This report reviews recent and current work on a variety of optical techniques applied to nondestructive evaluation (NDE) and carried out by the National Bureau of Standards. The optical methods discussed include holography, scattering from surfaces, microscopy, scattering from particles, and methods employing optical fibers. Much of this work is aimed at the development of accurate measurement methods for in-service inspection and for process monitoring in manufacturing, and at the development of standards and calibration procedures.
The demands for greater product quality and product reliability has created a need for better techniques of nondestructive evaluation. Each of the available techniques including ultrasonic, acoustical emissions, radiography, thermography, magnetic particles, eddy current, dye penetrant, and optical methods such as holography and shearography has its unique advantages and disadvantages. The selection of one technique depends on the particular objective of the evaluation. When selecting a technique, it is important to have a sound knowledge of the capabilities as well as limitations of each technique. This paper will give a thorough review of two major optical methods of nondestructive testing: holography and shearography.
Production line applications of non-destructive testing (NDT) methods demand detailed investigations not only of the problem to be solved but also of the suitability and capability of the chosen method. For this aim a classification of the NDT methods into relative and absolute ones is presented. The results of an experimental comparison of different NDT methods applied to defect detection in glass-fiber reinforced plastics tubes are demonstrated. Two methods are given special consideration with regard to their applicability in production line: the visual inspection and the holographic interferometry.
The use of holographic interferometry for nondestructive testing (NDT) in nuclear power plants and coal plants is discussed in terms of test code requirements, and is compared with conventional NDT techniques. Holography provides a useful laboratory tool to support the electric utility industry through measurement of deformation, movement, expansion, and vibration. Although optical holography may not have sufficient signal to noise ratio in the power plant environment for the detection of defects buried in thick walled pressure vessels or pipes, it offers a means to measure slight deformations so that pressure vessel integrity could be evaluated directly through response to pressure.
Holographic interferometry has developed into a powerful tool for the inspection of composite structures both at the manufacturing level in factory environments, as well as in damage assessment of aircraft in the field. Once performed only in the laboratory, holographic inspection is the preferred testing technology for certain bonded aircraft engine components. Holographic inspection applications of various composite structures are shown and test results presented.
A review of current applications of pulsed holographic interferometry is followed by a discussion of current limitations to widespread use of the technique. Existing work to overcome the limitations is discussed. A potential new source for pulsed NDT is
Methods of hologram interferometry are used to study load-deflection characteristics of small components. More specifically, fundamentals of the methods of double-exposure hologram interferometry, time average hologram interferometry, and heterodyne hologram interometry are discussed, procedures for quantitative interpretation of holograms are outlined, and their applications are illustrated by representative examples. Results presented in this paper indicate viability of the methods of hologram interferometry for quantitative studies of small components.
The medical device industry has seemed rather reluctant to integrate holographic technology into their product lines. While independent researchers have demonstrated the great potential for holography in imaging and display, it appears that holographic techniques may have the greater potential to solve some of the in-vitro testing and quality and process control problems peculiar to the medical device industry. Examples of these techniques include in-vitro testing of strain distributions surrounding bone plates, contouring for wear properties of artificial joints, and rapid nondestructive determination of leak rates in implantable electronics packages. With new advances in optics and electro-optics comes the hope that these and similar techniques may soon become integral parts of medical device manufacturing.
Electronic Speckle Pattern Interferometry (ESPI) is now fifteen years old. What has held it back in its development? Some recent developments enabling ESPI to become a commercial instrument capable of solving specific industrial tasks will be discussed. The use of this instrument to assist both the opto-mechanical engineer and the experimental engineer to help solve problems raised at product design level as well as later when "fire fighting" pre-production and in- situ crisis that require very short investigative response times.