Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Eingeladener Vortrag
- nein (443)
The need for steels with highest mechanical properties is a result of the increasing demands for energy and resource efficiency. In this context, high-strength low-alloyed (HSLA) structural steels are used in machine, steel and crane construction with yield strength up to 960 MPa. HSLA steels enable lightweight construction by thinner necessary plate thickness. However, welding of HSLA steels requires profound knowledge of three factors in terms of avoidance of hydrogen-assisted cracking (HAC): the interaction of (1) microstructure, (2) local stress/strain and (3) local hydrogen concentration. In addition to the three main factors, the used weld-arc process is also important for the performance of the welded joint, especially when using modern arc variants. In the past, the conventional transitional arc process (Conv. A) was mainly used for welding of HSLA grades. In the past decade, the so-called modified spray arc process (Mod. SA) was increasingly used for welding production. This modified arc enables reduced seam opening angles with increased deposition rates compared to the conventional process. Economic benefits of using this arc type are: a reduced number of necessary weld beads and a lower weld seam volume, which result in decreased total welding time and costs. Nonetheless, investigations on a high-strength S960QL showed significantly higher hydrogen concentrations in the weld metal at a reduced seam opening angle with Mod. SA. This indicates an increased risk for the susceptibility of the welded component to HAC. Hence, existing recommendations on HAC-avoidance cannot be transferred directly to the Mod. SA-process.
In the present study, the susceptibility to HAC of the HSLA steel S960QL with same type of filler material was investigated. For that purpose, both Conv. A and Mod. SA were used with same weld heat input at different deposition rates. For assessment of the HAC susceptibility, the externally loaded Implant-test was used. Both conducted test series with Conv. A and Mod. SA showed similar crack critical stress of about 280 MPa. Below this value, no delayed fracture appeared. The welds with Mod. SA showed higher hydrogen concentrations. The fracture occurred in the heat-affected zone (HAZ) or in the weld metal (WM). But in all specimens, cracks initiated at the notch root of the spiral notch of the implants within the coarse-grained HAZ. However, the test series with Mod. SA showed a significant extension of the time-to-failure of several hours compared to tests carried out with Conv. A. The reason is the deeper weld penetration in case of Mod. SA, which causes longer diffusion path for hydrogen. The fracture topography of the ruptured implant specimens with Conv. A was typical ductile in specimen center and quasi-cleavage like at the edge of the specimens. When using Mod. SA, the topography changed to primarily quasi-cleavage fracture topography with shares of intergranular fracture and secondary crack appearance.
Die Messung von Flüssigkeitskonzentrationen in Rohrsystemen ist von großem Interesse für viele unterschiedliche Anwendungen. Die meisten Messsysteme sind jedoch nicht in der Lage, die Flüssigkeit direkt im Rohr zu untersuchen und es muss eine zusätzliche Vorrichtung, wie z.B. einem Bypass, angebracht werden, welche den Kontakt zwischen Flüssigkeit und Sensor ermöglicht.
Um den Einbauaufwand gering zu halten und die Strömungseigenschaften des Rohres nicht zu beeinflussen, wird ein neuartiges Messsystem entwickelt, welches als Teil der Rohrwand ausgeführt werden kann. Dieses neuartige System ist angelehnt an die Idee der phononischen Kristallen (PnK). PnK’s bestehen im Allgemeinen aus einem Matrixmaterial, in welchem Streuzentren periodisch angeordnet sind. Dies führt beim Eintreffen einer akustischen Welle in definierten Frequenz-bereichen, sogenannter Bandlücken, zu zunehmender destruktiven Interferenz. Wird innerhalb einer solchen Bandlücke durch Einbringen einer flüssigkeitsgefüllten Kavität ein Resonanzverhalten erzeugt, kann dies genutzt werden, um die Flüssigkeit zu analysieren.
Im Rahmen von Voruntersuchung wird zunächst das akustische Verhalten des PnK’s, welcher für die Sensorentwicklung genutzt werden soll, unter Vernachlässigung der Flüssigkeit ausführlich untersucht. Hierbei werden zunächst die Bandlücken ermittelt und das Übertragungsverhalten betrachtet. Dieses wird im Anschluss experimentell überprüft. Hierzu werden neben herkömmlichen Fertigungsverfahren auch additive Verfahren zur Herstellung der Proben genutzt.
