The solid/fluid interface appears in many ultrasonic measurement systems. Models for the system must take account of the interface. Analytical models for wave phenomena at the interface (especially curved interfaces) are either difficult or subject to severe approximation. The finite element method is ideal for this especially when the problem domain is bounded. A survey of this subject has been given by Kalinowski [1]. In this paper, an axisymmetric finite element model is developed for a solid medium and a fluid medium in contact. Displacement is used as the primary variable in the solid media and pressure in the fluid. The scalar pressure in the fluid medium makes the total degrees of freedom less than if displacement is used. The global mass matrix and stiffness matrix are rendered symmetric by introducing a potential variable for the fluid medium [2]. The final finite element equations are solved by the explicit integration approach.

The growing complexity of inspection needs in the industrial workplace has contributed to an increasing interest in data fusion techniques. The interest in such methods have been fueled by a perception that classical approaches, involving the use of a single inspection methodology, are sometimes inadequate for capturing all the information necessary for characterizing the test specimen. It is often possible to employ two or more inspection techniques or measurement conditions for evaluating the specimen. Each test may provide a limited but slightly different perspective of the state the test object. Operators have traditionally combined the information from the test results “mentally” to draw conclusions. Recent developments in the field of data fusion, however, allow the information to be integrated on a more systematic basis. The use of such data fusion techniques can potentially improve the probability of detecting flaws and contribute to improved defect characterization results.

Binary acoustic Fresnel lenses (BAFLs) have recently emerged as possible replacements for spherical lenses for applications in acoustic microscopy. BAFLs are surface relief structures that are relatively easy to manufacture compared to conventional spherical lenses. While the latter requires careful grinding and polishing, the former can be easily fabricated to sub-micron dimension accuracy using existing VLSI etching technology. The term binary arises from the fact that each masking step during the lens production creates two phase levels. Therefore, a total of 2 n phase levels are created in n masking etching steps. A special case is when n = 1 (2 phase levels), which corresponds to the conventional Fresnel lens (zone plate).

The concept of data fusion has received significant attention recently. Work in the field has been motivated by the belief that a single NDE measurement is sometimes inadequate and does not offer sufficient information about the specimen under test [1–5]. The rationale underlying data fusion is that the use of multiple NDE methods, together with intelligent techniques for synergistically combining the data, can potentially lead to a better estimate of the desired information.

The detection, isolation, and characterization of flaws in components represent a critical need in manufacturing and quality control. Nondestructive testing (NDT) provides an effective way of inspecting materials for ensuring the quality and integrity of products and systems. Consequently, nondestructive inspection finds extensive application in several industries such as steel, nuclear and electronic industries for the evaluation of complex test objects with minimal interruption of routine operations[1].

In traditional ultrasonic imaging systems, a transducer is scanned across the surface of a specimen at constant intervals. Synthetic aperture focusing techniques (SAFT) have been utilized extensively to process the RF data in order to enhance the signal-to-noise ratio of the image [1]. However, the implementation of the algorithm using sampled RF data has the disadvantage of requiring large memory and high-speed devices. These requirements can be reduced by using the envelope of the RF signal which involves processing the baseband signal. The envelope detection can be easily implemented as part of the receiver circuit.

Synthetic aperture focusing techniques (SAFT) represent a special class of beam steering algorithms [1] that are used to improve the resolution and signal-to-noise ratio (SNR) of ultrasonic images. It has been traditionally performed using focused transducers, with the test material located in a region where the ultrasonic beam diverges beyond the focal point [2,3]. This is to ensure that the transducer collects enough data relating to the defect. Operating in the far-field of the transducer where the spatial field variations are slow minimizes the error.

The acoustic microscope, which utilizes focused ultrasound at hundreds of megahertz or even a few gigahertz, can have comparable wavelength and therefore resolution with the optical microscope since the velocity of ultrasound is five orders lower than that of light in fluid media [1, 2]. Since the appearance of the first acoustic microscope [3], extensive research has been devoted to its applications in the nondestructive testing of materials. With this instrument high contrast micrographs can be obtained which contain unique information not available in other imaging tools and the elastic properties of optically opaque materials can be determined. It is particularly suitable for detecting surface and subsurface defects in metal and ceramic materials and for examining integrated circuits and biological cells.