Three-dimensional assessment of low velocity impact damage in particle toughened composite laminates using micro-focus X-ray computed tomography and synchrotron radiation laminography

Results are presented studying the contribution of particle toughening to impact damage resistance in carbon fibre reinforced polymer materials. Micro-focus X-ray computed tomography and synchrotron radiation computed laminography were used to provide a novel, multiscale approach for assessing impact damage. Thin (1 mm thick) composite plates containing either untoughened or particle-toughened resin systems were subjected to low velocity impact. Damage was assessed three-dimensionally at voxel resolutions of 0.7 μm and 4.3 μm using SRCL and μCT respectively; the former being an innovative approach to the laterally extended geometry of CFRP plates. Observations and measurements taken from μCT scans captured the full extent of impact damage on both material systems revealing an interconnected network of intra- and inter-laminar cracks. These lower resolution images reveal that the particle-toughened system suppresses delaminations with little effect on intralaminar damage. The higher resolution images reveal that the particles contribute to toughening by crack deflection and bridging.     

任意角度入射的三维CT投影方法

本提出了一种主射线与旋转轴成任意入射角度的非常规的三维CT投影方法。在射线源能量较低的情况下，此法能够对大而扁的工业构件实施三维CT重建。中推导了这种投影方式下三维CT反投影重建算法，并通过计算机仿真进行了验证。     

Super resolution in CT

The general framework of super resolution in computed tomography (CT) system is introduced. Two data acquisition ways before or after the reconstruction respectively are described. Three models including the sinogram model, the in‐plane model and the z‐axis model, are addressed to adapt super resolution to CT system. The improved iterative back projection algorithm is used in this work. Experimental results based on simulated data, GE performance phantom scanned by GE LightSpeed VCT system, one patient volunteer scanned by TOSHIBA Aquilion system, and a special experimental apparatus demonstrate that super resolution is effective to improve the resolution of CT images. The sinogram model is suitable for future CT system; the in‐plane model is restricted to some special clinical diagnoses; and the z‐axis model is practicable for current general clinical CT images. © 2015 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 25, 92–101, 2015     

Analytic derivation of the longitudinal component of the three-dimensional point-spread function in coded-aperture laminography

Near-field coded-aperture data from a single view contain information useful for three-dimensional (3D) reconstruction. A common approach is to reconstruct the 3D image one plane at a time. An analytic expression is derived for the 3D point-spread function of coded-aperture laminography. Comparison with computer simulations and experiments for apertures with different size, pattern, and pattern family shows good agreement in all cases considered. The expression is discussed in the context of the completeness conditions for projection data and is applied to explain an example of nonlinear behavior inherent in 3D laminographic imaging.     

Computed tomography with linear shift-invariant optical systems

Optical systems capable of three-dimensional transmission imaging are considered; these systems employ a conventional tomographic setup with an added linear shift-invariant optical system between the sample and the detector. A theoretical analysis is presented of image formation and sample reconstruction in such systems, examples of which include diffraction tomography and phase-contrast tomography with the use of analyzer crystals. An example is introduced in which the image is obtained by scanning the beam along the line orthogonal to the optic axis and to the axis of rotation with a one-dimensional slit or grating parallel to the rotation axis. We show that under certain conditions the proposed system may allow quantitative local (region-of-interest) tomography.     

Bayesian reconstruction for SPECT: Validation with monte carlo simulation,experimental phantom,and real patient data

A method for Bayesian image reconstruction from projections is applied to Monte Carlo simulation, experimental phantom, and real patient data from a SPECT acquisition system. This statistical image reconstruction method has three distinct aspects: (1) it uses a priori information about image density distribution of a multinomial process; (2) it considers a spatial correlation of nearby image elements; and (3) it incorporates the Poisson nature of photon detection fluctuation. The Monte Carlo simulation data are generated by computer codes for selected mathematical phantoms containing hot and cold rods. The experimental phantom data are acquired with a Triad SPECT system using radioactive phantoms containing hot and cold rods. The real patient data are obtained from a patient brain scan using the Triad SPECT system. A parallel beam geometry is used. The data are acquired from 120 projection angles uniformly distributed from 0 to 360 degrees. At each projection angle, a 128 X 128 projection image is measured. This 128 X 128 projection samples are equally spaced along the axis of detector rotation and perpendicular to the axis, respectively. Each image slice is reconstructed using a 128 X 128 pixel array. Comparisons between this Bayesian method and maximum likelihood method and filtered backprojection method are give. An improvement in noise suppression is demonstrated using the Bayesian method while image resolution is preserved.     

