Wissenschaftliche Artikel der BAM
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- SAXS (5)
- Small-angle X-ray scattering (4)
- Corrosion (3)
- DNA (3)
- Fatigue strength (3)
- Femtosecond laser (3)
- Fracture mechanics (3)
- Ground vibration (3)
- Low energy electrons (3)
- Multiple crack propagation (3)
We report on a study in which plasmid DNA in water was irradiated with 30 keV electrons generated by a scanning electron microscope and passed through a 100 nm thick Si3N4 membrane. The corresponding Monte Carlo simulations suggest that the kinetic energy spectrum of the electrons throughout the water is dominated by low energy electrons (<100 eV). The DNA radiation damage, single-strand breaks (SSB) and double-strand breaks (DSB), was determined by electrophoresis. The median lethal dose of D1/2 = 1.7 ± 0.3 Gy was found to be much smaller compared to partially or fully hydrated DNA irradiated under vacuum conditions. The ratio of DSB to SSB was found to be (1:12) as compared to 1:88) found for hydrated DNA. Our method enables quantitative measurements of radiation damage to biomolecules (DNA, proteins) in solutions under varying conditions (pH, salinity, cosolutes) for an electron energy range which is difficult to probe by standard methods.
We propose the use of alternating pulse wavelengths in a direct-detection coherent optical time domain reflectometry (C-OTDR) setup not only to measure strain and temperature changes but also to determine the correct algebraic sign of the change. The sign information is essential for the intended use in distributed mode shape analysis of civil engineering structures. Correlating relative backscatter signal shifts in the temporal/signal domain allows for measuring with correct magnitude and sign. This novel approach is simulated, experimentally implemented and demonstrated for temperature change measurement at a spatial resolution of 1 m.
Halophilic bacteria use a variety of osmoregulatory methods, such as the accumulation of one or more compatible solutes. The wide diversity of compounds that can act as compatible solute complicates the task of understanding the different strategies that halophilic bacteria use to cope with salt. This is specially challenging when attempting to go beyond the pathway that produces a certain compatible solute towards an understanding of how the metabolic network as a whole addresses the problem. Metabolic reconstruction based on genomic data together with Flux Balance Analysis (FBA) is a promising tool to gain insight into this problem. However, as more of these reconstructions become available, it becomes clear that processes predicted by genome annotation may not reflect the processes that are active in vivo. As a case in point, E. coli is unable to grow aerobically on citrate in spite of having all the necessary genes to do it. It has also been shown that the realization of this genetic potential into an actual capability to metabolize citrate is an extremely unlikely event under normal evolutionary conditions. Moreover, many marine bacteria seem to have the same pathways to metabolize glucose but each species uses a different one. In this work, a metabolic network inferred from genomic annotation of the halophilic bacterium Halomonas elongata and proteomic profiling experiments are used as a starting point to motivate targeted experiments in order to find out some of the defining features of the osmoregulatory strategies of this bacterium. This new information is then used to refine the network in order to describe the actual capabilities of H. elongata, rather than its genetic potential.
Reliable nanomaterial classification of powders using the volume-specific surface area method
(2017)
The volume-specific surface area (VSSA) of a particulate material is one of two apparently very different metrics recommended by the European Commission for a definition of "nanomaterial" for regulatory purposes: specifically, the VSSA metric may classify nanomaterials and non-nanomaterials differently than the median size in number metrics, depending on the chemical composition, size, polydispersity, shape, porosity, and aggregation of the particles in the powder.
Here we evaluate the extent of agreement between classification by electron microscopy (EM) and classification by VSSA on a large set of diverse particulate substances that represent all the anticipated challenges except mixtures of different substances. EM and VSSA are determined in multiple labs to assess also the level of reproducibility. Based on the results obtained on highly characterized benchmark materials from the
NanoDefine EU FP7 project, we derive a tiered screening strategy for the purpose of implementing the definition of nanomaterials. We finally apply the Screening strategy to further industrial materials, which were classified correctly and left only borderline cases for EM.
On platelet-shaped nanomaterials, VSSA is essential to prevent false-negative classification by EM. On porous materials, approaches involving extended Adsorption isotherms prevent false positive classification by VSSA. We find no false negatives by VSSA, neither in Tier 1 nor in Tier 2, despite real-world industrial polydispersity and diverse composition, shape, and coatings. The VSSA screening strategy is recommended for inclusion in a technical guidance for the implementation of the definition.
The work aims at addressing the modelling and implementation of criteria for multiple crack propagation, including interaction and coalescence, for a more reliable fracture mechanics-based prediction of stress-life curves for weldments.
A large experimental work is presented in which micro-cracks have been made visible by heat-tinting at successive stages of fatigue life of the welded specimens. Here the correlation between the number of initiation sites and the applied stress level has been also investigated.
The criteria have been implemented in in-house software, which allows multiple fatigue crack propagation, and validated against selected experimental tests. The results have shown that the modelling of multiple crack propagation and interaction is crucial for the prediction of the fatigue strength of weldments, both in finite and infinite life regime.
