Nelson N. Hsu

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.

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.

Application of the Hilbert-Huang transform to machine tool condition/health monitoring

The machining process is highly dynamic, involving complicated interactions between a workpiece, cutting tool, and the machine tool itself. Appropriate vibration signal analysis can provide insight into the condition of both machine and process. In this paper, the utility of a new time-frequency technique for analyzing complicated time series data, known as the Hilbert-Huang Transform (HHT), is demonstrated. Preliminary analyses of vibration signals from end-milling tests are promising for future implementation of HHT in condition/health monitoring systems.

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.

Transient stress waves interaction with planar flaws

The NBS Center for Building Technology in the United States is developing sonar impact‐echo techniques for non‐destructive testing, notably to trace flaws in concrete, in which flaws reflect the sound waves generated by an impact on the surface. This paper deals with a fundamental aspect of the concept, namely the propagation of the waves within the mass and their interaction with the flaws. The results have validated the use of finite element analysis for studying transient wave propagation in elastic solids.

Response of two layers overlaying a half-space to a suddenly applied point force

Abstract Expressions for the elastodynamic displacements, generated by a normal point force acting on the top layer of two isotropic layers overlaying an isotropic half-space, are derived. Generalized ray theory is used to develop the displacement expressions. In particular, a combinatorial technique for counting the number of wave rays simultaneously arriving at a specific point on the free surface of the top layer is developed. Reflection and transmission coefficients are derived, the different travel paths are analyzed, the products of the reflection and transmission coefficients are generated, and the arrival times of signals traveling along various rays, including head waves, are obtained. A computer program has been developed for efficient numerical implementation of the formulation. Numerical results are shown.

Finite Element Studies of Transient Wave Propagation

The National Bureau of Standards (NBS) has been working to develop a nondestructive test method for heterogenous solids using transient stress waves [1-5]. The method is referred to as the impact-echo method. The technique involves introducing a transient stress pulse into a test object by mechanical impact at a point and measuring the surface displacement caused by the arrival of reflections of the pulse from internal defects and external boundaries. Successful signal interpretation requires an understanding of the nature of transient stress wave propagation in solids containing defects. A primary focus of the NBS program is on using the finite element method to gain this understanding.