The Largest Open Access Portal of Nondestructive Testing (NDT)

The comparison of many 3D volumes to find subtle differences is tedious, time-consuming and error-prone. Previously we presented Dynamic Volume Lines [1], a novel tool for the interactive visual analysis and comparison of ensembles of 3D volumes, which are linearized using Hilbert spacing-filling curve and represented as 1D line plots. In this paper we further demonstrate the usefulness and capabilities of our method by conducting a detailed visual analysis and evaluation of an artificial specimen from simulated 3D X-Ray Computed Tomography (XCT) and a real-world XCT titanium alloy specimen.

X-ray computed tomography (XCT) allows performing non-destructive measurements of fibers in fiber-reinforced polymeric materials. The accurate evaluation of fiber orientation and fiber length distribution is particularly useful as these factors have a strong influence on physical and mechanical properties of such materials. XCT data is already successfully used for fiber orientation analysis, whereas the fiber length measurement is more complex as it requires the individual fibers to be identified and segmented. Software tools able to cope with such tasks are available, but the accuracy of XCT fiber length measurement has not been thoroughly investigated so far. In this work, an experimental methodology was developed to determine the accuracy of XCT based fiber length measurements, using a reference object that was specifically designed, produced and calibrated for this purpose.

Using Talbot-Lau grating interferometer X-ray computed tomography (TLGI-XCT) we determine the fiber lay-up and quantify defects in loaded carbon fibre-reinforced polymer samples in three dimensions. In contrast to conventional XCT, TLGI-XCT provides three complementary image characteristics during image acquisition: a) attenuation contrast (AC), b) differential phase contrast (DPC), and c) dark-field contrast (DFC). Using a desktop TLGI-XCT system (Skyscan 1294), we visualize fibre bundles, resin rich areas, and defects (cracks and pores) in CFRP laminates that were subjected to low impact energies up to 15 Joules. The combined application of AC, DFC, and DPC volume data facilitates the intuitive visualization of fiber lay-up and the component´s microstructure, including resin-rich areas. Moreover, dark-field images yield a high contrast and a strong signal at interfaces improving the detection of microcracks in relatively large samples whereas these defects are barely detectable using standard XCT.

Because of their microstructure, carbon fibre reinforced polymers outperform conventional materials in many aspects. These materials are, given the low x-ray absorption contrast they provide, ideally suited for phase contrast x-ray computed tomography. For system prototyping and for benchmarking and testing acquisition and reconstruction procedures, computer simulations are a valuable tool. In this paper, we propose the application of an alternative simulation strategy that unites Monte Carlo simulations with numerical wave optics and involves detailed modelling of the internal carbon fibre reinforced polymer structure down to the individual fibre scale.

Ti6Al4V is a suitable titanium alloy for all kinds of medical implants and prostheses because of its high durability and biocompatibility. Furthermore, components of high complexity can be produced via additive manufacturing which allows for more flexibility and easy prototyping of patient specific implants. However, this flexibility implies the risk of internal defects resulting from the manufacturing process. The nondestructive investigation of critical components is therefore crucial to avoid premature failure. X-ray micro-computed tomography (XCT) is a method that can resolve internal structures three dimensionally in a non-destructive way. Nevertheless, the probability to detect defects is limited by the achievable resolution and image quality of a scan. In this contribution, we performed a systematic study to determine the pore size distribution in additively manufactured Ti6Al4V parts using XCT. We focused on the influence of scanning parameters such as voxel size, tube voltage and current on the image quality that determines the outcome of the porosity analysis. Image quality was assessed via contrast to noise ratio (CNR) and slanted-edge modulation transfer function (MTF) according to ISO 12233. Furthermore, we optimized the beam hardening correction for the scans and investigated influences of different image denoising algorithms. Results showed that tube voltage and current greatly influence the CNR of the data set while the MTF is, within limits, almost constant as long as the electron beam focus is optimized. With higher physical resolution, smaller defects can be detected, which leads to porosity values of 0.36, 1.35 and 2.54% at 10, 5 and 2.5 μm resolution respectively. Image post-processing can further influence porosity outcome because of the segmentation of noise induced particles. Different image denoising algorithms therefore can heavily reduce porosity values depending on spatial resolution.

