Time-of-Flight Diffraction (TOFD) is a reliable advanced ultrasonic inspection (UT) method for weld inspection. TOFD uses the "time of flight" of the ultrasonic pulse to find the location of the reflector. It can also be used in the heat-affected zone of overlays and other components such as pipes, pressure vessels, composites, tanks, and structural steel.
TOFD was originally developed to accurately monitor and determine the height of in-service discontinuities through walls in the nuclear industry. It has now been independently validated as one of the most effective techniques for locating and determining discontinuities in ferrite welds. It is often used in combination with phased-array ultrasonic inspection (PAUT) to perform a single-sided inspection of the weld.
There are two basic components, an illumination array and a receiver array, each consisting of multiple transducers (called transducers), and both are typically arranged on an XY Cartesian grid. The two arrays are separated by a known distance, usually a few centimeters, but it can be longer or shorter if needed, depending on the depth needs of the investigation.
HS 811 TOFD Flaw Detector
In TOFD, a pair of ultrasonic probes are located on opposite sides of the weld or area of interest. The transmitter probe emits an ultrasonic pulse that is picked up by the receiver probe on the other side. In the undamaged section, the receiver probe picks up signals from two types of waves: one that propagates along the surface (transverse wave) and another that reflects from the far wall (back wall reflection).
When discontinuities such as cracks are present, the ultrasonic waves are diffracted from the top and bottom tips of the cracks. Using the measured pulse time of flight, the depth of the crack tip can be automatically calculated by trigonometric applications. This method is even more reliable than conventional radiographic, pulse-echo manual UT, and automated UT weld inspection methods.
TOFD is best suited for characterizing powdered solids and powders, where diffraction and scattering are considered less important than for liquids and pastes.
HS 810 TOFD Flaw Detector
Major applications of TOFD technology include powder polymer characterization, crystalline materials, catalysts and products made from them, ceramics, metals, minerals, and related materials found in devices such as batteries, automobiles, electronic components, appliances, and industrial manufacturing facilities. Non-porous, continuous solid samples can also be tested with TOFD.
Reflectorless operation eliminates the need to reorient samples for different analysis areas, which saves a significant amount of time and allows each sample to be examined in multiple areas in a single test run.
With the TOFD, liquid-filled vessels with air headspace can be measured without having to empty or even flatten them during the test.
No minimum sample size restrictions.
Ability to measure wet samples.
Can handle heavy or fragile products, such as automotive parts that cannot be conveyed using a conveyor belt.
Data is displayed immediately upon inspection, so you don't have to wait for the scan to complete to know if you're getting good results. The entire measurement process can be completed in a matter of minutes.
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