An ultrasonic array is a single transducer that contains a number of individually connected elements. Recent years have seen a dramatic increase in the use of ultrasonic arrays for non-destructive evaluation. Arrays offer great potential to increase inspection quality and reduce inspection time. Their main advantages are their increased flexibility over traditional single element transducer methods, meaning that one array can be used to perform a number of different inspections, and their ability to produce immediate images of the test structure. These advantages have led to the rapid uptake of arrays by the engineering industry. These industrial applications are underpinned by a wide range of published research which describes new piezoelectric materials, array geometries, modelling methods and inspection modalities. The aim of this paper is to bring together the most relevant published work on arrays for non-destructive evaluation applications, comment on the state-of the art and discuss future directions. There is also a significant body of published literature referring to use of arrays in the medical and sonar fields and the most relevant papers from these related areas are also reviewed. However, although there is much common ground, the use of arrays in non-destructive evaluation offers some distinctly different challenges to these other disciplines.

Ultrasonic arrays for non-destructive evaluation: A review

http://www.me.sc.edu/research/lamss/pdfnew/Journals/J64_Ultrasonics_V48_N2_117-134_April-2008.pdf

In situ 2-D piezoelectric wafer active sensors arrays for guided wave damage detection.

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A review on non-destructive evaluation of rails: State-of-the-art and future development:

Ultrasonic arrays allow a given scatterer to be illuminated from a wide range of angles and hence are capable of extracting significant information about the scatterer. In this paper a general imaging methodology, termed multi-mode total focusing method, is proposed in which any combination of modes and reflections can be used to produce an image of the test structure. Like the total focusing method, this approach is implemented by post-processing the full matrix of array data to achieve a synthetic focus at every pixel in the image. A hybrid model is used to predict the array data and demonstrate the performance of the multi-mode imaging concept. This hybrid model combines far field scattering coefficient matrices with a ray-based wave propagation model. This allows the inclusion of longitudinal waves, shear waves and wave mode conversions. It is shown that, with prior knowledge of likely scatterer location and orientation, the mode combination and array location can be optimised to maximise the performance of array inspections. A practically relevant weld inspection application is then described and its optimisation is discussed.

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

Ultrasonic imaging using full-matrix capture, e.g., via the total focusing method (TFM), has been shown to increase angular inspection coverage and improve sensitivity to small defects in nondestructive evaluation. In this paper, we develop a Fourier-domain approach to full-matrix imaging based on the wavenumber algorithm used in synthetic aperture radar and sonar. The extension to the wavenumber algorithm for full-matrix data is described and the performance of the new algorithm compared with the TFM, which we use as a representative benchmark for the time-domain algorithms. The wavenumber algorithm provides a mathematically rigorous solution to the inverse problem for the assumed forward wave propagation model, whereas the TFM employs heuristic delay-and-sum beamforming. Consequently, the wavenumber algorithm has an improved point-spread function and provides better imagery. However, the major advantage of the wavenumber algorithm is its superior computational performance. For large arrays and images, the wavenumber algorithm is several orders of magnitude faster than the TFM. On the other hand, the key advantage of the TFM is its flexibility. The wavenumber algorithm requires a regularly sampled linear array, while the TFM can handle arbitrary imaging geometries. The TFM and the wavenumber algorithm are compared using simulated and experimental data.

The wavenumber algorithm for full-matrix imaging using an ultrasonic array

Processing of ultrasonic array data is traditionally based on having parallel transmission circuits that enable staggered firing of transmitter elements to produce the desired wavefront. This paper describes an alternative approach in which the full matrix of time domain signals from every transmitter–receiver pair is captured and post-processed. Various post-processing approaches are modelled and assessed in terms of their ability to image a point-like reflector. Experimental results are then presented which show good quantitative agreement with the modelled results. An advanced processing algorithm is also implemented which allows the array to be focused at every point in the target region in both transmission and reception. This approach is shown to offer significant performance advantages for NDE.

