Time of Flight Diffraction
Time of Flight Diffraction (TOFD) is an advanced method of ultrasonic testing. This technique is a sensitive and accurate method for the nondestructive testing of welds or defects.
TOFD is conducted from a system that is made up of two probes located on opposite sides of the weld. One of the probes is a transmitter which emits an ultrasonic pulse. The second opposing probe is the receiver, and it picks up the pulse. The probes are mounted on a movable cart, which uses a computer system to record data as it moves.
In a weld without any flaws or damage, once the pulse is emitted from the transmitter, it will split into two waves. One of the waves is known as the lateral wave and it travels along the surface of the part. The second is called the back wall reflection, which reflects off the back wall of the part. Both are picked up by the receiver, which it uses as a baseline.
If a discontinuity is present, a diffraction of the emitted wave will appear from each tip of the crack. This means that the wave splits into two parts which are detected by the receiving probe. Once discovered using the measured time of flight, the pulse depth and location of the defect can be calculated by comparing the diffracted waves to the lateral wave and back wall reflection.
TIME OF FLIGHT DIFFRACTION APPLICATIONS
Utilized for rapid detection and sizing of cracking and weld flaws
Typically used on heavy wall reactors or vessels in lieu of industrial radiography
Can be combined with Phased Array to provide higher POD and more accurate classification of defects
The detection of cracking
ADVANTAGES OF TIME OF FLIGHT DIFFRACTION TESTING
TOFD is very sensitive to cracks and is able measure their dimensions and position accurately. It is also able to detect cracks regardless of their orientation.
In most cases, only one scan is required to find defects in a weld. This makes TOFD one of the fastest methods available for detecting defects in welds. This method also has a good degree of repeatability and gives consistent results. This means that if TOFD is used on the same weld over time, it is able to track the growth of flaws. It is also capable of precisely detecting and sizing many different types of defects within a weld.