D Xiang

Ultrasonic Leaky Wave Measurements for Materials Evaluation

Ultrasonic leaky waves at a liquid/solid interface have been utilized by many investigators to characterize the properties of materials. The leaky surface wave, for example, has been used in acoustic microscopy [1], especially in Line-Focus-Beam (LFB) microscopy [2], for many years. Nearly all leaky-surface-wave measurements by acoustic microscopy rely on the interpretation of a V(z) curve, which is a record of the transducer’s voltage V as a function of the defocusing distance z between the transducer’s focal plane and the specimen surface while the transducer is operated in a tone-burst mode [3]. The focus of this V(z) analysis is the interference of two principal acoustic components — the direct reflection and the leaky surface wave. One of the challenges of this technique is to extract, from the complex V(z) interference patterns, the various leaky wave modes that may be simultaneously present in the material. Another challenge is the complexity of modeling and analyzing the V(z) curve, especially involving the pupil function that must be determined for the microscope’s particular lens [4].

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.

Time Domain Waveforms of a Line-Focus Transducer Probing Anisotropic Solids

Ultrasonic wave velocity measurements can be used to evaluate material properties such as elasticity, texture (crystalline structure), surface roughness, coating thickness, porosity, and residual stress. These properties may vary as a function of material sample position and orientation. Scanning acoustic microscope systems equipped with line-focus transducers have been successfully used to measure many of these inhomogeneous and anisotropic properties. These systems use a technique that relies on the measurement of the reflected high frequency (∼200 MHz) tone-burst echo amplitude, V, as a function of the defocus distance, z. Analysis of the interference minima in the V(z) curve yields the surface wave velocity in a direction perpendicular to the focal line, with a resolution of tens of micrometers in the propagation direction. Both theory and instrumentation are well documented for this technique [1–3].

Spatial Resolution with Time-and-Polarization-Resolved Acoustic Microscopy

Spatial resolution is an important factor in ultrasonic materials characterization. Scanning acoustic microscopy [1–2] has proved to be a useful tool for materials evaluation with micrometer-scale spatial resolution. Point-focus-beam (PFB) acoustic microscopy has high spatial resolution and is often used to produce images as well as to probe material inhomogeneity. However, a disadvantage of the PFB technique lies in its insensitivity to material anisotropy. In contrast, line-focus-beam (LFB) acoustic microscopy can provide a directional ultrasonic velocity measurement and is employed for characterization of anisotropic materials [3–5]. But the LFB technique, with its unidirectional spatial resolution, is generally incapable of producing images, and is therefore disadvantageous for probing inhomogeneous materials. In response to this need, a variety of lens designs [6–9] in acoustic microscopy have been proposed for measuring materials, which are both inhomogeneous and anisotropic.

Ultrasonics research in the NIST Manufacturing Engineering Laboratory

The Ultrasonic Standards Group in the Manufacturing Engineering Laboratory of the National Institute of Standards and Technology (NIST) performs research, develops standards, and offers calibration services in support of United States industry. In this paper, two areas of recent experimental research using a thin film piezoelectric polymer are briefly reviewed. One is the development and application of a large aperture lensless line-focus transducer for time-resolved pulsed-wave measurements. The second is the effect of nonlinear wave propagation on pulsed waveforms in industrial ultrasonic nondestructive testing applications.