Ludwig von Bernus

Broadband holography applied to eddy current imaging by using signals with multiplied phases

In this paper an eddy current imaging method for nondestructive testing purposes is presented which utilizes the concept of broadband holography. An eddy current coil which is used simultaneously as an antenna for eddy current generation and as a probe for detection of response of interaction between eddy currents and flaws, respectively, is moved along a synthetic aperture during the imaging procedure generating synthetic eddy current pulses by scanning a certain frequency range. In terms of wave propagation phenomena the penetration depth (range) of eddy currents in conducting media is small compared to the equivalent wavelength of this type of fields. Therefore, adequate resolution can only be obtained in the reconstructed cross-sectional images by phase multiplication of received multifrequency signals, which is equivalent to a fictitious reduction of wavelengths. Experimental results verify the imaging capability of this method with improved resolution compared to conventional eddy current testing methods.

Acoustical Imaging Using an Optimal Combination of Signal Prefil Tering and Pulse Compression

Resolution in acoustical imaging is characterized by the spectral properties of the transducer used in an imaging system, with the center frequency determining the lateral resolution (together with the aperture size and the object distance) and the bandwidth determining the axial resolution. As opposed to other diagnostic imaging methods (like X-ray imaging) the resolution of acoustical imaging systems usually needs to be optimized. Resolution can be increased by using high frequency transducers with a large bandwidth. But there are limitations in approaching higher frequencies and higher bandwidths due to the attenuation of the medium which has to be imaged or which is surrounding the objects of interest. This attenuation is limiting the allowable object distance as well as the (axial) size of the object areas.

Detection and Imaging of Defects Especially Materials with Small UT Transducers Using Broad-Band Holography

Since conventional single frequency acoustical holography provides only poor axial resolution, this concept was improved with the multifrequency holography to enhance the imaging quality. This leads to long data acquisition times because of the need to measure each frequency. A further step towards a fast imaging system with good spatial resolution is broadband holography. Here, one illuminates the object with broadband signals in a single measurement procedure.

Experiments and Experiences Using Filtering and Data Reduction Techniques for Imaging in Nondestructive Testing

The theory of quantitative ultrasonic imaging places high demands on the data to be collected. When using imaging techniques in the field, restrictions have to be faced which are imposed by the physical limitations of transducers or by limits on computer memory in terms of sampling density. In order to overcome these limitations it can be helpful to incorporate a priori knowledge in to the reconstruction algorithm, e.g., by application of filter techniques. Filtering is a convenient way to select the minimum necessary information to be sampled, stored or evaluated. The following filter techniques are investigated here: FIR filter in the time domain ALOK i, k filter K-space filter Image processing

Current in-service inspections of austenitic stainless steel and dissimilar metal welds in light water nuclear power plants

Abstract A tendency towards growing requirements for the inspection of austenitic piping can be observed in several countries. In Germany the revised KTA rule demands the UT inspection of austenitic and dissimilar metal welds in piping with diameters of 200 mm or more. On the basis of experience gained from austenitic piping with integranular stress corrosion cracking (IGSCC), longitudinal waves and mode conversion techniques are used. Depending on the geometry, material and grain orientation, spurious signals can be observed which require additional evaluation or analysis measurements. A promising new technique is based on horizontally polarized shear (SH) waves generated by electromagnetic acoustic transducers (EMATs). Investigations in the laboratory and field inspections showed that SH waves are well suited for the detection of longitudinal flaws, especially where the weld can be examined from one side only. For the complete solution of a given inspection problem SH waves can be combined with well-known standard techniques in order to provide redundant information for the characterization and sizing of indications. The investigation of possibilities of SH waves showed that the problem of cast austenitic steel inspection might not be solved using this technique. However, measurements using low frequency UT transducers showed promising results.