Forschungsprojekt zum Korrosionsverhalten von Edelstählen - BAM und VSM mit neuen Erkenntnissen
(2019)
Wie kann man verhindern, dass Edelstahl korrodiert? Und welchen Einfluss haben Kornarten und Körnungen von Schleifmitteln auf das Korrosionsverhalten? Diesen Fragen ist die Bundesanstalt für Materialforschung und -prüfung (BAM) mit Unterstützung der VSM AG zwei Jahre lang auf den Grund gegangen. Heute wurden im VSM Technical Center am Hauptsitz von VSM in Hannover die Ergebnisse vorgestellt. Die zentralen Erkenntnisse: Kornarten und Zusätze im Schleifmittel haben keinen Einfluss auf die Korrosion. Aber: Je feiner die Oberfläche geschliffen ist, desto besser ist der Korrosionsschutz. Dabei ist das Einhalten der Schleiffolge besonders wichtig.
We show that complex physical and chemical interactions between boehmite nanoparticles and epoxy drastically affect matrix properties, which in the future will provide tuning of material properties for further optimization in applications from automotive to aerospace. We utilize intermodulation atomic force microscopy (ImAFM) for probing local stiffness of both particles and polymer matrix. Stiff particles are expected to increase total stiffness of nanocomposites and the stiffness of polymer should remain unchanged. However, ImAFM revealed that stiffness of matrix in epoxy/boehmite nanocomposite is significantly higher than unfilled epoxy. The stiffening effect of the boehmite on epoxy also depends on the particle concentration. To understand the mechanism behind property alteration induced by boehmite nanoparticles, network architecture is investigated using dynamic mechanical thermal analysis (DMTA). It was revealed that although with 15 wt% boehmite nanoparticles the modulus at glassy state increases, crosslinking density of epoxy for this composition is drastically low.
Understanding the interaction between nanoparticles and the matrix and the properties of interphase is crucial to predict the macroscopic properties of a nanocomposite system. Here, we investigate the interaction between boehmite nanoparticles (BNPs) and epoxy using different atomic force microscopy (AFM) approaches. We demonstrate benefits of using multifrequency intermodulation AFM (ImAFM) to obtain information about conservative, dissipative and van der Waals tip-surface forces and probing local properties of nanoparticles, matrix and the interphase.
We utilize scanning kelvin probe microscopy (SKPM) to probe surface potential as a tool to visualize material contrast with a physical parameter, which is independent from the mechanics of the surface. Combining the information from ImAFM stiffness and SKPM surface potential results in a precise characterization of interfacial region, demonstrating that the interphase is softer than epoxy and boehmite nanoparticles. Further, we investigated the effect of boehmite nanoparticles on the bulk properties of epoxy matrix. ImAFM stiffness maps revealed the significant stiffening effect of boehmite nanoparticles on anhydride-cured epoxy matrix. The energy Dissipation of epoxy Matrix locally measured by ImAFM shows a considerable increase compared to that of neat epoxy. These measurements suggest a substantial alteration of epoxy structure induced by the presence of boehmite.
Carrier gas hot extraction (CGHE) is a commonly applied technique for determination of hydrogen in weld joints using a thermal conductivity detector (TCD) for hydrogen measurement. The CGHE is based on the accelerated hydrogen effusion due to thermal activation at elevated temperatures. The ISO 3690 standard suggests different specimen geometries as well as necessary minimum extraction time vs. temperature. They have the biggest influence on precise hydrogen determination. The present study summarizes the results and experience of numerous test runs with different specimen temperatures, geometries (ISO 3690 type B and small cylindrical samples), and factors that additionally influence hydrogen determination. They are namely specimen surface (polished/as-welded), limited TCD sensitivity vs. specimen volume, temperature measurement vs. effects of PI-furnace controller, as well as errors due to insufficient data assessment. Summarized, the temperature is the driving force of the CGHE. Two different methods are suggested to increase the heating rate up to the desired extraction temperature without changing the experimental equipment. Suggestions are made to improve the reliability of hydrogen determination depended on the hydrogen signal stability during extraction accompanied by Evaluation of the recorded data. Generally, independent temperature measurement with dummy specimens is useful for further data analysis, especially if this data is used for calculation of trapping kinetics by thermal desorption analysis (TDA).
