TOFD Inspection of Butt Welds

Fig. 1 - Example of a complex butt weld with dissimilar metals

TOFD is an ultrasonic inspection method developed for the nuclear industry It is highly effective in detecting cracks, Lack of side wall fusion, slag inclusions, and weld root corrosion and very accurate in defect sizing. This makes TOFD a powerful tool in defect monitoring, allowing for accurate defect expansion rates over time with multiple repeat inspections.

TOFD utilises a pair of probes and wedges in a send and-receive setup. The probes are placed on either side of the weld, facing each other at a specific distance apart. The transmitter introduces sound into the material, and the receiver picks up any tip-diffracted signals from defects in the material.

Fig. 2 - TOFD setup on a Butt weld sample in a multi-group setup with PA (Phased Array)

Tip Diffractions

TOFD inspection relies on the detection of ‘tip-diffracted’ signals emitted by defects (see Figure 3); all discontinuities within the material emit tip-diffracted signals when hit by an ultrasonic pulse; these signals are very low amplitude when compared to a reflected signal. Tip diffracted signals offer two main advantages when sizing and detecting defects:

1. Defects can be detected regardless of orientation in relation to the probe, this means inclusions and transverse defects can be detected with TOFD without having to run a separate transverse scan.
2. Tip diffractions emanate from the top and bottom of the defect allowing for highly accurate depth and through-wall sizing.

Fig. 3 - Diagram showing TOFD tip diffraction detection

TOFD Scanning on the VEO

TOFD scans on the VEO are simple to set up quickly from the start menu or by importing config files from UTStudio+. With all the settings needed to achieve a great overall data set with excellent signal-to-noise ratio and high scanning speeds. The Veo can also handle high Voltage at 400V, a 200 MHz acquisition rate, and in Multi scan mode 2 TOFD scans and up to 6 PA groups simultaneously.

Fig. 4 - TOFD Scanning on the VEO3

TOFD Data Review on UTStudio+

UTStudio+ is the Sonatest data analysis software which comes with specialist tools to help with defect identification and sizing. Annotations automatically provide the defect dimensions using hyperbolic cursor positions. PDF reports can be automatically generated holding all the inspection parameters and defect details.

Fig. 5 - TOFD data review on UTStudio+ post-analysis parameters, TOFD cursor contouring and annotation measurements

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