Post-processing of the full matrix of ultrasonic transmit-receive array data for non-destructive evaluation

Advanced post-processing for scanned ultrasonic arrays: Application to defect detection and classification in non-destructive evaluation

Ultrasonic arrays are increasingly widely used in nondestructive evaluation (NDE) due to their greater flexibility and potentially superior performance compared to conventional monolithic probes. The characterization of small defects remains a challenge for NDE and is of great importance for determining the impact of a defect on the integrity of a structure. In this paper, a technique for characterizing reflectors with subwavelength dimensions is described. This is achieved by post-processing the complete data set of time traces obtained from an ultrasonic array using two algorithms. The first algorithm is used to obtain information about reflector orientation and the second algorithm is used to distinguish between point-like reflectors that reflect uniformly in all directions and specular reflectors that have distinct orientations. Experimental results are presented using a commercial 64-element, 5-MHZ array on two aluminum test specimens that contain a number of machined slots and side-drilled holes. The results show that the orientation of 1-mm-long slots can be determined to within a few degrees and that the signals from 1-mm-long slots can be distinguished from that from a 1-mm-diameter circular hole. Techniques for quantifying both the orientation and the specularity of measured signals are presented and the effect of processing parameters on the accuracy of results is discussed.

Advanced Reflector Characterization with Ultrasonic Phased Arrays in NDE Applications

The use of ultrasonic arrays for NDT has increased significantly in recent years. This is due to the flexibility of array systems which can be electronically configured to produce plane, focused, steered and steer-focusedbeams. In this way, one array transducer can do the job of many standard single-element transducers. Currently, the approach is to have independently controlled parallel pulser-receiver channels connected to a number of array elements. These channels allow timing delays to be applied to each element on both transmission and reception. In this way a specific set of transmit-receive delays are applied to an array to create a specific beam profile. This paper describes an alternative approach in which the complete raw data set offline domain signals from every transmitter-receiver pair is collected, stored and post-processed. Theoretically, the time taken to acquire this data is approximately the same as that taken to perform a B-scan with the array. The key advantage is that post-processing of the complete raw data set enables any beam profile to be recreated. Additionally, this approach allows novel inspections to be performed which would be impossible with single-element transducers and impractical using the traditional phased array controller methodology. This paper concerns one such post-processing algorithm, the Total Focusing Method (TFM). In the TFM, an image is created in which the beam has been focused on every point within the field of view. This optimises the focusing performance of a given array. The use of the TFM is then demonstrated on a number of test structures and is proposed as a superior alternative to conventional array test techniques.

The post-processing of ultrasonic array data using the total focusing method

The full data set from an ultrasonic array comprises the time-domain signals from every possible transmitter-receiver pair in the array. The possibility of experimentally obtaining the full data set that is afforded by some new array controller systems massively increases the potential of ultrasonic arrays. Firstly, any conventional array imaging procedure (e.g. B-scans, angular scans, dynamic depth focusing) can be performed by post-processing the information in the full data set. More importantly, the full data set enables imaging techniques to be developed that have no direct counterpart in conventional ultrasonics. For example, the authors have previously shown that the total focusing method (TFM) outperforms other standard linear processing techniques when imaging a point reflector. This is because the TFM represents the complete array being focused in transmission and reception at every point in an image. In this paper two advanced versions of the TFM are described. The first called the vector TFM (VTFM) algorithm allows the angular reflectivity characteristics of any point in a sample to be obtained. This has obvious advantages for defect sizing and classification. The VTFM algorithm probes the scattering behaviour of a defect over a range of angles. The response is displayed as a vector that represents defect orientation and a scalar quantity that represents the specularity of the reflector. Both simulated and experimental results are shown from which the orientation of small reflectors is determined and visualised. The second algorithm is termed the diffraction TFM (DTFM) and uses a pair of arrays in a similar manner to time-of-flight diffraction (TOFD) systems. The immediate advantage of DTFM over TOFD is the ability to produce a B-scan image without moving the transducers. Potential additional advantages are superior sensitivity and the ability to overlay pulse-echo information from back-scattered signals with pitch- catch information from diffracted signals. Experimental results are presented from an industrial weld sample containing a number of realistic defects.

/pdf/enhanced-defect-detection-and-characterisation-by-signal-2g6vmthuyt.pdf

C. Holmes

Papers

Post-processing of the full matrix of ultrasonic transmit-receive array data for non-destructive evaluation

Advanced post-processing for scanned ultrasonic arrays: Application to defect detection and classification in non-destructive evaluation

Advanced Reflector Characterization with Ultrasonic Phased Arrays in NDE Applications

The post-processing of ultrasonic array data using the total focusing method

Enhanced Defect Detection and Characterisation by Signal Processing of Ultrasonic Array Data