8.5 Röntgenbildgebung
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- 2 Prozess- und Anlagensicherheit (3)
Eingeladener Vortrag
- nein (12)
Etalon features in transmittance spectra of films arise due to the coherent superposition of waves caused by reflections multiple times at the inner film boundaries. The fringe maxima occur at i𝜆 = 2nd cos 𝜑 with i - being an integer, 𝜆 - the wavelength, d - the film thickness, 𝜑 - the angle of incidence in respect to the film normal and n - the refractive index of the film. It is obvious that such features can hinder the interpretation of absorption bands when fringe amplitude and period are in the same range as the expected bands.
This poster discusses the suitability of two optical methods for the THz spectral range applied to Fourier Transform Spectroscopy.
Etalon effects in THz transmittance spectra, either taken in time- domain or frequency-domain, often hamper or even hinder the interpretation of film properties. In this paper we discuss the transferability and applicability of spectroscopic methods typically employed in the near and mid infrared spectral range to eliminate the fringes in the transmittance spectra in the THz spectral range.
We show how the combination of the spatial autocorrelation function and permeability calculations, applied to 3D X-ray computed tomography data, can yield quantitative information on the anisotropy of both meso-structure and fluid flow in Diesel Particulate Filter (DPF) materials, such as Cordierite and SiC. It was found that both the degree of anisotropy, and the orientation of the permeability and meso-structure are similar, but not identical. We confirm that the morphological anisotropy of cordierite materials is weak, and clearly influenced by the extrusion process that determines the main direction of anisotropy. Properties of the autocorrelation function are discussed and it is shown why estimating the characteristic length of real meso-structures (grain or ?pore? size) is not possible. Finally, we show that the autocorrelation function applied on grey-level images can give a good estimate of the degree of anisotropy even with limited resolution.
The ability to track electrode degradation, both spatially and temporally, is fundamental to understand performance loss during operation of lithium batteries. X-ray computed tomography can be used to follow structural and morphological changes in electrodes; however, the direct detection of electrochemical processes related to metallic lithium is difficult due to the low sensitivity to the element. In this work, 4-dimensional neutron computed tomography, which shows high contrast for lithium, is used to directly quantify the lithium diffusion process in spirally wound Li/SOCl2 primary cells. The neutron dataset enables the quantification of the lithium transport from the anode and the accumulation inside the SOCl2 cathode to be locally resolved. Complementarity between the collected neutron and X-ray computed tomographies is shown and by applying both methods in concert we have observed lithium diffusion blocking by the LiCl protection layer and identified all cell components which are difficult to distinguish using one of the methods alone.
In this presentation we discuss the online monitoring of metallic AM parts produced by the Laser Powder Bed Fusion (LPBF) process by using optical, thermographic and electromagnetic methods. In a first approach we present the detection of defects generated during the process and discuss how to improve these methods for the optimization of design and production of metallic AM parts.
Micromechanical response of multi-phase Al-alloy matrix composites under uniaxial compression
(2020)
Aluminum alloys are extensively used in the automotive industry. Particularly, squeeze casting production of Al-Si alloys is employed in the conception of metal matrix composites (MMC) for combustion engines. Such materials are of a high interest since they allow combining improved mechanical properties and reduced weight and hence improve efficiency. Being a multiphase material, most MMCs show complex micromechanical behavior under different load conditions. In this work we investigated the micromechanical behavior of two MMCs, both consisting of a near-eutectic cast AlSi12CuMgNi alloy, one reinforced with 15%vol. Al2O3 short fibers and the other with 7%vol. Al2O3 short fibers + 15%vol. SiC particles. Both MMCs have complex 3D microstructure consisting of four and five phases: Al-alloy matrix, eutectic Si, intermetallics, Al2O3 fibers and SiC particles.
The in-situ neutron diffraction compression experiments were carried out on the Stress-Spec beamline and disclosed the evolution of internal phase-specific stresses in both composites. In combination with the damage mechanism revealed by synchrotron X-ray computed tomography (SXCT) on plastically pre-strained samples, this allowed understanding the role of every composite’s phase in the stress partitioning mechanism. Finally, based on the Maxwell scheme, a micromechanical model was utilized. The model perfectly rationalizes the experimental data and predicts the evolution of principal stresses in each phase.
To assess the clinical course of a sheep stifle joint model for osteochondral (OC) defects, medial femoral condyles (MFC) were exposed without patella luxation using medial parapatellar skin (3–4 cm) and deep incisions (2–3 cm). Two defects (7 mm diameter; 10 mm depth; OC punch) were left empty or refilled with osteochondral autologous transplantation cylinders (OATS) and explanted after six weeks. Incision-to-suture time, anesthesia time, and postoperative wound or impairment scores were compared to those in sham-operated animals. Implant performance was assessed by X-ray, micro-computed tomography, histology, and immunohistology (collagens 1, 2; aggrecan). There were no surgery-related infections or patellar luxations. Operation, anesthesia, and time to complete stand were short (0.5, 1.4, and 1.5 h, respectively). The wound trauma score was low (0.4 of maximally 4; day 7). Empty-defect and OATS animals reached an impairment score of 0 significantly later than sham animals (7.4 and 4.0 days, respectively, versus 1.5 days). Empty defects showed incomplete healing and dedifferentiation/heterotopic differentiation; OATS-filled defects displayed advanced bone healing with remaining cartilage gaps and orthotopic expression of bone and cartilage markers. Minimally-invasive, medial parapatellar surgery of OC defects on the sheep MFC allows rapid and low-trauma recovery and appears well-suited for implant testing.
The relationship between residual stresses and microstructure associated with a laser powder bed fusion (LPBF) IN718 alloy has been investigated on specimens produced with three different scanning strategies (unidirectional Y-scan, 90° XY-scan, and 67° Rot-scan). Synchrotron X-ray energy-dispersive diffraction (EDXRD) combined with optical profilometry was used to study residual stress (RS) distribution and distortion upon removal of the specimens from the baseplate. The microstructural characterization of both the bulk and the nearsurface regions was conducted using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). On the top surfaces of the specimens, the highest RS values are observed in the Y-scan specimen and the lowest in the Rot-scan specimen, while the tendency is inversed on the side lateral surfaces. A considerable amount of RS remains in the specimens after their removal from the baseplate, especially in the Y- and Z-direction (short specimen Dimension and building direction (BD), respectively). The distortion measured on the top surface following baseplate thinning and subsequent removal is mainly attributed to the amount of RS released in the build direction. Importantly, it is observed that the additive manufacturing microstructures challenge the use of classic theoretical models for the calculation of diffraction elastic constants (DEC) required for diffraction-based RS analysis. It is found that when the Reuß model is used for the calculation of RS for different crystal planes, as opposed to the conventionally used Kröner model, the results exhibit lower scatter. This is discussed in context of experimental measurements of DEC available in the literature for conventional and additively manufactured Ni-base alloys.
