Overview - ISO 16826:2012 (Ultrasonic testing for discontinuities perpendicular to the surface)
ISO 16826:2012 is an International Standard that defines the principles for ultrasonic non‑destructive testing (NDT) using tandem and longitudinal‑longitudinal‑transverse (LLT) techniques to detect planar discontinuities oriented perpendicular to a scanned surface. It is primarily prepared for metallic materials with thicknesses between 40 mm and 500 mm and is intended for detection of discontinuities located more than 15 mm from the surface. The Standard also notes applicability to other materials or thinner sections when special measures are applied.
Key technical topics and requirements
- Techniques covered
- Tandem technique: two angled probes (commonly 45°) - one transmitter and one receiver - aligned so beams intersect in the examination zone.
- LLT technique: a longitudinal wave is launched, reflects from the back wall, converts to transverse at a perpendicular discontinuity and is detected by a separate receiver.
- Inspection geometry and probe setup
- Probe spacing, beam intersection depth and examination zone sizing are defined; probe spacing equations and nomograms are provided for planar and concentric (convex/concave) surfaces.
- Use of 45° probes is limited by curvature ratio (d/D) - the Standard specifies limits for convex and concave scanning surfaces and recommends changing incidence angles to avoid mode conversion.
- Sensitivity, calibration and DGS
- Sensitivity setting using backwall echoes, disk‑shaped reflectors (flat‑bottomed holes) and side‑drilled holes.
- Preparation of DGS/DAC diagrams for tandem and LLT setups, and required corrections for transfer losses, attenuation and edge sensitivity (typically up to 6 dB compensation).
- Depth and time‑base determination
- LLT depth determination requires calibration on reference blocks with disk reflectors; the Standard gives procedures and the governing relations between incident angles and wave velocities.
Practical applications and users
- Typical applications:
- Detection of planar flaws (e.g., delaminations, laminations, cracks) in thick-walled metallic components such as pressure vessels, forgings, large castings and pipeline sections.
- Inspection of components with parallel or concentric surfaces where through‑thickness coverage is required.
- Who uses this standard:
- NDT engineers, ultrasonic technicians, inspection planners and quality managers in oil & gas, power generation, heavy manufacturing and transportation sectors.
- Standards committees and labs developing inspection procedures and acceptance criteria.
Related standards (for cross‑reference)
- ISO 16810 - Ultrasonic testing: general principles
- ISO 16811 - Sensitivity and range setting (symbols and equations)
- ISO 16823, 16827, 16828 - Transmission, characterization/sizing, and TOFD methods
- ISO 5577, EN 1330‑4 - Ultrasonic testing vocabulary and terminology
ISO 16826:2012 is essential for practitioners implementing tandem or LLT ultrasonic testing to reliably detect and characterize discontinuities perpendicular to the surface in thick-section components.
Open preview (PDF)Non-destructive testing — Ultrasonic testing — Examination for discontinuities perpendicular to the surface