Ultrasonic Sampling Phased Array Testing as a Replacement for X-ray Testing of Weld Joints in Ship Construction

According to European Standard EN 1712 ultrasonic testing of thin-walled welded joints is mandatory for wall thicknesses of more than 8 mm only. Any thinner components do have to undergo X-ray inspection. Besides various advantages of X-ray testing viz. high sensitivity to smallest inclusions, high acceptance in the ship building sector and “automatic” documentation of inspection results, there are also several deficiencies (like radiation protection issues, inspection time expenditure etc.) creating a reasonable request for more costeffective alternatives. Sampling Phased Array technology introduced by Fraunhofer-IZFP provides significant improvement of flaw detectability also in thin-walled welded joints due to its tomographic approach in processing of signals obtained by ultrasonic phased array transducers. It allows high-quality imaging of welded joints and detection of relevant material flaws. Real-time ultrasonic imaging with tomographic quality offers a great alternative to Xray testing with respect to inspection speed and modern documentation of inspection results. The basic principles of Sampling Phased Array are presented in the paper and several application results obtained on welded joints of marine objects are presented. in general has the following advantages and disadvantages [2]. Table 2: Advantages and disadvantages of ultrasonic testing ___________________________________________________ Advantages Disadvantages _________________________________________________ Testing of thick-walled Acoustic coupling (surface components is possible contact) is required. without limitations Limitations due to surface Evaluation of flaw size, type, roughness are possible orientation can be obtained. High requirements on Fast, cost-effective testing with inspection staff due to immediate conclusion about rather complex calibration indication of UT instrument Automated or half-automated Limitations on flaw inspection and evaluation can detectability due to be implemented suboptimal insonification NEW!: Imaging techniques like position or flaw phased array allow orientation documentation and quantitative evaluation of inspection results _________________________________________________ For being able to replace x-ray by ultrasonic testing the following tasks must be solved:  Equal or better flaw detectability of relevant flaws compared to x-ray  Fast representation and evaluation of inspection results  Cost-efficient implementation of inspection system and inspection procedure  Mobile inspection system for in-situ applications 2 SAMPLING PHASED ARRAY TECHNIQUE The novel ultrasonic inspection technique rapidly coming into industrial application is phased array [3, 4]. Phased array testing offers significant advantages for ultrasonic testing (UT) of welded joints due to its extended information content provided by beam steering capability. Hence the combination of mechanical scanning and electronic beam steering increases flaw detectability, since it is being insonified from various angles of incidence. Phased array techniques may also have their limitations in certain applications with respect to spatial resolution in the far field of phased array transducers or inspection speed while beam steering over a big angle range and finite signal-to-noise ratio of the system can be seen as an advantage. Sampling Phased Array (SPA) technology developed by Fraunhofer IZFP is a next step in Phased Array technology. On the one side the technique is capable of fast synthesis of phased array ultrasonic signals for arbitrary angles of incidence with focusing in all the depths within the probe near field. On the other hand the back projection and overlapping in the volume the elementary wavelets obtained by SPA according to synthetic aperture focusing technique (SAFT) principles offers the best possible image reconstruction quality. The SPA technique offers the following practical advantages: 1 Ultra-fast virtual beam sweep for arbitrary angle range 2 Improved sensitivity and resolution in the near field of the transducer 3 Fast 2D / 3D imaging Unlike conventional Phased Array technique insonifying the inspection volume by directed sound fields under different angles of incidence, Sampling Phased Array performs data acquisition by exciting cylindrical or spherical waves that propagate in all directions. This can be implemented by firing single array elements or applying defocusing delay laws (Figure 1, left). Hence a very wide angle range can be covered after a single shot. The ultrasonic signals acquired and saved in each probe position for every single array element serve as an input data for image reconstruction. The reconstruction occurred according to the SAFT algorithm [5]. Since the sound field of array elements is very divergent, every time signal (A-scan) received contains overlapped echo-signals from available reflectors in different volume positions. The reconstructed image in one position of linear array visualizes a cut plane perpendicular to insonification surface, the so called sector-scan. For every point within this plane the propagation times from the transmitting elements and back to the receiving elements are calculated. The amplitude values from all A-scans with matching propagation times are added up in each image point [6]. Figure 1: Defocused transmission and sector image reconstruction by SPA Thus all angles of incidence and focal depths within the near field of the transducer can be realized even after one single transmitting/receiving act. Since the sound beam steering at each volume point, i.e. for all angles of incidence and focal depth, is performed not physically but virtually through the computer, a significant increase in inspection speed can be achieved by implementation of the SPA principle [7]. Furthermore the synthetic focusing in the near field of the UT transducer by the SAFT principle improves sensitivity and resolution (Figure 2). Figure 2: Principle of image reconstruction by SPA Thus for weld inspection the material flaws can be represented in tomographic quality that allows their exact sizing (Figure 3). Figure 3: Tomographic Image of an inclined lying crack 3 INSPECTION SYSTEMS FOR INDUSTRIAL APPLICATIONS Modern instrument engineering, e.g. latest signal processors and computers, offer sufficient computation power for performing SPA image reconstruction and processing, that outmatches conventional phased array systems in speed and quality. Versatile reconstruction techniques [8] can be implemented in a portable manual flaw detector (Figure 4). Figure 4: Manual ultrasonic tomograph A1550 IntroVisor by