Document Type

5. New Techniques and Phenomena

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Of interest in many engineering applications is the change in shape of a body or the change in strain distribution within a body when it is subjected to altered external conditions. To be of maximum use, methods for evaluating these conditions should not affect performance of the body; that is, they should be nondestructive. Optical interferometric techniques have been used extensively to determine the displacement of the surface of a body between two differing states. They are both nondestructive and offer a wide field of view. Depending upon the particular method that is chosen, displacement resolution can range from about 10-2 ~m (100 A) to 10 ~m (1 mm). Various forms of image shearing (displacement) interferometry are capable of measuring the derivative of the surface displacement between two states and hence can give directly the strain distribution. Many of these methods can be performed in real-time. In this case, an image of an appropriate reference state is formed and then used to compare with other states generated by a change in external conditions. We have found iaterferometric methods valuable for a number of specialized studies. Residual strain induced by cycling to increasingly higher pressures within the plastic flow regime has been measured in experimental pressure vessels. This information is being used to determine the optimum welding method and parameters for high energy rate forged stainless steel. Sub-surface defects and material inhomogenities frequently manifest themselves as an asymmetric or localized distortion of a body. We have used holography to search for localized deformation in fabricated components such as wound, fiber-epoxy pressure vessels. The elastic strain and thermal expansion of fabricated components have been measured using holography; this permits actual physical properties of the components to be compared with design or table values. These various applications of interferometry will be discussed in relation to practical problems and compared with other optical methods, in particular, speckle shearing interferometry.



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6 p.