Shi-Chang Wooh

Beam focusing behavior of linear phased arrays

One of the fundamental features of phased arrays is the ability to focus the propagating waves to a specific point within the load material by inducing a parabolic time delay. This required focusing time delay has been modified from the current formulation to incorporate either an odd or even number of elements. A brief procedure leading to the derivation of the pressure distribution for beam focusing is described, which gives rise to an unclosed form. Consequently, a numerical method is desirable for the analysis of beam focusing. Using this approach, beam directivity and pressure distributions are studied to predict the behavior of focusing as compared to steering. This shows a benefit of focusing over steering within the near field of the array, and that the directivity of focusing converges to that of steering in the far field.

Optimum beam steering of linear phased arrays

Abstract A model was developed to compute acoustic pressure distribution of the waves radiated from an ultrasonic linear phased array. Based on the model, beam directivity and steerability were studied for various transducer parameters such as number of elements, inter-element spacing, element width, and transducer frequency. Optimal transducer parameters were determined to obtain the best beam directivity by minimizing the main lobe width, eliminating grating lobes, and suppressing side lobes. It was shown that the solutions for the phased arrays can be reasonably approximated by the simple solutions of discrete line sources, if the element width is reasonably smaller than the wavelength or the steering angle is relatively small. The inter-element spacing as well as the number of elements plays an important role in determining transducer performances.

Influence of phased array element size on beam steering behavior

Certain characteristics of linear phased arrays were studied by investigating the influence of element width on the ultrasonic beam steering properties. The beam directivity patterns were analyzed and used as the criteria for determining optimal transducer design parameters. An approximate value of the critical interelement spacing, whilst squelching undesirable grating lobes, was obtained analytically using a Taylors series expansion. The effect of element size on the pressure in the steering direction was also studied. This work showed that the influence of element size on steering characteristics is marginal and a simple solution for discrete point sources can be used as a very close approximation to that of phased arrays. It was recommended to use the largest possible element width to produce the highest pressure while suppressing the grating lobe amplitude.

A simulation study of the beam steering characteristics for linear phased arrays

Ultrasonic beam steering characteristics for linear phased array transducers are simulated numerically by visualizing the full-field acoustic pressure field of the waves radiated from a linear phased array transducer. The influences of various transducer parameters on the beam steering properties are studied, including number of elements, inter-element spacing, element size, frequency of the transducer and the steering angle. In addition, the effects of these parameters on the near field characteristics are investigated by analyzing the acoustic pressure profile in the steering direction. The simulation results agree well with the analytical solutions which are valid only in the far field. A suggested scheme for optimal transducer design is presented.

Quantitative porosity characterization of composite materials by means of ultrasonic attenuation measurements

Ultrasonic techniques utilizing a pair of transducers in a combination of pulse-echo and through-transmission modes were developed. Two types of methods were discussed for determination of material attenuation in composites: direct or absolute methods for materials with low signal loss, and indirect or relative methods for materials with higher signal loss. In all cases, transfer functions of the transducers and specimen surfaces were taken into consideration so that the measurement system is self-calibrated. Void content was measured by means of microscopic image analysis of photomicrographs of the specimen cross-sections and a correlation of material attenuation with porosity was established.

Experimental Study of Phased Array Beam Steering Characteristics

The influence of several geometric parameters of linear phased arrays was studied. A systematic approach using an automated testing assembly was used to assess the steering performance of the array in a solid medium. In addition to calibrating the transducer with respect to its steering accuracy, this arrangement provided a detailed study of the effects of steering angle, number of elements, inter-element spacing and array aperture on the beam directivity. The experimental results show good agreement quantitatively with the predicted steering characteristics.

A new compression test method for thick composites

A test method was developed for determination of compressive properties of thick composite materials. A new fixture was designed and built based on the concept of transmitting the initial load through the tabs by shear loading and thereafter engaging the ends to apply the additional load to failure by end loading. This fixture was used to measure the longitudinal compressive properties of thick composite materials. It is shown that longitudinal compressive strengths of 1660 MPa (240 ksi) are obtainable experimentally for thick composites. Several factors were found to affect the results of these compression tests. They include alignment, specimen geometry, stress concentrations, specimen end-crushing, and timing of end-loading engagement.

Phased array element shapes for suppressing grating lobes

Most techniques for suppressing grating lobes in phased arrays while relaxing the interelement spacing requirement involve redistributing array elements in sparse aperiodic patterns, or varying the transmit-receive beam patterns. An alternative is presented which uses oversized array elements to eliminate grating lobes as a direct consequence of the element shape. It is shown that by using carefully shaped, overlapping elements, maximum scan angle can be exchanged for a reduced interelement spacing requirement.

Method and system for interpreting and utilizing multimode dispersive acoustic guided waves

A method and system of locating discontinuities, measuring distances, locating unknown sources, and measuring the thickness of multimode dispersive medium includes sensing a multimode dispersive acoustic guided wave and frequency decomposing that wave using spectral temporal analysis; selecting a frequency which identifies a group delay of the guided wave for each mode occurring at that frequency; selecting a mode from the group delays of the selected mode, determining the group velocity from the dispersion curves and computing the desired parameter.

THREE-DIMENSIONAL BEAM DIRECTIVITY OF PHASE-STEERED ULTRASOUND

A model is developed to compute the three-dimensional far-field acoustic pressure distribution of the waves radiated from an ultrasonic linear phased array with finite elemental dimensions. Based on the model, the three-dimensional beam directivity function is introduced and analyzed to investigate the effects of geometric parameters on the steering performance. It is confirmed that the elevation dimension does not influence the beam directivity in the azimuthal plane. This is not true, however, in nonazimuthal planes. In general, increasing the elevation dimension reduces the energy leaking into nonazimuthal directions. The influences of other transducer parameters, including the number of elements, interelement spacing, and aperture length, are also investigated. It is found that they have only trivial effects on the sideleaking, although they are the important factors that influence the directivity in the azimuthal plane.