NDT of vertical orientated cracks by mode conversion with optimized LLT probes

An ultrasonic single probe inspection method, based upon mode conversion, was developed for the detection of perpendicularly oriented cracks. In comparison to the cumbersome Tandem technique this "LLT-technique" has about the same sensitivity but has advantages in manual as well as automated inspection with regard to handling and applicability. Besides, the LLT-technique stands out for an improved accessibility of the inspection regions, a reduced crack inclination dependancy and less disturbances at cladded components. Since the specular reflections are exploited a reasonable amplitude evaluation is possible. The LLT-technique can be applied for the inspection of thick wall components as vessels or turbine shafts or thin wall components as tubes or sheets. The defect characterization is improved and there is the possibility for a defect classification, i.e. distinguish between cracks, pores and inclusions. Recently developed exact theoretical modells for calculating the scattering at finite flaws (strip like reflector, infinite long), confirm the high sensitivity of the LLT-technique also for finite flaws. Based upon a first approach of the above mentioned exact elastodynamic theory and calculations of the echo transmission between the probe wedge and surface under test, a set of three ultrasonic transducers for LLT-technique have been designed and the sensitivity areas calculated.