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Eingeladener Vortrag
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The spatial resolution in ultrasonic testing as a wave phenomenon in nature is limited to a
certain fraction of the wavelength, usually defined to be close to the Rayleigh criterion. In case of
complicated reflector surfaces – such as stress corrosion cracking - this limitation prevents an exact
visualization of the defect shape. There exist a few approaches to improve the spatial resolution, whose
reconstruction quality all in common also depend on the achieved signal-to-noise ratio of the raw data. In this
work we present a specific SAFT analysis, in which a high number of different angles of incidence produce a
sufficiently high number of different reflections at the crack edges resulting in an improved SNR. In doing
so, we reconstruct a coherent crack structure. First investigations were made at artificially simulated crack
configurations with different contours and curvatures in flat and cylindrical test blocks. The measurements
results visualized by representative scans – show details of crack design and crack orientation. We also will
present a comparison of the SAFT analysis between modelling and phased array measurements.
Weld inspection using ultrasonic pulse-echo techniques needs high skilled operators especially if geometrical indications must be taken into consideration. A clear separation between geometrical indications and defects like cracks must be guaranteed. This requirement needs sensitive techniques like the 45(deg) shear wave technique using the mirror effect. Since more than twenty years the Time Of Flight Diffraction technique (TOFD) is still under discussion for defect sizing. But on the other hand the potential of the TOFD technique for defect detection is described in technical papers of manufacturers. The main task of examinations carried out at BAM was the comparison about the detectability of surface breaking cracks between the common methods and the TOFD approach.
The comparison of probes for the inspection of cladding shows a remarkable fact: depending on the reflectors used, a different performance characterization of the probe must be expected. It seems important for performance demonstration trials - especially those including the underclad area - to use reflector types as close as possible to the real cracks to be detected. For different detection approaches, reflectors representing underclad cracks should have a reduced diffraction at the crack tip areas in order to check their potential to detect the indication based on surface reflections of the ultrasonic waves.
Ferroelectrets are promising materials for air-coupled ultrasonic transducers. A transducer made of polarized cellular polypropylene, including its electronic interface, was developed and compared with conventional air-coupled probes. Test pieces of fiber-reinforced polymer containing impact flaws and flat-bottom holes were inspected in transmission. The ferroelectret transducers achieved a considerably higher signal-to-noise ratio. The impacts were clearly visible with all transducers, but less noisy with ferroelectret transducers. The flat-bottom holes were better detectable than with a conventional probe with about the same focus size.
Guided waves travel in plates and hollow cylinders over large distances and propagate with multiple mode shapes. Therefore the waves can be used viably for integrity tests of large scale structures. The number of propagating modes increases with frequency. Due to their dispersive character the different modes are manageable only in a limited frequency range. Depending on the wave length and on the angle of impingement of the wave front to the coupling surface between transducer and structure, a trace wavelength is predefined and a selective excitation of single modes becomes feasible. By using phased array technique the excited wave mode can be selected by controlling the input signal of the transducer. Different modes are excitable with a sin-gle mechanical set-up. In a first step of the investigation, a calculation model is developed modelling the wave propagation and the selective excitation of guided wave modes depending on the control parameters. Dedicated experiments show the applicability of the method presented. The flaw detection of different sized cracks and of material thickness reductions is examined depending on the excitation wave mode.