What is time of flight diffraction (TOFD) in ultrasonic testing?
In this article:
• Understanding TOFD's Core Principles: Learn how Time-of-Flight Diffraction (TOFD) utilizes diffracted waves for precise defect detection and sizing, unlike conventional pulse-echo methods.
• Probe Configuration and Setup: Discover the specialized high-frequency, broadband probes used in TOFD, their through-transmission arrangement, and adherence to standards like EN ISO 10863.
• Interpreting Key Signal Components: Understand the significance of lateral waves and backwall reflections in A-scans, including hyperbolic depth calculation for accurate flaw localization.
• Addressing Near-Surface Limitations: Recognize TOFD's "dead zone" limitation near the surface and why it's often combined with other ultrasonic techniques for comprehensive inspection.
• Imaging and Evaluation: Explore how TOFD creates quasi-B-scans for detailed imaging and how measurement tools are used to determine indication length and depth for acceptance criteria.
• The Waygate Technologies Krautkrämer USM Go+ or USM 36 portable ultrasonic flaw detectors are ideal for TOFD, offering the high-frequency signal processing and display capabilities essential for accurate diffracted wave analysis.
What is time of flight diffraction (TOFD) in ultrasonic testing?
Time-of-Flight Diffraction (TOFD) is a powerful ultrasonic testing technique that offers distinct advantages over traditional pulse-echo methods—most notably, its ability to detect and size defects with high precision, regardless of flaw orientation. Originally developed for weld inspection, TOFD has become a trusted tool in non-destructive testing (NDT) due to its excellent sizing accuracy and high probability of detection for embedded planar defects such as cracks and lack of fusion.
Unlike conventional methods that rely on strong specular reflections, TOFD utilizes diffracted waves from the tips of discontinuities, making it highly effective for accurate depth profiling. This article explores how TOFD works, the physics behind its operation, typical probe configurations, signal interpretation techniques, and practical limitations. By understanding TOFD’s core principles and application strategies, inspectors can enhance flaw characterization, reduce false calls, and improve the overall reliability of weld evaluations.
How TOFD Differs from Conventional Pulse-Echo Techniques
In contrast to conventional pulse echo techniques, which evaluate reflections from flaws in the test object based on time-of-flight (distance) and amplitude, Time-of-Flight Diffraction (TOFD) utilizes a different physical property of waves: diffractions at the edges of discontinuities.
Probes specifically designed for TOFD have the highest possible frequencies (adapted to the material thickness), small probe diameters, and are broadband: High frequency and broad bandwidth ensure optimal (near) resolution, and small probe diameters generate the largest possible sound field divergence. The latter is important for complete in sonification of the region of interest.
Standards and Setup Geometry
TOFD is described in the standards EN ISO 10863 (2020) and EN ISO 16828 (2014). Here, one can also find corresponding recommendations for the selection of probe parameters depending on the wall thickness t of the test object.
TOFD is almost exclusively applied to planar, parallel-sided components for the inspection of welds, typically employing two longitudinal wave angle beam probes in a through- transmission arrangement, fig. 51. The ultrasonic instrument evaluates all signals received by the receiver probe R.
Probe Placement and Beam Interaction
The probes are positioned for the application (weld inspection) so that the acoustic axes intersect at approximately 2/3 of the wall thick- ness2 (fig. 52).
With the beam angle β, this results in the distance PCS (PLrWobe Center SeparatioBnR = center- to-center distance between the probes to the two sound exit points):
This distance must remain constant during the application, and therefore the probes are fixed in a scanner (fig. 58).
Key Signal Components in the A-Scan
Two indications appear on the screen of the test instrument: The lateral wave (LW) with a sound path along the surface and the backwall reflection (BR) (fig 52). The signal representation in the A-scan is dis- played in unrectified mode with the time of flight in μs on the horizontal axis and the amplitude on the vertical axis (fig. 53). When observing the signals, it is immediately apparent that they are 180° out of phase: The first maximum half-wave of the lateral wave is positive, while that of the backwall reflection is negative. The phase positions of TOFD signals play an important role in the evaluation.
