Uwe Ewert

Terahertz birefringence of liquid crystal polymers

The authors present terahertz time-domain spectroscopy (TDS) measurements on injection molded liquid crystalline polymers (LCPs). Pure LCP yields a strong birefringence of Δn=0.2. They find that the direction of the optical axis varies within the injection molded sample. By terahertz (polarization) imaging domains of certain directions of the optical axis could be observed. These strongly depend on the mold geometry. Since the alignment of particles and rodlike polymer molecules tend to follow the melt flow during injection molding, terahertz TDS can be used to analyze injection molded LCP parts and control the process performance.

Handheld Terahertz Spectrometer for the Detection of Liquid Explosives

We present a handheld fiber-coupled terahertz spectrometer operating at a center wavelength of 1550 nm. The key elements are a fs-fiber laser, a fiber stretcher delay line and fiber-coupled antennas, which contain novel InAlAs-InGaAs multi layer chips. First experimental data obtained with this system demonstrates its great potential and robustness. In addition, we investigate different hazardous and harmless liquids in reflection geometry. These experiments show that liquids are in principle distinguishable by terahertz spectroscopy. Finally, first steps towards an algorithm that allows for an extraction of the liquids dielectric properties are discussed. The algorithm works for the analysis of reflection data even if the liquid is located inside a container.

Cross-sectional imaging of building elements by new non-linear tomosynthesis techniques using imaging plates and 60Co radiation

The quantitative measurement of the positions and diameters of steel reinforcements in buildings is based on the concept of non-linear backprojection (NLBP). Advantages are 3D-image reconstructions from a small number of views, fast numeric algorithms and the consideration of the statistical, spectral and other characteristics of the object. For the radiographic inspection a 60 Co source was used and both X-ray films and imaging plates for detection. The imaging plates were combined with special filter screens for contrast optimization. The digitized images from the plates have a high dynamic range and can be read into a computer without a photographic developing process. A numerical preprocessing procedure was developed as space filtering in different directions followed by a combination of the image angle components. A generalized Hough or Radon transformation and a statistical evaluation of the resulting image was employed. The special advantage of this method is the reconstruction of direction-selective structures.

Paradigm Shift in the Holistic Evaluation of the Reliability of NDE Systems

Abstract The aim of this publication is to provide an overview of new methodologies for evaluating the reliability of NDE systems in accordance with the specific requirements of industrial applications. After a review of the substantive issues of the previous decades, the go forward guidance is concluded. For high safety demands a quantitative POD (Probability of Detection) created from hit/miss experiments or signal response analysis and ROC (Receiver Operating Characteristics) are typically created. The modular reliability model distinguishes between the pure physical-technical influence, industrial application factors, and human factors. It helps to learn which factors can be determined by modeling and by open or blind trials. A new paradigm is offered to consider the POD or reliability of the system as a function of the configuration of input variables and is used for optimization rather than for a final judgement. New approaches are considered dealing with real defects in a realistic environment, affordable but precisely like the Bayesian approach or model assisted methods. Among the influencing parameters, human factors are of high importance. A systematic psychological approach helps to find out where the bottlenecks are and shows possibilities for improvement.

OPTIMUM EXPOSURE CONDITIONS FOR COMPUTED RADIOGRAPHY DEPENDING ON FIXED PATTERN NOISE AND EFFICIENCY OF IMAGING PLATE‐SCANNER SYSTEMS

The presently active standards on Computed Radiography (CR) need a major revision. It was observed by many users that the image quality for class B of EN 14784‐2 is not achievable under the same exposure conditions as used for film exposure. A mathematical model was developed and tested, which allows the calculation of the image quality, proven by image quality indicators (IQI), depending on the fixed pattern noise and the efficiency of the used imaging plate (IP) scanner system. All tested IP scanner systems provide a fixed correlation between the measured signal/noise ratio (SNR) and the grey values in the digital images. The maximum achievable SNRmax depends on the fixed pattern noise of the plate (high dose limit). Depending on the exposure dose an optimum visibility of IQIs can be predicted by calculation and measured considering the attenuation coefficient for calculation of the contrast/noise ratio (CNR). The diameter of the just visible wire is proportional to 1/sqrt(CNR). The optimum tube voltage...

