Processing of ultrasonic array signals for characterizing defects. Part I: signal synthesis.

This work presents a novel procedure to characterize damage using an array of ultrasonic measurements in a generalized model-based inversion scheme, which integrates the complete information recorded from the measurements. In the past, we proposed some idealized nondestructive evaluation test methods with emphasis on the numerical results, but it is necessary to develop the techniques in greater detail in order to apply the techniques to real conditions. Our detection principle is based on the measurement and inversion of frequency-domain data combined with a reduced set of output parameters. The approach is developed and tested for the case of an aluminum specimen with a synthetic array of point contact ultrasonic transmitters and receivers. The first part of this two-part paper is focused on numerical synthesis of the experimental measurements using the boundary element method for a general ultrasonic propagation model. This part also deals with the deconvolution by comparing the data measured from the damaged and undamaged specimens. The deconvolution technique allows us to calibrate the data by taking into account the uncertainties due to mechanical properties, input signal, and other coherent noise. The second part of the paper presents the inversion of the measurements to obtain the parameters and ultimately to predict the position and size of the real defect.

Processing of ultrasonic array signals for characterizing defects. Part II: experimental work.

This is Part II of the two-part paper aimed at integrating the numerical synthesis and experimental investigation of the ultrasonic wave propagation model for quantitative nondestructive evaluation. The first part of the paper focused on synthesizing and predicting measured signals using the boundary element method and the deconvolution technique based on the comparison between the signals obtained from defective and undamaged (reference) specimens. In the second part, we present an inversion technique which allows us to obtain the position and size of the defect. The inversion scheme is processing the frequency domain information rather than time-domain time-of-flights or vibration eigenmodes. This technique is tested experimentally for the case of a side-drilled hole with a non-trivial location in terms of standard pulse-echo techniques. It is shown that the scheme is particularly effective when the information of the defect is masked by other predominant signal components.

Design of ultrasonic wedge transducer.

Cones and wedges inserted between an ultrasonic transducer and the specimen provide the transducer (circular or rectangular shape) with enhanced capability for point or line contact with the specimen. Such an arrangement is useful in that the transducer can be used for transmitting to and receiving from a point (or line) source, and that it can eliminate the undesirable aperture effect that makes the transducer blind to waves traveling in certain directions and those of certain frequencies. In this paper, a comprehensive numerical analysis based on a wave propagation model is carried out for the study of characteristics and parameters of cones and wedges influencing their performance. We study the effect of the dimensions, shape and aperture on the frequency response and the angle of incidence of the wave. For computational accuracy and efficiency, the boundary element method is used in the analysis.

Laser-ultrasonic generation of Lamb waves in the reaction force range.

The Laser-ultrasonic generation of Lamb waves in an elastic plate is investigated theoretically and experimentally for a laser source whose intensity is high enough to create reaction forces (normal tractions) on the illuminated surface of the specimen. The analytical solutions for transient waves are derived using the integral transform method first by considering an arbitrary source shape and time excitation function, and then specifically for circular and line source shapes. The simulation study allows us not only to predict the behavior of individual wave modes but also to construct the overall responses; thus it helps us better understand the wave excitation mechanisms. The dispersive and multi-modal nature of laser-generated Lamb waves is presented by showing the spatiotemporal Fourier transform of displacements obtained by the simulation study. The transform, displayed in the frequency-wave number domain, enunciates the characteristics of the propagating individual Lamb wave modes. The simulation results are then compared with the 2-D Fourier transform of a set of experimental data obtained by scanning an aluminum plate specimen.

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.