This work introduces methods for analyzing the three imaging modalities delivered by Talbot-Lau grating interferometry X-ray computed tomography (TLGI-XCT). The first problem we address is providing a quick way to show a fusion of all three modalities. For this purpose the tri-modal transfer function widget is introduced. The widget controls a mixing function that uses the output of the transfer functions of all three modalities, allowing the user to create one customized fused image. A second problem prevalent in processing TLGI-XCT data is a lack of tools for analyzing the segmentation process of such multimodal data. We address this by providing methods for computing three types of uncertainty: From probabilistic segmentation algorithms, from the voxel neighborhoods as well as from a collection of results. We furthermore introduce a linked views interface to explore this data. The techniques are evaluated on a TLGI-XCT scan of a carbon-fiber reinforced dataset with impact damage. We show that the transfer function widget accelerates and facilitates the exploration of this dataset, while the uncertainty analysis methods give insights into how to tweak and improve segmentation algorithms for more suitable results.

This paper presents multi-modal image data of different fibre reinforced polymer samples acquired with a desktop Talbot-Lau grating interferometer (TLGI) X-ray computed tomography (XCT) system and compare the results with images acquired using conventional absorption-based XCT. Two different fibre reinforced polymer samples are investigated: (i) a carbon fibre reinforced polymer (CFRP) featuring a copper mesh embedded near the surface for lightning conduction and (ii) a short glass fibre reinforced polymer (GFRP) sample. The primary goal is the non-destructive detection of internal defects such as pores and the quantification of porosity. TLGI provides three imaging modalities including attenuation contrast (AC) due to absorption, differential phase contrast (DPC) due to refraction and dark-field contrast (DFC) due to scattering. In the case of the CFRP sample, DPC is less prone to metal streak artefacts improving the detection of pores that are located close to metal components. In addition, results of a metal artefact reduction (MAR) method, based on sinogram inpainting and an image fusion concept for AC, DPC and DPC, are presented. In the case of the GFRP sample, DPC between glass fibres and matrix is lower compared to AC while DPC shows an increased contrast between pores and its matrix. Porosity for the CFRP sample is determined by applying an appropriate global thresholding technique while an additional background removal is necessary for the GFRP sample.

This paper presents a case study of two selected beam hardening correction methods and their effects on dimensional measurements of multi-material objects. The methods under test are empirical cupping correction (ECC) and empirical dual energy calibration (EDEC). These methods were originally developed for medical applications and their potential for the reduction of artefacts is typically only analysed based on grey value images. For testing and benchmarking of the mentioned methods for dimensional metrology, a dedicated multi-material reference standard—a multi-material hole cube—is used. This reference standard was originally developed for acceptance testing of CT systems. This paper shows a second application of this standard. The reference standard has been calibrated by tactile measurements to assess centre–centre distance errors as well as patch-based bidirectional length measurement errors on beam hardening corrected data and on uncorrected data. For the application of the method also to industrial multi-material scenarios, slight modifications of the ECC method are proposed. Practical aspects of both the ECC and the EDEC approaches as well as measurement results are analysed and discussed in detail. ECC was able to significantly improve dimensional measurements and was especially able to reduce extreme errors occurring in particular in multi-material scenarios by a factor of more than 4. EDEC, the dual-energy approach, reduced grey value inhomogeneities caused by artefacts even more. Its performance for dimensional measurements was however a little worse than ECC. EDEC data resulted in a slightly larger total range of residual measurement errors, mainly due to an elevated noise level.

