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Eingeladener Vortrag
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Elastic wave propagation of ultrasound in bituminous road surfaces – simulations and measurements
(2015)
Maintenance costs of road infrastructure are increasing steadily. Adverse environmental impacts on infrastructure get more and more important as well. Therefore, it is important to determine how limited financial resources can be directed with an optimum pay-out. The present study takes first steps towards the usage of low-frequency ultrasound as a tool to evaluate the road condition.
We investigated the efficiency of phase gratings, i.e., the visibility upon variation of different parameters. Rotating around an axis parallel to the grid lines of the phase grating changes the grating’s shape and thereby the initial distribution of phase shifts. This yields high visibilities for shorter propagation distances than derived from box shapes. Tilting the grating in the scattering plane allows continuous tuning of the grating’s height that corresponds to an ideal phase shift for a particular photon energy. This opens the way for tuning the design energy suitable for the material under investigation. Our study included simulations for monochromatic sources with the sampling procedure for rectangular and triangular phase gratings.
We report on the in situ investigation of mechanochemical syntheses of metal phosphonates. The metal phosphonates are formed in milling reactions starting from a metal acetate and a phosphonic acid. The conversions are observed by synchrotron PXRD and Raman spectroscopy to shed light on the reaction mechanisms including possible intermediates.
We present an in situ investigation of the mechanochemical formation of cocrystals using synchrotron XRD and Raman spectroscopy. This combination allows to study milling processes on the level of the molecular and crystalline structure thus obtaining reliable data for mechanistic studies. Thereby, mechanochemical syntheses can be optimized to isolate new crystal structures.
Characterization of residual stress state by neutron diffraction and residual magnetic field mapping
(2016)
Based on the residual stress characterization of tungsten inert gas welded S235JRC+C plates by means of neutron diffraction, the evaluation of residual stress with high spatial resolution GMR (giant magneto resistance) sensors is discussed. The experiments performed indicate a correlation of residual stress changes and local residual magnetic stray fields.
Additive Manufacturing (AM) by Selective Laser Melting (SLM) offers ample scope for producing geometrically complex parts as compared to the traditional subtractive manufacturing strategies. However, the residual stresses developed during the processing can reduce the load bearing capacity as well as induce unwanted distortion, limiting the application of SLM parts.
IN718 manufactured by SLM process can show high tensile residual stresses in the surface as high as the yield strength of the wrought alloy. On the other hand, residual stresses in the bulk Show considerably lower stress values, even in compression, indicating a stress re-distribution during deposition of the SLM layers.
Selective Laser Melting (SLM) technique allows to produce parts with complex geometry. Due to the rapid solidification and heat gradient during production, SLM results in presence of residual stress. The present study shows the influence of manufacturing parameters on surface residual stress of Ti-6Al-4V SLM parts. High tensile stresses in the front surface are found. Heat Treatment conditions relax residual stresses almost to zero. High scanning speed during manufacturing results in higher tensile stresses in the surface.
Ermüdungsverhalten, Lebensdauer und Schädigungsprozesse von P92 unter zyklischen Betriebsbedingungen
(2016)
Schnelle Lastwechsel mit hohen Temperaturgradienten führen zu thermischen und mechanischen Beanspruchungen v.a. in dickwandigen Bauteilen. Daraus ergibt sich ein komplexes Lastkollektiv, bei dem der Werkstoff sowohl durch Kriechermüdung als auch durch thermomechanische Ermüdung (TMF) geschädigt wird. Welche Auswirkungen hat eine gleichzeitige Änderung von Temperatur, mechanischer Beanspruchung und Arbeitsmedium auf die Schädigungsprozesse?
Were prepared four unsubstituted Pc - C60 fullerene systems, using two different synthetic strategies, via Prato (1,3 - dipolar cycloaddition) or Bingel reactions. The precursor Pcs 1 and 2 were successfully synthesized by cyclotetramerization reaction between TBDPS - protected and unsubstituted phthalonitriles. The surface studies are currently ongoing.
Ethylene-propylenene-diene copolymer rubbers (EPDM) are used in a wide range of sealing applications e.g. in automotive industry or in containers for dangerous goods. Investigations with regard to the ageing behavior and lifetime prediction of commercial EPDM rubber seals consisting of 48 % polypropylene (PP) and 4.1 % of 5-ethylidene-2-norbornene (ENB) were conducted [1]. However, structural parameters (monomer ratio, diene type, curing agent etc.) and additives (filler, plasticizer etc.) are known to affect the ageing behavior of rubber compounds [2, 3]. The aim of the current study was to elucidate the influence of each component on the overall deterioration of material properties after thermal ageing. Therefore, different EPDM mixtures were prepared at BAM, the reference being an EPDM with 48 % PP and 4.1 % ENB reinforced with Sillitin. Five more formulations were prepared in order to assess the effect of (i) filler type (carbon black or sillitin), (ii) curing agent (sulfur or peroxide), (iii) plasticizer, (iv) ethylene-propylene ratio and (v) ENB content. Initially, the properties of the unaged materials were investigated by tensile testing, hardness and density measurements, TGA, DSC and DMA. Sulfur vulcanization resulted in higher elongation at break due to the flexibility of the sulfidic crosslinks. Carbon black resulted in better reinforcement compared to Sillitin. A higher ethylene and ENB content lead to higher hardness due to higher crystallinity and higher crosslink density, respectively.
