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Erscheinungsjahr
- 2010 (3) (entfernen)
Dokumenttyp
Schlagworte
- D-nozzle (1)
- Phased array (1)
- Phased array technique (1)
- Real and simulated cracks (1)
- Round bar testing (1)
- SAFT reconstruction (1)
- Stress corrosion cracking (1)
- Ultrasonic testing (1)
Within a know-how transfer project funded by the government conventional ultrasonic
technique was replaced by phased array technique for automated round-bar testing. Instead of
applying a great number of conventional probes to achieve acceptable volume coverage we used
curved linear arrays. The benefits of phased array technique such as programmable skew angles,
beamforming and beam positions, led not only to a significant decrease in inspection time, but also the
number of probes could be substantially reduced . Finally, the testing parameters for a large range of
bar-diameters could be adapted by software control instead of time-consuming mechanical
replacement. The probe-design was carried out by a proprietary modelling program. Both the
theoretical calculations as well as the latter experimental verifications revealed significant advantages
of curved arrays versus the planar types. A radial oriented probe offers perfect adaption to the
cylindrical shape of the specimen allowing wide variations of the sound field. Thus beam direction,
beam size and beam position could be optimized with respect to a minimum of inspection cycles, as
inspections have to be executed in-line during the production. A number of laboratory tests were
carried out on special test components. In order to achieve an optimal performance of the reference
rod we implemented three different types of reference reflectors: (i) flat-bottom-holes with diameters
of 0.8 mm and 1.2 mm, (ii) side-drilled-holes with a diameter of 0.7 mm for the detection of
volumetric flaws, and (iii) notches with a depth of 0.2 mm and 0.5 mm for the detection of surfaceoriented
defects. All laboratory tests were carried out with the COMPAS-XXL inspection system, a
proprietary development of BAM.
Bei der automatisierten Prüfung von Stangen mit Ultraschall-
Gruppenstrahlertechnik besteht grundsätzlich die Möglichkeit, Prüfkopfparameter
wie Einschallwinkel, Fokustiefe und Schallbündelbreite programmgesteuert an sich
verändernde Prüfbedingungen anzupassen. Zu diesen zählt die Temperatur des
Koppelmediums (Wasser), die in einem Bereich von 10°C bis 50°C liegen kann, was
zu unterschiedlichen Schallgeschwindigkeiten führt und den Einschallwinkel
verändert. Mit der Folge, dass unter Umständen die geforderte 100%-Abdeckung
des Prüfvolumens nicht mehr gewährleistet ist.
Auch geometrische Abweichungen, wie sie durch die fertigungsbedingte Streuung
des Prüflings gegeben sind, können Einfluss auf das Prüfergebnis nehmen. So
führen Abweichungen vom idealen Geradheitszustand zu Fehlpositionierungen des
Prüfkopfs, was wiederum Einfluss auf die Lage und Beschaffenheit des Schallbündels
in der Stange hat. Ähnliches gilt für nicht vollkommen kreisförmig ausgebildete
Stangendurchmesser, die unterschiedliche Schalleintrittsbedingungen bewirken und
Abweichungen von den theoretisch ermittelten Werten zur Folge haben.
Im Rahmen eines vom BMWi geförderten Forschungsvorhabens wurde der Einfluss
dieser Parameter untersucht und ihre Auswirkungen für den Betrieb einer
automatisierten Prüfanlage unter realen Betriebsbedingungen ermittelt. Dies betrifft
vor allem den maximal möglichen Schallbündelversatz innerhalb des Arrays, der die
Anzahl der erforderlichen Prüftakte bestimmt und damit die maximale
Vorschubgeschwindigkeit des Prüflings. Des Weiteren werden Möglichkeiten zur
programmgesteuerten Kompensation der untersuchten Temperatur- und
Geometrieabweichungen durch eine flexible Anpassung der Gruppenstrahler-
Prüfkopfparameter aufgezeigt.
Stress corrosion cracking is a transcrystalline or intercrystalline crack formation in materials which
occurs under the influence of static tensile stress or residual stress and a specific aggressive
medium such as chloride containing substances.
This special sort of crack formation is involved in complex crack configurations, which may
only insufficiently be captured by conventional ultrasonic probes with fixed angles of incidence.
Only a number of different beam angles produce sufficient reflection to reconstruct the complete
defect shape from measured ultrasonic data. The SAFT algorithm, which was recently successfully
used in many industrial NDT-applications, is a promising tool for the reconstruction process. In
addition, the combination of phased array technique and SAFT was developed in several projects
by BAM, Berlin. Investigations using phased array equipment were performed to look into crack
configurations in test blocks with different surface curvatures. UT-SAFT has been used for the
analysis of spark-eroded notches simulating stress corrosion cracks at the thermo sleeve weld of a
nozzle. For comparison, UT-SAFT has also been applied for the analysis of real reflectors at the
same position in a nozzle used in a power plant, which was repaired later on. SAFT-scans received
from reconstructed ultrasonic measurement data confirm the practical usefulness of the SAFTalgorithm
developed by BAM.