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.