Fraunhofer Institute for Large Structures in Production Engineering IGP

For the validation of approaches like fracture mechanics used for the description of crack propagation in high-strength bolting assemblies, it is often necessary to determine the size of an existing crack in the tested assembly. However, since the fatigue crack typically initiates in the root of the first load bearing thread, it is not directly accessible during fatigue testing. The crack size determination method presented here achieves further propagation of the original fatigue crack after the fatigue test by hydrogen-induced cracking under constant load. The characteristic microstructure of the resulting fracture surface then allows for the determination of the original crack size. In the present study, the fatigue crack sizes in M12 bolting assemblies are determined and a corresponding linear elastic fracture mechanics model is validated. The presented method reliably leads to a failure in the plane of the original fatigue crack and allows for a precise measurement of the crack length. The validated fracture mechanics model describes the crack propagation reasonably well, taking into account the overall service life. Overall, the presented method is a valuable addition to existing crack size determination methods and a versatile tool in the further development of more advanced fracture mechanics models for high-strength bolting assemblies.

Abstract The use of aluminum alloys is becoming increasingly important, particularly in special shipbuilding, where the low density and high corrosion resistance of the materials offer decisive advantages. In practice, however, there are considerable technical challenges. Compared to steel, the use of aluminum alloys implies a significantly greater welding-related distortion of the structure. This distortion must then be reduced using costly and time-consuming processes. The most commonly used process for this is the acetylene-oxygen flame straightening process. However, the knowledge and skills of experienced flame straighteners are largely subjective, which limits the possibility of a general process characterization. In order to make the process accessible to inexperienced prospective flame straighteners, the idea was born to systematize the process and work out promising straightening strategies in advance. This simulation-based approach not only reduces the probability of wrong decisions during the process, but also creates a basis for making the process more efficient and easier to plan. It is shown how the FE method can be used to optimize a straightening strategy in order to reduce distortion on larger assemblies. A series of experimental investigations help to validate the virtual simulation model.

ABSTRACT Hybrid joints, which combine adhesive bonding with pretensioned bolts, offer significant advantages in terms of enhanced strength and simplified manufacturing processes, if compared to simply bonded. However, bonding on galvanized steel presents unique challenges due to the presence of corrosion-resistant coatings that can negatively impact adhesive performance. This study investigates the performance of hybrid joints using two epoxy adhesives, Scotch-Weld7240 (SW7240) and SikaDur-370 (S370), focusing on the differences in load-bearing capacity between galvanized and non-galvanized steel substrates. Experimental results reveal that both adhesives exhibit substantially lower load capacities on galvanized surfaces, with reductions of approximately 26% for SW7240 and 57% for S370. Predictions with a probabilistic approach underestimated the experimental values by approximately 9% for SW7240 and 10% for S370. These findings underscore the critical influence of surface treatment on hybrid joint performance and highlight the necessity for optimized adhesive formulations tailored for galvanized steel applications to improve bonding efficacy and overall reliability.

This publication covers experimental investigations on the design resistance of arc-brazed fillet welds (CuAl7) on low-alloyed structural steel (S355) subject to predominantly static loading and regarding steel construction regulations (Eurocode). In current steel construction regulations, there is no standardized design approach for arc-brazed fillet welds available, so arc-brazed connections are rarely used despite the benefits they offer in several regards compared to conventionally welded connections. Therefore, a resistance model for arc-brazed fillet welds was calibrated based on tensile tests that were conducted on gas metal arc-brazed specimens with transverse and longitudinal fillet welds. Based on the statistical evaluation of the test results according to Annex D of EN 1990, a newly determined correlation factor βb is proposed, which can be used for the static design of arc-brazed fillet welds made of CuAl7. This approach leads to a significantly higher calculated design resistance than previous non-standardized design approaches allowed. Also, it was found that the failure behavior of the fillet welds is critical for the design resistance of the joints and that there is a need for further investigations with regard to a targeted joint failure, which, analogous to welded fillet welds, should take place along the throat of the weld and not along the less resistant diffusion zone of the joint. Thus, the results underscore the potential for the use of arc-brazed connections in steel construction in regard to their load-bearing capacity, but also highlight the necessity of continued research regarding factors influencing their structural integrity.

