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Paper des Monats
- ja (32)
Die wasserstoffunterstützte Schädigung von Komponenten und Bauteilen aus Stahl ist ein Phänomen, welches seit vielen Jahrzehnten bekannt ist und untersucht wird. Eine Vielzahl von Quellen (z.B. Schutzgas oder Feuchtigkeit beim Schweißen, Reinigung von Metallen in Säurebädern, galvanischer oder kathodischer Schutz) ermöglicht die Wasserstoffaufnahme in den Stahl. Der im Gitter gelöste oder an Wasserstoffhaftstellen (Versetzungen, Grenzflächen, Poren, etc.) getrappte Wasserstoff diffundiert aufgrund von Konzentrationsgradienten oder getrieben durch Spannungs- bzw. Dehnungsgradienten durch das vorliegende Gefüge, wo er in Kombination mit einwirkenden Beanspruchungen (äußere Last oder Eigenspannungen) eine lokale, signifikante Degradation der mechanisch-technologischen Eigenschaften bewirken kann.
Die dazu entwickelten und allgemein anerkannten Schädigungsmodelle gehen unter anderem von einem Einfluss des Wasserstoffs auf die Versetzungsentstehung und Versetzungsbeweglichkeit aus. Des Weiteren wird angenommen, dass Wasserstoff nicht nur durch Diffusion im Gitter transportiert wird, sondern auch an Versetzungen angehaftet ist und sich mit diesen im Falle plastischer Verformung mitbewegt.
Wasserstoff hat im Vergleich zu den meisten üblichen Legierungselementen von Stahl (Eisen, Chrom, Kohlenstoff) einen großen Wechselwirkungsquerschnitt für kalte und thermische Neutronen, was Neutronenradiographie und -tomographie zu geeigneten bildgebenden Verfahren zur lokalen Detektion von Wasserstoffakkumulationen in Stahl macht.
Elektrochemisch mit Wasserstoff beladene Zugproben aus supermartensitischem Stahl wurden vor und nach dem Zugversuch an der ANTARES beamline am FRM II polychromatisch radiographiert bzw. tomographiert. Die Bruchoberfläche der Probe wurde zusätzlich rasterelektronenmikroskopisch charakterisiert. Die Fraktographien der Bruchoberfläche in Verbindung mit den durch die Tomographie gewonnenen Informationen zu Wasserstoffansammlungen zeigen, dass nach dem Bruch auch in duktilen Versagensbereichen untypischerweise vermehrt Wasserstoff zu finden ist.
A novel approach for the reconstruction of an equivalent volumetric heat source from a known weld pool shape is proposed. It is based on previously obtained weld pool geometries from a steady-state thermo-fluid dynamics simulation. Hereby the weld pool dimensions are obtained under consideration of the most crucial physical phenomena, such as phase transformations, thermo-capillary convection, natural convection and temperature-dependent material properties. The algorithm provides a time and calibration efficient way for the reproduction of the weld pool shape by local Lamé curves. By adjusting their parameters, the identification of the finite elements located within the weld pool is enabled. The heat input due to the equivalent heat source is assured by replacing the detected nodes’ temperature by the melting temperature.
The model offers variable parameters making it flexible and adaptable for a wide range of workpiece thicknesses and materials and allows for the investigation of transient thermal effects, e.g. the cooling stage of the workpiece. The calculation times remain acceptably short especially when compared to a fully coupled process simulation. The computational results are in good agreement with performed complete-penetration laser beam welding experiments.
A novel approach for the reconstruction of an equivalent volumetric heat source from a known weld pool shape is proposed. It is based on previously obtained weld pool geometries from a steady-state thermo-fluid dynamics simulation. Hereby the weld pool dimensions are obtained under consideration of the most crucial physical phenomena, such as phase transformations, thermo-capillary convection, natural convection and temperature-dependent material properties. The algorithm provides a time and calibration efficient way for the reproduction of the weld pool shape by local Lamé curves. By adjusting their parameters, the identification of the finite elements located within the weld pool is enabled. The heat input due to the equivalent heat source is assured by replacing the detected nodes’ temperature by the melting temperature.
The model offers variable parameters making it flexible and adaptable for a wide range of workpiece thicknesses and materials and allows for the investigation of transient thermal effects, e.g. the cooling stage of the workpiece. The calculation times remain acceptably short especially when compared to a fully coupled process simulation. The computational results are in good agreement with performed complete-penetration laser beam welding experiments.
Messing unter Druck
(2019)
The weld pool dynamics and shape play a fundamental role in keyhole mode welding. The presented work aims the experimental and numerical investigation of the influence of the weld pool characteristics on the formation of hot cracking. The experimental procedure allows recording the molten pool in the longitudinal section of a butt joint configuration of 15 mm thick structural steel and transparent quartz glass by using a high-speed video camera and two thermal imaging cameras. The observations show that the dimensions of the weld pool vary depending on the depth. The regions close to the surface form a teardrop-shaped weld pool. A bulge-region and its temporal evolution are observed approximately in the middle of the depth of the weld pool, where hot cracking appears. A numerical framework including models for the weld pool dynamics, global temperature field, transient stress state, crystal growth, diffusion and macro-segregation and subroutines for their one-way couplings is developed. The numerically obtained and experimentally observed results are in a good agreement. It is shown that the bulge-region leads to a delay in the solidification behavior, increased temporal tensile stresses and accumulation of impurities in the defect region and hence enhance the probability of hot cracking formation.
The geometrical characteristics of the weld end crater are commonly used for the validation of numerical results in welding simulation. A semi-analytical model calculating the cooling stage of the welding process after the moving energy source is turned off has been developed. A solution for various combinations of heat sources and workpieces has been found. The theoretical limits for the heat transfer of the absorbed energy during cooling in a thin plate and a semi-infinite body were studied. It is shown that after turning off the energy source, an additional melting of the base material in longitudinal direction may occur. The developed technique is applied to complete-penetration keyhole laser beam welding of a 2 mm thick austenitic chromium-nickel 316L steel plate at a welding speed of 20 mm/s and a laser power of 2.3 kW. The results show a theoretical increase of the weld end crater length in comparison to the length of the steady-state weld pool of up to 19 %. A shift of the centre of the end crater, in which the solidification of the liquid metal ends, towards the tail of the end crater relative to the axis of the heat source at the time of its termination, was computed. The speed and the direction of crystallization of the molten material in the weld pool and the end crater were found to be different. A good agreement between the computational results and the welding experiments was achieved.
