Advances in Imaging of Ndt Results

Several developments in the past have greatly helped to enhance ultrasonic detection and sizing. An important step forward was the introduction of Time of Flight Diffraction (ToFD), making sizing considerably less dependent of defect orientation. Optimized pulse-echo techniques, advances in transducer technology and software support combined with ToFD now enable the relatively high reliability and accuracy of Automated Ultrasonic Testing (AUT). However, despite advanced imaging techniques, even the most advanced AUT approach still uses a series of one-dimensional measurements. This is even the case when phased arrays are used, because these 'emulate' the conventional zonal concept. Apart from the development of the Synthetic Aperture Focusing Technique (SAFT), the basic principles of AUT have not changed in more than 50 years. RTD bv, together with Delft University of Technology, are working on a concept that uses a phased array in combination with so-called 'inverse wave field extrapolation'. This concept is based on fundamental wave theory and has not yet been used for NDT applications. The principles have been used in applications outside NDT, such as seismic exploration. This development in fact goes a step further than SAFT. By taking all possible reflections, diffraction’s and wave mode conversions into account, this real-time imaging technique will present a true image of the defect, without being restricted by defect shape or orientation. The image will present the maximum possible amount of information on defect nature, shape, orientation and size. The only limitation is presented by physics. In this paper, the principles and first results will be presented. Introduction: Phased array technology has been applied in the field of non destructive testing (NDT) for several years. Mostly these applications were in the medical and nuclear field, but phased array technology is becoming more attractive for industrial applications such as pipeline girth weld inspections. Girth weld inspection with phased array technology makes use of the same inspection method as a conventional multiple probe inspection. This method is based on zonal discrimination: the cross section of the weld is divided into several zones and each zone is inspected with a corresponding ultrasonic beam configuration. The beam configurations can be generated with a phased array transducer. Since the beam generated by a phased array transducer can be controlled by the computer, one phased array transducer can be used for the inspection of each zone on one side of the weld. This flexibility provides great benefits in preparation time and probe ‘consumption’. Despite these benefits, the results and data display are almost identical compared to conventional inspection, because the same inspection method is used. In the following we introduce another inspection method developed in the field of seismic exploration of the subsurface of the earth or ocean. Seismic exploration is based on measurements of echoes caused by acoustic reflection from layers in the subsurface. The acoustic signals are generated at the surface for example by a dynamite explosion or an air gun at sea. The responses from the subsurface are then recorded by an array of microphones. With the data and basic wave theory (such as inverse wave field extrapolation), a reconstruction of the layers in the subsurface can result in an image. RTD b.v., TU Delft and TNO TPD carried out a joined research project to apply the imaging concept used in seismic exploration on girth weld inspection. The research has been subsidised by the Dutch government. In the following, the imaging principles based on the already patented inverse wave field extrapolation (IWEX) method are explained. Differences with the zonal discrimination concept will be explained and also the benefits. Similarities and differences with other ultrasonic inspection methods will be discussed. Finally the first results will be shown and conclusions will be drawn. Principle of creating an image using inverse wave field extrapolation (IWEX): Generating a beam with a phased array transducer is done by firing adjacent elements with a given time delay (phase shift) corresponding to a desired angle and focal spot. As opposed to beam generation, the elements for the imaging process are used without time delays. A data set is created by firing one single element and receiving the response on all the available elements. For example, if two phased array transducers are used with 64 elements each, a data set with 128 x 128 A-scans (or traces as they are called in the field of seismic exploration) can be acquired. The data can be stored in a three dimensional matrix, the co-ordinates of the transmitters, the co-ordinates of the receivers and the recorded time base, see figure 1. Usually, the data in the matrix is transformed to the frequency domain, so that the third dimension represents all the frequency components rather then the time base.