiles confirm the ability of our algorithms to refocus the beam after passing through a scattering and refractive sample. Different excitation signals and windowing options introduced through ULA-OP were examined and compared.

4I-2 Ultrasonic Pulse-Echo NDE of Adhesive Bonds in Sheet-Metal Assemblies

The development of a 20 MHz pulse-echo method for nondestructive evaluation (NDE) of adhesive bonds was undertaken to provide assurance of bond integrity in vehicle body assemblies. This new NDE method features improvements over previous methods implemented in production, and extends the range of bond evaluation effectiveness. The NDE is accomplished by the acquisition and analysis of acoustic echoes that return from bond joints that have interfaces between layers with large acoustical impedance mismatch. These echoes reverberate in the multilayered joint structures and are captured for a computer-automated analysis that provides a rapid interpretation of the indications, and subsequently yields a simplified display of the inspection results

Distortion of shear waves passing through a friction coupled interface

Friction coupled interfaces closed under pressure are considered as a possible source of nonlinearity for the case of horizontally polarized shear waves. The distortion of a normally incident harmonic wave is calculated. From the Fourier transform, it is explicitly shown that only odd harmonics will be produced.

Theoretical and Experimental Study of the Acoustic Nonlinearities at an Interface with Poor Adhesive Bonding

The problem of characterizing adhesion bonds nondestructively is of ever increasing importance, as industry moves towards the use of adhesive technology as a means of efficiently bonding all types of materials. While conventional ultrasonic methods seem to provide adequate inspection for delaminations or missing glue, it doesn’t appear capable of distinguishing poorly cured adhesion, “kissing” delaminations, or weak adhesive bonds. It has long been speculated that nonlinear ultrasonic inspection might provide a solution to these problems. Numerous recent papers have been devoted to these proposals, dealing with both the experimental and the theoretical difficulties they pose, which are significant1, 2, 3, 4. It is our view that the main difficulties are inherent in the interpretation of the origin of tiny harmonic distortions being produced in a complicated structure with numerous frequency dependent effects such as resonance, attenuation, reflection, diffraction, and possibly dispersion, as well as filtration in the generating and recieving electro-mechanical systems. The approach we propose is therefore to couple a simple and flexible time-domain theory, hopefully capable of providing recognizable predictions, with a minimalist experimental approach which is clear and interpretable, based on powerful input signals and sizeable distortion of the waveform.

Development of a Nondestructive Method for Ultrasonic Evaluation of Adhesive Bond Joint Performance

Adhesives are a group of substances with chemical composition and structure specially formulated for joining the surfaces of materials together. These substances are generally applied to specific areas of substrate materials that are to be bonded in order to fabricate an assembly. The region between the two or more substrate materials being joined together by the adhesive, including the surface interfaces, is the adhesive bond joint (ABJ). Irrespective of the chemical nature and of the mechanisms of adhesion in a bonded assembly, the adhesive material is usually a thin layer between two substrate surfaces of the components of the assembly. The components of the assembly and the adhesive material are generally composed of materials that are substantially different in many of their physical, chemical and mechanical characteristics. Therefore, regardless of the chemical composition of the adhesive and adherent materials, all ABJ’s are multilayered structures, composed of highly inhomogeneous materials, in which the adhesive must contribute to the structural integrity of the whole assembly. In other words, the adhesives are not just a means of joining structural elements, but are load-bearing components of structural systems; therefore their failure could cause the structure to fail.

Resolving the Location of Acoustic Point Sources Scattered Due to the Presence of a Skull Phantom

This paper considers resolving the location of a foreign object in the brain without the removal of the skull bone by detecting and processing the acoustic waves emitted from the foreign object modeled as point source. The variable thickness of the skull bone causes propagation acoustic waves to be scattered in such a manner that the acoustic wave undergoes a variable time delay relative to its entry point on the skull. Matched filtering can be used to detect the acoustic wave front, the time delay variations of the skull can be corrected for, and matched filtering time reversal algorithms can then detect the location of the acoustic source. This process is examined experimentally in a water tank system containing an acoustic source, custom-made skull phantom, and receiver. The apparatus is arranged in transmission mode so that the acoustic waves are emitted from the source, scattered by the phantom, and then received by a second transducer. The skull phantom has been designed so that the acoustic properties (velocity, density, and attenuation correspond approximately to those of a typical human skull. In addition, the phantom has been molded so that the surface closest to the acoustic source has smoothly oscillating ridges and valleys and a flat outer surface, approximately modeling a real-world skull bone. The data obtained from the experiment is processed to detect and extract the scattered acoustic wave front and correct for the time of flight variations in the skull. This re-creates the approximate wave front of a point source, whose location can be resolved via a matched filtering time reversal algorithm. The results of this process are examined for cases where there is no phantom present (no scattering), and with the phantom present. Comparison of these results shows a correlation between the calculated locations of the acoustic source and the expected location.