Accelerator-based neutron tomography cooperating with X-ray radiography

Neutron resonance absorption spectroscopy (N-RAS) using a pulsed neutron source can be applied to time-of-flight (TOF) radiography, and the obtained parameters from the peak shape analysis can be reconstructed as the tomograms of nuclide distributions using computed tomography (CT). The problem is that the available spatial resolution is not sufficient for radiography imaging. In this study, we combined neutron and X-ray radiographies to improve the quantitative reconstruction of the neutron tomogram. The accelerator-based neutron source emits X-rays (or gamma-rays) at the same time the neutron pulse is emitted. We utilized the X-ray beam from the neutron source to obtain X-ray radiogram on the same beam line with neutron radiography and then reconstructed the neutron tomogram quantitatively with the help of a detailed sample internal structure obtained from the X-ray radiogram. We calculated the nuclide number density distribution tomogram using a statistical reconstruction procedure, which was easy to include in the structure model during the reconstruction. The obtained result of nuclide number density distribution showed good coincidence with the original object number density.     

A method for measuring tissue-equivalent dose using a pin diode and activation foil in epithermal neutron beams with EN < 100 keV

Silicon (Si) pin diodes can be used for neutron dosimetry by observing the change in forward bias voltage caused by neutron induced displacement damage in the diode junction. Pin diode energy response depends on Si displacement damage KERMA (K(Si)). It is hypothesised that tissue-equivalent (TE) neutron dose could be expressed as a linear combination of K(Si) and foil activation terms. Monte Carlo simulations (MCNP) of parallel monoenergetic neutron beams incident on a cylindrical TE phantom were used to calculate TE dose, K(Si) and Au, Cu and Mn foil activations along the central axis of the phantom. For spectra with neutron energies <100 keV, it is possible to estimate the TE kerma based on silicon damage kerma and Cu or Mn foil measurements. More accurate estimates are possible for spectra where the maximum neutron energy does not exceed 30 keV.     

Application of Different X-ray Techniques to Improve In-Service Carbon Fiber Reinforced Rope Inspection

Carbon fiber reinforced polymer ropes are gaining in significance in the fields of civil engineering and hoisting applications. Thus, methods of non-destructive testing (NDT) need to be developed and evaluated with respect to new challenges and types of defects. Particularly important is the development of in-service testing solutions which allow the integration in global online monitoring systems. Conventional methods like electrical resistivity or strain measurements using optical fibers are already in use. This study investigates the possibility of using various X-ray techniques to increase the reliability and significance of NDT and their applicability to in-service testing. Conventional film radiography is the most common technique; however, even after image enhancement of the digitized film, this technique lacks contrast sensitivity and dynamic range compared to digital detector array (DDA) radiography. The DDA radiography is a highly sensitive method; yet, the limitation is that it delivers 2D images of 3D objects. By the use of co-planar translational laminography the detectability of planar defects is superior to 2D methods due to multiple projection angles. Apart from this, it can be used on-site due to a rather simple setup and robust equipment. In this work two photon counting detectors (PCD) with different sensor materials (Si and CdTe) were used. The results show that the resolution and defect recognition is lower in case of DDA radiography and laminography using PCDs compared to high-resolution computed tomography. However, the DDA radiography and laminography are sensitive enough to both fiber breakage and delaminations and can be significantly advantageous in terms of measurement time and adaptability for on-site monitoring.     

Rotations of silica tetrahedrons in first oxide layer on Si(1 0 0): A quantum chemical study

Optimization of large SiO₂/Si clusters was carried out by a semiempirical method. The major result is that the compressive stress which is generated during oxidation induces an organized rotation of silica tetrahedrons. For the first oxidized layer, the tetrahedrons rotate by 20° around the axis parallel to the dimer. In order to obtain reliable energies and density of states (DOS), this model was also computed with an ab initio Hartree–Fock method employing periodic boundary conditions. Using a network connectivity concept, we explain how the influence of a chemical species on the silicon oxidation process depends on the valency of the species. A new technology computer aided design (TCAD) model of oxidation is proposed on the basis of these results.     