A powerful tool to understand, demonstrate and explain the limits of the pulsed technique in terms of detectability and localizability of AM keyhole pores has been assessed by comparing the active thermographic approach (both experimental and FEM simulations) to Computed Tomography results;
✓ µCT results demonstrate that the intended defect geometry is not achieved; indeed a network of voids (microdefects consisting of small sharp-edged hollows with a complicated, almost fractal, inner surface) was found;
✓ both Exp-PT and FEM results explains clearly why no indication of defect related to the thermal contrasts could be found during the investigation of an uncoated surface. However, the application of further data evaluations focusing on the thermal behavior and emissivity evaluation (PPT post data processing) enable the detection of some defects;
✓ coating facilitates a closer inspection of inner defects, but inhomogeneities of the coating could impair the spatial resolution and lead to the emergence of hotspots (the FEM simulation reached its limit with this extreme geometry where a 25 µm thin disc is considered at a 1 cm thick specimen in millisecond time resolution);
✓ both Exp-PT and FEM results allow the conclusion that very short pulses of 200 ms or shorter should be sufficient to detect these defects below, but near the surface; besides a short duration of the thermal phenomenon it should be emphasized, about 0.04 s (high frame rate camera);
Functional materials for energy conversion are important technology drivers needed for the implementation of low carbon energy. Therefore, researchers commonly focus on improving the intrinsic properties of a functional material. However, for applications, the extrinsic properties are at least as important as the intrinsic ones. Consequently, it is important to investigate and understand the external and internal structure of semi-finished products and especially defect dependent properties. The extrinsic properties may change during application and the life cycle of the material as well as through processing and molding steps. Our studies show how X-ray tomographic (XCT) investigations can contribute to structure investigations in composites and massive samples using the example of magnetic materials for energy conversion.
Eine kritische Aufgabe im Rahmen der Etablierung von Prozess-Struktur-Eigenschafts-Performance-Beziehungen bei der additiven Fertigung (AM) von Metallen ist die Ermittlung von zuverlässigen und gut dokumentierten Kennwerten zum Materialverhalten sowie das Schaffen von Wissen über die Struktur-Eigenschafts-Korrelation. Schließlich ist dies die Grundlage für die Entwicklung gezielterer Prozessoptimierungen und zuverlässigerer Lebensdauer-Vorhersagen. In diesem Zusammenhang zielt dieser Beitrag darauf ab, Daten und Erkenntnisse über das Kriechverhalten des austenitischen Edelstahls 316L zu liefern, der mittels Laser-Powder-Bed-Fusion (L-PBF) hergestellt wird. Um dieses Ziel zu erreichen, wurden Proben aus konventionellem warmgewalztem sowie AM-Material gemäß den bestehenden Normen für konventionelles Material geprüft und vor und nach dem Versagen mikrostrukturell charakterisiert. Die Probekörper wurden aus einzelnen Blöcken des AM-Materials gefertigt. Die Blöcke wurden mit einer Standard-Scan- und Aufbaustrategie hergestellt und anschließend wärmebehandelt. Das Kriechverhalten wird anhand der Kriechlebensdauer und ausgewählter Kriechkurven und Kennwerte beschrieben und vergleichend bewertet. Der Einfluss von Defekten und Mikrostruktur auf das Materialverhalten wird anhand von zerstörenden und zerstörungsfreien Auswertungen an ausgewählten Proben analysiert. Der AM-Werkstoff zeigt kürzere Kriechlebensdauern, erreicht das sekundäre Kriechstadium deutlich schneller und bei geringerer Dehnung und weist eine geringere Kriechduktilität im Vergleich zu seinem konventionellen Gegenstück auf. Das Kriechschädigungsverhalten des AM-Werkstoffs ist eher mikrostruktur- als defektgesteuert und ist durch die Bildung intergranularer Kriechrisse gekennzeichnet. Als kritische Merkmale werden die Versetzungsdichte sowie die Versprödung der Korngrenzen identifiziert. Die Mikro-Computertomographie (µCT) erweist sich als Alternative zur Metallographie, um die Kriechschädigung zu analysieren.
Creep and fracture behavior of conventionally and additively manufactured stainless steel 316L
(2020)
A critical task within the frame of establishing process-structure-property-performance relationships in additive manufacturing (AM) of metals is producing reliable and well-documented material behavior’s data and knowledge regarding the structure-property correlation, including the role of defects. After all, it represents the basis for developing more targeted process optimizations and more reliable predictions of performance in the future. Within this context, this contribution aims to close the actual gap of limited historical data and knowledge concerning the creep behavior of the widely used austenitic stainless steel 316L, manufactured by Laser-Powder-Bed-Fusion (L-PBF). To address this objective, specimens from conventional hot-rolled and AM material were tested under application-relevant conditions according to existing standards for conventional material, and microstructurally characterized before and after failure. The test specimens were machined from single blocks from the AM material. The blocks were manufactured using a standard scan and build-up strategy and were subsequently heat-treated. The creep behavior is described and comparatively assessed based on the creep lifetime and selected creep curves and characteristic values. The effect of defects and microstructure on the material’s behavior is analyzed based on destructive and non-destructive evaluations on selected specimens. The AM material shows shorter creep lives, reaches the secondary creep stage much faster and at a lower strain, and features lower creep ductility compared to its conventional counterpart. The creep damage behavior of the AM material is more microstructure than defect controlled and is characterized by the formation and accumulation of single intergranular damage along the whole volume. Critical features identified are the grain morphology and the grain-boundary as well as the dislocation’s density. Micro-computed tomography (µCT) proves to be an alternative to metallography to analyze the creep damage.