Especially in aeronautic and space industry it is important to produce carbon fibre-reinforced polymers (CFRP) with very low porosity since there is a direct relation between porosity and mechanical properties, such as shear strength. Typical acceptable porosity values are in the range of < 2.5 vol.%. The most common non-destructive method for measuring the porosity is ultrasonic testing (UT) assuming there is a linear correlation between ultrasonic attenuation and porosity. However, the ultrasonic attenuation coefficient depends not only on the porosity, but also on the shape and distribution of the pores, the presence of other material inhomogeneity’s and the individual material system. This can lead to significant errors in the determination of the porosity. Acid digestion and materialography are mainly used as reference method, but both methods are destructive, time-consuming and inaccurate. This paper deals with the application of X-ray computed tomography (XCT) methods as reference for quantitative evaluation and characterization of porosity in carbon fibre-reinforced composites. The degree of accuracy strongly depends on several material and XCT parameters: • Material parameters: woven fabric or unidirectional material, additional glass fibres or copper wires for lightning protection, higher dense epoxy used for adhesive bonding, etc. • XCT parameters: resolution and voxel size (VS), used XCT device and modality (e.g.: cone beam XCT vs. Talbot Lau grating interferometer XCT), threshold method, etc. In this contribution several mentioned parameters influencing the quantitative evaluation of porosity in CFRP are discussed and different types of CFRP with porosity are presented. Porosity values gained with XCT methods are compared with UT, acid digestion or materialography.

In this contribution, high-resolution X-ray computed tomography (XCT) results of various inhomogeneous samples including a poplar wood sample, a carbon fibre reinforced polymer sample and an AlSiCu light metal alloy are presented. These samples have been acquired with a new lab-based nano-XCT device equipped with a nano-focus X-ray tube and two different detector systems. Depending on the material system and the measurement task, the user has to choose between these two detector types that can be exchanged by a quick mounting system. Limitations on resolution and contrast-to-noise ratio (CNR) are discussed qualitatively and quantitatively for selected measurements and evaluation tasks. Structures between 500 and 700 nm could be clearly resolved. In addition the positioning accuracy of the exchangeable flat panel detector is investigated. Detector repositioning shows high reproducibility, but systematic measurement errors are in the range of half of a voxel for repeated scans of a calibrated ball bar phantom with a system voxel size of (3 µm)³.

Components manufactured from titanium alloys are the state-of-the-art solution for medical implants due to their high strength and biocompatibility. However, the stiffness of the material can create stress shielding between the bone-implant interface. With additive manufacturing (AM) patient-specific implants can be produced with a controlled porosity in order to match the stiffness of human bone more closely. In this paper we investigate a sample of seven Ti6Al4V pedicle screws manufactured by selective laser melting (SLM) in relation to pore size distribution and mechanical properties using finite element analysis (FEA). Screws were scanned using X-ray microcomputed tomography (XCT) at isometric voxel sizes between 2.5 µm and 16 µm. Since the neck of the pedicle screw is exposed to high stress, e.g. during mechanical pull-out tests, this region was investigated in more detail via FEA. µFE models were generated based on XCT data and elastic moduli estimated by analysis of eight regions of interest respectively. Results show an average porosity of 1.16 ±0.12% with a higher porosity towards the core of the material compared to its surface and a marked decrease in stiffness in relation to the level of porosity.

Polymeric foams are used intensively for a widespread of different applications. In order to extend the range of applications further, especially for novel polymeric materials, knowledge about microstructure and change in microstructure during mechanical loading is needed. This contribution uses X-ray tomography to characterise the cell structures while interrupted compression testing. The investigated specimens were punched out of 3 mm thick sheets of polypropylene and had cylindrical shape. From a first compression test without stopping the experiment the positions on the force-extension curve were defined for interrupting. Figure 1 shows the curve of the interrupted compression test. After the desired force was reached, a delay time was applied to prevent relaxation during CT scanning. The resolution is depending on the cell size and the cell wall thickness. For the investigated materials a voxel edge length of 2.5 µm was reasonable. CT scans were performed at a laboratory CT device. The slice images in Figure 2 (left) show collapsing of cells which is pronounced in the centre of the sheet. The segmentation of each separated cell is done using MAVI (Fraunhofer ITWM, Kaiserslautern) and a watershed approach. The image processing of deformed cells requires adapted pre-processing and parameter optimization compared to the undamaged state. Quantitative results in terms of changes in cell size distributions for several materials tested by interrupted in-situ CT will be shown.