HIGH TEMPERATURE SHORT FATIGUE CRACK BEHAVIOUR IN A STAINLESS STEEL

Abstract— Continuous low cycle fatigue (LCF) tests with-and without-hold time in push-pull and torsion loading modes and sequential LCF tests in push-pull mode were carried out at 650°C in air on thin tubular specimens of 316 stainless steel; the sequential tests involving pure fatigue (PF) and creep-fatigue (CF) loadings. The growth of short fatigue cracks was studied by taking several replicas from the specimen surface which were subsequently observed under a scanning electron microscope. An analysis was done with respect to both crack density and the orientation of microcracks and macrocrack(s) which led to failure.
Crack density was higher on the surface of a CF tested specimen than that of a PF tested specimen. Mainly short cracks oriented at 45° to the specimen axis were observed on a torsion fatigue tested specimen surface. For push-pull specimens the microcracks propagated perpendicular to the specimen axis to form macrocracks that propagated in the same direction. On the other hand, for torsion specimens the microcracks which initially propagated at 45° to the specimen axis linked to form macrocracks oriented parallel and perpendicular to the specimen axis. However, the macrocrack responsible for the final fracture was always oriented parallel to the specimen axis.
Cumulative damage was dependent on the type of loading (PF or CF) in the first part of sequential tests. In particular microcracks initiated during an initial damage phase observed under sequential LCF tests in PF were found to be healed by oxide formation during the hold times applied in the subsequent CF loading and this produced a total damage summation significantly larger than one.     

Feasibility Study for Improving the Image Characteristics in Digital Tomosynthesis (DTS) Using a Compressed-Sensing (Cs)-Based Pre-Deblurring Scheme

Digital tomosynthesis (DTS) has been widely used in both industrial nondestructive testing and medical x-ray imaging as a popular multiplanar imaging modality. However, although it provides some of the tomographic benefits of computed tomography (CT) at reduced dose and imaging time, the image characteristics are relatively poor due to blur artifacts originated from incomplete data sampling for a limited angular range and also aspects inherent to imaging system, including finite focal spot size of the x-ray source, detector resolution, etc. In this work, in order to overcome these difficulties, we propose an intuitive method in which a compressed-sensing (CS)-based deblurring scheme is applied to the projection images before common DTS reconstruction. We implemented the proposed deblurring algorithm and performed a systematic experiment to demonstrate its viability for improving the image characteristics in DTS. According to our results, the proposed method appears to be effective for the blurring problems in DTS and seems to be promising to our ongoing application to x-ray nondestructive testing.     

Bone fracture analysis on the short rod chevron-notch specimens using the X-ray computer micro-tomography

Mechanical fatigue of bone leads to micro-cracking which is associated with remodeling, establishing a balance in the microcrack population of the living tissue, thus, in the steady-state, the microstructure of bone provides sites of discontinuity acting as stress raisers. Hence fracture toughness plays a decisive role in bone functionality by determining the level to which the material can be stressed in the presence of cracks, or, equivalently, the magnitude of cracking which can be tolerated at a given stress level. Cortical bone, which behaves as a quasi-brittle solid when fractured, was tested as short-rod chevron-notched tension specimens (CNT). The main features of the CNT specimen are its geometry and the V shaped notch. The notch leads to steady-state crack propagation whilst the requested geometry allows a diameter 40% smaller than the thickness of a standard compact tension specimens (CT). These features are essential to distinguish the inhomogeneties in the fracture properties of materials like bone. Bone structure and crack propagation of the CNT specimens were analyzed using X-ray computed micro-tomography (XMT), which is a non-invasive imaging technique. The unique feature of the micro-CT is the high resolution three-dimensional image which consists of multi-sliced tomographs taken in a fine pitch along the rotational axis. Fracture toughness (K _IC) computed according to the peak load was 4.8 MNm^-3/2 while that derived from experimental calibration tests using XMT was 4.9 MNm^-3/2.     