Influence of the scanning strategy on the residual stress state in IN718 additive manufactured parts
(2020)
Laser Powder Bed Fusion (L-PBF) is an additive manufacturing technique enabling the design of complex geometries that are unrivalled by conventional production technologies. Nevertheless, L-PBF process is known to induce a high amount of residual stresses (RS) due to the high temperature gradients present during powder melting by laser. High tensile residual stresses are to be found the edges whereas the bulk material shows balancing compressive RS. Literature shows that the RS is highly sensitive to the process parameters. In particular, this study presents the characterization of the RS state in two L-PBF parts produced with a rastering scan vector that undergoes 90° or 67° rotation between subsequent layers.
Modern air-liners and wind turbine rotor blades are made up primarily of fiber reinforced plastics. Failure of these materials heavily impairs the serviceability and the operational safety. Consequently, knowledge of the failure behavior under static and cyclic loads is of great interest to estimate the operational strength and to compare the performance of different materials. Ideally, the damage evolution under operational load is determined with in-situ non-destructive testing techniques. Here, we report in-situ synchrotron X-ray imaging of tensile stress induced cracks in carbon fiber reinforced plastics due to inter-fiber failure. An inhouse designed compact tensile testing machine with a load range up to 15 kN was integrated into the beamline. Since conventional radiographs do not reveal sufficient contrast to distinguish cracks due to inter-fiber failure and micro cracking from fiber bundles, the Diffraction Enhanced Imaging (DEI) technique is applied in order to separate primary and scattered (refracted) radiation by means of an analyzer crystal. This technique allows fast measurements over large fields-of-view and is ideal for in-situ investigations.
Rapid cooling rates and steep temperature gradients are characteristic of additively manufactured parts and important factors for the residual stress formation. This study examined the influence of heat accumulation on the distribution of residual stress in two prisms produced by Laser Powder Bed Fusion (LPBF) of austenitic stainless steel 316L. The layers of the prisms were exposed using two different border fill scan strategies: one scanned from the centre to the perimeter and the other from the perimeter to the centre. The goal was to reveal the effect of different heat inputs on samples featuring the same solidification shrinkage. Residual stress was characterised in one plane perpendicular to the building direction at the mid height using Neutron and Lab X-ray diffraction. Thermography data obtained during the build process were analysed in order to correlate the cooling rates and apparent surface temperatures with the residual stress results. Optical microscopy and micro computed tomography were used to correlate defect populations with the residual stress distribution. The two scanning strategies led to residual stress distributions that were typical for additively manufactured components: compressive stresses in the bulk and tensile stresses at the surface. However, due to the different heat accumulation, the maximum residual stress levels differed. We concluded that solidification shrinkage plays a major role in determining the shape of the residual stress distribution, while the temperature gradient mechanism appears to determine the magnitude of peak residual stresses.
Ti-6Al-4V alloy is intensively used in the aerospace industry because of its high specific strength. However, the application of Laser Powder Bed Fusion (LPBF) Ti-6Al-4V alloy for structurally critical load-bearing components is limited. One of the main limiting factors affecting the structural integrity, are manufacturing defects. Additionally, the high cooling rates associated with LPBF process result in the formation of large residual stress (RS) with complex fields. Such RS can cause cracking and geometrical distortions of the part even right after production. Also, the microstructure of LPBF Ti-6Al-4V in the as-built condition is significantly different from that of the conventionally produced alloy. All these factors affect the mechanical behavior of the material. Therefore, to improve the material performance it is important to evaluate the individual effect of RS, defects, and microstructure on fatigue life. To this aim Ti-6Al-4V LPBF material in as-built condition and subjected to different post-processing, including two heat treatments (for stress relief and microstructural modification) and Hot Isostatic Pressing (HIP, for densification), were investigated.
Prior to Low Cycle Fatigue (LCF) tests at operating temperature (300°C), the microstructure (phases, crystallographic texture, and grain morphology), the mesostructure (defect shape and distribution), and subsurface RS on the LCF samples were investigated. It was found that the fatigue performance of HIPped samples is similar to that of conventionally produced Ti-6Al-4V. The tensile RS found at the surface of as-built samples decreased the fatigue life compared to heat-treated samples. Additionally, the modification of the microstructure (by heat treatment) did not affect the fatigue performance in the regime of mostly elastic strain. This shows that in the absence of tensile RS the manufacturing defects solely control the failure of LPBF components and densification has the strongest effect on the improvement of the mechanical performance.
Integration of fibre reinforcement in high-performance cementitious materials has become widely applied in many fields of construction. One of the most investigated advantages of steel fibre reinforced concrete (FRC) is the deceleration of crack growth and hence it’s improved sustainability due to e.g. decrease of permeability of concrete by aggressive substances. Additional benefits are associated with the structural properties of FRC, where fibres can significantly increase the ductility and the tensile strength of concrete. In some applications, such as tunnel linings or industrial slabs, it is even possible to entirely replace the conventional reinforcement, leading to significant logistical and environmental benefits. Fibre reinforcement can, however, have critical disadvantages and even hinder the performance of concrete, since it can induce an anisotropic material behaviour of the mixture if the fibres are not appropriately oriented. For a safe use of FRC in the future, reliable non-destructive methods need to be identified to assess the fibres’ orientation in hardened concrete. In this study, ultrasonic material testing, electrical impedance testing, and X-ray computer tomography have been investigated for this purpose using specially produced samples with biased or random fibre orientations. This paper demonstrates the capabilities of each of these NDT techniques for fibre orientation measurements and draws conclusions based on these results about the most promising areas for future research and development using these techniques.
In-situ Prozessüberwachung in der additiven Fertigung von Metallen mittels optischer Verfahren
(2020)
Einer der aussichtsreichsten Ansätze, die Qualität und Sicherheit der gefertigten Teile in der metallbasierten additiven Fertigung (AM) zu erhöhen und die Notwendigkeit aufwändiger und zeitintensiver, zerstörender oder zerstörungsfreier Prüfungen (ZfP) nach der Fertigung zu verringern, liegt in dem Einsatz von in-situ Prozessüberwachungstechniken. Bereits jetzt werden erste Messsysteme zur Kontrolle der Energiequelle, des Bauraums, des Schmelzbades und der Bauteilgeometrie kommerziell angeboten. Weitere ZfP Verfahren, wie z.B. die aktive und passive Thermografie, werden in der Literatur als geeignet für die in-situ Anwendung angesehen, allerdings gibt es noch wenig konkrete praktische Umsetzungen, da die Möglichkeiten und individuellen Grenzen dieser Methoden, angewendet auf AM, noch nicht ausreichend erforscht sind. Aus diesem Grund verfolgt die BAM mit dem Projekt „Process Monitoring of AM“ (ProMoAM) im Themenfeld Material das Ziel, Verfahren des Prozessmonitorings zur in-situ Bewertung der Qualität additiv gefertigter Metallbauteile weiterzuentwickeln.