The characteristics of TiO2 coatings can greatly influence their final performance in large-scale applications. In the present study self-assembly of TiO2 nanoparticles (NPs) in multiple layers was selected as a deposition procedure on various substrates. For this, the main prerequisite constitutes the surface modification of both NPs and substrate with e.g. silane coupling agents. A set of functionalized TiO2 NPs has been produced by reaction with either (3- aminopropyl)triethoxysilane (APTES) or (3-aminopropyl)phosphonic acid (APPA) to functionalize the NP surface with free amino-groups. Then, the complementary functionalized NP set can be obtained from an aliquot of the first one, through the conversion of free surface amino-groups to aldehydes by reaction with glutaraldehyde (GA). Several types of TiO2 NPs differing in size, shape and specific surface area have been functionalized. FTIR, TGA, SEM/EDS, XPS, Auger electron spectroscopy (AES) and ToF-SIMS analyses have been carried out to evaluate the degree of functionalization, all the analytical methods employed demonstrating successful functionalization of TiO2 NP surface with APTES or APPA and GA.
Susceptibility of 304 stainless steel to crevice corrosion in electrochemically active fluids
(2019)
The susceptibility of AISI 304 stainless steel to crevice corrosion on the effect of contact with electrochemically active fluids was investigated using exposure and stepwise potentiostatic polarisation. Crevice materials made up of 304 SS and Polyether ether ketone (PEEK) forming two kinds of crevices including 304 SS-to-PEEK and 304 SS-to-304 SS were tested.
Susceptibility of 304 stainless steel to crevice corrosion in electrochemically active fluids
(2019)
The susceptibility of AISI 304 stainless steel to crevice corrosion on the effect of contact with electrochemically active fluids was investigated using exposure and stepwise potentiostatic polarisation. Crevice materials made up of 304 SS and Polyether ether ketone (PEEK) forming two kinds of crevices including 304 SS-to-PEEK and 304 SS-to-304 SS were tested.
New reference material candidates for traceable size measurement of non-spherical nanoparticles
(2019)
New model nanoparticles with well-controlled shape were synthesized within the EMPIR project nPSize - Improved traceability chain of nanoparticle size measurements. Their systematic characterization takes place by the traceable methods scanning/transmission electron microscopy, atomic force microscopy and small angle X-ray scattering. Following reference nanoparticle candidates are under investigation with respect to their homogeneity and stability: titania nanoplatelets (10-15 nm x 50-100 nm), titania bipyramides (~60 nm x 40 nm), titania acicular particles (100 nm x 15-20 nm; aspect ratio 5.5/6), gold nanorods (~10 nm x 30 nm), and gold nanocubes (~55 nm x 55 nm x 55 nm).
Susceptibility of 304 stainless steel to crevice corrosion in electrochemically active fluids
(2019)
The susceptibility of AISI 304 stainless steel to crevice corrosion on the effect of contact with electrochemically active fluids was investigated using exposure and stepwise potentiostatic polarisation. Crevice materials made up of 304 SS and Polyether ether ketone (PEEK) forming two kinds of crevices including 304 SS-to-PEEK and 304 SS-to-304 SS were tested.
Two epoxy-based nanocomposites with different nanofillers (layered double hydroxide and boehmite) were investigated employing temperature modulated DSC, flash DSC and broadband dielectric spectroscopy. Detailed investigation on the molecular mobility of the two systems showed the effect of the fillers on the structure of the bulk epoxy matrix and the interface formed at the polymer/particle interface
Based on the three-phase model of semi-crystalline polymers, we determined all phase fractions of the NiAl-LDH/PLLA nanocomposites in dependence on the concentration of the nanofiller. Moreover, the rigid amorphous fraction (RAF) was separated into the RAFcrystal and the RAFfiller unbiasedly. A detailed comparison to the related nanocomposite system MgAl-LDH/PLLA was made considering that Mg and Ni have different atomic weights. As a first result is was found that NiAl-LDH/PLLA displays a higher crystallization rate compared to MgAl-LDH/PLLA, which is related to the different morphologies of the two nanocomposite systems. For both systems RAFcrystal increases with increasing concentration of the nanofiller. This means in the case of the nanocomposite not each crystal produces the same amount of RAF, as often assumed. Also, RAFfiller increases with the concentration for both systems but in a different way. This is discussed considering again the different morphologies of both nanocomposites.
For a better understanding of the damage of glued wood tensile tests on miniature specimens made of glued spruce wood with acoustic emission and in-situ synchrotron-based X-ray computer microtomography has been monitored. As adhesive, urea-formaldehyde resin was used. For comparison purposes, tensile tests were carried out on solid wood and bonded miniature tensile shear samples with acoustic emission. The acoustic emission signals of all experiments were with classified pattern recognition. This resulted in two classes of signals for each two frequency peaks. In one class was the low-frequency, in the other the higher frequency peak of higher intensity, but this essentially independent from the structure (solid wood or plywood) and the size scale of the test specimens. The influence of the adhesive layers was determined on plywood test specimens in laboratory scale and on miniature test specimens with an adhesive layer and selected fiber orientations examined. This gave evidence that the sound emission signals from the failure of the adhesive layer presumably of the class with low frequency signals peak in the range of services can be assigned.
Nach einer Übersicht zu den immer schneller aktualisierenden Rahmenbedingungen von Politik und Gesellschaft folgt eine Übersicht zu materialwissenschaftlichen Problemen des Recyclings von Kunststoffen. Lösungsansätze aus der Forschung reichen von einfacher Optimierung bis hin zur radikalen Neukonstruktion der polymeren Werkstoffe. Aus dem bereits möglichen Ansatz "performance-by-design" wird ein neuer Weg des "recycling-by-design" adressiert. Dies inkludiert methodisch eine skalenübergreifende Modellierung und die Depolarisation bis zum Monomer.