X-ray imaging methods such as conventional X-ray computed tomography (XCT) based on absorption are essential techniques in various domains, e.g. medicine and materials science. In the last 15 years an important innovation in X-ray imaging technology has emerged through the introduction of Talbot-Lau grating interferometry (TLGI) [1-4]. Using this imaging technique with three different gratings the extraction of attenuation contrast (AC), differential phase contrast (DPC), dark-field contrast (DFC) information has become available in lab-based X-ray computed tomography systems. In this contribution, we demonstrate the usefulness of TLGI based XCT for different applications in industry and materials science. DPC has advantages for the discrimination of materials with similar X-ray attenuation like water and epoxy and is less probe to metal artefacts. In contrast, DFC is beneficial for the characterization of polymeric foams, for the detection of carbon yarn structures and for the detection of damage (cracks, micro-voids etc.) in different kinds of polymers. DFC´s anisotropy can be used to combine the results of two successive DFC measurements in 0 and 90 degrees allowing the visualization and quantification of the weaving pattern of a carbon fabric in 3D.

The open source software open_iA is presented, which facilitates non-destructive testing tasks such as feature extraction, quantification and visualization involving industrial computed tomography datasets. open_iA provides a general framework for visualizing and processing volumetric datasets, including a 3D view, axis-aligned slicers, as well as a wide variety of image processing algorithms for pre-processing of the data for the subsequent analysis. Pluggable modules provide tailored analysis tools for specific scenarios. In this work the structure of the tool open_iA itself and a selection of its modules are outlined.

X-ray computed tomography (XCT) provides a volumetric map of a specimen in three dimensions, generated from a set of radiographs. Continuous improvements in the quality and performance of X-ray tubes and devices have led to cone beam XCT systems which can now achieve spatial resolutions below 500 nm. We report on the application of high-resolution XCT for the nondestructive characterization of various materials with a special focus on fiber-reinforced polymers. From the XCT-data quantitative data can be extracted like the fibre orientation and fibre length distribution. For this purpose the XCT-data are processed by several consecutive steps: pre-processing, segmentation, template matching, medial axis extraction, individual fibre extraction including cluster analysis and final fibre characterisation. In this work the fiber length distribution of glass fibers in a polymer matrix are determined by XCT and light optical microscopy. The main advantages of XCT are: It is a nondestructive method with high reproducibility, you can measure many fibers (up to 90.000) per sample and only one method is needed for the measurement of fiber length, diameter and orientation. As an example the fiber length distribution functions of injection moulded polypropylene reinforced with 10 %, 30 %, 40 %, 50 % and 60 % glass fibers were measured quantitatively by XCT. It is clearly recognisable that the fibre length distribution function is shifted to lower values with increasing fibre content. 4DXCT or in-situ XCT is applied for the characterization of glass fiber reinforced polymers during tensile testing. An in-situ tensile testing stage is placed in a high resolution XCT-device, so that the damage introduced during tensile testing and the damage mechanisms can be clearly detected by XCT analyses. The study of the evolution of various defects in short fibre reinforced composite material is conducted for damage characterisation. Quasi static tensile force is applied from 85% to 99 % relative to the pre-evaluated final fracture force. Damages are analysed after every step of the force. The main damage mechanisms are fiber pull-out, fiber fracture, fiber-matrix deboning and matrix cracks. The XCT-resolution is high enough so that the different mechanisms can be distinguished clearly and a quantitative classification is possible.