多排螺旋CT小剂量预扫描门静脉血管成像的临床应用

目的:探讨16排螺旋CT小剂量预扫描门静脉造影的可行性.材料与方法:将100例疑有肝脏疾病进行16排CT门静脉造影的患者分成两组,50例采用常规扫描即注射对比剂后延时25-27s行动脉期扫描,延时45-55s行门脉期扫描,延时80-120s行肝实质期扫描.造影剂浓度370mg/ml,剂量为2ml/kg,注射速率3.5ml/s.50例采用小剂量预扫描即检查均行小剂量预扫描,预扫描造影剂的用量为20ml,注射速率为3.5ml/s.扫描采用轴扫,扫描层厚为5mm,层间距为0mm,时间间隔为2s,扫描层数为8-10层,延时时间为30-35s.获取门静脉时间浓度曲线.确定门静脉扫描的延时时间来进行门脉血管造影.运用容积重建法(VR)、多层面重建法(MPR)和最大密度投影法(MIP)对门静脉及其分支进行重建,评价门静脉成像的效果.成像效果由两位高年资的医生进行评估.结果:采用小剂量预扫描方法的门静脉成像比常规扫描门脉成像成功率高,显示血管细小分支级别高.结论:门静脉小剂量预扫描的方法能大大提高门静脉成像的成功率和较高的门脉成像满意度.具有很好的临床应用价值.     

Radiography and partial tomography of wood with thermal neutrons

The effective high neutron scattering absorption coefficient of hydrogen (48.5 cm²/g) due to the scattering allows neutrons to reveal hydrocarbon structures with more contrast than X-rays, but at the same time limits the sample size and thickness that can be investigated. Many planar shaped objects, particularly wood samples, are sufficiently thin to allow thermal neutrons to transmit through the sample in a direction perpendicular to the planar face but not in a parallel direction, due to increased thickness. Often, this is an obstacle that prevents some tomographic reconstruction algorithms from obtaining desired results because of inadequate information or presence of distracting artifacts due to missing projections. This can be true for samples such as the distribution of glue in glulam (boards of wooden layers glued together), or the course of partially visible annual rings in trees where the features of interest are parallel to the planar surface of the sample. However, it should be possible to study these features by rotating the specimen within a limited angular range. In principle, this approach has been shown previously in a study with fast neutrons [2]. A study of this kind was performed at the Antares facility of FRM II in Garching with a 2.6×10⁷/cm² s thermal neutron beam. The limit of penetration was determined for a wooden step wedge carved from a 2 cm×4 cm block of wood in comparison to other materials such as heavy metals and Lucite as specimens rich in hydrogen. The depth of the steps was 1 cm, the height 0.5 cm. The annual ring structures were clearly detectable up to 2 cm thickness. Wooden specimens, i.e. shivers, from a sunken old ship have been subjected to tomography. Not visible from the outside, clear radial structures have been found that are typical for certain kinds of wood. This insight was impaired in a case where the specimen had been soaked with ethylene glycol. In another large sample study, a planar board made of glulam has been studied to show the glued layers. This study shows not only the limits of penetration in wood but also demonstrates access to structures perpendicular to the surface in larger planar objects by tomography with fast neutrons, even with incomplete sets of projection data that covers an angular range of only 90° or even 60°.     

Dispersion of linearly polarized light propagating in a thin birefringent plate

We analyze theoretically the dispersion of linearly polarized light propagating in a uniaxial anisotropic medium where multibeam interference is present. Explicit expressions of the group-delay dispersion for transmitting waves are derived for the simplest situation, and the effect of dispersion on pulse broadening is analyzed for a few selected cases. Our results reveal that at normal incidence and in the situation where the optic axis is parallel to the surface of birefringent plate (in the x-y plane), the dispersion of the refracted wave decreases with the extent of birefringence. In particular, the dispersion for the electric field parallel to the polarization direction of the incident light changes with the rotation angle between the optic axis and the polarization direction of the incident field, whereas the dispersion for the refracted field whose direction is vertical to the polarization of incident light is independent of this angle. For oblique incidence, dispersion varies substantially for different incident angles. In the situation where the optic axis is in the x-z plane at either normal or oblique incidence, the dispersion increases in a periodically oscillating manner as a function of the relative thickness of the birefringent plate.     