Cell-to-cell variation and specialization in sugar metabolism in clonal bacterial populations
(2017)
While we have good understanding of bacterial metabolism at the population level, we know little about the metabolic behavior of individual cells: do single cells in clonal populations sometimes specialize on different metabolic pathways? Such metabolic specialization could be driven by stochastic gene expression and could provide individual cells with growth benefits of specialization. We measured the degree of phenotypic specialization in two parallel metabolic pathways, the assimilation of glucose and arabinose. We grew Escherichia coli in chemostats, and used isotope-labeled sugars in combination with nanometer-scale secondary ion mass spectrometry and mathematical modeling to quantify sugar assimilation at the single-cell level. We found large variation in metabolic activities between single cells, both in absolute assimilation and in the degree to which individual cells specialize in the assimilation of different sugars. Analysis of transcriptional reporters indicated that this variation was at least partially based on cell-to-cell variation in gene expression. Metabolic differences between cells in clonal populations could potentially reduce metabolic incompatibilities between different pathways, and increase the rate at which parallel reactions can be performed.
Corrosion fatigue (CF) of duplex stainless steel (DSS) X2CrNiMoN22-5-3 was investigated in in-situ geothermal conditions of the Northern German Basin (NGB). The influence of the load type on CF behavior of the alloy was examined by applying push/pull and rotating bending cyclic load to failure. The hour-glass shaped specimens with both polished and precisely turned surface finish were tested in corrosion chambers allowing for the permanent circulation of Northern German Basin electrolyte heated to 369 K. Additionally, the influence of a protective cathodic potential on the CF life expectancy was investigated by employing a potentiostat. The rotating bending cyclic load led to a higher CF life expectancy, which decreased with increasing duration of the experiment due to a higher deterioration rate of the material. The application of a cathodic potential caused a significant increase of the CF lifespan from 4.7 x 105 (open-circuit potential (OCP)) to 107 cycles (preset threshold cycle number) for the potential range from USHE = – 450 to – 900 mV. Microstructural analysis of the CF damage revealed horizontal grain attack within corrosion pit cavities, multiple fatigue cracks and preferable deterioration of the austenitic phase.
The paper provides an overview on the results of a German cluster project on the use of fracture mechanics to the determination of the fatigue strength of weldments with fatigue cracks originating at the weld toes. The approach includes (a) a concept for short crack propagation for which the common ΔK concept is not applicable and the crack closure effects are still being gradually build-up, (b) a method for determining fatigue life relevant initial crack sizes as they are needed in any fracture mechanics analysis and (c) multiple cracking and crack coalescence at load levels higher than the endurance limit. The analyses are stochastically performed. Both, the endurance limit and the finite life branch of the S-N curve are determined.
Besides a brief introduction into the approach, validation examples are presented. These comprise different weldment types (butt welds, cross joints and longitudinal stiffened plates), two steels (S355NL and S960QL) of quite different strengths, different weld geometries due to different welding techniques (WIG, MAG), as-welded and stress relieved welds and different stress ratios varying from R = -1 to R = 0.5.
The additive manufacturing technology of Selective Laser Melting (SLM) experiences a rapid development within an increasing marked of quite different application fields. The properties of SLM materials and structures are influenced by a number of tech-nological parameters such as the metal powder (particle size, homogeneity, cleanliness), the laser tool (power, beam diameter, pulse lengths), the scanning operation (speed, sequence and orientation of melting paths), parameters of the over-all equipment (design and preheating of the base plate, currents and turbulence in the protective gas atmosphere) and, last not least, the hatching strategy including the build-up direction of the structure with respect to the loading direction of the component.
For the perspective use of SLM structures as load carrying, safety-relevant components the knowledge of their mechanical properties is necessary. It is essential to understand these in the context of the manufacturing-related features and at the back-ground of the basic characteristics of metallic materials: crystal lattice, microstructure and material defects. The paper provides an overview on factors which affect the mechanical parameters stiffness, strength, ductility, toughness, fatigue crack propagation and fatigue strength in the context of selective laser melting.
For the application of the time-temperature superposition principle a suitable relation is needed to describe the time-temperature shift factor α. Therefore, the Arrhenius equation is widely used due to its simple form and often leads to suitable results. Where, the Arrhenius equation presents a linear relation for the temperature-dependent shift factor in logarithmic scale ln(α) with the absolute inverse temperature (1/ϑ). However, in cases with a large temperature range which eventually include more complex reaction processes, the functional relation between ln(α) and (1/ϑ) is nonlinear in the ‘Arrhenius plot’. In those cases, the monotone change of the nonlinear range in the ‘Arrhenius plot’ can be interpreted as a transient range between two approximately linear or constant regions. An extended application of the modified Arrhenius equation from Nakamura (1989) is presented in this study for this transient range. The introduced method was applied to describe the time-temperature equivalence in the relaxation analysis of restoring seal force of metal seals, which are used in lid-systems of transport and interim storage casks for radioactive materials. But, the method is widely valid and can be used for different objectives which are characterized by thermorheologically simple behavior with nonlinear sensitivity to inverse temperature.