Im Beitrag wird zunächst das Projekt vorgestellt und anschließend der Fokus auf eine Messserie gelegt, in der Probekörper aus dem austenitischen Edelstahl 316L mit lokal variierenden Prozessparametern mittels selektiven Laserschmelzen (L-PBF) aufgebaut wurden. Der Bauprozess wurde hierbei durch das maschineneigene, koaxial arbeitende Photodiodensystem (Melt-Pool-Monitoring), einer Mittelwellen-Infrarotkamera und einer optischen Tomografiekamera im sichtbaren Wellenlängenbereich (Langzeitbelichtung für die Dauer eines Lagenaufbaus mit einer CMOS-Kamera mit hoher Ortsauflösung) simultan überwacht. Als Referenz für diese Methoden wurden die Probekörper mittels Computertomografie untersucht. Für die dabei anfallenden teils großen Datenmengen wurden Algorithmen für ein effizientes Preprocessing entwickelt. Es wurden Merkmale der Messdaten in Korrelation zu Fehlern und Inhomogenitäten extrahiert, welche für die einzelnen Methoden vergleichend vorgestellt und diskutiert werden.
The understanding of dynamic processes in Li-metal batteries is an important consideration to enable the full capacity of cells to be utilised. These processes, however, are generally not directly observable using X-ray techniques due to the low attenuation of Li; and are challenging to visualise using neutron imaging due to the low temporal resolution of the technique. In this work, complementary X-ray and neutron imaging are combined to track the dynamics of Li within a primary Li/SOCl2 cell. The temporal challenges posed by neutron imaging are overcome using the golden ratio imaging method which enables the identification of Li diffusion in operando. This combination of techniques has enabled an improved understanding of the processes which limit rate performance in Li/SOCl2 cells and may be applied beyond this chemistry to other Li-metal cells.
This article highlights Peter Paufler’s academic genealogy on the occasion of his 80th birthday. We describe the academic background since 1776, which covers 11 generations of scientists: Ritter, Ørsted, Hansteen, Keilhau, Kjerulf, Brøgger, Goldschmidt, Schulze, Paufler, Meyer, and Leisegang. The biographies of these scientists are described in spotlight character and references to scientists such as Dehlinger, Ewald, Glocker, Röntgen, Vegard, Weiss, and Werner are given. A path is drawn that begins in the Romanticism with electrochemistry and the invention of what is probably the first accumulator. It leads through the industrialization and the modern geology, mineralogy, and crystallography to crystal chemistry, metal and crystal physics and eventually returns to electrochemistry and the aluminum-ion accumulator in the era of the energy transition. The academic genealogy exhibits one path of how crystallography develops and specializes over three centuries and how it contributes to the understanding of the genesis of the Earth and the Universe, the exploration of raw materials, and the development of modern materials and products during the industrialization and for the energy transition today. It is particularly characterized by the fields of physics and magnetism, X-ray analysis, and rare earth compounds and has strong links to the scientific landscape of Germany (Freiberg) and Scandinavia, especially Norway (Oslo), as well as to Russia (Moscow, Samara, St. Petersburg). The article aims at contributing to the history of science, especially to the development of crystallography, which is the essential part of the structural science proposed by Peter Paufler.
Modern air-liners and wind turbine rotor blades are made up primarily of fiber reinforced plastics. Failure of these materials heavily impairs the serviceability and the operational safety. Consequently, knowledge of the failure behavior under static and cyclic loads is of great interest to estimate the operational strength and to compare the performance of different materials. Ideally, the damage evolution under operational load is determined with in situ non-destructive testing techniques. Here, we report in-situ synchrotron X-ray imaging of tensile stress induced cracks in carbon fiber reinforced plastics due to inter-fiber failure. An inhouse designed compact tensile testing machine with a load range up to 15 kN was integrated into the beamline. Since conventional radiographs do not reveal sufficient contrast to distinguish cracks due to inter-fiber failure and micro cracking from fiber bundles, the Diffraction Enhanced Imaging (DEI) technique is applied in order to separate primary and scattered (refracted) radiation by means of an analyzer crystal. This technique allows fast measurements over large fields-of-view and is ideal for in-situ investigations. Imaging and the tensile test are run at the highest possible frame rate (0.7 s-1 ) and the lowest possible strain rate (5.5∙10-4 s -1 ). For 0°/90° non-crimp fabrics, the first inter-fiber cracks occur at 380 MPa (strain 0.8 %). Prior to failure at about 760 MPa (strain 2.0 %), we observe the evolution of nearly equidistant (1 mm distance) cracks running across the entire sample in the fully damaged state.
X-Ray computed tomography and traditional analysis of a capillary absorption test in cement pastes
(2020)
Water absorption testing has the attraction of being simple, relatively quick and use water as a fluid which is the key substance in the possible concrete deterioration. However, in spite of the advantages its application has remained empirical, except for the testing of resistance to frost, through the identification of the critical degree of saturation. Rilem TC-116 studied this test, among others, for the characterization of concrete resistance against transport through the concrete pore network. Results indicated a low discriminating ability of the absorbency for qualifying concrete resistance and then, it seemed necessary to try to move forward by making the link between the absorption testing and the concrete pore microstructure. In the present work, thanks to the X-Ray computed tomography, it is described the evolution of the water front in three paste-specimen with different w/c ratios (different pore radius distributions). The observations allowed us to deduce that all the pores start to fill simultaneously, but the bigger ones fill quicker and rise less. The saturation degree is measured along the height of the sample, showing that each pore size has a different rate which becomes constant after the first hour. Washburn’s equation was applied to the progressive advance of the water front, resulting in a pore radius much smaller than the one expected.
