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  - ToFD

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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 principle


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 200C) 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 applications


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.