ISO 16826:2025

Overview - ISO 16826:2025 (Ultrasonic testing for discontinuities perpendicular to the surface)

ISO 16826:2025 defines principles and practical guidance for ultrasonic non-destructive testing (NDT) using the tandem technique and the longitudinal-longitudinal-transverse (LLT) technique to detect discontinuities that are perpendicular - or nearly perpendicular - to the testing surface. The standard focuses on metallic materials with thicknesses between 40 mm and 500 mm and covers objects with parallel or concentric surfaces. Phased array ultrasonic testing can be used with these techniques but may require extra verification steps.

Key topics and technical requirements

• Scope and applicability: Tandem and LLT techniques for embedded planar discontinuities; provisions for other materials or thinner sections with a written testing procedure.
• Test equipment and personnel: Applies ISO 16810 principles for equipment, qualification and competence of test personnel.
• Tandem technique essentials:
  • Normally uses two similar 45° angle-beam transverse-wave probes (transmitter and receiver).
  • Guidance on probe spacing, probe movement (fixed spacing or simultaneous opposite movement), and avoidance of beam angles causing mode conversion.
  • Sensitivity setting options: back-wall echo, disc-shaped (flat-bottom) reflectors, side‑drilled holes.
  • Determination and sizing of test zones, allowance for beam divergence, and acceptable sensitivity variation (edge sensitivity limits).
• LLT technique essentials:
  • Principles for generating and interpreting LLT wave interactions to detect perpendicular discontinuities, with time‑base and sensitivity setting guidance.
• Depth-gain and sensitivity diagrams: Depth‑gain diagrams and DGS-derived guidance help set gain and assess detectability across the thickness.
• Special cases: Procedures for concentric (convex/concave) surfaces and recommendations when probe sizes differ (e.g., >160 mm wall thickness).
• Annexes and tools: Nomograms and graphical methods for probe spacing and zone determination; formulas and diagrams provided for practical setup.

Practical applications and users

ISO 16826:2025 is used by:

• NDT/ultrasonic inspectors and technicians performing in-service inspection and shop testing
• QA/QC engineers, welding inspectors, and integrity specialists
• Industries with thick metallic components - heavy fabrication, pressure vessels, power generation, petrochemical, pipeline and structural steel Benefits include reliable detection of embedded planar defects (e.g., cracks, lack-of-fusion) oriented perpendicular to the surface and standardized setup procedures for repeatable results.

Related standards

• ISO 16810 - Ultrasonic testing - General principles
• ISO 16811 - Sensitivity and range setting
• ISO 16823 - Through‑transmission technique
• ISO 16827 - Characterization and sizing of discontinuities
• ISO 16828 - Time‑of‑flight diffraction (TOFD) technique

Keywords: ISO 16826:2025, non-destructive testing, ultrasonic testing, tandem technique, LLT technique, phased array, test zones, sensitivity setting, depth-gain diagram.