SAFT FOR ANISOTROPIC MEDIA

As the mechanical behavior of these materials is more complex than their isotropic counterparts, it should be expected that anisotropic materials would be more difficult to test and characterize. Yet, nondestructive test techniques for these materials are all too often simple extensions of isotropic approaches.

Laser UT Applications

Laser based ultrasonic inspection offers many intriguing features for composite inspection. This approach uses the rapid thermal expansion of a material due to the laser beam to launch acoustic waves into a material and an interferometric approach to accurately measure the resulting particle displacements at the part surface. It is rapid, cost-effective ( once the initial system cost has been recovered ) and non-contact, a fact that is particularly important for composites sensitive to moisture degradation. While laser generation is a good way to generate acoustic waves within a material, laser ut has a major drawback in its lack of sensitivity as a detector, at least an order of magnitude less sensitive than conventional piezoelectric transducers. This insensitivity is particularly critical in composites which are by nature highly attenuative due to their heterogeneous microstructure. In order to be more useful in practical situations, some means of improving sensitivity must be developed.

Synthetic Aperture Focusing Technique (SAFT )

In virtually all areas of scientific investigation, digital processing techniques have developed rapidly in recent years due to a rapid

increase in electronics performance and a strong need to enhance signal resolution. Ultrasonics is no exception. The synthetic aperture focusing technique ( SAFT ) offers a unique way to collect and process acoustic data to electronically simulate the action of a physical lens. Focussing can be used to improve signal to noise ratio is improved by bringing multiple signals into coincidence a) physically due to the geometry of a lens or b) electronically by applying an appropriate time delay to the received signals from a given image plane. The use of SAFT has been shown to improve the signal to noise ratio by as much as 20 dB in isotropic applications. Similar improvement may be expected in anisotropic media, provided that a suitable correction is made for the intrinsic directional property variations within the material.

Energy Considerations

There is one unique aspect of ultrasonic testing of anisotropic which must also be addressed as it plays a critical role in signal interpretation – energy propagation. Unlike isotropic media where the group and phase velocities coincide with one another, energy propagation in an anisotropic media will not coincide with the wave normal unless the propagation direction is along a symmetry axis. This phenomena, known as energy flux deviation or beam skew. Acoustic energy does not necessarily propagate in the direction normal to the face of the transducer (wavenormal direction) as one might expect from experience with isotropic media, but with rather be skewed at an oblique angle relative to the wavenormal. This phenomena has important ramifications in both experimental design and the interpretation of ultrasonic data. In particular, any SAFT reconstruction algorithm must account for both beam skew effects as well as the intrinsic variations in acoustic velocity with propagation direction due to anisotropy.

Program Objectives

The principal objective of this research effort is the design and implementation of a reliable SAFT. Software will be designed to calculate the time shift factors for acoustic imaging in anisotropic media and implement then in a SAFT algorithm. This utility of this algorithm in improving signal to noise will be evaluated first for synthetic data with random noise superimposed to simulate experimental error and then data from laboratory experiments.

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