Time Of Flight Diffraction
Time of Flight Diffraction (TOFD) is an advanced ultrasonic nondestructive testing method that uses diffracted ultrasonic waves from flaw tips, rather than only reflected echoes, to locate and size internal discontinuities in welds. Two probes on the same surface, one transmitting and one receiving, scan along a joint to build a grayscale cross-sectional image from the time-of-flight of the lateral wave, back-wall echo, and diffracted tip signals.
On critical fabricated assemblies, pressure parts, pipe welds, structural welds, and repair welds, TOFD is used to verify whether a discontinuity is present and estimate its through-wall height from measured arrival times of tip diffracted signals. The probes sit in a pitch-catch arrangement with a fixed probe center separation matched to material thickness, so the ultrasonic beam covers the weld volume and heat-affected zone. The result is a digitized B-scan-like image that helps inspectors distinguish the lateral wave, back-wall echo, and flaw response, supporting weld acceptance decisions and post-repair verification. In production environments, TOFD is chosen when objective records, high probability of detection for planar defects, and repeatable sizing are needed for QA/QC documentation. It is an inspection technique for metal weldments or structural components, not a machining-process measurement method for toolpath or part geometry.
- Dead zone near the surface or root: The lateral wave and near-surface signals can mask small indications, so very shallow flaws may be missed or under-characterized during TOFD imaging and setup design.
- Incorrect probe spacing or wedge setup: A wrong probe center separation, wedge angle, or scan position can fail to insonify the weld volume, leading to poor coverage and inaccurate defect sizing on thick or thin sections.
- Rough or irregular weld geometry: Coarse surface conditions, weld cap irregularity, or cluttered geometry generate strong noise that overwhelms the weak diffracted tip signals, reducing signal clarity and confidence in interpretation.
What physical phenomenon does TOFD use?
TOFD uses diffraction from crack tips. The time difference between received diffracted signals is used to infer defect depth and extent, which is why it can locate and size internal discontinuities in welds.
Why is TOFD better than conventional pulse-echo for sizing?
Because TOFD measures the timing of tip-diffracted waves, it provides more accurate through-wall sizing and has less dependence on defect orientation than reflection-only ultrasonic methods.
What does PCS mean in TOFD?
PCS stands for Probe Center Separation, the distance between the two probe centers. It is adjusted to the inspected material thickness to optimize coverage and signal geometry for reliable TOFD inspection.