TOFD

Principle of the TOFD process

Manually guided probes

Typical measurement signal of a TOFD test

The TOFD method ( Time of Flight Diffraction ) is an ultrasonic test method developed in the United Kingdom in the 1970s , which is mainly used to test weld seams. While flaws in the material are usually found by analyzing reflected signals, TOFD primarily uses the diffraction signals that originate from discontinuities . TOFD is one of the few testing techniques that is able to describe a defect or a discontinuity relatively precisely with length, depth and height. All other ultrasonic testing methods describe a discontinuity due to the reflection behavior and not due to the real expansion.

Alternative naming

The term diffraction transit time technology exists as a translation of TOFD in the German regulations . In practice, however, the English abbreviation is used.

Standards for the TOFD process

The TOFD procedure is standardized on an international and European level.

German Institute for Standardization (DIN)

DIN EN 583-6, Non-destructive testing - Ultrasonic testing - Part 6: Time of diffraction, a technique for finding and measuring inhomogeneities

DIN EN 15617, Non-destructive testing of welded joints - Time-of-flight diffraction technique (TOFD) - Acceptance limits

DIN EN ISO 10863, non- destructive testing - ultrasonic testing - application of the time-of- flight diffraction technique (TOFD)

Procedure

Be used Prüfkopfpaare (transmitter and receiver), the broadband possible longitudinal ultrasonic waves transmit. The TOFD method is primarily used for testing weld seams, as it is particularly suitable for testing longitudinal defects.

The test is partially or fully mechanized, using a scanner, both test heads are guided along the weld seam and send ultrasonic signals into the weld seam at fixed intervals. An integrated incremental encoder records the distance covered.

This creates sound field gradients. The shortest connection between the transmitter and receiver probe corresponds to the surface wave. It is also referred to in the specialist literature as the Lateral Wave (LW). The longest sound path that can occur between the two probes belongs to the path over the back wall. This is why this signal is also called the back wall echo (RW). In a display-free (i.e. fault-free) weld seam, these two waves describe the surface and the rear wall of the component to be tested; there are no additional signals between the two echoes. However, if additional echoes occur, these are caused by discontinuities in the volume to be tested. The ultrasonic impulse hitting the discontinuities creates new ultrasonic waves at their edges, the so-called diffraction or diffraction waves (BW). These spread out spherically, also reach the reception probe and generate a corresponding display.

The diffraction waves generated can be assigned to the upper and lower peaks of the discontinuities due to a phase shift of 180 ° and the respective transit time. Since these processes run very quickly and are imperceptible to the human eye, the individual ultrasound images are strung together depending on the distance covered. For better readability, the amplitudes of the individual signals are then converted into gray levels so that a scan image is created.

Application within the ASME code

The ASME Boiler and Pressure Vessel Code in Section VIII prescribes radiography as the primary test method for thick-walled vessels . In the early 2000s, attempts were made to replace radiography with mechanized ultrasonic testing. The ASME Code Case 2235 was created for this. The requirements of this ASME Code Case are, however, quite high. The entire volume of a weld seam must be checked and, depending on the wall thickness, an area of 25 mm or 50 mm on both sides next to the weld seam must also be checked. This is to ensure that areal defects are found in the heat affected zone. In addition, a validated method for evaluating the echo height of reflectors (at least binding errors) must be provided.

literature

Engineering Applications of Ultrasonic Time-of-Flight Diffraction, 2nd ed., JP Charlesworth and JAG Temple, Research Studies Press, 2002.
American Society of Mechanical Engineers: Use of Ultrasonic Examination in Lieu of Radiography , Case Code 2235-4, Section I and Section VIII, Divisions 1 and 2, ASME, USA, 2001

Individual evidence

^ JP Charlesworth, JAG Temple: Engineering Applications of Ultrasonic Time-of-Flight Diffraction Second Edition , Research Studies Press LTD, 2001, p. 2554, ISBN 0-86380-239-7 .
↑ DIN EN 583-6: 2009, No. 3.2.
↑ DIN EN 583-6: 2009: No. 4.1 (short description) - Basic arrangement for the TOFD method.
↑ American Society of Mechanical Engineers : Use of Ultrasonic Examination in Lieu of Radiography , Case Code 2235-4, Section I and Section VIII, Divisions 1 and 2, ASME, USA (2001).

[1] JP Charlesworth, JAG Temple: Engineering Applications of Ultrasonic Time-of-Flight Diffraction Second Edition , Research Studies Press LTD, 2001, p. 2554, ISBN 0-86380-239-7 .

[2] DIN EN 583-6: 2009, No. 3.2.

[3] DIN EN 583-6: 2009: No. 4.1 (short description) - Basic arrangement for the TOFD method.

[4] American Society of Mechanical Engineers : Use of Ultrasonic Examination in Lieu of Radiography , Case Code 2235-4, Section I and Section VIII, Divisions 1 and 2, ASME, USA (2001).