Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Eingeladener Vortrag
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The fatigue behaviour of cast iron is usually investigated on machined specimens. Components of cast iron, however, have a casting skin; therefore the investigation of the influence of the casting skin on the lifetime is of interest. To study this influence isothermal fatigue tests were carried out on heat-resisting spheroidal graphite cast iron EN GJS SiMo 4.05 in 4–point-bending set-up at 400 °C. Specimens with and without casting skin were investigated comparatively. The number of cycles to failure was significantly lower for specimens with casting skin. Metallographic investigations underline the reduction of lifetime caused by casting skin.
Micrometric surface topologies are required for a wide range of technical applications.
While lowered surface features have been used for many years to improve the tribological behavior of contacting surfaces, there are also other fields of application, where the potential of elevated surface features is known, e. g. for metal forming tools. However, the demand for a high wear resistance of these structures often inhibits an industrial application. A solution is offered by structuring techniques that use additional material. A promising approach is the localized dispersing of hard ceramic particles by pulsed laser radiation, the so-called laser implantation. This paper describes the potential to adjust the geometry as well as the mechanical properties of laser implanted surfaces by means of microstructural and topological investigations. Afterwards, results of a wear test are given and different applications for this structuring technique are discussed. It can be shown that dome-shaped or ring-shaped structures on a micrometric scale can be produced with high hardness and wear resistance.
Seitens der Industrie besteht großes Interesse, Polyethylenterephthalat (PET) als Werkstoff für Verpackungen zum Transport von Gefahrgütern einzusetzen, da aufgrund der hohen Fes-tigkeit und Steifigkeit von PET die Wanddicken und somit die Kosten der Verpackungen reduziert werden können. Die Prüfung der Spannungsrissbeständigkeit von Polyethylenformstoffen als Werkstoffe von Verpackungen erfolgt mit Labormethoden unter Verwendung einer Standardflüssigkeit als Prüfmedium für die Spannungsrisse auslösende Wirkung auf Polyethylen (PE), wodurch Zeit und Kosten der Prüfungen reduziert werden.
Ziel dieser Arbeit war es, eine Laborprüfmethode zum Vergleich der Spannungsrissbeständigkeit von PE und PET auf ihre Anwendbarkeit zu prüfen, wie z.B. den Full Notch Creep Test (FNCT). Es wurde untersucht, ob die Prüfkörper aus PE und PET mit umlaufender Kerbe in dieser durch die chemische Industrie entwickelten und von der BAM konzipierten Apparatur auf der Basis des FNCT zu messbaren Ergebnissen unter Einfluss eines Ölsäureamidethoxylates als Netzmittel bei 50 °C führen.
Die Testergebnisse bestätigten die Eignung des Prüfverfahrens für die acht eingesetzten Formstoffe aus PE. Dieses Prüfverfahren konnte nicht für PET angewandt werden, da die Prüfkörper aufgrund der hohen Festigkeit und Steifigkeit des PET beim Kerbvorgang zerbrachen. Die gleiche Aussage konnte für den Nachweis der der Spannungsrissbeständigkeit von Verpackungen, der in der BAM Gefahrgutregel BAM-GGR 015 beschrieben wird, getroffen werden. Die einzige Möglichkeit zum Nachweis der Spannungsrissbeständigkeit von PET besteht in der Durchführung von Stapeldruckprüfungen.
Welding is the most important joining technique and offers the advantage of customizable plate thicknesses. On the other hand, welding causes residual stresses and deformations influencing the load carrying capacity. Their consideration in the design requires simple and fast models. Though welding simulation has contributed to accurately access to these values nowadays, their application to large components remains still in a less practicable range. Nevertheless, many studies emphasized the need to make corrections in recently available simplified models. Especially the influence of residual stresses seems somewhat overestimated in many cases if comparing conventional structural steel S355 and high-strength steel S690. In the age of computer-aided design, an improved procedure to implement weld-inducted imperfections appears overdue. This will be presented in two parts. The first part illustrates the potential influence of post-welding imperfections exemplified for weak axis buckling in comparison with the general method in accordance with Eurocode 3. Residual stresses and initial crookedness were varied systematically in order to produce a scatter band of capacities. An approach to characterize the borders of these imperfections was untertaken before that. The excessive scattering of reduction factors for the load bearing capacity demonstrates the importance of these variables. Results were finally evaluated against advanced simulation models which will be further detailed in part two of this contribution.