Thermal ageing for up to 30 days (with 10 days intervals) took place in circulating hot air ovens at 125 °C. Pronounced crosslinking was found to take place in all peroxide-cured materials, as the unreacted double bonds of the ENB units acted as starting points for oxidation and crosslinking. Increasing the ethylene content resulted in an increase in the ageing resistance of EPDM. This improvement was attributed to the higher crystallinity that inhibits oxygen diffusion and to the smaller number of chain scissions which occur in the PP units. Although significant loss of the plasticizer was observed, the remaining plasticizer adequately improved the tensile properties of the material. The filler type did not significantly affect the ageing behavior of EPDM. Finally, the lower thermal stability of the sulfidic crosslinks resulted in desulfurization and, thus, in an increase in the crosslinking density.
To sum up, it has been shown that the monomer composition, curing agent and additives used in EPDM formulations greatly influence the properties and ageing resistance of these materials.
Bei der Bewertung der Verlässlichkeit der im Labor gewonnenen Prüfergebnisse spielen Ringversuche eine entscheidende Rolle. Die Bundesanstalt für Materialforschung und -prüfung (BAM) unterstützt deshalb den weiteren Ausbau des Ringversuchsprogramms des im Jahr 2007 gegründeten Kompetenzzentrums zur Qualitätssicherung für Prüfungen von Gefahrgütern und Gefahrstoffen auf physikalische Gefahren (Centre for quality assurance for testing of dangerous goods and hazardous substances, CEQAT-DGHS).
Bei allen bisher untersuchten Prüfmethoden besteht ein Verbesserungsbedarf. Die RV müssen daher zunächst auf die Methodenentwicklung, -verbesserung und -validierung abzielen und nicht auf Leistungstests.
Application of 4Q-BSE detector and dedicated software enables 3D surface measurements of fracture samples in SEM. 3D surface measurement is independent from magnification and material invariant. Quantitative height measurements require careful detector adjustment and system calibration. Live 3D view allows a fast assessment of topographical features. In addition to SE and BSE imaging, 3D surface data help to analyse and interpret fracture samples.
Pressurisation of full-containment pressure vessels in fire is known to be driven by thermal stratification. The predominant mode of heat transfer to the contents (convection from the shell to the liquid phase) results in formation of „hot“ boundary layers. Sub-cooled boiling may also be present. The warm layer rises to the surface through buoyancy and bubble flow, increasing the surface of the liquid above that of the bulk temperature, and hence driving a pressure rise.
For reliable prediction of the complex effects governing vessel pressurization a three-dimensional numerical model is required. Work is being undertaken on such a model by other institutions in cooperation with this project.
The arrangement of the measuring sensors allowed the
capture of air flow induced temperatures and material
temperatures on various points of the cylinder body. Four
significant temperature levels could be observed: Inside the cylinder (far from the wall, close to the wall), inside the aluminium liner and outer surface. Figure 5 shows the temperature distribution after a regular filling process with a filling time of ten minutes. The temperature sensors T1 - T9 are marked with different colors. The corresponding measurement points are shown in Figure 4. Due to the high thermal conductivity of the aluminum liner the induced temperature peaks are quickly derived. A flow-induced exceeding of the maximum approval temperature is not expected. Regarding the safety assessement the results show that the pressure and temperature profilesmustbe taken into account.
A new tank design for Hazmat
(2016)
Initial studies have shown pre-deformed honeycomb structure
pressure vessels to have comparable burst pressure to straight steel vessels.
It was shown that honeycomp structured tanks have a higher energy absorption than straight steel tanks.
Future studies will investigate optimization of the deformed plate, to determine if increased payloads or a reduced wall thickness can be achieved.
Mg-Y,RE-Zn systems present second phases with high thermal stability, promoting the increase of strength and creep resistance at high temperatures.
Mg and its alloys have a potential application as biomaterials due to their biocompatibility and degradation behaviour. The dominant corrosion mechanism in those alloys is the microgalvanic corrosion through coupling of the more noble second phases with the Mg matrix, and therefore a low concentration of those was sought.
In order to reduce CO2 emissions fossil fuelled power stations with high combustion efficiency are being developed. The increase of the operating temperature, a common way to improve combustion efficiency, leads to enhanced corrosion of heat exchange steel tubes in the power plants. Within the framework of the European project “Production of Coatings for New Efficient and Clean Coal Power Plant Materials” (POEMA)", high temperature corrosion protection coatings are now under investigation.
Thin porous sol-gel alumina films are promising candidate coating materials. Coatings were prepared by applying boehmite sols on grinded steel P92 and subsequent heat treatments at temperatures up to 650 °C. Thus a porous layer of worm-like particles was formed consisting of nano-crystallites and amorphous alumina. A dense interface with satisfying adhesion resulted from diffusion of chromium and iron ions out of the steel into the porous coating. However, the film locally exhibited some cracks caused by steep edges in the grinded steel surface. An appropriate substrate pretreatment should avoid this problem.
The protection of the steel relies on the barrier function of the alumina coating and the formation of a dense chromia layer at the steel surface.