B.R. Tittmann

Method of measuring the dynamic viscosity of a viscous fluid utilizing acoustic transducer

The dynamic viscosity of a viscous medium is measured by positioning an acoustic transducer in the temperature and pressure environment of the medium and spaced from the medium, then measuring a first resonant frequency and bandwidth for acoustic shear wave propagation within the transducer. The transducer is then positioned in surface contact with the medium, and a second resonant frequency and bandwidth are measured. The viscosity of the medium is calculated from the difference between the first and second resonant frequencies and bandwidths. The step of measuring a first resonant frequency and bandwidth involves applying a first input signal to the transducer to generate acoustic shear waves within the transducer, measuring the frequency and amplitude of the output signal produced by the transducer in response to the acoustic shear waves, and repeating the steps of applying and measuring for a range of first input signal frequencies to determine a first resonant frequency and bandwidth for the transducer. Similarly, the step of measuring a second resonant frequency and bandwidth includes applying a second input signal to the transducer to generate acoustic shear waves within the transducer, measuring the frequency and amplitude of the output signal produced by the transducer in response to the acoustic shear waves, and repeating the steps of applying and measuring for a range of second input signal frequencies to determine a second resonant frequency and bandwidth for the transducer.

Sonic gas pressure gauge

A method and an apparatus are provided for measuring the pressure of a gas within a sealed vessel. A sonic transducer is used to apply an oscillating force to the surface of the vessel. The frequency of the ultrasonic wave is swept through a range which causes resonant vibration of the gas in the vessel. A receiving transducer measures the amplitude of the resultant vibration at the vessel surface and reveals the resonant frequency of the gas as peaks in the amplitude of the sweep. The resonant frequency obtained depends upon the composition of the gas, its pressure and temperature, and the shape of the confining vessel. These relationships can be predetermined empirically so that the pressure inside the vessel can be calculated when the composition of the gas, its temperature, and shape of the confining vessel are known. The output of the receiver is fed into a computer which is programmed to calculate the pressure based upon these predetermined relationships which are stored in the computer.

Dissipation of elastic waves in fluid saturated rocks

Experimental results are presented on the dissipation of elastic waves in porous rock. These results show the combined effects of frequency, pressure, temperature, and fluid viscosity on attenuation and modulus in fluid‐saturated sandstones. An attenuation peak and an associated modulus defect have been mapped. The data are consistent with a linear relaxation mechanism involving the flow of a viscous intergranular fluid, with a characteristic relaxation time proporational to fluid viscosity.

Synthesis of an Ultrasonic Array Using Laser-Based Techniques

Both the thermoelastic generation of ultrasound using a laser and the interferometric detection of ultrasound a re well known. The extension of these techniques to the formation of arrays of sources and receivers will be described. These techniques a r e of particular interest for t he rapid inspection of materials without t he need for con tac t or immersion of the material. W e will report t he results of experiments t o study the ultrasonic source characterist ics as a function of laser spot s ize and shape, and as a function of the surface boundary constraints. The requirements of t he detector such as the sensitivity, bandwidth, acceptance angle and suitability for use in a n array will be described. Results of a n analysis of the suitability of different interferometers will be presented.

Ultrasonic Attenuation in Carbon-Carbon Composites and the Determination of Porosity

Ultrasonic spectroscopy together with optical microscopy were used to study the distribution of porosity in twodimensional carbon-carbon composites. In the frequencydependent attenuation (0.5-3.0 MHz), characteristic features are found which lead to separate estimates of mean size and volume fraction of pores. Four samples of two-dimensional carbon-carbon composites were studied, corresponding to each of four key steps in the fabrication process. A method was developed to analyze the data. A correlation between the degree of porosity and ultrasonic attenuation was found in qualitative agreement with theory. The absence of quantitative agreement is thought to be a result of the nonideal (nonspherical) geometry of the pores.

Infrared Sensing of Ultrasonically Generated Thermal Waves for NDE Applications

The ultrasonic nondestructive evaluation (NDE) of composites (e.g., graphite reinforced epoxy GR/E) for delamination and porosity is difficult because of high a ttenuation of the material, which ultimately converts the ultrasonic energy to heat via the generation of thermal waves. The technique described here seeks to take advantage of this energy conversion by detecting and characterizing surface temperature contours produced by anomalies in thermal diffusivity and/or acoustic propagation in the subsurface and interior regions of the material. A similar technique has recently been proposed for material characterization studies (e.g., dislocation density in metal single crystals). This presentation will show the e xperimental approach and the results of exploratory experiments on GR/E with two types of flaws: porosity and simulated impact damage. cussion will include a conceptual approach to the rapid inspection of large structures. This dis

Laser-Induced Acoustic Shock Waves

The use of laser pulses to induce elastic waves in materials has many applications, ranging from weapons damage to noncontact NDE. This paper presents preliminary results on the generation of high-pressure nonlinear acoustic waves by the use of high-intensity laser pulses. The laser used was a Nd:YAC Q-switched laser with a wavelength of 1.06 microns, peak energy of 700 mj, peak power of - 108 W/cm2, Gaussian intensity profile and pulsewidth of 10 ns. In the first phase of the experiments, a water volume was chosen as a model propagation medium to simulate a homogeneous, isotropic medium supporting only longitudinal waves. The beam was focussed onto the water to achieve the intensity necessary to cause dielectric breakdown, evidenced by optical emissions. The resulting acoustic shock waves were detected with a commercial hydrophone PKI4 (2.54 mm diameter) with a frequency response flat to 300 kHz. This was used to measure the frequencydependent ultrasonic radiation pattern at a distance of 1 m as a function of elevation angle, laser intensity and depth of focal point below the surface of the water. Similar but somewhat sharper results were obtained when the focus was changed from near surface to 30 mm below the surface. The data substantiate that the effective source of the shock waves is in the shape of a small cylindrical column approximately 20 mm long in length, oriented perpendicular to the surface, and some distance below it, depending upon the focal point of the laser beam.

Laser-Based Ultrasonics on Gr/Epoxy Composite a Systems Analysis

Critical issues are examined in the application of laser generation and detection of ultrasound to the inspection of large area air-frame composites. Among these issues are surface roughness, signal-to-noise ratio, insensitivity to the path length between the part and detector, and wide-band versus narrow-band generation. Supporting experiments are reported on broad-band and narrow-band generation in Gr/Epoxy panels and angular reflectance measurements on painted and unpainted Gr/Epoxy. On the basis of these measurements, a laser-in/laser-out systems analysis is carried out for a 10 mm diameter delamination about 1 cm deep. The analysis assumes that a Spherical Fabry-Perot interferometer is used for detection and a 10 nsec laser pulse with a peak power of 13 MW/cm2 for generation. The estimates indicate a S/N ≈ 20 dB for a detection probe laser power of about 400 mW.

Elastic Anisotropy of Carbon-Carbon Composites During the Fabrication Processes

The elastic coefficients of carbon-carbon composites were measured at four steps during the fabrication process. An ultrasonic materials characterization testbed was used to show that the carbonization-induced porosity drastically affects the anisotropy factor 2C66/Cz z-C1 2. These and other results suggest that the use of ultrasonic waves (by noncontact methods such as photoacoustics) could provide a me

Acoustic measurement of near surface property gradients

ningful technique for monitoring the fabrication process and thereby allow optimization of the control parameters and ultimately the quality and processing speed of the final product.