Wave equation-based imaging of mode converted waves in ultrasonic NDI, with suppressed leakage from nonmode converted waves

Abstract

The value of imaging techniques in ultrasonic nondestructive inspection (NDI) to find and characterize defects in steel components has already been demonstrated. The imaging techniques based on the integral representation of the wave equation, the Rayleigh integrals for wave field extrapolation, are becoming feasible and attractive due to advances in array technology and due to faster computers. Known implementations are the total focusing method (TFM), the synthetic aperture focusing method (SAFT), and the inverse wave field extrapolation method (IWEX). In principle, these techniques compensate propagation effects from sources to a scatterer such as a defect and propagation effects from the scatterer to receivers. Currently, this approach is applied to wave fronts of single modes (pure longitudinal or pure transversal). In practice, multiple wave fronts from the scatterer will be received as a result of mode conversion. These arrivals will not have the same arrival time because of the difference in sound velocity between longitudinal and transversal waves. Images of mode converted waves are obtained by choosing the appropriate sound velocity that corresponds with the mode-converted scattered wave in the imaging process. Therefore, the nonmode converted waves will image as leakage artifacts in the mode-converted images, and vice versa. This may lead to false interpretations. In this paper, such artifacts will be identified and explained with the help of an analytical example. Measurements from steel test pieces with a 4 MHz linear array transducer with 64 elements will be used to demonstrate the artifacts. Furthermore, a procedure to predict the artifacts and the subsequent suppression from the input measurements will be presented and demonstrated.

Plane Wave Imaging for ultrasonic non-destructive testing: Generalization to multimodal imaging

“…In this situation, the cylindrical wave emitted by an element, radiating mainly perpendicularly to the transducer plan, is not the most fitted type. In some cases, this is highlighted by the existence of nonphysical indications, also called image artifacts, that may lead to misinterpretations [10,11]. The second drawback is the framerate limit due to the number of transmissions (N) and the storage and processing of the N Â N signals.…”

On the Use of the Geometric Median in Delay-and-Sum Ultrasonic Array Imaging

“…As the recorded ultrasonic data contains a superposition of wave modes, imaging with one mode leads to the creation of artefacts in the image caused by the others [9]. Existing artefact filtering techniques are based on the identification (manually or with a threshold) then suppression (with zeroing or subtraction) of the signal which creates artefacts in other views [9], [10].…”

“…As the recorded ultrasonic data contains a superposition of wave modes, imaging with one mode leads to the creation of artefacts in the image caused by the others [9]. Existing artefact filtering techniques are based on the identification (manually or with a threshold) then suppression (with zeroing or subtraction) of the signal which creates artefacts in other views [9], [10]. Another artefact filtering strategy consists in reducing the influence of unphysical ultrasonic paths in the reconstruction using appropriate weights, which has been done in composite materials [11] and for planar defects [10].…”

Defect detection using ultrasonic arrays: The multi-mode total focusing method

“…Based on the wavenumber imaging approach is used in synthetic aperture radar and sonar [12], Hunter et al [13] developed a frequency domain implementation of the TFM. Portzgen et al [14,15] described a similar frequency domain algorithm termed inverse wave-field extrapolation that included the possibility of using mode conversions to image scatterers. Wilcox et al [16] extended the TFM to provide defect characterisation capability.…”