The lateral wave always appears first because the distance between the transmitter and receiver is the shortest. The corresponding time of flight at the maximum of the first half-wave represents the zero point with depth “0”. The minimum of the first half-wave of the backwall signal corresponds to the thickness (t) of the test object or calibration block. This means that all diffraction signals from the overlapping region of the sound beams appear between the lateral wave and the backwall reflection. However, the relationship between the time of flight and the corresponding depth is not linear but follows a hyperbolic function (eqn. 54, fig. 56.)
Hyperbolic Depth Calculation
where:
z = depth [mm]
PCS = Probe Center Separation [mm] c = sound velocity [mm/μs]
∆t = time of flight difference relative to the lateral wave [μs]
Even at the first reflection on the opposite wall, in addition to the reflection of the longitudinal wave, wave conversion into a shear wave occurs. This shear wave propagates at ap- proximately half the velocity of the longitudinal wave, so these signals are received much later. It is recommended to display the shear wave signals in the A-scan as well, but the evaluation of diffraction signals from discontinuities is always performed in the region between the lateral wave and the backwall reflection.
The decisive advantage over pulse echo ultrasonic testing techniques is that diffraction signals are virtually independent of the orientation and shape of dis- continuities. Therefore, if an inclusion, crack, or other reflector is located between the probes, diffraction occurs at the edges of the reflector. These edges become the source points of diffracted, spherical elementary waves, which are detected by the receiver probe and appear in the A-scan at high instrument gain (fig. 54).
The signal of the upper edge is 180 degrees out of phase with the lateral wave, while the signal from the lower edge has the same phase. For precise localization of the reflector, the time of flight ∆t1 at the minimum of the first half-wave of the upper edge and ∆t2 at the maximum of the first half-wave of the lower edge are measured, both relative to the zero point (fig. 54). The depth (z) of the respective diffraction centre is calculated using eqn. (54).
From the calculated depths for the upper and lower edges, the depth extent h = z2 - z1 of the discontinuity can be determined—one of the criteria for determining the acceptance level of indications. Note, however, that the value z has a tolerance due to the unknown trans- verse coordinate of the diffraction centre, which lies on an elliptical curve for a given time of flight ∆t (fig. 55).
The possible deviation increases with the lateral displacement of the diffraction centre, smaller PCS, and decreasing time of flight differences (i. e., smaller depths). Through parallel scanning, where the probes are moved parallel to the deflection plane, the trans- verse coordinate of the diffraction centre can be precisely determined. This allows for accurate depth de- termination, as the measured time of flight is minimized when the diffraction centre lies exactly midway between the two probes.
Near-Surface Limitations and Dead Zones
For discontinuities that approach the coupling surface of the probes, the diffraction indications eventually overlap the lateral wave and can no longer be clearly detected and measured. Additionally, this overlap causes changes in the amplitude and phase of the lateral wave. This creates a ‘’dead zone’’ approximately equal to the pulse width of the lateral wave, ideally one oscillation period (fig.56).
Example:
For a PCS of 50 mm, this results in dead zones of at least 3,9 mm at 10 MHz and 5,5 mm when testing with 5 MHz.
This poor near resolution is the big- gest disadvantage of TOFD, as near-surface discontinuities are hardly detected at all or only indirectly! Consequently, further testing of the near-surface region is re- quired to achieve the necessary test sensitivity. In practice, therefore, TOFD inspection is combined with conventional pulse-echo techniques or phased array testing. The inspection is carried out using ap- propriate scanners that integrate all probes and a test instrument that operates both testing techniques simultaneously. In this way, a weld can be reliably inspected with a single scan5.
5According to EN ISO 10863, welds with a thick- ness > 50 mm must be inspected in two or more depth zones. Depending on the scanner used, this may require multiple scans.
Imaging and Scan Interpretation
TOFD is considered an imaging inspection method. For this purpose, the amplitudes of the A-scan are first converted into gray values and displayed on the time axis (fig. 57).
