ISO 16826:2012 defines the principles for tandem‑ and longitudinal-longitudinal-transverse (LLT) wave‑examination for the detection of discontinuities perpendicular to the surface.
The general principles required for the ultrasonic examination of industrial products are described in ISO 16810. A list of symbols and equations is given in ISO 16811.
The tandem‑ or LLT‑examination should be used for the detection of planar discontinuities with distance to the surface greater than 15 mm. ISO 16826:2012 has been prepared for the examination of metallic materials with a thickness between 40 mm and 500 mm with parallel or concentric surfaces. It can, however, be used for other materials and smaller thickness provided special measures are taken.