Residual stresses and distortions in welded I-girders for steel construction are relevant when evaluating the stability of steel beams and column members. The application of high strength steels allows smaller wall thicknesses compared to conventional steels. Therefore, the risk of buckling has to be considered carefully. Due to the lack of knowledge concerning the residual stresses present after welding in high strength steel components conservative assumptions of their level and distribution is typically applied. In this study I-girders made of steels showing strengths of 355 MPa and 690 MPa were welded with varying heat input. Due to the dimension of the I-girders and the complex geometry the accessibility for residual stress measurement using X-ray diffraction was limited. Therefore, saw cutting accompanied by strain gauge measurement has been used to produce smaller sections appropriate to apply X-ray diffraction. The stress relaxation measured by strain gauges has been added to residual stresses determined by X-ray diffraction to obtain the original stress level and distribution before sectioning. The combination of both techniques can produce robust residual stress values. From practical point of view afford for strain gauge application can be limited to a number of measuring positions solely to record the global amount of stress relaxation. X-ray diffraction can be applied after sectioning to determine the residual stresses with sufficient spatial resolution.
Estimation of heat flux parameters during static gas tungsten arc welding spot under argon shielding
(2017)
A multi-physics modelling of a static Gas Tungsten Arc Welding (GTAW) operation has been established in order to estimate the heat flux exchanged between the arc plasma and the work-piece. The heat flux was described with a Gaussian function where two parameters required to be estimated: process efficiency and radial distribution. An inverse heat transfer problem (ihtp) has been developed in the aim to estimate these parameters from experimental data. Levenberg-Marquardt algorithm was used as the regularization method in addition to an iterative process. The experiment consisted in a static spot weld with GTAW process. The weld spot was on for 5 s under Argon shielding gas, 2.4 mm pure tungsten electrode on a SS304L disc. Temperatures were measured with thermocouples and weld pool growth monitored with a high speed camera. The experimental data were used to solve the ihtp what led to values such as 0.7 for process efficiency and average radial distribution of 1.8 mm.
This paper describes austenitic-ferritic duplex stainless steels, SAF 2205, in the presence of hydrogen. The duplex stainless steels (DSS) properties include excellent resistance to stress corrosion cracking, high strength and good weldability. Those steels are preferably used in industries combining hydrogen and loads. Hydrogen location in addition to hydrogen binding energy with the steel's defects are of great importance for the analysis of hydrogen embrittlement model in that steel. It is known from previously published works that the susceptibility to hydrogen embrittlement will depend on the competition between reversible and irreversible traps; meaning a direct relation to the hydrogen's state and position in the steel. In this work, we examine the local hydrogen concentration, trapping and distribution by two modern and advanced techniques: thermal desorption spectrometry (TDS) and we support it by time of flight-secondary ion mass spectrometer (ToF-SIMS). In this paper, we support and give for the first time new insights and better understanding to the hydrogen embrittlement mechanism in SAF 2205. The trapping energies levels were calculated using TDS and Lee and Lee's model. This model revealed reversible in addition to irreversible trapping sites. Also the trapping controlling mechanism was found to be a combination of detrapping controlled mechanism and diffusion controlled mechanism. The use of ToF-SIMS for local imaging the distribution of hydrogen species supports the discussion of the different hydrogen traps in this type of steel. The hydrogen embrittlemet phenomenon in SAF 2205 will be discussed in details in that paper.
Neutron Bragg-edge imaging was applied for the visualization of a γ-Austenite to α'-martensite phase transformation. In the present study, a super martensitic
stainless steel sample was heated until complete austenitization and was subsequently cooled down to room temperature. The martensitic phase Transformation started at Ms = 190 °C. Using a monochromatic neutron beam with λ = 0.390 nm, the transmitted intensity was significantly reduced during cooling below Ms, since the emerging martensitic phase has a higher attenuation coefficient than the austenitic phase at this wavelength. The phase Transformation process was visualized by filming the transmission images from a scintillator screen with a CCD camera with a temporal resolution of 30 s and a spatial
resolution of 100 µm.
The residual stress distribution of tungsten inert gas welded S235JRC+C plates was determined by means of neutron diffraction(ND). Large longitudinal residual stresses with maxima around 600 MPa were found. With these results as reference, the evaluation of residual stress with high spatial resolution GMR (giant magneto resistance) sensors was discussed. The experiments performed indicate a correlation between changes in residual stresses (ND) and the normal component of local residual magnetic stray fields (GMR). Spatial variations in the magnetic field strength perpendicular to the welds are in the order of the magnetic field of the earth.
Mechanochemistry is increasingly used as a ‘green alternative’ for synthesizing various materials including pharmaceutical cocrystals. Herein, we present the mechanochemical synthesis of three new cocrystals containing the API carbamazepine (cocrystals CBZ:Indometacin 1:1, CBZ:Benzamide 1:1, and CBZ:Nifedipine 1:1). The mechanochemical reaction was investigated in situ documenting a fast and complete reaction within one minute. Online NMR spectroscopy proved the direct influence of the dissolution behaviour of the coformers to the dissolution behaviour of the API carbamazepine. The dissolution behaviour of the organic cocrystals is compared to the behaviour of the pure drug indicating a general applicability of this approach for detailed cocrystal dissolution studies.