CEQAT-DGHS Interlaboratory Test Programme for Chemical Safety - Need of Test Methods Validation -
(2019)
Safety experts, manufacturers, suppliers, importers, employers or consumers must be able to rely on the validity of safety-related test methods and on correct test results and assessments in the laboratory. Via the eChemPortal lots of data from the REACH registration dossiers are available. However, the quality and correctness of the information remains in the responsibility of the data submitter. Unfortunately, we found more or less appropriate information on physicochemical properties and concluded that more quality or adequacy of any data submitted will be needed.
Interlaboratory tests play a decisive role in assessing the reliability of test results. Interlaboratory tests on different test methods have been performed by Bundesanstalt für Materialforschung und –prüfung (BAM) and Physikalisch-Technische Bundesanstalt (PTB) in collaboration with the QuoData GmbH during the last 10 years. Significant differences between the results of the participating laboratories were observed in all interlaboratory tests. The deviations of the test results were not caused only by laboratory faults but also by deficiencies of the test method.
In view of the interlaboratory test results the following conclusions can be drawn:
• To avoid any discrepancy on classification and labelling of chemicals it should become state of the art to use validated test methods and the results accompanied by the measurement uncertainty.
• A need for improvement is demonstrated for all examined test methods. Thus, interlaboratory tests shall initially aim at the development, improvement and validation of the test methods and not on proficiency tests.
• The laboratory management and the practical execution of the tests need to be improved in many laboratories.
• The term "experience of the examiner" must be seen critically: A "long experience with many tests" is not necessarily a guarantee for correct results.
In Vivo Biotransformations of Indium Phosphide Quantum Dots Revealed by X‑Ray Microspectroscopy
(2019)
Many attempts have been made to synthesize cadmium-free quantum dots
(QDs), using nontoxic materials, while preserving their unique optical properties. Despite impressive advances, gaps in knowledge of their intracellular fate, persistence, and excretion from the targeted cell or organism still exist, precluding clinical applications. In this study, we used a simple model organism (Hydra vulgaris) presenting a tissue grade of organization to determine the biodistribution of indium phosphide (InP)-based QDs by X-ray fluorescence imaging. By complementing elemental imaging with In L-edge X-ray absorption near edge structure, unique information on in situ chemical speciation was obtained. Unexpectedly, spectral profiles indicated the appearance of In−O species within the first hour post-treatment, suggesting a fast degradation of the InP QD core in vivo, induced mainly by carboxylate groups. Moreover, no significant difference in the behavior of bare core QDs and QDs capped with an inorganic Zn(Se,S) gradient shell was observed. The results paralleled those achieved by treating animals with an equivalent dose of indium salts, confirming the preferred bonding type of In3+ ions in Hydra tissues. In conclusion, by focusing on the chemical identity of indium along a 48 h long journey of QDs in Hydra, we describe a fast degradation process, in the absence of evident toxicity. These data pave the way to new paradigms to be considered in the biocompatibility assessment of QD-based biomedical applications, with greater emphasis on the dynamics of in vivo biotransformations, and suggest strategies to drive the design of future applied materials for nanotechnology-based diagnosis and therapeutics.
Semiconductor nanocrystals (quantum dots, QDs) are well known for their superior photophysical properties and enabled advancements in several key technologies of the 21st century and numerous technological applications. However, the most studied II-VI semiconductor nanocrystals contain the toxic heavy metal element cadmium, which is limiting their utilization in commercial applications. This has drawn the interest to alternative materials with less toxicity but having similar photophysical features.
The newest generation of TV screens based on QDs have shown that there is a promising environmentally friendly alternative with similar optoelectronic properties, namely indium phosphide (InP) QDs. InP QDs possess a bulk band gap of 1.35 eV with an exciton Bohr radius of ca. 10 nm and thus allow to tune their photoluminescence (PL) from the visible to the near-infrared. Tuning the size and shape of InP QDs and thus tailor their optoelectronic properties can be achieved by different strategies, which range from different types and concentrations of precursors, synthesis temperature or post-synthetic manipulations like etching. The incorporation of other elements like Gallium within the InP core synthesis is another possibility. Using a GaP intermediate layer before growing a ZnS shell has been shown to increase the PL quantum yield, which has been attributed to reduced lattice strain and the removal of phosphor vacancies. Different Ga precursors were investigated but a thorough investigation in terms of their reactivity, localization in the QD and influence on the photophysical properties is lacking to date.
In this contribution we will present the detailed investigation of the presence of two different Ga precursors within the InP core synthesis. Photophysical characterizations (steady-state and PL life-time measurements), transmission electron microscopy, XRD and EDX gave insights into the reactivity of the Ga precursors, the Ga localization in the InP core and influences on the photophysical properties. The variation of the precursor and surfactant concentration and the utilization of different ligands for the Ga precursor allowed tuning the PL emission towards the blue or the red. Depending on the used precursor type we observed the formation of larger-sized InP/GaP core/shell nanocrystals or the formation of InGaP alloy structures enabling to assess the blue range of emission (475 nm).
Bei der Marktüberwachung im Bereich Explosivstoffe und Pyrotechnik stehen die Behörden der Länder immer wieder vor dem Problem, mit ausreichenden Kenntnissen über die Vielfalt der Produkte ausgestattet zu sein. Darüber hinaus lassen sich nur in seltenen Fällen im Rahmen der Marktüberwachung Prüfungen mit der regulären Funktion des Explosivstoffs oder der Pyrotechnik durchführen. Dadurch konzentriert sich die Überwachung vor allem auf äußere Merkmale und die Kennzeichnung. In dem Vortrag werden anhand von Beispielen die technischen und rechtlichen Hintergründe für entsprechende Kontrollen dargestellt und mit den Erfahrungen der an der Marktüberwachung Beteiligten abgeglichen.