The mechanisms controlling the fatigue response of an A357-T6 cast aluminium alloy under cyclic torsional loading are investigated. Surface crack monitoring coupled with Electron BackScattering Diffraction (EBSD) analysis is used to study crack initiation. Determination of S-N curve combined with interrupted in situ fatigue testing using synchrotron tomography allows the study of the propagation behaviour. It is observed that fractographic morphologies depend on the stress level. At intermediate-low stress levels ( τmax < 100 MPa), the grain structure controls the crack initiation and propagation periods. Cracks are usually nucleated in mode II from slip planes close to the specimen axis or perpendicular to it. Mode II crack growth dominates the early stages of crack propagation as mode III inward crack growth is rapidly decelerated. This behaviour leads to the formation of characteristic shallow surface cracks. Once the crack is long enough for the mode II driving force to be overtaken by mode III, propagation branches into ~45 ◦mode I.
The goal of the Special Issue “Brittle Materials in Mechanical Extremes” was to spark a discussion of the analogies and the differences between different brittle materials, such as, for instance, ceramics and concrete. Indeed, the contributions to the Issue spanned from construction materials (asphalt and concrete) to structural ceramics, reaching as far as ice. The data shown in the issue were obtained by advanced microstructural techniques (microscopy, 3D imaging, etc.) and linked to mechanical properties (and their changes as a function of aging, composition, etc.). The description of the mechanical behavior of brittle materials under operational loads, for instance, concrete and ceramics under very high temperatures, offered an unconventional viewpoint on the behavior of brittle materials. This is not at all exhaustive, but a way to pave the road for intriguing and enriching comparisons.
The present work offers an explanation on how the long-range interaction of dislocations influences their movement, and therefore the strain, during creep of metals. It is proposed that collective motion of dislocations can be described as a fractional Brownian motion. This explains the noisy appearance of the creep strain signal as a function of time. Such signal is split into a deterministic and a stochastic part. These terms can be related to two kinds of dislocation motions: individual and collective, respectively. The description is consistent with the fractal nature of strain-induced dislocation structures predicated in previous works. Moreover, it encompasses the evolution of the strain rate during all stages of creep, including the tertiary one. Creep data from Al99.8% and Al- 3.85%Mg tested at different temperatures and stresses are used to validate the proposed ideas: it is found that different creep stages present different diffusion characters, and therefore different dislocation motion character.
X-ray computed tomography has many applications in materials science and non-destructive testing. While the standard filtered back-projection reconstruction of the radiographic data sets is fast and simple, it typically fails in returning accurate results from missing or inconsistent projections. Among the alternative techniques that have been proposed to handle such data is the Direct Iterative REconstruction of Computed Tomography Trajectories (DIRECTT) algorithm. We describe a new approach to the algorithm, which significantly decreases the computational time, while achieving a better reconstruction quality than that of other established algorithms.
Intensified Alkali-Silica Reaction (ASR) damage has occurred on German roadways in recent years, leading to requirements for compulsory pre-construction investigation of aggregate alkali sensitivity using concrete tests with external alkali supply. However, since these tests are time-consuming and cost-intensive, there is interest in replacing them with a solubility test on pure aggregate in 0.1 M KOH solution at 80°C with a defined NaCl content (1wt.-%). In this context, the influence of aggregate pore structure on SiO2 and Al2O3 solubility was investigated in this project. This paper compares the results of porosity studies with X-ray Computed Tomography (3D-CT) and the Brunauer-Emmett-Teller (BET) method on individual quarried and river gravel granules of both rhyolite and greywacke. For visualization and quantification of both externally accessible and fully enclosed surfaces of granules using X-ray 3D-CT, special software tools were developed. The results demonstrated that the river gravel granules had significantly larger externally accessible surfaces than the quarried granules. BET measurements on individual stones showed, as expected, that measured surfaces were about three orders of magnitude larger than those from the X-ray 3D-CT analyses due to the higher spatial resolution of BET. There was no apparent correlation between the X-ray 3D-CT and BET surface areas. Mercury porosimetry measurements indicate that this may be due to the presence of significant porosity below the spatial resolution of the X-ray 3D-CT. A comparison of SiO2 and Al2O3 solubility measurements with the X-ray 3D-CT and BET surface area data resulted only in weak, inconclusive correlations, indicating the need for further experimental investigation.
As opposed to reviewing results on experimental determination of residual stress by diffraction, this paper discusses the open issues when dealing with residual stress determination in additive manufactured parts, in particular those manufactured with laser powder bed fusion techniques. Three points are addressed in detail: (a) the proper determination of the strain-free reference d0, (b) the problem of the determination of the principal axes, and (c) the use of the correct diffraction elastic constants. It is shown that all methods to determine the strain-free reference d0 suffer from caveats, and care must be taken in evaluating the most suitable for the problem being tackled. In addition, it is shown that, in some systems, the principal axes do correspond to the geometrical axes of the specimen, but this needs to be systematically checked, especially in the case of uni- or bidirectional hatching strategies. Finally, the need to experimentally determine the proper diffraction elastic constants is underlined, especially in the case of strongly textured specimens, which again depends on the deposition strategy.
The 5G technology promises real time data transmission for industrial processes, autonomous driving, virtual and augmented reality, E-health applications and many more. The Low Temperature Co-fired Ceramics (LTCC) technology is well suited for the manufacturing of microelectronic components for such applications. Still, improvement of the technology such as further miniaturization is required. This study focuses on the characterization of inner metallization of LTCC multilayer modules, especially on the vertical interconnect access (VIA). Critical considerations for this characterization are delamination, pore clustering in and at the edge of the VIA, deformation, and stacking offset. A LTCC multilayer consisting of a glassy crystalline matrix with silver based VIAs was investigated by synchrotron x-ray tomography (CT). The aim of this study is to propose a multitude of structural characteristic values to maximize the information gained from the available dataset. Data analysis has been done with the open source software ImageJ as well as several additional plugins. The high-resolution CT data was evaluated through 2D slices for accessibility reasons. The segmentation of all 2000 slices to assess the different regions e.g. pores, silver and glass ceramic was done by a supervised machine learning algorithm. A quantitative evaluation of shape, deformation, and porosity of the VIA with respect to its dimensions is presented and the suitability of the characterization approach is assessed.