Since the early 19th century microorganisms were studied on their capabilities of causing microbiologically influenced corrosion (MIC) of metals. The most studied ones are sulfate-reducing bacteria (SRB), but others can corrode metals as well, e.g. acid-producing bacteria or methanogenic archaea (MA). However, these studies were mostly focused on metals related to the petroleum industry but metals for other industries, e.g. dentistry, are also susceptible to corrosion. The inert Titanium (Ti) is often used as an implant material, but it is a base metal. The formation of a passivating oxide layer allows Ti to be corrosion resistant at normal conditions.
Nonetheless, scanning electron microscope images on dental implants from patients with acute peri-implantitis showed clear signs of corrosion. Currently, the corrosion mechanism of dental implants is unknown, but many indications suggest that oral microorganisms, including MA (Methanobrevibacter oralis) and SRB (Desulfomicrobium orale), could be involved.
To determine if MA or SRB can corrode Ti (pure Ti or Ti-6Al-4V alloy), corrosion rate, methane and sulfide concentrations were analyzed. Electrical potential measurements using in-house developed electrochemical cells indicated a potential change on Ti in the presence of a corrosive MA strain compared to an abiotic control.
Microbial composition comparison will be analyzed using samples from dental pockets of 150 infected patients by considering the quality of the implant and 50 healthy people by means of amplicon sequencing. Enrichments and isolation of pure cultures from the dentals samples are also examined for their corrosion behavior. Overall, this is the first study investigating the susceptibility of dental implant material to corrosion using human related MA.
In the present study, the susceptibility to HAC of the HSLA steel S960QL with same type of filler material was investigated. For that purpose, both Conv. A and Mod. SA were used with same weld heat input at different deposition rates. For assessment of the HAC susceptibility, the externally loaded Implant-test [ ] was used. Both conducted test series with Conv. A and Mod. SA showed similar crack critical stress of about 280 MPa. Below this value, no delayed fracture appeared. The welds with Mod. SA showed higher hydrogen concentrations. The fracture occurred in the heat-affected zone (HAZ) or in the weld metal (WM). But in all specimens, cracks initiated at the notch root of the spiral notch of the implants within the coarse-grained HAZ. However, the test series with Mod. SA showed a significant extension of the time-to-failure of several hours compared to tests carried out with Conv. A, see Fig. 1a. The reason is the deeper weld penetration in case of Mod. SA (Fig. 1b and Fig. 1c), which causes longer diffusion path for hydrogen. The fracture topography of the ruptured implant specimens with Conv. A was typical ductile in specimen center and quasi-cleavage like at the edge of the specimens. When using Mod. SA, the topography changed to primarily quasi-cleavage fracture topography with shares of intergranular fracture and secondary crack appearance.
Die Verwendung von LTT-Zusatzwerkstoffen stellt einen alternativen Ansatz zu den sonst üblichen Verfahren der Schweißnahtnachbehandlung zur Schwingfestigkeitserhöhung geschweißter Stahlkonstruktionen dar. Der wesentliche Wirkmechanismus beruht auf der Beeinflussung des Eigenspannungszustands durch die niedrige Martensitstarttemperatur bereits während des Schweißens. Dadurch werden die Druckeigenspannungen aus der behinderten Volumenausdehnung infolge Phasenumwandlung voll wirksam. Weiter weist die Schweißnaht eine hohe Härte auf, die die Schwingrissbildung verzögern kann, allerdings auch zu einer niedrigen Kerbschlagarbeit führt.
Im ersten Teil dieses Beitrages werden grundlegende Untersuchungen zur schweißtechnischen Verarbeitung eines LTT-Zusatzwerkstoffes präsentiert. Dies beinhaltet zunächst die Sicherstellung der Schweißbarkeit sowie die Charakterisierung der Schweißgefüge und deren Härte mit dem Ziel, reproduzierbare Verbindungseigenschaften für die nachfolgende Schwingfestigkeitsprüfung herzustellen.
Der zweite Teil des Beitrages beschäftigt sich mit den dazugehörigen Schwingfestigkeitsuntersuchungen an den Stählen S355J2 und S960Q unter Verwendung konventioneller Zusatzwerkstoffe im Vergleich zum LTT-Zusatz.
