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Neutrons are absorbed particularly by hydrogen containing materials so
they can be used as a tool for visualising heterogeneous density distributions of organic materials. Penetration limits are set by the layer thicknesses and the neutron energies applied. In case of specimens with a flat shape the situation may be encountered that the object only could be penetrated in selected directions. In addition, the overall size may exceed the beam diameter and thus the viewing window if a certain region of interest should be studied by tomography without destroying the integrity of the specimen.
This study shows the capabilities and limits of thermal and fast neutrons to investigate flat wooden specimens such as boards and girders by neutron tomography under such circumstances. Taking projections was impaired either by the limits of penetrability or by the total size of the object. As a consequence, projections were included for reconstruction only from a limited angular range of 90°. It could be shown that an approach based on the slice theorem was capable to visualise structural features along the beam directions while simply omitting the perpendicular ones without causing additional artefacts.
Small samples with a thickness of up to 2 cm but several times broader could be studied with the ANTARES facility of the FRM II neutron source in Garching providing thermal neutrons while larger objects required a beam of higher energy as available in the NECTAR facility of the same institution. The fast (fission) neutrons (1.5 – 2 MeV) of this site allowed investigating an area of interest inside a girder with a cross section of 23.5 x 49 cm². Internal features such as inclusions could be detected as well as a heterogeneous density distribution in glue layers.
A novel approach to strongly suppress artifacts in radiography and computed tomography caused by the effect of diffuse background Signals (backlight) of 2D X-ray detectors is suggested. Depending on the detector geometry the mechanism may be different, either based on the optical scattering of the fluorescent screen materials into optical detection devices or Compton or X-ray fluorescence scattering by the detector components. Consequently, these erroneous intensity portions result in locally different violations of Lambert Beers law in single projections (radiographs).
When used as input data for computed tomography these violations are directly observed via modulation of the projected mass as a function of the rotation Phase and the samples aspect ratio (dynamics). The magnitude of the diffuse Background signal depends on the detector area covered by the projected sample. They are more pronounced the smaller the shaded area and the stronger the total attenuation.
Moreover, the local intensity mismatch depends on the attenuation of the sample.
We present very basic reference data measured with multiple metal foils at a synchrotron radiation source. Beam hardening artifacts can be excluded due to the monochromatic radiation. The proposed correction procedure assumes a constant (non-local) scattering mechanism.
X-ray computed tomography (CT) is a volumetric (3D) Imaging diagnostic method, well established in the medical field, and in industrial NDE as well. Developments in industrial CT aim to extent the applicability to complex structures, which do not allow the access of all directions. This are e.g. limited view, data and angle CT applications. New reconstruction algorithms are required on one
side, and the accuracy has to be improved on the other side. Numerical Simulation can support such developments by providing well defined data sets for the testing of reconstruction algorithms. This approach of virtual CT is realized within the radiographic simulator aRTist, developed by BAM. The poster shows the possibilities of this tool to consider complex scan paths. Simulated data sets have been reconstructed by an versatile backprojection algorithm.
POD is the most often used approach for the evaluation of the reliability of non-destructive testing methods in critical areas. But especially for those cases the requirements for a sufficient amount of data, adequate type of defects and the statistical requirements are high. The claim for checking these requirements is not new, but never the less often ignored. Especially the large amount of data for critical real defects seems too often to be not really solvable. In this presentation we show an approach to use real defect data with knowledge from former testing results with the same equipment, in a useful adequate way. Furthermore we introduce a method to evaluate the real defect description parameters for the radiographic testing. This approach was used for a joint project with the company POSIVA, which is building a final depository for spent nuclear fuel in Finland. Radiographic testing is one of the NDT-methods they use to test the electron beam weld of the copper canister. The copper canister will be used in the deposit as a corrosion barrier within the deposit concept. Through the high sophisticated manufacturing methods of the canister there are almost no material defects, which could be used for reliability evaluation. In this context the idea of Bayesian Updating of the POD with real defect data with former results from artificial defect data was born to get a useful evaluation.