The scanner (fig. 58) is equipped with a position encoder, so that during non-parallel scanning of the weld, the A-scans are recorded and stored as grayscale lines at defined intervals (scan step). After scanning, they are available for evaluation as a quasi-B-scan (fig. 59).
Measurement Tools and Evaluation Criteria
For the evaluation of the TOFD scan, there are typically three different cursors available in the ultrasonic instrument:
1. A-scan cursor (vertical), for selecting the x-coordinate at which the stored A-scan is displayed (test data cusor).
2. Start cursor (crosshairs) positioned at the beginning x1 and the corresponding depth z1 of an indication (→ measurement cursor A).
3. End cursor (crosshairs) positioned at the end x2 of an indication and the corresponding depth z2 of the indication (→ measurement cursor B).
The result of the evaluation is the indication length l = x2 - x1 and the depth extent h = z2 - z1. The test data cursor assists in the precise positioning of the crosshairs by displaying the corresponding A-scans, taking the phase into account. The indication length l and depth extent h are used to determine the acceptance level according to EN ISO 15626, considering whether the indication is connected (open) to the surface or to the opposite side, or whether it is embedded. The amplitudes of indications are not evaluated. Indications that are open to the surfaces have smaller permissible lengths and depth extents.
Indication Characterization and Depth Accuracy
TOFD indications can often be characterized relatively well. An experienced inspector can determine with reasonable certainty whether a discontinuity is embedded or open to a surface. In such cases, a significant change in the respective signal of the lateral wave or backwall reflection is observed; for example, the lateral wave is completely interrupted at the crack in fig. 59. The slightly downward-shifted signal with the same phase as the lateral wave is already the diffraction indication of the lower crack edge and thus directly provides the crack depth.
The depth extents (h) are determined with a small tolerance, approximately ±0,2 mm depending on the wall thickness/depth zone, for diffraction centers positioned centrally between the probes (avoiding the tolerance from lateral displacement). The indication length (l) is determined with a resolution equal to the set scan step. Depending on the wall thickness, the scan steps are 1–3 mm. Therefore, realistic dimensions of the detected discontinuities are obtained, whereas pulse echo techniques work with equivalent reflector sizes, which can deviate significantly from the actual reflector size due to the uncertainty of the shape, orientation, and surface condition of a natural material flaw.
The combined application of TOFD and pulse echo techniques/phased array provides high test reliability, optimizing the probability of detection (POD) of discontinuities because the advantages of the two inspection techniques complement each other very well and compensate for their respective disadvantages. In inspection practice, this can eliminate the need for previously required additional X-ray inspection of the weld, resulting in significant time and cost savings, see ASME-Code Case 2235-9 (2010).
Further mathematical algorithms for improving image quality and more precise indication evaluation are included in most ultrasonic instruments but will not be discussed further here.
In summary
Time-of-Flight Diffraction (TOFD) stands out in the field of ultrasonic testing for its precise defect sizing and orientation-independent detection capabilities. Through the use of high-frequency, broadband probes in a through-transmission setup, TOFD captures diffraction signals from flaw tips, enabling accurate depth measurements and visualization of embedded discontinuities. While it does face limitations—most notably poor near-surface resolution—it excels in sizing internal defects and complements other techniques like pulse-echo and phased array.
In practice, TOFD is rarely used in isolation. Its greatest value lies in being part of a combined inspection strategy that leverages the strengths of multiple methods to achieve high test reliability. When properly implemented with appropriate scanners and instrumentation, TOFD can replace more invasive techniques like radiography, reducing cost and inspection time without compromising quality. For inspectors and engineers aiming to ensure the structural integrity of welds, mastering TOFD is essential for achieving both regulatory compliance and operational confidence.
For more information, please refer to the operating manuals of the respective instruments and relevant technical literature, e. g.:
[30] Ultrasonic Time of Flight Diffraction, Eclipse Scientific Products Inc. (2013)