Das verwendete Schweißdetail ist eine Verbindungsschweißung am Stumpfstoß unter Variation der Schweißnahtausführung (DY-Naht und V-Naht). Weitergehend wird eine bauteilähnliche Probe (die Längssteife) untersucht, bei der der LTT-Zusatzwerkstoff als zusätzliche Schweißlage aufgebracht wurde. Neben der Schwingfestigkeit werden die Schweißnähte hinsichtlich der Eigenspannungen, der Eigenspannungsstabilität im Schwingversuch sowie metallografisch charakterisiert. Die Ergebnisse zeigen, dass LTT-Zusatzwerkstoffe unter Einhaltung wesentlicher Konstruktionsrichtlinien zu einem deutlichen Anstieg der Schwingfestigkeit führen. Dadurch kann das Leichtbaupotential hochfester Stahlgüten genutzt werden.
This book provides a comprehensive and thorough guide to those readers who are lost in the often-confusing context of weld fatigue. It presents straightforward information on the fracture mechanics and material background of weld fatigue, starting with fatigue crack initiation and short cracks, before moving on to long cracks, crack closure, crack growth and threshold, residual stress, stress concentration, the stress intensity factor, J-integral, multiple cracks, weld geometries and defects, microstructural parameters including HAZ, and cyclic stress-strain behavior. The book treats all of these essential and mutually interacting parameters using a unique form of analysis.
Laminography is a widely used NDT technique for large flat object which cannot be investigated by traditional computed tomography. However, due to the limited scanning angle of laminography, the reconstructed image has more artifact interference, which seriously affects the reconstructed image quality. Reducing artifacts of the laminography image and enhancing the images have become important research effort. In this paper, we present dual-energy materials characterization methods based on photon counting detectors to reduce artifacts and enhance image for laminography. The photon counting detector used in this study allows the setting of two independent energy thresholds in order to acquire dual-energy images for laminography from a single scan. The dual energy imaging methods of basis material decomposition (BMD) and weighted logarithmic subtraction (WLS) were studied in the paper with respect to laminography image enhancement. A fast decomposition algorithm on laminographic projection domain with approximating the inverse dual-energy equations to calculate the thickness of basic materials was used in the BMD dual-energy imaging methods. The experimental results show that the BMD method can characterize materials and enhance features of the basic material within the laminographic dataset. In the WLS method, a linear operation was applied on dual-energy images reconstruction directly, which can eliminate the attenuation of one specific material in the resultant image by setting an appropriate weighting factor. In our experiments. WLS method was used successfully to eliminate the strong artifacts generated by the special material and enhance the images. Dual-energy materials characterization methods based on photon counting detectors show potential applications in laminography.
Polymers at interfaces play a major role in a broad variety of applications ranging from engineering purposes (for instance polymer based nanocomposites) to high tech implications (for instance light emitting diodes). Here the glass transition and glassy dynamics is considered for epoxy-based nanocomposite with Layered Double Hydroxide nanofiller and for thin films of a misicble polymer blend of PVME/PS with thicknesses down to 7 nm. The materials are investigated by spectroscopic techniques (broadband and specific heat spectroscopy), as well as by fast scanning calorimetry and small- and wide-angle X-ray scattering.
For inorganic/polymer nanocomposites a so-called Rigid Amorphous Phase (RAF) is formed in the interfacial region by adsorption of polymer segments onto the nanoparticles. The segmental dynamics of RAF is expected to be altered, as compared to the pure matrix, which might percolate into the entire system, affecting the overall nanocomposite properties. A combination of two relaxation spectroscopy techniques (Broadband Dielectric Spectroscopy (BDS) and Specific Heat Spectroscopy (SHS) in a form of Temperature Modulated DSC (TMDSC)) as well as Fast Scanning Calorimetry (FSC) was employed to investigate the structure and molecular mobility of nanocomposites based on Epoxy and Layered Doubled Hydroxides with different nanoparticle content.
First, BDS investigations proved the existence of a process, which is present only for nanocomposites, assigned to the dynamics of polymer segments within RAF. Second, the amount of RAF was quantified by analyzing the change of specific heat capacity step of nanocomposites, comparing to the pure material.
Thirdly, the glass transition of nanocomposites was studied with FSC, applying high heating rates (500-10 000 K/s). Considering that all techniques probe essentially the same molecular process, an activation plot was constructed, delivering a complete picture of the molecular mobility and structure of the polymer nanocomposites including RAF.
