Gerald V. Blessing

The design, construction and application of a large aperture lens-less line-focus PVDF transducer

A large aperture lens-less line-focus transducer for materials characterization is described. The transducer design is based on a time-domain Greens function formalism, which also yields good theoretical corroboration of the experimental results. The transducer construction is readily achieved by conforming commercial polyvinylidene fluoride (PVDF) film to a cylindrical (convex) surface, followed by casting a tungsten-powder-loaded epoxy resin backing material into an attached housing. This transducer can be used to characterize sample material properties by simultaneously measuring surface wave speeds and bulk wave transit times, from which the thickness and anisotropy may be deduced.

Acoustic emission monitoring of high speed grinding of silicon nitride

Acoustic emission (AE) monitoring of a machining process offers real-time sensory input which could provide tool condition and part quality information that is critical to effective process control. However, the choice of sensor, its placement, and how to process the data and extract useful information are challenging application-specific questions which researchers must consider. Here we report an effort to resolve these questions for the case of high speed grinding of silicon nitride using an electroplated single-layered diamond wheel. A grinding experiment was conducted at a wheel speed of 149 m s-1 and continued until the end of the useful wheel life. AE signal data were then collected for each complete pass at given grinding times throughout the useful wheel life. We found that the amplitude of the AE signal monotonically increases with wheel wear, as do grinding forces and energy. Furthermore, the signal power contained in the AE signal proportionally increases with the associated grinding power, which suggests that the AE signal could provide quantitative information of wheel wear in high-speed grinding, and could also be used to determine when the grinding wheel needs replacement.

Ultrasonic measurements of surface roughness

Pulsed ultrasound propagating in water was used at megahertz carrier frequencies (nominally 10-50MHz) to reflect and scatter from rough surfaces in the same way as light. We have considered noncontact ultrasonic techniques as complementary to optical techniques in several ways: (a) for specificapplications such as wet surfaces, (b) for rougher surfaces with average roughness, R(a) ≥ 0.1 µm, and (c) for (simultaneous) profilometry by time-of-flight measurements. Stylus and ultrasonic data are compared. An example of application to the manufacturing environment is for on-line, real-time sensor feedback and process control in the cutting or grinding of metals and ceramics.

Time and polarization resolved ultrasonic measurements using a lensless line-focus transducer

We have developed a transducer which allows the benefits of Line-Focus Beam (LFB) acoustic microscopy to be realized over large areas, using a conventional pulser-receiver. Experimental evidence is presented to show that the transducer is correctly modeled in detail by Greens function theory, and that all relevant wave speeds can also be predicted using a much simpler geometrical ray model. Data obtained by simply rotating the transducer a fixed distance above the specimen are presented using grey-scale plots which establish the ease with which anisotropy can be revealed. Finally, a grey scale plot of rotational-scan data recast in terms of velocity is shown to demonstrate the simultaneous detection of both surface and pseudo-surface waves in the same crystallographic orientation of a silicon specimen.

Imaging of acoustic surface wave slowness

An experimental method has been devised for imaging the acoustic surface wave slowness (inverse of the phase velocity) in anisotropic solids. This technique utilizes a specially designed broadband, line-focus ultrasonic transducer to sense the leaky surface waves as well as leaky pseudosurface waves emanating from a solid immersed in water. By rotating such a transducer about its symmetric axis normal to the solid surface, the orientation-dependent time wave forms can be obtained. These wave forms are readily transformed into a surface wave slowness image with a simple algorithm.

Materials Characterization by a Time-Resolved and Polarization-Sensitive Ultrasonic Technique

Since first developed by Lemons and Quate in 1973 [1], scanning acoustic microscopy has been able to obtain images comparable to those from a high quality optical microscope [2]. In the meantime, many investigators [3–7] have developed that technology to determine the microscopic properties of materials. Among those developments, the line-focus-beam (LFB) acoustic microscopy work of Kushibiki and Chubachi [6–7] in the early 1980’s has been most widely recognized [8–10]. Since the LFB technique is a directional measurement, it can be used to study material anisotropy and stress.

