Accurate, Reliable and Fast Ultrasound Testing
Time of Flight Diffraction (TOFD) is an advanced ultrasonic testing technique that provides a very cost-effective inspection of welded joints and materials.
Many studies have compared TOFD to the more traditional radiographic, manual, and automated pulse echo methods. The results demonstrated TOFD has a higher probability of detection (POD) and a lower false call rate (FCR) on most of the flaws.
TOFD is suitable for pre-service and in-service inspections. It is excellent for acquiring base line data on welds and progressively monitoring the growth of service induced flaws.
Equipment calibration is relatively quick and simple. Detection and sizing are done simultaneously. Scanning rates of two to six linear inches per second are common. These factors all contribute to the ability of TOFD to be a fast and efficient method for examining a lot of weld in a very short amount of time.
Baker Testing will work with you to determine if TOFD is the best inspection method for your job.
The most common application for TOFD is weld inspection. In many instances, Time of Flight Diffraction can be applied where conventional ultrasonic testing is typically used. Services Baker Testing offers using TOFD include:
- Pipe and Pipelines
- Steam Piping
- Process Piping
- Boiler Tubes
- Storage and Water Tanks
- Pressure Vessels
- Paper Mills
- Detection and Sizing of IGSCC
- Detection and Sizing Hot Hydrogen Attack
- Overlay / Cladding & Babbitt Disbond
TOFD applies a pitch-catch method with broad beam probes, one on each side of the weld, to flood a large inspection area with ultrasound. (In many cases, only a single linear scan is required to examine the entire weld joint and adjacent base material). The method detects the forward scattered or reflected signals from the extremities of the flaws, rather than back reflected waves as is done with the conventional pulse-echo technique. This allows flaw detection that is practically independent of defect orientation.
The transmitter probe emits an ultrasonic pulse, which is picked up by the receiver probe on the opposite side. In a flawless piece, signals picked up by the receiver probe are from two waves – one that travels along the surface (lateral wave) and one that reflects off the opposite wall (back-wall reflection).
When a discontinuity such as a crack is present, there is a diffraction of the ultrasonic sound wave from the top and bottom extremities of the crack. Using the measured time of flight of the signals, the system software can apply basic trigonometry functions to calculate the vertical positions of the crack tips.
Probes are mounted to a scanning mechanism along with an encoder, which tracks the position of the probes relative to the test piece. As a linear scan is being performed, analog signals are digitized and recorded by the inspection system.
The transducers used have unique characteristics – longitudinal wave, highly damped, broadband, small diameter element that provide high sensitivity and excellent resolution.
A Practical Alternative to Radiography
TOFD is a safe, fast and economical alternative to radiography. The danger of radiation is eliminated and there is no need to evacuate large areas when testing welds. In most cases there is no need to stop the welding process, allowing higher production rates. The ability of TOFD to accurately size and position unacceptable flaws aids in the weld repair process. Like radiography, TOFD provides a permanent record. The data is digitally recorded and stored for future reference.
Advantages of TOFD
- High probability of flaw detection and low false call rate
- Accurate flaw sizing and characterization
- Flaw detection is practically independent of defect orientation
- Flaw sizing is not dependent on signal amplitude
- Wide range of material thickness can be examined (6mm – 300mm) (.24in – 1.18in)
- Relatively quick and simple calibration process
- High scanning speed results in large number of welds examined in a short period of time
- Quick analysis and examination results
- Able to measure flaw length, height and through thickness position
- Reduced costs compared to other examination methods
- Portable equipment
- Reproducible results are beneficial for flaw growth monitoring
- Data is digitally stored for future reference or second opinion
- Compliant with most international and local codes/standards