X‐ray imaging methods such as microcomputed-tomography (XCT) are essential techniques to reveal and quantify internal defects in materials. Conventional absorption-based contrast (AC) provides information on the attenuation of the X-ray beam intensity and is an invaluable tool in various domains, e.g. medicine and materials science. In the last decade however an important innovation in X‐ray technology has emerged by the introduction of Talbot-Lau grating interferometry. This method provides three complementary characteristics in a single scan of the specimen: a) the attenuation contrast, b) the differential phase contrast (DPC), and c) the dark-field contrast (DFC). DPC is related to the index of refraction and DFC reflects the total amount of radiation scattered at small angles, e.g. caused by microscopic inhomogeneities represented by cracks and pores. Using a novel Talbot-Lau grating interferometer XCT system for laboratory applications we visualize crack-like defects in carbon fiber reinforced laminates that were subjected to impact forces up to 30 Joules. Using DFC we are able to detect cracks in samples that were subject to low impact forces whereas these defects are merely detectable using AC. Secondly, we investigate polypropylene test specimens that are loaded in tensile testing until final failure. Lower grey levels near the fracture surface in the AC and DPC images indicate pores. Due to increased scattering in these regions DFC images provide a high signal intensity even though the defects are smaller than the spatial resolution. Due to the fact that DFC delivers morphological information in the sub‐pixel regime depending on the local scattering power, dark field imaging delivers information that may otherwise be inaccessible using conventional XCT. Using a Talbot-Lau XCT we show that dark field images yield a high contrast and a strong signal at interfaces, e.g. for cracks and pores. Additionally we present multi-modal image data of of carbon fibre reinforced polymers with a copper mesh near the surface used for lightening conduction. The scanning task includes the detection and characterization of internal defects such as pores. We show that the differential phase contrast modality is less prone to metal streak artefacts, thus improving the detection of pores in CFRP located close to metal components.

Tannin- and Lignin-based foams are novel materials that find increasing attention in the insulation and automotive industry due to their versatile properties such as high fire resistance and remarkably low thermal conductivity. While foam density can be estimated with high precision, it is difficult to non-destructively determine pore morphology since the size distribution of foam pores differs in three dimensions. It is therefore challenging to obtain reliable information using standard methods like optical light microscopy or scanning electron microscopy, and to differentiate between the strut system and cell walls. In this contribution we non-destructively investigate three samples of tannin foam that were produced in different moulds along with one lignin specimen using X-ray microcomputed tomography (XCT). The tannin samples show porosities between 89% and 93.5% and a mean wall thickness between 15.2 µm and 16.6 µm. The lignin sample is characterized by a porosity of 79.9% and a mean wall thickness of 360.6 µm. We found that differences in mould dimensions influence the pore morphology not only in pore size but also in shape, sphericity and anisotropy. Additionally, we quantified the porosity of added wood particles that were identified inside the Lignin cell matrix.

An interrupted in-situ X-Ray computed tomography (XCT) was used to investigate the influence of different Carbon fibre reinforced polymers (CFRP) material systems on the crack progression and propagation of adhesive jointed CFRP plates. The damage behaviour of adhesive jointed CFRP is decisive for the mechanical strength. For this paper the material systems (I) CFRP fabric plate with another CFRP fabric plate and (II) a CFRP fabric plate with a unidirectional (UD) CFRP plate were insitu investigated with a high resolution X-Ray computed tomography (XCT) with a voxel size of (6 µm)³ at different load steps. The damage volumes of both material systems were segmented and compared with each other. The results confirmed the stress concentration at the end of the specimens [14]. In general the CFRP-cracks in the plates can be identified as major damage mechanism. The different strength of the two material systems is caused by the missing damage in the UD-plate. This lack of damage is caused by the high crack resistance of the UD-plies in respect to the force direction and results in a higher strength of the system fabric-UD.

In this work a simulation-based approach is used to optimize a certain high-resolution inspection task of an AlSiCu material system. The actual scans that are also used for the verification of the simulation results are performed on a new lab-based nanofocus X-ray computed tomography (nano-XCT) system that is equipped with two different detector systems. Depending on the material system and the measurement task, the user has to choose between these two detector types. In order to simplify user operation and optimize scan quality, numerical simulations with the tool SimCT are performed to find suitable scanning parameters and strategies without relying on long and time-consuming scanning series. Theoretical limits on resolution and contrast-to-noise ratio are compared with experimental results.