Time-resolved ultrasonic body wave measurements of material anisotropy using a lensless line-focus transducer

For plate-like sample geometries, a line-focus transducer can be used to detect back-reflected echoes through the thickness of the sample. The interaction of the convergent cylindrically focused probing wave with the material anisotropy produces multiple echoes which can be interpreted as the reflected and mode converted waves. These echoes are time-resolved and their arrival times are polarization dependent. A simple polar display of the rotationally scanned time waveforms reveals intriguing details that resemble slowness curves. We present both experimental and theoretical results for body wave measurements using our line-focus transducer on various crystals.

Instrumented indentation and ultrasonic velocity techniques for the evaluation of creep cavitation in silicon nitride

Instrumented indentation and ultrasonic wave velocity techniques combined with precise density change measurements and transmission electron microscopy (TEM) were used to investigate the changes of elastic moduli in silicon nitride after tensile deformation up to 3%. Linear dependencies on strain were also found for the degradation of the indentation modulus, longitudinal and transverse ultrasonic wave velocities, Youngs, shear and bulk moduli and Poissons ratio. The results obtained by indentation technique and ultrasonic method were essentially identical. TEM observation confirmed that multigrain junction cavities were responsible for the density changes and the elastic moduli degradation. The density changes were linearly proportional to tensile strain with the slope of 0.75. Thus, cavitation is the dominant creep mechanism in silicon nitride studied. Instrumented indentation and ultrasound velocity techniques are suitable for non-destructive monitoring of creep damage accumulation in ceramic components.

Ultrasonic Measurement of the Dynamic Elastic Moduli of Small Metal Samples

Ultrasonic velocity measurements were used to determine the dynamic elastic moduli of small metal samples to a 2σ measurement uncertainty of better than 1%. The samples were cylindrical in shape, possessing nominal diameters of 12 mm and thicknesses of 6 mm. Aluminum, titanium, and stainless steel compositions were examined. In the longitudinal wave measurements, a 20-MHz quartz piezoelectric element transducer was employed in pulse-echo mode using a tone-burst excitation. These longitudinal time-of-flight measurements were made with a repeatable 2σ measurement uncertainty of ≤1.0 ns. Shear time-of-flight measurements, using a 10-MHz contact ceramic piezoelectric element transducer subjected to tone-burst excitation, were made with a repeatable 2σ measurement uncertainty of ≈2.0 ns. NIST calibration facilities provided measurements for the density and lengths of these samples, as needed for the moduli calculations. Results indicate that the measurement uncertainties for elastic moduli measurements of this type can be kept quite small.

Statistical error analysis of time and polarization resolved ultrasonic measurements

The time and polarization resolved ultrasonic technique which we previously developed has been demonstrated to simultaneously provide measurements of the wave velocity in the coupling liquid, and the leaky surface wave and leaky longitudinal wave velocities in solid samples. To document the measurement precision associated with this technique, a statistical method is employed for the data fit and error analysis. With the help of statistical analysis, the simple ray model used to determine wave velocities in this technique is first confirmed by theoretical data which are predicted by the Greens function. Error analysis is then applied to the experimental data. The results show that this technique has a relative expanded uncertainty (equal to twice the standard deviation) of 0.03% for the wave velocity in water, and an uncertainty less than 0.2% and 2%, respectively, for the leaky surface and leaky longitudinal wave velocities in a crown glass sample. The uncertainty in the repeatability for leaky surface wave measurements is observed to be much less than the expanded uncertainty of a single measurement set. This methodology also has been applied to a set of steel samples. The results allow that the expanded uncertainty for leaky surface wave velocities is less than 0.07%, enabling a correlation of the measured velocities with specific sample heat treatments.