<p>Fatigue analysis is a critical design aspect for slip-resistant connections subjected to cyclic loading. Slip-resistant connections are assumed to have a high fatigue strength comparable to the level of the cut base material due to the uniform and planar force transmission resulting from the preloaded bolting assemblies. For this reason, most commonly, the fatigue design of slip-resistant connections is based on the Δσ-concept according to DIN EN 1993-1-9, regardless of the manufacturing conditions, such as various cutting processes. To investigate whether the various cutting processes have an impact on the fatigue strength, the presented study has been carried out.</p><p>The presented study was conducted on slip-resistant connections manufactured by water jet cutting, oxy-fuel cutting (without and with post-treatment), and plasma cutting. The results of the fatigue tests demonstrated that all investigated cutting processes yielded higher fatigue resistances than those specified in DIN EN 1993-1-9 and FprEN 1993-1-9 of the second generation. Additionally, no significant influence of the cutting process on the fatigue behaviour was observed.</p>

In the current work, twin-wire arc-sprayed copper coatings are investigated to reduce the spread of pathogenic germs in broiler farming. Compressed air and nitrogen are used as process gasses, while the coating torches are varied. The results demonstrate a reduction of 99% pathogenic load due to the presence of coatings in comparison with the uncoated nickel-chromium-steel. This accounts especially for the bacterial strains E.coli, S.aureus and E.cecorum, which are the predominant bacteria in broiler farming. Moreover, posttreatment processes like cold plasma, tungsten inert gas arc processing and shot peening are investigated to further increase the bactericidal properties and abrasion resistance characteristics of the coatings. Further investigations involve the microstructure and the electrical conductivity of the coatings. In this work, it is demonstrated that copper-coated surfaces have an inhibitory effect on bacterial growth of the three investigated bacterial strains compared to the uncoated bulk nickel-chromium-steel material.

Abstract Die Tragfähigkeit gleitfest vorgespannter Verbindungen hängt neben der Haftreibungszahl maßgeblich von der Vorspannkraft und ihrem Erhalt ab. Für zyklisch beanspruchte Stahltragwerke wie Türme für Windenergieanlagen (WEA) sind wartungsfreie/‐arme Schraubverbindungen von besonderem Interesse. Die Ausführung mechanisch wartungsfreier Verbindungen erfordert zwingend die Kenntnis der über die Lebensdauer der betreffenden Struktur auftretenden Vorspannkraftverluste, da diese bereits bei der Bemessung der Verbindung zu berücksichtigen sind. Im stahlbaulichen Kontext werden die mechanisch gefügten Bauteile aus Korrosionsschutzgründen jedoch oftmals beschichtet. Aus diesem Grund ist anstelle einer analytischen Berechnung häufig eine versuchsgestützte Ermittlung der Verluste notwendig. Umfang und Aufwand entsprechender Versuchskampagnen im Labor oder im Feld können jedoch unter Zuhilfenahme bestehender Erfahrungen entscheidend reduziert werden. Der vorliegende Beitrag thematisiert daher Laboruntersuchungen in Anlehnung an die IEC‐Richtlinie 61400‐6, in denen erstmals systematisch die Sensitivität der Vorspannkraftverluste gegenüber relevanten Einflussgrößen bewertet wurde. Hierbei wurden neben Langzeitsetzversuchen auch Versuche unter schwingender Beanspruchung durchgeführt, die aus für Windenergieanlagen typischen Belastungssituationen abgeleitet wurden.

Abstract Die Fertigung kundenindividueller elektronischer Bauteile ist durch volatile Losgrößen und heterogene Automatisierungsgrade geprägt. Trotz verfügbarer Assistenzsysteme können zeitbezogene Abweichungen oft erst mit hohem Aufwand in der Produktionsleitung identifiziert und geeignete Maßnahmen verzögert eingeleitet werden. Um eine proaktive Steuerung auf Feldebene zu realisieren, bedarf es einer teilautomatisierten Lösung. Process Mining stellt einen datenbasierten Ansatz dar, um den organisatorischen und zeitlichen Produktionsablauf aufzudecken. Anhand prädiktiver Ergebnisse im operativen Betrieb können somit geeignete Maßnahmen bereitgestellt werden. In diesem Beitrag wird ein anwendungsorientierter Ansatz zur vorhersagenden Prozesssteuerung von Fertigungsabweichungen unter Einsatz von Process Mining vorgestellt.