Microwave Metrology for Nondestructive Evaluation

Electromagnetic energy propagating at microwave frequencies can be dealt with in many respects in a manner similar to that of visible light. The principal difference between visible light and microwave "light" is that microwave wavelengths can be between 1,000 and 10,000 times longer than the wavelength associated with visible light. In some cases, these long wavelengths can provide potential advantages for certain types of remote inspection processes. For example, work was completed this year on a remote but quantitative method for determining the macroscopic RMS urface roughness and correlation lenghts of a conductive surface. By measuring the complex amplitude variations in the speckle pattern of microwave light scattered from a rough surface, the nature of the roughness could be directly quantified. Using 60 GHz microwave energy, it was possible to confirm experimentally the ability of this technique to make quantitative measurements over a range of surface roughnesses orresponding to sandpaper grits over a range from 16-80 grit.

In addition to roughness measurements, work is under way to determine the practical limits with which microwave energy might be substituted for visible light energy for interferometric pickup of ultrasonic signals. Part of the difficulty in making adequately sensitive vibration measurements using visible laser light to detect ultrasonic and acoustic emission signals lies in the fact that signal-to-noise ratio improves as the square root of the amount of light which can be collected after reflection from the surface of an object. When an object's surface is optically diffuse, light is scattered in all directions and very little of it can be recovered into the interferometer for sensitive surface vibration measurements. A surface can be considered to be fully optically rough when fluctuations in peak height exceed 1/4 of the wavelength of the incident radiation. Thus, for visible light most sensitive measurements are made when the surface is polished to a near mirror-like state. On the other hand, if microwave energy is used instead of visible laser light, surfaces can have far greater roughness but yet behave as a near mirror-like surface for microwave "light." Thus, materials whose surfaces appear optically rough and diffuse in the visible spectrum would, in the microwave regime, appear effectively to be highly polished.

The use of microwave energy for interferometric detection of sound, however, though facilitating efficient light collection, presents other instrumentation and noise challenges. Perhaps greatest among these is the fact that conventional detection methods for microwave energy have effective quantum efficiencies many orders of magnitude poorer than photon sensitive detectors used in the visible light regime. As part of this program alternative sensors, such as high speed bolometric devices, will be investigated as possible means to overcome the quantum inefficiencies of conventional detectors and thus facilitate the use of microwaves as an alternative means for remote and noncontact detection of ultrasound.

Practical Application

The ability to perform remote measurement of surface roughnesses should assist in making critical measurements relative to wear and degradation of materials in service. It should be possible, for example, to measure roughening owing to corrosion of pipelines and vessels coated with a nonconducting insulation material. In principle, it should be possible to look for similar roughening resulting from corrosion on the interior surfaces of aircraft skins. Such measurements could be made without removing insulation or upholstery from the cabin area of commercial or military aircraft.

Practical developments which could make feasible the use of microwave systems for ultrasound detection are of great potential interest since such systems should permit measurements on as-machined and optically diffuse surfaces. Because of their history in military and communications applications, microwave components are more rugged and, in many cases, less expensive than lasers and related optical components currently used for noncontact detection of ultrasound.