Rapid cooling rates and steep temperature gradients are characteristic of additively manufactured parts and important factors for the residual stress formation. This study examined the influence of heat accumulation on the distribution of residual stress in two prisms produced by Laser Powder Bed Fusion (LPBF) of austenitic stainless steel 316L. The layers of the prisms were exposed using two different border fill scan strategies: one scanned from the centre to the perimeter and the other from the perimeter to the centre. The goal was to reveal the effect of different heat inputs on samples featuring the same solidification shrinkage. Residual stress was characterised in one plane perpendicular to the building direction at the mid height using Neutron and Lab X-ray diffraction. Thermography data obtained during the build process were analysed in order to correlate the cooling rates and apparent surface temperatures with the residual stress results. Optical microscopy and micro computed tomography were used to correlate defect populations with the residual stress distribution. The two scanning strategies led to residual stress distributions that were typical for additively manufactured components: compressive stresses in the bulk and tensile stresses at the surface. However, due to the different heat accumulation, the maximum residual stress levels differed. We concluded that solidification shrinkage plays a major role in determining the shape of the residual stress distribution, while the temperature gradient mechanism appears to determine the magnitude of peak residual stresses.
Modern air-liners and rotor blades of wind turbines are basically made of fiber reinforced plastics (FRP). Their failure heavily impairs the serviceability and the operational safety. Consequently, knowledge of the failure behavior under static and cyclic loads is of great interest to estimate the operational strength and to compare the performance of different materials. Ideally, the damage evolution under operational load is determined with in-situ non-destructive testing techniques. Here, we report on in-situ synchrotron X-ray imaging of tensile stress induced cracks in carbon fiber reinforced plastics (CFRP) due to inter fiber failure. An in-house designed compact-tensile testing machine with a load range up to 15 kN was integrated into the beam path. Since conventional radiographs do not reveal sufficient contrast to distinct cracks due to inter fiber failure and micro cracking from fiber bundles, the Diffraction Enhanced Imaging technique (DEI) is applied in order to separate primary and scattered (refracted) radiation by means of an analyzer crystal. In the laboratory, scanning X-ray refraction topography of CFRP has been applied long before but it comes along with several disadvantages: the long total measuring time hampers real time (in-situ) measurements and the required small beam size hinders end-to-end imaging. The introduced technique overcomes both drawbacks. Imaging and tensile test rig are run unsynchronized at the greatest possible frame rate (0.7 s-1 at 28.8 µm pixel size) and smallest possible strain rate (5.5∙10-4 s-1). For 0°/90° non-crimped fabrics (ncf) the first inter fiber cracks occurred at 380 MPa (strain 0.7 %). Prior to failure at about 760 MPa (strain 2.0 %) we observe the evolution of a nearly equidistant 1 mm grid of cracks running across the entire sample in the fully damaged state before total failure.
In this work, which is part of a larger research program, a framework called “virtual data fusion“ was developed to provide an automated and consistent crack detection method that allows for the cross-comparison of results from large quantities of X-ray Computed Tomography (CT) data. A partial implementation of this method in a custom program was developed for use in research focused on crack quantification in Alkali-Silica Reaction (ASR)-sensitive concrete aggregates. During the CT image processing, a series of image analyses tailored for detecting specific, individual crack-like characteristics were completed. The results of these analyses were then “fused” in order to identify crack-like objects within the images with much higher accuracy than that yielded by any individual image analysis procedure. The results of this strategy demonstrated the success of the program in effectively identifying crack-like structures and quantifying characteristics, such as surface area and volume. The results demonstrated that the source of aggregate has a very significant impact on the amount of internal cracking, even when the mineralogical characteristics remain very similar. River gravels, for instance, were found to contain significantly higher levels of internal cracking than quarried stone aggregates of the same mineralogical type.
The effect of two types of scanning strategies on the grain structure and build-up of Residual Stress (RS) has been investigated in an as-built IN718 alloy produced by Laser Powder Bed Fusion (LPBF). The RS state has been investigated by X-ray diffraction techniques. The microstructural characterization was performed principally by Electron Backscatter Diffraction (EBSD), where the application of a post-measurement refinement technique enables small misorientations (< 2°) to be resolved. Kernel average misorientation (KAM) distributions indicate that preferably oriented columnar grains contain higher levels of misorientation, when compared to elongated grains with lower texture. The KAM distributions combined with X-ray diffraction stress maps infer that the increased misorientation is induced via plastic deformation driven by the thermal stresses, acting to self-relieve stress. The possibility of obtaining lower RS states in the build direction as a consequence of the influence of the microstructure should be considered when envisaging scanning strategies aimed at the mitigation of RS.
Due to the wide range of applications, the easy production and the large field of use, reinforced concrete (RC) is a widespread building material. This variety of applications is reflected in a wide range of physical material properties. Not only therefor it still is a technical challenge to provide all necessary test conditions for experimentally reproducing dynamic effects under impact loading of RC structures. In this paper we present investigations on the thicknesses of RC plates under low and medium high velocity impact loading by a flat-tipped impactor. The planar tomography setup at BAM is used to visualize the impact damage and to characterize the damage features such as cracks, scabbing and spalling. Further, the comparison of tomography results with those of an applied numeric simulation analysis is used to verify the numeric models for future damage prognosis under impact loading. Using the results of both, the tomographic as well as the FE analysis, different damage features were investigated and compared regarding their validity. Crack damage plays a leading part and the significance of summarized crack values as well as their distribution is analyzed. The total damage value but also the determined damage distribution both provide an input for describing damage as a function of the impactor velocity and plate thickness.
Herein, we present a detailed investigation of the electrochemically triggered formation and dissolution processes of α- and β-sulfur crystals on a monolithic carbon cathode using operando high-resolution synchrotron radiography (438 nm/pixel).
The combination of visual monitoring with the electrical current response during cyclic voltammetry provides valuable insights into the sulfur formation and dissolution mechanism. Our observations show that the crystal growth process is mainly dictated by a rapid equilibrium between long-chain polysulfides on one side and solid sulfur/short-chain polysulfides on the other side, which is consistent with previous studies in this field. The high temporal and spatial resolution of synchrotron imaging enables the observation of different regimes during the sulfur formation and dissolution process. The appearance of short-chain polysulfides after the first anodic CV peak initiates a rapid dissolution process of α-sulfur crystals on the cathode. The increase in the long-chain lithium polysulfide concentration at the cathode surface during charge results in an increased crystal growth rate, which in turn produces imperfections in α- and β-sulfur crystals. There are strong indications that these defects are fluid inclusions, which may trap dissolved polysulfides and therefore reduce the electrochemical cell capacity.