The present study deals with the experimental characterization of short crack propagation in SLM (selective-laser-melting) manufactured stainless steel. More specifically, the determination of cyclic R-curves is discussed. This describes the dependency of the crack propagation threshold on crack growth during the short crack propagation stage. For metals, the threshold, starting at a material-intrinsic value, increases until it reaches a value independent of the crack length due to crack closure phenomena which build up at that stage. The cyclic R-curve, when used in the frame of a cyclic R curve analysis, characterizes the resistance of a material to fatigue crack growth and the ability to arrest a physically short crack. Thus, it is the link between classical fatigue and fracture mechanics. In the high-cycle-fatigue range, the short crack propagation stage dominates the overall lifetime, i.e., the number of cycles until failure. Below the fatigue limit crack arrest of hitherto propagable micro-cracks will occur. The effort for the experimental characterization of the short fatigue crack propagation behavior and the cyclic R-curve is very high compared to experiments on long crack propagation. A very exact measurement of crack extension is required, since small increments need to be depicted. Pre-cracking must leave a closure free initial crack, since closure must be build up only by the cyclic R-curve. The closure-free status is achieved by compression pre-cracking. The aim of the present study is an insight into the influence of an AM process on the short crack propagation threshold. Cyclic R-curves are experimentally determined at different load-ratios for 316L austenitic steel specimens produced by SLM and conventional manufacturing. Residual stresses are measured in the crack plane and their influence on the cyclic R-curve is discussed.
Residual stresses count among the most limiting factors in the application of additively manufactured materials in safety relevant components subject to cyclic loading. The source of such stresses is inherent in the manufacturing Laser Beam Melted (LBM) process due to rapid cooling and solidification and their distribution in the specimen or component is not homogeneous, but it is usually characterized by high gradients. Moreover, the magnitude of the residual stress field depends very much on the orientation, being higher in the build direction.
Many works in the literature advise to carry out a post-LBM treatment to relieve residual stresses, even though particular attention should be paid in choosing the parameters for the thermal treatment in order to avoid microstructural/phase transformations, which would affect greatly the mechanical properties of the material.
Particularly in the case of the austenitic stainless steel 316L, it has been shown that annealing at high temperatures (above 900°C), besides relieving the residual stresses, may cause recrystallization, grain growth and even phase transformation. In contrast, if a too low annealing temperature is chosen in order to preserve the microstructure, the residual stresses cannot be completely relieved.
This work aims to address the effect of residual stresses on short and long crack propagation for SEN(B) specimens made of 316L fabricated by LBM, in which notches have been machined by electro-discharge machining (EDM) on the mid-plane, perpendicular to the build direction. The specimens underwent different annealing treatments in inert atmosphere, in which the maximum temperature has been varied up to 900°C. The amount of residual stresses and their distribution has been measured by X-ray and neutron diffraction and the specimens have been subject to cyclic loading in a resonant testing machine. The tests show a massive influence of the residual stresses in the build direction on the resistance to fatigue crack propagation of additively manufactured 316L. Finally, a comparison with conventionally manufactured 316L is presented.
The present study deals with the experimental characterization of short crack propagation in SLM (selective-laser-melting) manufactured stainless steel. More specifically, the determination of cyclic R-curves is discussed. This describes the dependency of the crack propagation threshold on crack growth during the short crack propagation stage. For metals, the threshold, starting at a material-intrinsic value, increases until it reaches a value independent of the crack length due to crack closure phenomena which build up at that stage. The cyclic R-curve, when used in the frame of a cyclic R curve analysis, characterizes the resistance of a material to fatigue crack growth and the ability to arrest a physically short crack. Thus, it is the link between classical fatigue and fracture mechanics. In the high-cycle-fatigue range, the short crack propagation stage dominates the overall lifetime, i.e., the number of cycles until failure. Below the fatigue limit crack arrest of hitherto propagable micro-cracks will occur. The effort for the experimental characterization of the short fatigue crack propagation behavior and the cyclic R-curve is very high compared to experiments on long crack propagation. A very exact measurement of crack extension is required, since small increments need to be depicted. Pre-cracking must leave a closure free initial crack, since closure must be build up only by the cyclic R-curve. The closure-free status is achieved by compression pre-cracking. The aim of the present study is an insight into the influence of an AM process on the short crack propagation threshold. Cyclic R-curves are experimentally determined at different load-ratios for 316L austenitic steel specimens produced by SLM and conventional manufacturing. Residual stresses are measured in the crack plane and their influence on the cyclic R-curve is discussed.
The assessment of high cycle fatigue in additive manufactured (AM) components is a challenge due to complex microstructure, anisotropic material behavior, residual stresses and porosity / lack-of-fusion defects. Due to the statistical distribution of defects, a high scatter band of S-N-curves is expected. The fracture mechanics-based fatigue assessment of additive manufactured components must consider the propagation of short cracks emanating from defects.
In this work, the fatigue crack propagation resistance in the short and large crack regimes of additive and conventionally manufactured AISI 316L stainless steel is examined experimentally based on the cyclic R-curve. However, remaining residual stresses in the AM specimen lead to unexpected and dramatic crack-growth during the pre-cracking procedure.
Microbial modelling of sulfate-reducing bacteria (SRB) and methanogenic archaea (ME) using iron
(2019)
Sulfate reducing bacteria (SRB) and methanogenic archaea (MA) are commonly found in the oil and gas environments. The formation of hydrogen sulfide (HS-) is particularly concerning for the petroleum industry due to its corrosiveness. However, the activities of SRB are limited to the concentration of sulfate present in the environment, whereas methanogens can utilize substrates such as H¬2 for methanogenesis. MA is commonly found in sulfate-free environments, such as deep sediments, and are known to form interspecies electron transfer relationships with SRB. Recently, SRB and MA capable of microbiologically influenced corrosion (MIC) by using elemental iron as a direct electron source (EMIC) have gained increased attention. On the iron surface, EMIC-SRB can outcompete EMIC-MA in the presence of sulfate, but this changes as sulfate depletes. The formation of FeS on the metal surface can be further utilized by MA for methanogenesis as it provides a conductive path. However, the possible kinetics involved of the overall process are currently unknown.
We obtained a co-culture of EMIC-SRB and EMIC-MA to investigate the growth rates and electrical potential changes under different environmental conditions, including changes in pH, temperature and salinity. Results indicate that under neutral conditions and using iron as the sole substrate, methane production (up to 5 mM) starts after sulfate was depleted. Electrochemical measurements will be conducted on the co-culture under different conditions to determine the changes in the electrical potential in correlation with the sulfate and methane concentration. Fluorescence and electron microscope images of the biofilm structure will be used to visualize cell distribution and morphology. This study embarks the first step of understanding the relationship between EMIC-SRB and EMIC-MA. Such knowledge is important for the field of microbial electrophysiology and can be further explored for industrial applications.
