Time of Flight Diffraction Ultrasonics
The TOFD technique relies on the interaction of ultrasonic waves with the tips of flaws. When an ultrasonic wave interacts with a planar flaw it results in the production of diffracted waves from the tips of the flaw in addition to the normal reflected wave. This diffracted energy is emitted over a wide angular range and is assumed to originate from the flaw extremities.
Probes having a wide beam divergence angle are generally used, since the diffraction of ultrasonic waves from flaw tips is less dependent on the orientation of the flaw than in the case of a reflection from the face of a flaw. Fig.2 shows a typical received A-scan signal from an embedded planar flaw.
The first signal to arrive at the receiver is the lateral wave which travels just beneath the upper (scan) surface of the specimen. In the absence of any flaws, the second signal to arrive is that reflected from the far (backwall) surface, commonly referred to as the backwall echo.
When a planar flaw exists, diffracted energy from the upper tip or extremity will be received before that from its lower tip. These two compression wave signals from the flaw tips will occur between the lateral wave and backwall echo responses. Therefore it is possible to determine not only the through-wall height of the flaw, but also its location within the thickness of the specimen. Note that Fig.2 shows a phase reversal between the lateral wave and backwall echo, and between the upper and lower flaw tip signals. This characteristic effect can greatly assist interpretation of TOFD data and is why TOFD A-scans are acquired unrectified, i.e. they are RF waveforms.
Copyright © 2012, TWI Ltd
Information and advice are provided in good faith. No warranty expressed or implied is given regarding the results or effects of applying information or advice from the website, nor is any responsibility accepted for any consequential loss or damage. Accessibility statement.