Assessment of Hot Cracking Behaviour in Welds

Abstract Hot crack assessment during production and processing of metallic materials is an essential prerequisite for the safety of welded structures. The hot cracking investigations presented here were carried out as part of a study aimed at the development of Cr/Ni low transformation temperature filler materials. Low transformation temperature alloys open up the possibility for welding high strength steels. The externally loaded Modified Varestraint Transvarestraint hot cracking test was employed in the experiments described. The hot cracking resistance was evaluated with the help of light microscopy applied at the specimen surface. The proportionality between hot cracking susceptibility and Cr/Ni alloy content was explained by the altered solidification kinetics and by the enlarged solidification interval. The internal crack paths and the three-dimensional structure of the crack net in the material volume were examined using X-ray computer tomography. The total crack lengths for different material depths and circumjacent rectangular volumes, respectively, were be determined. An increasing hot cracking susceptibility with increasing Cr/Ni alloy content was also be established for the specimen volume.

Observer POD for radiographic testing

The radiographic testing (RT) is a non-destructive testing (NDT) method capable of finding volumetric and open planar defects depending on their orientation. The radiographic contrast is higher for larger penetrated length of the defect in a component. Even though, the detectability of defects does not only depend on the contrast, but also on the noise, the defect area and the geometry of the defect. The currently applied Probability of Detection (POD) approach uses a detection threshold that is only based on a constant noise level or on a constant contrast threshold. This does not reflect accurately the results of evaluations by human observers. A new approach is introduced, using the widely applied POD evaluation and additionally a detection threshold depending on the lateral area and shape of the indication. This work shows the process of calculating the POD curves with simulated data by the modeling software aRTist and with artificial reference data of different defect types, such as ASTM E 476 EPS pl...

Terahertz Time Domain Spectroscopy for Non-Destructive Testing of Hazardous Liquids

Abstract Hazardous liquids, liquid explosives and flammable liquids are characterized by means of terahertz time domain spectroscopy (THz-TDS) in analogy to existing non-destructive testing (NDT) strategies. Various polar liquids (alcohols, acetone, hydrogen peroxide, nitro methane) and non-polar aircraft and automobile fuels as well as organic solvents are characterized in the practically relevant, non-contact and non-destructive reflection geometry in the time domain. Absorption coefficients and indices of refraction of a representative set of these liquids are investigated in the spectral range between 0.1 THz and 1.1 THz in transmission configuration by two different set-ups either suitable for strongly or weakly absorbing liquids.

Measurement and modeling of scatter ratios at high energies

Determining scatter ratios at high energies (>∼1 MeV) presents challenges not immediately obvious from measurements at conventional X‐ray energies. This includes reduced attenuation even in heavy elements as well as a stronger forward bias of scattering. In order to check the reliability of different measurement setups with regard to material thicknesses, distances, and collimation, attenuation and scattering were separately simulated using a Monte Carlo model. The simulation results help in understanding the sources of radiation scattered into the detection area, thereby aiding in eliminating undesired contributions.

Modelling based radiography for NDE of subsea pipelines

This work presents the use of limited experimental measurements to develop a set of calibrated simulation parameters that can then be used for reliable simulation of subsea pipeline inspections. The modelling software aRTist is used as the simulation tool, and the calibration is through comparison with experimental images of a well characterised sample in a water tank. Image quality parameters such as signal-to-noise ratio, contrast and basic spatial resolution are compared with the aim of matching simulated values to experimental results. Currently the model is partially calibrated, with signal-to-noise ratio successfully matched while differences are still found in contrast-to-noise ratio comparisons. This means that measurements depending on absolute intensity are not accurate enough in the simulation at this stage. However, the simulation is found to be accurate for wall thickness measurements in tangential images, which are not based on absolute intensity, with simulated and experimental cases produc...