Lloyd A. Ahlberg

Dynamic Viscosity Measurements of Fluids Employing Resonance Characteristics of a Piezoelectric Element Vibrating in the Shear Mode

Many material processes may be usefully characterized by measurement of the effective viscosity during the solidification of the material. For example, monitoring the viscosity of a resin during the curing of a composite material allows the optimization of the cure strategy to minimize porosity in the final product. Also, the flow of fluids is essentially a function of their viscosity and well-regulated flow is often required, such as in an automated painter or in a printing press. Thus, it is important to have a method to measure viscosity continuously during the entire process time. It is well-known that viscosity can be determined by using ultrasonic techniques.1 Usually, this is done by measuring the attenuation of longitudinal waves propagating in the viscous medium, but this method requires many assumptions and many conditions to be fulfilled that obviously limit its applicability. Some of these assumptions and conditions are: (1) all the other causes of ultrasonic attenuation such as diffraction, dispersion, thermoelastic loss, must be negligible in comparison to the viscous loss; (2) the sum of the volume and the shear viscosities is measured, rather than only the shear viscosity;2 (3) usually, the ratio of the imaginary part of the stiffness coefficient to the real part of this same coefficient must be assumed to be much lower than unity; (4) the viscous medium must have sufficient thickness that the different echoes in the pulse-echo train are resolved, yet be thin enough that the first echoes are detectable; and (5) internal reflections inside composite laminates must be assumed negligible.

Attenuation and Grain Noise Parameters in Ni-Base Alloys

The frequency dependent ultrasonic attenuation and grain noise were determined for samples of three alloys often used in jet aircraft engine turbine discs: Waspalloy, IN-100, and Ti-6246. In addition to propagation of longitudinal waves, also shear waves were considered. The frequency dependence was extracted from broadband echos received through a low attenuation buffer. A key feature of the results for IN-100 is the presence of a low concentration of micropores which appear to influence the scattering of ultrasound and therefore the attenuation and material noise values. Of the three alloys, Waspalloy was found to have the highest attenuation value and Ti-6246 the lowest.

Q Measurements under Confining Pressure

Birch and Bancroft [1] demonstrated the feasibility of resonating a rock sample under hydrostatic pressure by vibrating the fully encapsulated samples in the torsional mode. They used this approach to obtain some Q data and extensive data on the modulus of rigidity as a function of pressure for a wide variety of rock samples. We have extended their approach with the additional stipulation that the rock sample environment approach lunar conditions; i.e., a vacuum. The motivation has been to provide guidance and aid in the interpretation of lunar seismic experiments which have yielded anomalously low attenuation values. Our technique and some of the results for our torsional mode measurements are summarized below. Also described are preliminary results on measurements in the extensional (Young’s modulus) mode. Most of our Q measurements have been carried out in the pressure range 0 to 2 kbar, because this is the range where partial crack closure produces the most dramatic increases in the velocity and where any changes in Q might also be expected to be most pronounced.

Processing Resin Matrix Composites Using High Intensity Ultrasound

ABSTRACl Experiments were performed to demonstrate several aspects of the use of high intensity ultrasound (HIUS) in the place of heat and pressure as the sole source of energy in processing graphite/epoxy composite laminates. Self-limiting o f the sonic energy absorption was observed in small specimens as curing of the epoxy proceeded, and fully cured, we1 1 consolidated, and pore-f ree materi a1 was produced. Extension to larger specimens requires consideration of how to obtain t he desired spatial distribution of sonic energy in the metal f orm which supports the composite. A special case occurs when the composite part has a nonuniform thickness over its area. A method was developed for uniform consolidation of a 76 m square laminate that was 12-plies thick over half its area and 20-plies thick over the other half, and methods were conceived for extending this processing to full-scale production parts.

Ultrasonic Monitoring of Thermally Curing Resins with the Use of Shear Wave Reflectivity

A technique is presented for estimating t he state of viscosity of a thermally curing resin. The resins complex shear modulus at 1 MHz is derived from the measured reflection coefficient of shear wave pulses at the tool-resin interface. A speci a1 transducer-buffe r assembly that operates at high temperature and provides a reference cal i bration signal has been developed. With this assembly, absolute d eterminations a re made throughout the cure cycle o f the storage (real) and loss (imaginary) components of the shear modulus, from the latter of which the h igh frequency dynamic viscosity is calculated. Comparison with data obtained at low shear rates with a 10 Hz torque viscometer indicates the feasibility of using the high frequency technique to monitor the rheological properties of thermally c uring r esins.