With the goal to improve their photostability, InP-based QDs are passivated with three types of inorganic shells, namely (i) a gradient ZnSexS1−x shell, (ii) an additional ZnS shell on top of the gradient shell with two different thicknesses (core/shell/shell, CSS), (iii) an alumina coating on top of ZnS. All three systems have photoluminescence Quantum yields (PLQY) > 50%and similar PL decay times (64–67 ns). To assess their photostability they are incorporated into a transparent poly (methyl methacrylate) (PMMA) matrix and exposed to continuous irradiation with simulated sunlight in a climate chamber. The alumina coated core/shell system exhibits the highest stability in terms of PLQY Retention as well as the lowest shift of the PL maximum and lowest increase of the PL linewidth, followed by the CSS QDs and finally the gradient shell system. By means of XPS studies we identify the degradation of the ZnS outer layer and concomitant xidation of the emissive InZnP core as the main origins of degradation in the gradient structure. These modifications do not occur in the case of the alumina-capped sample, which exhibits excellent chemical stability. The gradient shell and CSS systems could be transferred to the aqueous phase using surface ligand exchange with penicillamine. Cytotoxicity studies on human primary keratinocytes revealed that exposure for 24 h to 6.25–100 nM of QDs did not affect cell viability. However, a trend toward reduced cell proliferation is observed for higher concentrations of gradient shell and CSS QDs with a thin ZnS shell, while CSS QDs with a thicker ZnS shell do not exhibit any impact.
Ambient water influences sub-critical crack growth (SCCG) from microscopic surface flaws, leading to stress corrosion at the crack tip. The complex influence of humidity accelerating slow crack propagation (region I) is well studied only for dry commercial NCS glass (< 1000 ppm water). To shed light on this influence, the effect of water is mimicked by studying SCCG water-bearing glasses. For this purpose, water-bearing silicate glasses of 8 wt% total water were synthesized at 0.5 GPa and compared to dry glasses. SCCG was measured in double cantilever beam geometry. For dry glasses, 3 trends in crack velocity vs. stress intensity, KI, curve were found. The slope in region I increases in the order NCS < NBS < BaCS < NZnS < NAS glass. The velocity range of region II, reflecting the transition between corrosion affected and inert crack growth (region III), varies within one order of magnitude among these glasses. The KI region of inert crack growth strongly scatters between 0.4 and 0.9 MPam0.5. For hydrous glasses, it is found that water strongly decreases Tg, form a new sub-Tg internal friction peak caused by molecular water, and makes the glasses more prone to SCCG. The observed trends will be discussed in terms of the effects of Youngs Modulus on the strain energy release rate and energy dissipation related to mechanical glass relaxation phenomena.
The particle size distribution is considered the most relevant information for nanoscale property identification and material characterization. The current OECD test guideline on particle size and size distribution (TG 110) is not applicable to ‘nano-sized’ objects. In this project we thus develop a new OECD test guideline for the measurement of the size and size distribution of particles and fibers with at least one dimension in the nanoscale. A fiber is defined as an object having an aspect ratio of length/diameter l/d >3. The width and length of each fiber should be measured concurrently.
In order to measure the particle size distributions, many techniques are available. 9 methods for particles and 2 methods for fibres have been tested in a prevalidation study and appropriate methods will be compared in an interlaboratory round robin test starting in February 2019.
Predictive Methods for Determining the Thermal Decomposition Properties of Hazardous Substances
(2019)
Due to the fast development and availability of computers, predictive approaches are increasingly used in the evaluation process of hazardous substances complementary to experiments. Their use was recommended as alternative to experimental testing by the REACH regulation to complete the lack of knowledge on properties for existing substances that must be registered before 2018 (upon quantities). Among the proposed predictive approaches, Quantitative Structure Property Relationships (QSPR) are powerful methods to predict macroscopic properties from the only molecular structure of substances.
In that context, the HAZPRED project (2015-2018, founded by the SAF€RA consortium) aims to develop theoretical models (e.g. QSPR) and small-scale tests to predict complex physico-chemical properties (e.g. thermal stability, explosivity) of hazardous substances to complete the lack of knowledge on these hazardous substances quickly or to understand their decomposition behaviour better.
In particular, this contribution will present the work done in this project on the physical hazards of organic peroxides and self-reactive substances: gathering of existing experimental data, new experimental campaigns, review of existing models and proposition of new estimation methods.
The properties of nanomaterials are influenced not only by their chemical composition but also by physical properties (such as size, geometry and crystal structure). For the reliable determination and assessment of behaviour and effects of nanomaterials as well as for the determination of the exposure of humans and environment a comprehensive physical-chemical characterization of nanomaterials is essential. This is an important prerequisite to identify them as nanomaterials and to interpret and compare test results and - in future – to forecast interaction and effects of nanomaterials.
In 2006, the OECD launched a sponsorship program for the testing of nanomaterials in which 11 nanomaterials were thoroughly investigated using a variety of methods. The aim of the project was, among other things, to find out where problems occur and where there are gaps in the measurement and test procedures and where are changes required. An important outcome of the sponsorship program was the finding that the OECD Test Guidelines should in several cases be extended to the specific needs in testing of nanomaterials. The existing standardized test methods of the OECD for physical-chemical characterization have not been developed for nanomaterials in particular. A high demand for an extension of the test guidelines was identified. Germany complied with the OECD's request in 2017 and has agreed to extend the “Test Guideline on Particle Size Distribution / Fiber Length and Diameter Distributions Test Guideline” for Manufactured Nanomaterials (MN). UBA commissioned BAM and BAuA with the preparation of the Test Guideline. The aim of the project is the development of a harmonized test protocol for a valid and reproducible determination of particle size and size distribution which is one of the most relevant physical-chemical properties for MNs.
Different measuring methods provide different results for the size distribution of the particles. This is caused by the different measuring principles of the methods. Each method measures a specific parameter that ultimately determines particle size. First, the measured quantity differs for each method (Scattered light intensity, 2D image / projection, electric mobility, etc.). Second, the calculated diameters of the MN may differ (Feret Diameter, Area Projection, Mobility Diameter, Aerodynamic Diameter, Hydrodynamic Diameter). Third, a measuring method provides a size distribution which is measured either mass-based, surface-based or number-based. A conversion between the results requires additional parameters and thus possibly increases the measurement error.