The contour scan strategies in laser powder bed fusion (LPBF) of Ti-6Al-4V were studied at the coupon level. These scan strategies determined the surface qualities and subsurface residual stresses. The correlations to these properties were identified for an optimization of the LPBF processing. The surface roughness and the residual stresses in build direction were linked: combining high laser power and high scan velocities with at least two contour lines substantially reduced the surface roughness, expressed by the arithmetic mean height, from values as high as 30 μm to 13 μm, while the residual stresses rose from ~340 to about 800 MPa. At this stress level, manufactured rocket fuel injector components evidenced macroscopic cracking. A scan strategy completing the contour region at 100 W and 1050 mm/s is recommended as a compromise between residual stresses (625 MPa) and surface quality (14.2 μm). The LPBF builds were monitored with an in-line twin-photodiode-based melt pool monitoring (MPM) system, which revealed a correlation between the intensity quotient I2/I1, the surface roughness, and the residual stresses. Thus, this MPM system can provide a predictive estimate of the surface quality of the samples and resulting residual stresses in the material generated during LPBF.
The creep strengthening mechanisms in (age-hardenable) aluminum alloys are analyzed on the basis of a new microstructural study of powder samples, an analysis of a comprehensive revision of creep data from the literature, and a new modeling approach. A strategy based on the strength difference (SD) method to separate the contributions of solid solution atoms and precipitates to creep strengthening is proposed. The new methodology considers the combination of the two contributions avoiding the need of a threshold stress term in the creep equation. The contribution of both precipitates and solid solution is taken into account by means of the analysis of the lattice parameter variation with aging time. For this study, powders of two commercial AA2xxx alloys have been analyzed using diffraction methods. The experimental results are modeled using Lubarda’s approach combined with the SD method.
The paper presents the results of a series of combined mechanical and in-situ morphological investigations on highstrength strain-hardening cement-based Composites (SHCC). Tension and compression experiments were performed in a CT scanner employing a dedicated mechanical testing rig. The in-situ microtomographic scans enabled correlating the measured specimen response with relevant microstructural features and fracture processes. The microstructural segmentation of SHCC was performed in the framework of Deep Learning and it targeted an accurate segmentation of pores, fibers and aggregates. Besides their accurate volumetric representation, these phases were quantified in terms of content, size and orientation. The fracture processes were monitored at different loading stages and Digital Volume Correlation (DVC) was employed to spatially map the strains and cracks in the specimens loaded in compression. The DVC analysis highlighted the effect of loading conditions, specimen geometry and material heterogeneity at the mesolevel on the strain distribution and fracture localization.
The paper is motivated by some inconsistencies and contradictions present in the literature on the calculation of the so-called diffraction elastic constants. In an attempt at unifying the views that the two communities of Materials Science and Mechanics of Materials have on the subject, we revisit and define the terminology used in the field. We also clarify the limitations of the commonly used approaches and Show that a unified methodology is also applicable to textured materials with a nearly arbitrary grain shape. We finally compare the predictions based on this methodology with experimental data obtained by in situ synchrotron radiation diffraction on additively manufactured Ti-6Al4V alloy. We show that (a) the transverse isotropy of the material yields good agreement between the best-fit isotropy approximation (equivalent to the classic Kröner’s model) and the experimental data and (b) the use of a general framework allows the calculation of all components of the tensor of diffraction elastic constants, which are not easily measurable by diffraction methods. This allows us to extend the
current state-of-the-art with a predictive tool.
Using SXRR to Probe the Nature of Discontinuities in SLM Additive Manufactured Inconel 718 Specimens
(2020)
The utilization of additive manufacturing (AM) to fabricate robust structural components relies on understanding the nature of internal anomalies or discontinuities, which can compromise the structural integrity. While some discontinuities in AM microstructures stem from similar mechanisms as observed in more traditional processes such as casting, others are unique to the AM process. Discontinuities in AM are challenging to detect, due to their submicron size and orientation dependency. Toward the goal of improving structural integrity, minimizing discontinuities in an AM build requires an understanding of the mechanisms of formation to mitigate their occurrence. This study utilizes various techniques to evaluate the shape, size, nature and distribution of discontinuities in AM Inconel 718, in a non-hot isostatic pressed (HIPed) as-built, non-HIPed and direct age, and HIPed with two step age samples. Non-destructive synchrotron radiation refraction and transmission radiography (SXRR) provides additional information beyond that obtained with destructive optical microscopy. SXRR was able to distinguish between voids, cracks and lack of melt in, due to its sensitivity to the orientation of the discontinuity.
The understanding of process-microstructure-property-performance (PMPP) relationships in additive manufacturing (AM) of metals is highly necessary to achieve wide-spread industrial application and replace conventionally manufactured parts, especially regarding safety-relevant applications. To achieve this understanding, reliable data and knowledge regarding material’s microstructure-property relationships (e.g. the role of defects) is needed, since it represents the base for future more targeted process optimizations and more reliable calculations of performance. However, producing reliable material data and assessing the AM material behaviour is not an easy task: big challenges are e.g. the actual lack of standard testing methods for AM materials and the occasional difficulties in finding one-to-one comparable material data for the conventional counterpart.
This work aims to contribute to end this lack of reliable material data and knowledge for the low cycle fatigue behaviour of the most used titanium alloy in aerospace applications (Ti-6Al-4V). For this purpose, two sets of test specimens were investigated. The first set was manufactured from cylindrical rods produced by an optimized DED-L process and the second was manufactured from a hot formed round bar. The test specimens were cyclically loaded until failure in the low-cycle-fatigue (LCF) regime. The tests were carried out according to ISO 12106 between 0.3 to 1.0 % axial strain amplitude from room temperature up to 400°C. The LCF behaviour is described and compared between materials and with literature values based on cyclic deformation curves and strain-based fatigue life curves. Besides, the parameters of Manson-Coffin-Basquin relationship were calculated. The microstructures (initial and after failure) and fracture surfaces were comparative characterized. Thereby, the focus lied on understanding the role of grain morphology and defects on the failure mechanisms and fatigue lifetimes. For this latter characterization, optical microscopy (OM), scanning electron microscopy (SEM) and micro computed tomography (µCT) were used.