Die warmfeste austenitische Gusseisenlegierung EN-GJSA-XNiSiCr35-5-2 (häufig auch als Ni-Resist D-5S bezeichnet) wurde hinsichtlich ihres mechanischen Verhal-tens bei hoher Temperatur charakterisiert. Dazu wurden (isotherme) niederzyklische (LCF-) und (nicht-isotherme) thermomechanische Ermüdungsversuche (TMF) zwischen Raumtemperatur und 900 °C durchgeführt. Diese Ergebnisse dienten (zu-sammen mit weiteren Versuchsdaten) der Kalibrierung werkstoffmechanischer Modelle. Bei den höchsten Prüftemperaturen wurde Schädigung in Form von Kriechen beobachtet und metallographisch dokumentiert.
Metalle
(2019)
Dieses Standardwerk bietet eine Darstellung der Struktur und Eigenschaften der Metalle und ihrer Anwendungen als Werkstoffe. Im ersten, wissenschaftlichen Teil werden der atomare und mikroskopische Aufbau, die thermodynamischen und die grundlegenden physikalischen und mechanischen Eigenschaften der Metalle beschrieben sowie die Grundlagen der thermisch aktivierten Reaktionen und der Phasenumwandlungen. Die wichtigsten experimentellen Untersuchungsmethoden werden erläutert, wobei die mikroskopischen und Beugungsverfahren einen Schwerpunkt bilden.
Im zweiten, technischen Teil werden die Werkstoffgruppen der Metalle und ihre anwendungsbezogenen Eigenschaften behandelt. Dabei wird auf die zugrunde liegenden Legierungen und die Verfahren zur gezielten Einstellung ihrer Eigenschaften eingegangen. Schwerpunkte bei der Darstellung der Werkstoffe sind die Stähle, die teilchengehärteten Legierungen, die Magnetwerkstoffe und die pulvermetallurgisch hergestellten Werkstoffe. Außerdem werden die Oberflächeneigenschaften und die Verfahren zur Oberflächenbehandlung dargestellt.
Overview about the surface initiated microstructure formation in glass surfaces. Samples which are exposed to a temperature treatment, can develop a crystalline microstructure above Tg at the surface. These separated crystals can be preferably oriented towards the surface of the sample. First experiments about the origin of these orientation phenomenon as well as the potentially causing mechanisms are presented and discussed within the presentation.
The aim of this research is to study the influence of moisture absorption at low moisture contents on the creep behaviour of an epoxy adhesive in steel bonded joints. Single lap joints were manufactured using high strength steel adherends and a two-component epoxy adhesive. The single lap joints were tested at load Levels corresponding to average lap shear stresses of±5%, 15%, 30% and 45% of the dry lap shear strength in both 40 °C air and 40 °C distilled water. Specimens were not pre-aged to be able to analyse the coupled effect of moisture and loading. The test results show that an increase in the load level resulted in an increase in the instantaneous strain and in the creep strain rate. The creep strain of single lap joints loaded in water was generally larger than for the ones loaded in air. For joints loaded in water the creep behaviour was found to be dependent on the moisture concentration in the adhesive. At low moisture percentages creep was suppressed, resulting in a lower instantaneous strain. At higher moisture percentages creep was promoted, resulting in a larger strain rate. The suppression of creep at low moisture percentages is attributed to water molecules bonding to the epoxy macromolecules, resulting in a reduction in molecular mobility and a smaller creep strain. At higher moisture percentages the plasticizing effect of the water dominates, resulting in a larger creep strain. The Maxwell threeelement solid model and Kelvin-Voigt three-element solid model were used to simulate the creep behaviour of the single lap joints loaded in air and water. The models gave good representations of the creep Response across the different load levels in both water and air, they were however unable to give a correct representation of the instantaneous strain of the single lap joints loaded in water. This is attributed to the models being unable to account for the present short-term relaxation process that is dependent on the moisture concentration.
Ergebnisse aus dem AiF Forschungsvorhaben IGF 18823 N/1 "Optimierung von Korundschleifprozessen"
Ein Referenzwerkstoff mit einem definierten Oberflächenzustand wurde zunächst umfassend mittels elektrochemischer Korrosionsuntersuchungen, KorroPad-Untersuchungen und oberflächenanalytischer Verfahren charakterisiert. Anhand definierter Schleifversuche an diesem Referenzzustand wurden anschließend verschiedene Schleifparameter variiert, deren Auswirkungen auf die Korrosionsbeständigkeit mit den gleichen Methoden beurteilt wurden. Somit war es möglich, den Einfluss einzelner Schleifparameter auf das Korrosionsverhalten aufzuzeigen.