In addition to the technical differences, the individual parameters are strongly influenced by the structure and material of the nanoparticles. For example, a surface functionalization can lead to very different results in the size distribution. The suitability of measurement methods differs with the material of the MN. As a result, two very different results can be measured for the particle size distribution using two different methods, which are nevertheless both correct. Several large projects in recent years therefore concluded that nanomaterials should be characterized by at least two complementary method. Imaging techniques are regarded as one of these methods for the characterization, the complementary methods are supposed to be statistical methods.
The different results for the size distribution of nanomaterials become problematic for the registration of new MN. A comparable and reproducible size distribution is a prerequisite for a standardized registration. In the future, the particle size distribution in the EU will also decide on the classification of a substance as a nanomaterial or as a non-nanomaterial. Especially in borderline cases, a standardized and comparable measurement methodology is therefore essential.
The new OECD test guideline will address the following four main steps in the determination of the length and width distributions of fibers: sample preparation, image acquisition, data evaluation and uncertainty analysis. As the sample preparation has to be optimized for each material, general quality criteria will be given in the protocol. For full visibility of a fiber the appropriate resolution has to be chosen. In the data evaluation the length and diameter of each fiber will be determined concurrently to allow for application of different regulatory definitions. The quality of the results critically depends on the sample preparation as well as the data evaluation. In this step the classification rules have to be formulated and followed accurately in order to optimize reproducibility of the method. The SOP will be validated in an international round robin test, which is planned for 2018/2019.
Die Entstehung von Erstarrungsrissen stellt ein in der Schweißtechnik lange bekanntes Problem dar. Diese prozessseitige Herausforderung ist selbst nach dem heutigen Wissenstand schwer zu beherrschen und gefährdet die Sicherheit der gefügten Bauteile. Dieses Problem erweist sich als besonders schwierig beim einlagigen Hochleistungslaserstrahlschweißen von Metallteilen hoher Blechstärke, welche eine essentielle Rolle in vielerlei industriellen Anwendungen spielen.
Gegenstand des hier bearbeiteten Forschungsprojekts war die numerische Untersuchung der Entstehungsmechanismen des Mittelrippendefekts beim Laserstrahlschweißen niedriglegierter Stähle. Mittels mehrerer gekoppelter Simulationen, für welche sowohl kommerziellen Softwares, wie COMSOL Multiphysics und ANSYS, als auch in-house entwickelte Programme eingesetzt worden sind, wurden die am stärksten ausgeprägten Mechanismen der Erstarrungsrissbildung und deren Zusammenspiel aufgeklärt. Begleitend zur Simulation wurden mehrere experimentelle Untersuchungen für den niedriglegierten Baustahl S355 an den Blechdicken 12 mm und 15 mm zu deren Validierung durchgeführt. Die Ergebnisse der Simulation des Kristallwachstums, der Diffusion und der Makroseigerung von Schwefel zeigen, dass die Risse für bestimmte Werkstoffe mit einer hohen Konzentration an Verunreinigungen
hauptsächlich metallurgisch bedingt entstehen können. Allerdings zeigte die Untersuchung des Einflusses der Ausbauchung (Bulging) im Mittenbereich der Schweißnaht auf das transiente Temperaturfeld und die transienten Transversalspannungen im risskritischen Bereich auch, dass die Rissbildung nicht nur metallurgisch bedingt und insbesondere auch für aus metallurgischer Sicht eigentlich rissunkritische Materialien auftreten kann.
Der Bulging-Bereich hat sich in der Untersuchung als besonders kritisch in allen Teilen der Simulationskette gezeigt. Die Einflüsse dieser geometrischen Besonderheit konnten bei allen physikalischen Teilaspekten der Problematik als heißrissinduzierend identifiziert werden. Von besonderer Bedeutung wird an dieser Stelle das erkannte Zusammenspiel dieser Einflüsse hervorgehoben. Bisher wurde die Bildung der Erstarrungsrisse hauptsächlich durch einen einzelnen physikalischen Aspekt, wie z.B. rein metallurgisch oder rein thermomechanisch bedingt, isoliert betrachtet. Die gewonnen Erkenntnisse über die Einflüsse des Bulgings auf die unterschiedlichen Teilaspekte zeigen deutlich, dass alle Entstehungsmechanismen auch die Schmelzbadgeometrie als Ursprung haben. Von dieser hängen die transienten Temperatur- und Spannungsfelder, die Kristallwachsrichtung, die Erstarrungsgeschwindigkeit und die Konzentration von Verunreinigungen, welche zur Entstehung von niedrigschmelzenden Phasen führt, ab.
Die Ergebnisse des Forschungsvorhabens sind vielfach in referierten Zeitschriften und auf Fachtagungen veröffentlicht worden. Hervorzuheben dabei ist der „Best Presentation Award“ auf der LANE Conference 2018 in Fürth, wodurch die Thematik einem breiten Publikum bekannt gemacht werden konnte.
Microbiologically influenced corrosion (MIC) of iron is usually attributed to sulfate-reducing microorganisms (SRM) in offshore industries, such as the oil and gas pipelines, due to the high concentrations of sulfate in the seawater. SRM act upon the metal by the reactiveness of hydrogen sulfide (HS-), and by withdrawal of the available electrons (Fe --> Fe2+ + 2e-; E° = -0.47 V) in electrical contact with the metal (EMIC). However, methanogenic archaea can also cause MIC. Because they do not produce HS-, withdrawal of electrons may be their main corrosive mechanism; however, mechanistic details and kinetics of the overall process are poorly understood.