How to organize a neutron imaging user lab? 13 years of experience at PSI, CH

PSI has a relatively long tradition in neutron imaging since the first trials were done at its formerly existing research reactor SAPHIR with film methods. This reactor source was replaced after its shutdown in 1994 by the spallation neutron source SINQ in 1996, driven by the 590 MeV cyclotron for protons with presently up to 2.3 mA beam current.One of the first experimental devices at SINQ was the thermal neutron imaging facility NEUTRA, which was designed from scratch and has been the first device of its kind at a spallation source. Until now, NEUTRA has been successfully in use for many investigations in a wide range of studies covering fuel cell research, environmental behavior of plants, nuclear fuel inspection and the research on cultural heritage objects. It has been the host of PhD projects for students from all over Europe for years. In a previous meeting it has been offered as a European reference facility. Some of its features were really adapted to the layout of new installations.In 2004, it was possible to initiate the project of a second beam line at SINQ for imaging with cold neutrons. Previous studies have shown the potential of this option in order to broaden the user profile and to extend the scientific basis for neutron imaging. It was inaugurated with a workshop at PSI in 2005. The user service was started at the facility ICON in 2006.Beside the setup, installation and optimization of the facilities, the organization of the user program plays an important role. The two neutron imaging beam lines are equal installations at SINQ among the 14 scientific devices. Therefore, the user approach is organized via “calls for proposals”, which are sent out each half year via the “Digital User Office (DUO)” (see http://duo.web.psi.ch). The evaluation of the proposals is done by the “Advisory Committee for Neutron Imaging (ACNI)” consisting of 6 external and PSI internal members.Further requests are given by industrial collaborations. This beam time allocation is handled more directly and in time in order to fulfill the companies’ demands. Here, the confidentiality plays a more important role than in scientific studies that are done with the aim of a free publication. It has been possible to earn money regularly from the industrial projects in order to cover the salary cost of some positions within the NIAG group.The permanent improvement of the methodology and performance in neutron imaging is a third major activity of the NIAG team. Running projects in this direction are the permanent insert of a grating interferometry device, improved energy selection with the help of single graphite crystals and utilization of the beam line BOA at SINQ for the energy range between 4 and 15 Å.     

Biaxially textured Mo films with diverse morphologies by substrate-flipping rotation

A class of nanostructured Mo thin films was grown by DC magnetron sputtering using a robust substrate rotation mode called 'flipping rotation'. In this rotation mode, the substrate is arranged to rotate continuously at a fixed speed around an axis lying within and parallel to the substrate. The incident flux is perpendicular to the rotational axis, and the incident flux angle changes continuously. Mo nanostructured films, grown under different rotation speeds with three orders of magnitude spread (ranging from 0.008 to 24?rotation?min( - 1)), different flipping directions (clockwise and counter-clockwise), and different ending deposition angles, were characterized using scanning electron microscopy (SEM) and reflection high energy electron diffraction (RHEED) surface-pole-figure techniques. Despite their very different morphologies, such as 'C'-shaped, 'S'-shaped, and vertically aligned nanorods, the same [Formula: see text] biaxial texture with an average out-of-plane dispersion of ～ 15° was observed. In contrast, we showed that only a fiber-textured Mo film was obtained by using the conventional rotation mode where the oblique incident flux angle was fixed with the substrate rotating around the surface normal.     

Parity-Violating Neutron Spin Rotation in a Liquid Parahydrogen Target

Our understanding of hadronic parity violation is far from clear despite nearly 50 years of theoretical and experimental progress. Measurements of low-energy parity-violating observables in nuclear systems are the only accessible means to study the flavor-conserving weak hadronic interaction. To reduce the uncertainties from nuclear effects, experiments in the few and two-body system are essential. The parity-violating rotation of the transverse neutron polarization vector about the momentum axis as the neutrons traverse a target material has been measured in heavy nuclei and few nucleon systems using reactor cold neutron sources. We describe here an experiment to measure the neutron spin-rotation in a parahydrogen target (n-p system) using pulsed cold-neutrons from the fundamental symmetries beam line at the Spallation Neutron Source under construction at the Oak Ridge National Laboratory.     

Anisotropic Creep Rupture Properties of a Nickel-base Single Crystal Superalloy at High Temperature

The creep rupture properties of a single crystal superalloy were tested at 975℃/255 MPa as a function of the deviation degrees from [001].The misorientation of the specimens away from [001] distributed approximately along a line between [001]-[011] and [001]-[111] boundaries in the triangle of the stereographic projection.Creep rupture lifetimes of the specimens were not sensitive to the misorientation until the deviation degree exceeded ～30 deg.Two steps of lattice rotation were found in all specimens during creep,first towards the [001]-[111] boundary,and then to [001] or [111] along the boundary.Single slip and strong asymmetric deformation were observed during the first stage of lattice rotation in specimens with large misorientation.The rotation mechanism was associated with the activated slip systems according to the calculated Schmid factors.The impact of lattice rotation on the rupture properties was also discussed.