Wire Arc Additive Manufacturing (WAAM) features high deposition rates and, thus, allows production of large components that are relevant for aerospace applications. However, a lot of aerospace parts are currently produced by forging or machining alone to ensure fast production and to obtain good mechanical properties; the use of these conventional process routes causes high tooling and material costs. A hybrid approach (a combination of forging and WAAM) allows making production more efficient. In this fashion, further structural or functional features can be built in any direction without using additional tools for every part. By using a combination of forging basic geometries with one tool set and adding the functional features by means of WAAM, the tool costs and material waste can be reduced compared to either completely forged or machined parts. One of the factors influencing the structural integrity of additively manufactured parts are (high) residual stresses, generated during the build process. In this study, the triaxial residual stress profiles in a hybrid WAAM part are reported, as determined by neutron diffraction. The analysis is complemented by microstructural investigations, showing a gradient of microstructure (shape and size of grains) along the part height. The highest residual stresses were found in the transition Zone (between WAAM and forged part). The total stress range showed to be lower than expected for WAAM components. This could be explained by the thermal history of the component.
Virtuelle CT
(2020)
Das Computerprogramm aRTist ist ein praktisches Simulationswerkzeug zur Erzeugung realistischer Radiographien virtueller Durchstrahlungsaufbauten. Mit Durchstrahlungssimulationen können Bauteile, repräsentiert durch STL-Geometrien, wie in einem Computertomografen gescannt werden. Dabei werden verschiedenste Parameter gezielt und exakt berücksichtigt.
Die Virtuelle Computertomografie bietet neue Möglichkeiten der Untersuchung von Parametereinflüssen. Neben der Möglichkeit, physikalische Effekte an- und abzuschalten, z.B. die Streustrahlung, können auch Scanbewegungen getestet werden, bevor diese technisch realisiert werden.
Das für die Forschung entwickelte Simulationsprogramm aRTist bietet mit seiner intuitiven grafischen Benutzeroberfläche vielfältige Anwendungsmöglichkeiten auch in Praxis und Entwicklung, beispielsweise zur Parameteroptimierung, für Machbarkeitsstudien oder zur Abschätzung der Unsicherheit dimensioneller Messungen.
The Stone Age site Riņņukalns, Latvia, is the only well-stratified shell midden in the Eastern Baltic. In this paper, we present new interdisciplinary results concerning its dating, stratigraphy, features, and finds to shed light on the daily life of a fisher population prior to the introduction of domesticated animals. The undisturbed part of the midden consists of alternating layers of unburnt mussel shell, burnt mussel shell and fish bone, containing artefacts, some mammal and bird bones, and human burials. Two of them, an adult man and a baby, are discovered recently and date to the calibration plateau between 3350 and 3100 cal BC, and to the later 4th millennium, respectively. Stable isotopes suggest a diet based heavily on freshwater fish, and this is supported not only by ten thousands of identified fish remains, but also by a fish bone concentration nearby the skull of the man, which is interpreted as remain of a grave gift (possible fish soup). Of special interest are the baby’s stable isotope values. It shows that the mother’s diet was atypical (perhaps because she was non-local), and/or that dietary stress during pregnancy increased fractionation between the mother’s diet and her bloodstream
Connecting Diffraction-Based Strain with Macroscopic Stresses in Laser Powder Bed Fused Ti-6Al-4V
(2020)
The laser powder bed fusion (LPBF) production process often results in large residual stress (RS) in the parts. Nondestructive techniques to determine RS are badly needed. However, a reliable quantification of macro-RS (i.e., stress at the component level) by means of diffraction-based techniques is still a great challenge, because the link between diffraction-based strain and macro-RS is not trivial. In this study, we experimentally determine (by means of in-situ synchrotron radiation diffraction) this link for LPBF Ti-6Al-4V. We compare our results with commonly used models to determine the so-called diffraction elastic constants (DECs). We show that LPBF materials possess different DECs than wrought alloys, simply because their microstructural and mechanical properties are different. We also show that the existing models can be used to calculate DECs only if high accuracy of the RS values is not required. If the peculiarities of the microstructure have to be taken into account (as is the case of additively manufactured materials), a radically new approach is desirable.
Explaining Deviatoric Residual Stresses in Aluminum Matrix Composites with Complex Microstructure
(2020)
The residual stresses in multiphase metal Matrix composites with both random planar-oriented short fibers and particles were studied by neutron diffraction and by a model based on the reformulation of classic Maxwell’s homogenization method. Contrary to common understanding and state-of-the-art models, we experimentally observed that randomly oriented phases possess non-hydrostatic residual stress. The recently developed modeling Approach allows calculating the residual stress in all phases of the composites. It rationalizes the presence of deviatoric stresses Accounting for the interaction of random oriented phases with fibers having preferential orientation.
An ultrasonic vibration post-treatment procedure was suggested for additively manufac-tured lattices. The aim of the present research was to investigate mechanical properties andthe differences in mechanical behavior and fracture modes of Ti6Al4V scaffolds treated withtraditional powder recovery system (PRS) and ultrasound vibration (USV). Scanning electronmicroscopy (SEM) was used to investigate the strut surface and the fracture surface mor-phology. X-ray computed tomography (CT) was employed to evaluate the inner structure,strut dimensions, pore size, as well as the surface morphology of additively manufacturedporous scaffolds. Uniaxial compression tests were conducted to obtain elastic modulus,compressive ultimate strength and yield stress. Finite element analysis was performedfor a body-centered cubic (BCC) element-based model and for CT-based reconstructiondata, as well as for a two-zone scaffold model to evaluate stress distribution during elasticdeformation. The scaffold with PRS post treatment displayed ductile behavior, while USVtreated scaffold displayed fragile behavior. Double barrel formation of PRS treated scaffoldwas observed during deformation. Finite element analysis for the CT-based reconstructionrevealed the strong impact of surface morphology on the stress distribution in comparisonwith BCC cell model because of partially molten metal particles on the surface of struts,which usually remain unstressed.
The material and damage behaviour of additively manufactured polyamide 12 under cyclic loading was characterized by cyclic tests and microstructure analysis by using microscopy, X-ray refraction, and computed tomography. The results were used to determine parameters for the viscoplastic material model by Chaboche and a damage model by Gurson-Tvergaard-Needleman. The temperature was monitored during the experiments and the self-heating effect was observed. By including this effect, a higher accuracy could be achieved with the results of mechanical experiments.