Synthetic aperture focusing technique for industrial applications
Detection and evaluation of defects rely strongly on ultrasonic inspection techniques in the data acquisition procedure and the processing technique. The performance of those techniques can be improved if high-frequency ultrasonic signals are stored and evaluated by tomographic algorithms like "Synthetic Aperture Focussing Technique - SAFT". Special algorithms allows one to use forward- or backward scattered signals, to apply the inverse algorithm not only for plane surfaces or cylindrical components but in addition - CAD- based - to arbitrarily shaped components. The contribution covers the principle of SAFT and demonstrates the capabilities in different application fields. Due to the change of the frequency within the range of 100 kHz up to 10 MHz problems could be tackled like imaging of tendon ducts or reinforcement in concrete slabs, half penny shaped fatigue cracks in a cladded segment of a ferritic pipe or inspection of axial or radial bore holes in turbines or generators. One important aspect is the 3D-imaging of 3D-defects. It will be demonstrated that despite the possibility of rotating data cubes after a 3D-inversion scheme, the information displayed is restricted to the insonified part of the defect. With respect to practical purposes the 3D-data cube will be not rotated but sliced into orthogonal planes to evaluate size and orientation of defects quantitatively. In addition there is an improved possibility to characterize a defect if it is surface connected or if a subsurface crack may start from one of the layers of a cladded vessel or from the interface between the cladding and the base material. In automatic inspection systems, x-y-scanners are used for flat components or pipe scanner for cylindrical components. A special probe holding device for contact technique probes is used to expand the sizing technique to arbitrarily shaped components. This device measures the vertical movement of a probe with an accuracy of 0.01 mm. The surface profile can be extracted to build up the surface contour in a CAD-drawing. This is important to take care of the change in insonification direction at the crown of a weld or on grinded surfaces with irregular shapes. In the reconstruction process it is important to know the insonification angle relative to the actual position of the probe. This is realized by the simulation of the start of the multiple step-size and the end of the mechanical scan at the surface of the CAD-component. Knowing the normal to the surface of each probe position, all insonification angles inside the 3D shear wave beam can be calculated and used to apply the SAFT algorithm correctly. The efficiency of this technique will be demonstrated at a component of an engine for heavy cargo ships.