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Time of Flight Diffraction (TOFD)
Time of Flight Diffraction is a rapidly increasing technique with unique features. High accuracy defect sizing is combined with high inspection speed. This combination provides a fast and high quality technique for the inspection of welds and other critical areas in installations. Other than most NDT techniques, TOFD is able to measure defect length and through thickness height, which are the key parameters in mechanical strength calculations.
TOFD inspection applies two ultrasonic
probes on either side of the weld as
pitch-catch. The transmitter probe emits
a wide angle beam covering the complete
weld volume and heat affected zone in one
scan. Thicker components (>50mm) are
subdivided in several depth zones.
Possible defects diffract the sound beam
at the upper and lower defect tips and
the receiver probe picks up diffraction signals.
Accurate defect sizing
Defect sizing is based on position mapping
of the diffraction signals. The defect height
can be readily measured by straightforward
subtraction of lower and higher defect tip
positions. A typical figure for the accuracy
that may be achieved in through thickness
height measurement is 0,5 mm. Surface
breaking defects are discriminated from
embedded defects; volumetric defects
from planar ones. An important advantage
of TOFD, compared to conventional ultrasonic
and radiography, is a high probability of
detection (POD), virtually independent on it defect orientation.
TOFD weld inspection
The preparation required for a TOFD scan is minimal, which makes the technique attractive even when only a small number of welds have to be inspected. TOFD may be applied during construction, where time constraints exist. TOFD allows examination directly after welding (up to 200°C) without
hold up of production speed; acceptance results are directly available. TOFD is a fast, safe and cost effective alternative to radiography.
On stream inspection with TOFD
In contrast with radiography, for TOFD examination only external access to the object is required. In the service stage of process installations (e.g. vessels) and pipe-work, TOFD may be applied 'on stream' to inspect equipment, while it remains in service. 'Fingerprints' of the object are taken to monitor initially acceptable defects and reveal and progressively monitor service induced defects. It can be used for the inspection of welds, HAZ and base material.
TOFD is applicable for weld inspection of pipes or plates in a thickness range of 6 mm upward (also heavy wall vessels up to 300 mm can be examined), pipe to plate nozzles, complex joints etc. All types of welding defects including lack of side wall fusion and root penetration can easily be detected and sized. For TOFD inspection, the feasibility of coarse-grained materials such as several types of austenitic steel, is usually assessed on a case to case basis by means of test specimen. Varying concentrations Ni and Cr alloys as well as duplex materials can be inspected successfully.