Using the Modular Reliability Model the three different main influencing elements, i.e. intrinsic capability (IC), application parameters (AP) and the human factors (HF), are, in the first instance, investigated separately. The intrinsic capability stands for the pure physical-technological process of the signal detection caused by the waves or the rays from a material defect in the presence of noise (driven by the material and the devices). This intrinsic capability is the upper bound of the possible reliability. Already when measuring this intrinsic capability for thick walled components the original one-parameter POD must be extended to a multiparameter POD, where, in addition to the defect size, a number of additional physical parameters, such as the grain size distribution (or attenuation), defect depth, and angle or surface roughness, must be considered. For real life cycle assessments it is necessary to evaluate the signal response from real defects. The industrial application factors, e.g. coupling conditions, limited accessibility, heat and environmental vibrations, diminish the reliability. The amount of reduction can be determined quantitatively, if the underlying conditions are controlled. In case they are not controlled it is necessary to count for a fluctuation in the reliability in the field anyway. The third group of important influencing factors are the human factors, which do not only cover the individual performance capability of the inspectors but also the design of the working place,the procedure, the teamwork quality, interaction with systems, the organization, and finally, the relationship between the companies involved in the inspection process and to which extend the responsible parties are aware of it. When comparing an “ideal inspection” with a “real inspection” it is worth while to look how the existing practices, rules and standards support reliable testing and where the “delta” is. In the context of vigor, with respect to the industrial end user, it needs to be shown how the level of reliability of NDE, influenced by the different factors, has an impact on acceptance or rejection of safety critical parts.
Nondestructive testing (NDT) methods are applicable in many ways for defect detection as well as optimization of packaging production processes. The visual inspection method is the technique which is mostly used, although neither the customer nor a skilled inspector may be aware of this.
Techniques like ultrasound for wall thickness measurement, the application of x-ray techniques or the use of dye penetrant techniques for crack detection are rather well known to the NDT family. However, acoustic emission techniques can also be helpful for the characterization of packaging materials. In this paper the employment of x-ray computer tomography (CT) for measurement of wall thickness in plastic jerrycans, especially on the edges, and its possible optimization of production processes will be discussed.
A comparison of the CT and ultrasonic examinations of wall thickness and the reliability of this measured data are also important points for the discussion, especially if some safety factors can be derived from the results.
The leakage test is also good for jerrycans, IBCs or other kinds of drums for the transportation and storage of dangerous goods. The disadvantage of this kind of test is that a lack of quality, e.g. wall thickness deterioration, cannot be detected if the wall thickness withstands the internal pressure. Therefore, especially for Quality assurance, the other, previously mentioned techniques are better for a statistical assessment of a batch. This paper is focused on the comparison of different NDT results with special emphasis on the large range of material mechanical properties.
Time reversal techniques are used in ocean acoustics, medical imaging and non-destructive evaluation to backpropagate recorded signals to the source of origin. We demonstrate experimentally a technique which improves the temporal focus achieved at the source location. The experiment consists of propagating a signal from a transducer within a concrete block to a single receiver on the surface, and then applying time reversal or deconvolution to focus the energy back at the source location. The results show that we are able to generate a focus in time at the correct location. The proposed method is simple and proven to be robust. Additionally, its costs are negligible due to deconvolution being a preprocessing step to the recorded data. The technique can be applied for detailed investigation of the source mechanisms (e. g. cracks) but also for monitoring purposes.
At BAM a multi-sensor robot system BetoScan is used for the investigation of reinforced concrete floors affected by corrosion in parking garages.
Potential maps, as well as the distribution of concrete cover and moisture can be assessed simultaneously and data can be collected contactlessly. In order to evaluate the extent of degradation adequately and to divide the investigated structure into zones with defined damage classes, large data sets have to be collected and interpreted manually. Thus, to promote an efficient data evaluation framework, which could speed up and simplify the evaluation of large data sets, an unsupervised data fusion is of major interest. However, taking into account that collected data do not certainly coincide in space, a scattered data interpolation method should be applied prior data fusion.
In the paper, a case study involving a BetoScan data set acquired from a reinforced concrete floor of a parking garage in Germany is presented. The data set includes potential mapping, covermeter based on eddy current, as well as microwave moisture measurements. Among the examined methods for interpolation of scattered data, kriging shows to yield smooth interpolated data plots even in the case of very sparse data. In the post-processing step, the investigated structure is efficiently segmented into zones using clustering based data fusion methods, which prove to be robust enough also for handling noisy data. Based on the minimization of the XB validity index, an unsupervised selection of optimal segmentation into damage classes is derived.