To investigate the corrosion potential of methanogens, we studied the EMIC methanogenic strains isolated from marine sediments (Methanobacterium-affiliated strain IM1) and crude oil tanks (Methanococcus maripaludis Mic1c10), in an in-house developed flow-through cell to simulate a fluctuating environment. A co-culture of M. maripaludis and D. alaskensis was also established to study the effect of syntrophic growth on metal corrosion that may occur in industrial pipelines. Results indicate that the rates of iron corrosion due to coupled methanogenesis (up to 0.4 mm/yr) are higher to that caused by the marine SRM Desulfovibrio alaskensis (0.15 mm/yr). Surface analyses of the metal showed severe pitting with high methane production. Genomic analysis of the EMIC methanogen M. maripaludis Mic1c10 will provide an insight on the mechanisms of MIC. Such knowledge and deeper understanding also from an electrokinetic point of view may not only provide further models in microbial electrophysiology, but also contribute to mitigation strategies in MIC
Microbiologically influenced corrosion (MIC) of iron is usually attributed to sulfate-reducing microorganisms (SRM) in offshore industries, such as the oil and gas pipelines, due to the high concentrations of sulfate in the seawater. SRM act upon the metal by the reactiveness of hydrogen sulfide (HS-), and by withdrawal of the available electrons (Fe --> Fe2+ + 2e-; E° = -0.47 V) in electrical contact with the metal (EMIC). However, methanogenic archaea can also cause MIC. Because they do not produce HS-, withdrawal of electrons may be their main corrosive mechanism; however, mechanistic details and kinetics of the overall process are poorly understood.
To investigate the corrosion potential of methanogens, we studied the EMIC methanogenic strains isolated from marine sediments (Methanobacterium-affiliated strain IM1) and crude oil tanks (Methanococcus maripaludis Mic1c10), in an in-house developed flow-through cell to simulate a fluctuating environment. A co-culture of M. maripaludis and D. alaskensis was also established to study the effect of syntrophic growth on metal corrosion that may occur in industrial pipelines. Results indicate that the rates of iron corrosion due to coupled methanogenesis (up to 0.4 mm/yr) are higher to that caused by the marine SRM Desulfovibrio alaskensis (0.15 mm/yr). Surface analyses of the metal showed severe pitting with high methane production. Genomic analysis of the EMIC methanogen M. maripaludis Mic1c10 will provide an insight on the mechanisms of MIC. Such knowledge and deeper understanding also from an electrokinetic point of view may not only provide further models in microbial electrophysiology, but also contribute to mitigation strategies in MIC
In order to characterise the material and damage behaviour of additively manufactured polyamide 12 (PA12) under quasi-static load and to implement it in a numerical model, experiments under quasi-static load as well as microstructural investigations were carried out. Selective laser sintering (SLS) was used as the manufacturing process. For the classification of the material behaviour, quasi-static cyclic tests with holding times as well as tensile tests were performed. X-ray refraction and computed tomography (CT) were used to investigate the damage behaviour. The Chaboche model, which has already been applied for metallic materials under thermomechanical loading, served as the basis for the selection of the numerical material model. The same procedure was used for the selection of the damage model, where the Gurson–Tvergaard–Needleman (GTN) model was chosen, which was already used for porous metallic materials. The Chaboche model shows very good agreement with experimental results. Furthermore, the coupling with the GTN model allows a very good modelling of the damage behaviour. Finally, it could be shown that the selected models are suitable to simulate the material and damage behaviour of 3D printed PA12.
Virtual-lab-based determination of a macroscopic yield function for additively manufactured parts
(2019)
This work aims for an yield function description of additively manufactured (AM) parts of S316L steel at the continuum-mechanical macro-scale by means of so-called virtual experiments using a crystal plasticity (CP) model at meso-scale. Additively manufactured parts require the consideration of the specific process-related microstructure, which prevents this material to be macroscopically treated as isotropic, because of crystallographic as well as topological textures.
EBSD/CT-Scans from in-house additively manufactured specimen extract the unique microstructural topology which is converted to a representative volume element (RVE) with grain structure and crystal orientations.
Crystal plasticity model parameters on this RVE are calibrated and validated by means of mechanical testing under different texture angles. From virtual experiments on this RVE, yield loci under various loading conditions are simulated. The scale bridging from meso- to macro-scale is realised by the identification of the simulated yield loci as a modified anisotropic Barlat-type yield model representation.
The talk will give an overview of past, ongoing and future activities at BAM welding divisions, specifically those regarding prospects and challenges of the ongoing digital transformation and the move to more accessible research data (open science) with concerns to welding research. We will discuss current ways to publish and share research results inside the welding community and highlight approaches and advancements from other scientific fields to improve accessibility and reproducibility. We would also like to discuss the feasibility of integrating open science principles into the current IIW landscape of meetings, publications and education. In Addition we will introduce our upcoming series of workshops organized by BAM focusing on defining and implementing an open source file format specifically designed to publish and exchange high quality welding research data.
WelDX is a newly started research project at BAM publicly funded by the German Federal Ministry of Education and Research. Over the course of three years the project aims to build the foundation for a publicly accessible file format and to foster research collaboration in arc welding on a national and international level.
The talk will motivate benefits and discuss challenges of using a common file format designed to describe, store and share arc welding research data in the scope of "Open Science". By using common open source software and tools, welding data will be made more accessible and reusable so that new scientific practices may emerge.
The proposed file format aims to be easy to use for common welding applications while also offering the possibility to describe complex experiments for state of the art welding research.
In addition the talk will illustrate how in the future other facilities and researchers will be able to use experimental arc welding data generated at BAM for their own research, for example to conduct their own data analysis or welding process and thermo-mechanical simulations.
The WelDX research project aims to foster the exchange of scientific data inside the welding community by developing and establishing a new open source file format suitable for documentation of experimental welding data and upholding associated quality standards. In addition to fostering scientific collaboration inside the national and international welding community an associated advisory committee will be established to oversee the future development of the file format. The proposed file format will be developed with regards to current needs of the community regarding interoperability, data quality and performance and will be published under an appropriate open source license.
By using the file format objectivity, comparability and reproducibility across different institutes and experimental setups can be improved.
Ziel des WelDX Forschungsvorhabens ist es, den Austausch wissenschaftlicher Forschungsdaten aus dem Bereich der Schweißtechnik auf Basis eines einheitlichen Open Source Dateiformates zu ermöglichen und damit die Einhaltung und Dokumentation geeigneter Qualitätsstandards zu fördern. Im Zuge der Projektarbeit wird die Zusammenarbeit und der Wissenstransfer schweißtechnischer Institute zunächst auf nationaler Ebene gefördert und die Bildung eines von der Fachcommunity getragenen Kuratoriums zur Betreuung und Weiterentwicklung des Dateiformates und der Ausarbeitung zugehöriger Qualitätsstandards angestrebt.
