🔩 Ultrasonic testing using the TOFD (time-of-flight diffraction) technique — detection of discontinuities and determination of their height from the time of flight of waves diffracted at their tips, mainly in welded joints of low-alloyed steels, PN-EN ISO 16828

Materials and NDT PN-EN ISO 16828

Overview

WHAT THE STANDARD COVERS. Scope from the PKN catalogue card of PN-EN ISO 16828:2026-01 (English version), in full (our translation): “This document specifies the general principles for the application of the time-of-flight diffraction (TOFD) technique for both detection and sizing of discontinuities in low-alloyed carbon steel components. This document also applies to other types of materials, provided the application of the TOFD technique is performed with necessary consideration of geometry, acoustical properties of the materials and the sensitivity of the test. Although this document is applicable, in general terms, to discontinuities in materials and applications covered by ISO 16810, it contains references to the testing of welds. This approach has been chosen for reasons of clarity as to the ultrasonic probe positions and directions of scanning. Unless otherwise specified in the referencing documents, the minimum requirements of this document are applicable. Unless otherwise specified, this document is applicable to the following test object classes as defined in ISO 16811: class 1, without restrictions; classes 2 and 3, specified restrictions apply (see Clause 10); classes 4 and 5 require special procedures, which are also addressed (see Clause 10). NOTE Techniques for the use of TOFD for testing welds are described in ISO 10863, and the related acceptance criteria are given in ISO 15626”. We do not have the text of the 2025 edition. We read the text of the previous edition — the sample of ISO 16828:2012 (introduced by PN-EN ISO 16828:2014-06) from the cover to 6.1 with the table of contents; we do not have Clauses 6.2–13 or Annex A (reference blocks).

ACCORDING TO THE TEXT OF ISO 16828:2012 (foreword, introduction, Clauses 1–6.1, table of contents). The standard was prepared by ISO/TC 135 “Non-destructive testing”, SC 3 “Ultrasonic testing”; the introduction states that it is based on EN 583-6:2008 and belongs to the series ISO 16810 (general principles), 16811 (sensitivity and range setting), 16823 (transmission technique), 16826 (discontinuities perpendicular to the surface), 16827 (characterization and sizing), 16828 (TOFD). Scope (1): general principles of TOFD for detection and sizing of discontinuities in low-alloyed carbon steel components, also in other materials with consideration of geometry, acoustic properties and sensitivity; references to welds for clarity of probe positions; product class 1 without restrictions, 2 and 3 with restrictions, 4 and 5 — special procedures (Clause 9). References (2): ISO 9712, ISO 16810, ISO 16811, EN 12668-1, -2, -3. Terms (3): scanning-surface dead zone (indications obscured by the lateral wave), back-wall dead zone (obscured by the back-wall echo), A-scan (amplitude versus time), B-scan (time of flight versus probe displacement), non-parallel scan (perpendicular to the beam direction) and parallel scan; symbols: x, y, z, Δx (discontinuity length), Δz (height), d (depth of a tip), Dds and Ddw (dead zones), c (sound velocity), R (spatial resolution), t, Δt, td, tp (pulse duration measured at 10 % of the peak amplitude), tw (time of flight of the back-wall echo), s (half the distance between probe index points), W (wall thickness). Principle (4.1): the technique relies on the interaction of waves with the tips of discontinuities, which emit diffracted waves over a large angular range; the dimension of a discontinuity is always determined from the time of flight of the diffracted signals, the signal amplitude is not used for sizing. Basic configuration: separate transmitter and receiver, normally wide-angle compression-wave probes; the first signal is the lateral wave, the second (without discontinuities) — the back-wall echo; signals from discontinuities arrive between them (neglecting mode conversion), from the upper tip earlier than from the lower one; there is a phase reversal between the lateral wave and the back-wall echo and between the echoes of the upper and lower tips; scanning from both surfaces improves precision for near-surface discontinuities. Surface and couplant (4.2): surface condition at least according to ISO 16810 — diffracted signals may be weak, so a poor surface severely reduces reliability; couplant compatible with the material (water with additives, paste, oil, grease, cellulose paste), with constant characteristics and suitable for the temperature. Materials (4.3): routinely unalloyed and low-alloyed steel and their welds, also fine-grained austenitic steel and aluminium; coarse-grained and strongly anisotropic materials (cast iron, austenitic welds, high-nickel alloys) require additional validation and data processing; by agreement inspectability may be confirmed on a specimen with artificial or natural defects, bearing in mind that artificial defects diffract differently from real ones. Personnel (5): qualified at least according to ISO 9712 plus additional training and examination in TOFD for the product classes tested, according to a written practice. Equipment (6.1): at least according to EN 12668-1, -2 and -3.

