Time of Flight Diffraction (TOFD) is an advanced inspection technique that detects and accurately sizes an asset’s anomalies.
The non-destructive testing method known as Time of Flight Diffraction (TOFD) is an advanced technique commonly used in weld inspection. As part of the Ultrasonic testing family, TOFD provides a highly accurate solution to flaw detection and sizing for both pre-service weld inspection and for in-service inspection.
TOFD is performed using a pair of probes which are placed on either side of the weld, or item, being inspected. One of these probes acts as a transmitter sending out a pulse and the other, the receiver, receives this pulse. TOFD is an ultrasonic diffraction technique very different from a reflective techniques of other ultrasonic methods. When a flaw is present, this flaw or crack’s tip will create a diffraction of the ultrasound pulse. Trigonometry can be used the determine the crack’s depth through the time of flight measurement of the pulse.
TOFD is typically utalised for weld quality assessment, either on its own or in combination with PAUT. As TOFD uses diffracted signals, the detection of defects is not affected by geometry, making it very useful for detection of volumetric and planar flaws. Other applications beyond weld quality assessment include:
- Pipe scanning for the detection of material anomalies, and in-service defects.
- Pressure vessel scanning for detection of MIC, SCC and other process related defects.
- Bonded clad materials to identify disbonds and cracking in the cladding or parent material.
- Storage tanks for detection of SCC in the welds or parent material.
- Accurate monitoring of defect growth, with repeatable scans for comparison.
TOFD testing is often performed in conjunction with other advanced NDT techniques, specifically with Phased Array techniques given PAUT’s ability to access other weld elements such as root and caps.
Advantages of TOFD
The advantages of TOFD include:
- TOFD accurately detects and sizes many sub-surface features and is not affected by defect orientation or geometry.
- TOFD is a rapid and highly reliable testing method that can be used to detect cracks and defects and monitor these defects over time.
- It can be carried out whilst production is occurring, which removes any downtime and efficiency concerns.
- TOFD can be used up to 120°C with special probe wedges and couplant.
- Produces result quickly along with accompanying graphics and full data recording.
- A permanent record of scan data can be maintained.
The ALS Approach
ALS continuously looks ahead and regularly invests in the latest technologies available to our industry. With continual advancement and superior customer value a priority, including comprehensive training, certification and regular competency assessments, ALS is able to continually deliver the most advanced, efficient and effective inspection methods available in the asset care field.