Zunächst erfolgt die Erstellung und Veröffentlichung eines geeigneten Dateiformates zum Austausch großvolumiger, detaillierter Forschungsdaten im Bereich mechanisierter und automatisierter Schweißverfahren. Durch die Nutzung des Dateiformates zur Dokumentation der Forschungsarbeiten soll auch für komplexe Schweißaufgaben und Versuchsaufbauten eine deutlich gesteigerte objektive Vergleichbarkeit der Messdaten und Ergebnisse im Sinne der guten wissenschaftlichen Praxis zwischen den Instituten erzielt werden.
Laser powder bed fusion (L-PBF) is one of the most promising additive manufacturing (AM) technologies for the production of complex metallic real part components. Due to the multitude of factors influencing process conditions and part quality and due to the layer-wise characteristic of the process, monitoring of process signatures seems to be mandatory in case of the production of safety critical components. Here, the iterative process nature enables unique access for in-situ monitoring during part manufacture. In this talk, the successful test of the synchronous use of a high-frequency infrared camera and a camera for long time exposure, working in the visible spectrum (VIS) and equipped with a near infrared filter (NIR), will be introduced as a machine manufacturer independent thermal detection monitoring set-up. Thereby, the synchronous use of an infrared camera and a VIS NIR camera combines the advantages of high framerate and high spatial resolution. The manufacture of a 316L stainless steel specimen, containing purposely seeded defects and volumes with forced changes of energy inputs, was monitored during the build. The measured thermal responses are analysed and compared with a defect mapping obtained by micro X-ray computed tomography (CT).
The first results regarding methods for data analysis, derived correlations between measured signals and detected defects as well as sources of possible data misinterpretation are presented in this talk.
Um den wachsenden Anforderungen an den stofflichen und konstruktiven Leichtbau sowie den Forderungen nach Ressourceneffizienz Rechnung zu tragen, werden in vielen Industriebranchen zunehmend höherfeste Feinkornbaustähle mit Streckgrenzen über 690 MPa eingesetzt. Allerdings werden mit zunehmender Festigkeit deutlich höhere Anforderungen an deren schweißtechnische Verarbeitung gestellt, da die Sensibilität gegenüber einer Herabsetzung der mechanischen Eigenschaften durch den beim Schweißen aufgenommenen diffusiblen Wasserstoff mit steigender Festigkeit zunehmen kann. In den vergleichenden Untersuchungen von konventionellem Übergangslichtbogen und modifiziertem Sprühlichtbogen bei reduziertem Nahtöffnungswinkel konnte gezeigt werden, dass die Schweißprozessparameter die in das Schweißgut eingebrachte Wasserstoffkonzentration beeinflussen. Grundsätzlich ist den erarbeiteten Ergebnissen zu entnehmen, dass im Schweißgut von Stumpfstoßverbindungen mit reduziertem Nahtöffnungswinkel erhöhte mittlere Wasserstoffkonzentrationen vorliegen. Diese können mit geeigneten Wärmeführungen signifikant reduziert werden. Dabei erwies sich eine Nachwärmprozedur aus der Schweißwärme heraus als zielführend. Zudem konnte der Effekt der Nachwärmprozedur anhand fremdbeanspruchter Kaltrissprüfungen nachgewiesen werden.
In der aktuellen RL-SIB wie auch im Gelbdruck der Instandhaltungsrichtlinie ist bei Anwendung des Instandsetzungsprinzips W-Cl bzw. Verfahren 8.3 der Erfolg der Maßnahme durch Einbau und Messung von Korrosionsstrom-Mess-Einrichtungen nachzuweisen. Auch wird bei der Instandsetzung von chloridbelasteten Rissen bei Stahlbetonbauteilen durch Verpressen der Risse bzw. Aufbringen einer Rissbandage ein Korrosionsmonitoring empfohlen. Anerkannte Regelwerke oder Normen über Art und Aufbau des Monitoring-Systems sowie Kriterien zur Bewertung der Messergebnisse im Hinblick auf den Erfolg der Instandsetzungsmaßnahme gibt es derzeit nicht. Auch sind keine Untersuchungen zur Qualität der Messergebnisse veröffentlicht.
Ziel des Vorhabens ist die Ausarbeitung eines Regelwerks-Entwurfes für die Art und den Aufbau von Korrosionsmonitoring-Systemen sowie für Kriterien zur zuverlässigen Bewertung der Messergebnisse, mit denen der Erfolg bei verschiedenen Instandsetzungskonzepten überprüft werden kann.
The advantages of treating prefabricated components made of Ultra-High-Performance Concrete (UCPC) at 90 °C are largely recognized, while hydrothermal treatment at 185 °C and the corresponding saturation pressure of 1.1 MPa, in contrast, is not applied in building practice so far and was studied only by a few researchers.
In the research presented, the parameters pre-storage-time before treatment and dwell time in the autoclave were systematically varied. The results illustrate in which way compressive strength increases with the duration of treatment. Already samples which were only heated up and immediately cooled down show an increase in strength compared to the 28-day reference of about 10 %. After 20 h the maximum increase of about 25 % is reached. Interestingly the compressive strength of samples treated very long hardly decreases. It can be therefore concluded that for the composition investigated the achievable mechanical strength is not severely sensitive regarding the duration of the treatment.
Also, pre-storage times before autoclaving longer than 0.5 days showed no significant impact on strength development. The phase composition measured by XRD exhibits significant changes compared to the reference. The amount of ettringite and portlandite have vanished, the cement clinker phases are substantially decreased and hydroxylellestadite and hydrogarnet are formed as new phases. The increase in strength can be assigned to an improved pozzolanic and hydraulic reaction. Contradictory to literature, there was found no obermorite, a mineral which is known to be susceptible for disintegration at Long autoclaving duration. This explains why no significant strength loss appeared.
Alkali-silica reaction (ASR) is a serious Problem concerning the deterioration of concrete. This leads to significant maintenance and reconstruction costs for concrete infrastructures all over the world. The geographical regions, which play a major role in concrete deterioration, are for example coastal Areas or in general, areas with a high air humidity/salinity due to closeness of rivers or lakes.
These exemplary results show, the application of Raman spectroscopy establishes a valuable approach for characterizing the chemical and structural composition of ASR-products. The results prove the potential to trace the kind of reaction products, developing in dependence on the aggregate and to follow up its local alteration from origin of the gel.