STATUS (PKN catalogue cards as of 02.10.2026). The card of PN-EN ISO 16828:2026-01 (English version, published 08-01-2026, 34 pages, price group Q) has no “Withdrawn” header: Sector of Basic Issues and Management Systems, KT 7 Non-Destructive Testing, ICS 19.100, “Introduces: EN ISO 16828:2025 [IDT], ISO 16828:2025 [IDT]”, “Replaces: PN-EN ISO 16828:2014-06 – English version”. The card of PN-EN ISO 16828:2014-06 (32 pages; it introduced EN ISO 16828:2014 and ISO 16828:2012) has the “Withdrawn” header: withdrawal date 08-01-2026, “Replaced by: PN-EN ISO 16828:2026-01”; it itself replaced PN-EN 583-6:2009. In the “Scope” field of that card, Note 3 refers to “ISO 10683” — the text of the standard has ISO 10863 (TOFD standards for welds); on the card it is a typing error. The PKN search engine (query “PN-EN ISO 16828”, 02.10.2026) does not show a Polish-language version of any edition. According to the EVS catalogue, ISO 16828:2025 is valid from 10.07.2025, and ISO 16828:2012 is the previous edition in the history of the standard. What changed for the result: we do not know — we do not have the text of the 2025 edition, and comparing the “Scope” fields of both cards shows only that the reference to the restrictions for classes 2–5 moved from Clause 9 to Clause 10, which indicates a change in the clause structure.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database, data up to 17.09.2026, read 02.10.2026). The number 16828 appears in 9 records of 8 laboratories: AB 001, AB 313, AB 608, AB 1336, AB 1425, AB 1429, AB 1767, AB 1927. In the current scopes on the PCA website (read 02.10.2026, issues from 23.04.2025 to 08.07.2026) seven laboratories cite the 2014-06 edition (AB 1927 written as “PN EN ISO 16828:2014-06”, without a hyphen), and AB 608 — the designation alone without a date; none cites the 2026-01 edition. The “Laboratories” tab (“PN-EN ISO 16828”) shows 7 laboratories — without AB 1927, whose entry begins with “PN EN” — checked with a query of the tab on the labcoda.pl production server on 02.10.2026.

WHAT THE LABORATORIES TEST (PCA entries). “Internal discontinuities” and “Discontinuities” of welded joints (AB 001: welded joints and steel products of thickness from 6 mm, steel and building structures; AB 1336: range from 6,0 mm), “Welding imperfections” (AB 1429); technique — “ultrasonic method”.

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. During measurement: the height of a discontinuity is determined only from the difference in times of flight of the signals from its tips — the size of the amplitude says nothing about the size of the discontinuity (4.1). A discontinuity just below the scanning surface can hide in the lateral wave, and one near the bottom — in the back-wall echo (dead zones, 3.1.1, 3.1.2); without scanning from the other side, no indication does not mean no discontinuity. With the material: in coarse-grained and anisotropic materials (austenitic welds, cast iron, high-nickel alloys) the technique requires additional validation — without it the result may look clean (4.3). During validation: artificial defects in a reference specimen diffract waves differently from real ones (4.3). With the surface: weak diffracted signals are lost with a poor surface and unstable coupling (4.2). With the edition: scopes cite the 2014-06 edition, withdrawn on 08-01-2026; evaluation according to acceptance criteria is given by ISO 15626, not by this standard.

WHAT WE DO NOT GIVE. We do not have the text of the 2025 edition or Clauses 6.2–13 of the 2012 edition: requirements for probes and scanning mechanisms, probe selection and separation, time-window setting, sensitivity, scan resolution and speed, formulae for depth and height, evaluation of errors and dead zones, requirements for reference blocks (Annex A) or the content of the test report — so we give none of these values or formulae.

Method principle

Transmitting and receiving ultrasonic probes on either side of the tested area: waves diffracted at the tips of a discontinuity arrive between the lateral wave and the back-wall echo; the presence of a discontinuity is detected from the diffracted signals, and its height and depth are determined from the times of flight (difference in times from the upper and lower tips) — the amplitude is not used for sizing.

Applications

• Welded joints and steel products — detection and sizing of internal discontinuities; 8 laboratories in PCA scopes
• Steel and building structures (in PCA scopes from a thickness of 6 mm)
• Techniques for welds according to ISO 10863 and evaluation against the acceptance criteria of ISO 15626 (referenced in the standard)

Key parameters

Standard

Standard number

PN-EN ISO 16828:2026-01 (wersja angielska; EN ISO 16828:2025 IDT, ISO 16828:2025 IDT)

Title (PL)

Badania nieniszczące — Badania ultradźwiękowe — Dyfrakcyjna technika czasu przejścia do wykrywania i wymiarowania nieciągłości

Title (EN)

Non-destructive testing — Ultrasonic testing — Time-of-flight diffraction technique for detection and sizing of discontinuities

Step-by-step procedure

1. Preparation

   Personnel qualified according to ISO 9712 with additional TOFD training; surface at least according to ISO 16810; couplant with constant characteristics. PN-EN ISO 16828:2026-01 current, while scopes cite the withdrawn 2014-06 edition (PKN cards as of 02.10.2026).

2. Setting up probes and system

   Probe selection and separation, time window, sensitivity, scan resolution and speed according to Clause 7 (we do not have its content); checking system performance on a reference block (Annex A).

3. Scanning

   Non-parallel scan along the reference line (e.g. the weld axis) and, where necessary, parallel scan; for near-surface discontinuities — from both surfaces, if accessible.

4. Analysis

   Indications between the lateral wave and the back-wall echo; position, length, depth and height from the times of flight of the signals from the tips (Clause 8 — we do not have the formulae); do not use the amplitude for sizing.

5. Limitations and report

   Take into account dead zones and measurement errors (Clause 10); test report according to Clause 13; evaluation of defects according to ISO 15626.

Required equipment and apparatus

Health and safety (OHS)

• The standard contains no safety warnings in the part we read; testing on structures and pipelines requires rules for work at height and in confined spaces set by the laboratory
• TECHNICAL NOTE: no indication in the dead zone near the surface or near the bottom does not mean there is no discontinuity

Frequently asked questions