Interfaces de MATLAB con DLLs genéricas 1 INTRODUCCIÓN

Ultrasonic TOFD has received strong attention since the late 1970s as a technique to improve the sizing accuracy of flaws. Since then, lots of research work was car- ried out on fields like describing TOFD technique, discussing its reliability, TOFD systems development and hardware design and signal processing and analysis of TOFD data. However, work in the field of automatic TOFD interpretation is only recent, and a few number of research papers and articles can be found in liter- ature. The discussion below shows a brief development life cycle of TOFD with some focus on automatic techniques.

The National NDT Centre at Hartwell (then part of the UK Atomic Energy Authority-UKAEA) asked Dr Maurice Silk to try developing an ultrasonic siz- ing technique that is more accurate than the conventional methods (PE at that time). In the early 1970s, Silk started the experimental work, with a focus on using the diffracted signals and the transit time delay to accurately measure the size and depth of the defects in steel structures. This work resulted in a new ultra- sonic testing technique that was called TOFD. Then, Silk further developed the technique to improve sizing accuracy and the early results were very promising on artificial defects (the obtained accuracy was ±0.2 mm) [7, 8]. Furthermore, Silk applied TOFD to inspect austenitic welds [9, 10, 11, 12] and the achieved sizing accuracy was±0.3 mm with defects deeper than 1.7 mm. Details of Silk’s work can be found in a series of publications which focused on the potential improvement of defect sizing [12, 13, 14, 15, 16, 17, 18, 19]. The work described how TOFD can be used in accordance with the standard codes such as ASME XI code [20]. An im- portant advancement was in 1990s when Silk proposed the use of advanced signal processing techniques and synthetic aperture focusing technique (SAFT) [21] to

flaws in noisy TOFD data and high probability was obtained at moderate levels noise [23].

After Silk, many researchers worked on studying TOFD principles and to expose its potential further like Trimborn [24], Mondal [25] and Brown [26]. Geus [27] gave an overview on the advances in transducers technology (the hardware side of TOFD) and also on the modelling and interpretation tools.

In the late of 1990s, some researchers demonstrated that TOFD can not be used as a stand-alone method to provide substantial precision in defect detection and sizing. Just [28], Erhard [29, 30] and Hecht [31] demonstrated that TOFD can be used as an additional inspection method for verification of PE or radiography techniques. Kreier et al. [32] confirmed this also by performing many tests on artificial and natural defects and reached to a conclusion that TOFD is not an alternative to PE but can be an addition to it in order to be able to detect and size a wide range of defects including the surface ones.

The researcher who had faith in TOFD at that time tried to prove otherwise. Geus [27] and Brown [26, 33, 34] demonstrated that TOFD can be used as a stand-alone inspection method and went even further to say that it can replace conventional techniques such as PE and radiography. Betti [35, 36] also demon- strated this by experimental comparison. Trimborn [37] measured the performance of TOFD and concluded that TOFD is an efficient technique for defect detection and sizing. Webber [38] and Krutzen [39] also demonstrated this by examples and

comparisons with other ultrasonic sizing techniques to prove that TOFD is an ac- curate sizing technique and that it can be used for detection and characterisation of flaws with high accuracy as well.

Baby et al. [40, 41, 42] was another researcher who highly contributed in the development of TOFD’s sizing capabilities by investigating the sizing of cracks embedded in sub-cladding in B-scan images and obtained an accuracy of±0.2 mm with crack heights ranging from 1.68 mm to 19.04 mm in steel structures surfaces.

Some automatic attempts then started to emerge. Lawson [43, 44, 45, 46, 47] used image processing and multi-layer perceptron artificial neural network (ANN) to automate detection of weld defects. His focus was to perform the detection at elevated temperatures of up to 250◦ C. For that purpose, very high temperature probes are developed to give much better results in these high temperatures as the SNR was found to be decreased sharply [48, 49, 50].

Voon et al. [51] then developed a genetic-based inverse voting Hough transform method for automatic detection of parabolas in B-scan images to be able to perform automatic sizing. Maalmi et al. [52] had similar work in this area. Baskaran et al. [53, 54, 55] and Swamy et al. [56] proposed an automatic siz- ing method for vertical and inclined defects in thin plates (6−10 mm) in B-scan images by developing a TOFD system that was integrated with a developed embed- ded signal identification technique (ESIT) and point source correlation technique (PSCT). The achieved results were quite good compared to manual inspection and the performance in accuracy was 93% compared to 84% for manual inspection.

Moura et al. [57, 58] worked on automatic defect classification by using hierar- chical and non-hierarchical linear classifiers to be able to classify different defect

data, with emphasis on accuracy, reliability, efficiency, minimum user interaction and simple output presentation in order to be suitable for in-site interpretation, thereby shifting part of the interpretation burden from a trained human operator to a machine.

Automatic TOFD data interpretation methods are of significance and interest to many researchers because of their benefits in terms of reducing the burden on human operators and, hence, reducing measurement errors, and ultimately, saving time and cost. The main interest in this area is to develop reliable and robust algorithms that are capable of producing results from data collected in different operating conditions and components that are characterised by having different shapes and sizes.