Biaxiale Ermüdung reibrührgeschweißter Strukturelemente

Biaxial fatigue of friction stir welded stiffened panels Within the framework of the European WelAir project, cruciform specimens made from stiffening FSW overlap joints were fatigued in the DLR biaxial test rig. To resemble the loading situation of pressurized fuselage structures, proportional loading without any phase shift, but with different load ratios λ between the loading components in both directions was applied.Natural crack initiation and subsequent crack growth were governed by the stiffness gradient caused by introducing the stringer. Cracks initiated and propagated at run‐in and run‐out locations in a direction perpendicular to the weld seam. The shortest fatigue life was observed for uniaxial loading in welding direction (λ = 0). An additional stress component perpendicular to the joint line (λ > 0) resulted in a higher number of cycles to failure. Similar to single stringer panels, increasing the load ratio also increased the number of cycles to failure for FSW clip‐stringer structural members, but additionally gives a different location of the fatal crack.     

Identification of local cyclic stress‐strain curves at material inhomogeneities on the basis of digital image correlation

Low‐cycle fatigue (LCF) tests on butt‐welded joints revealed that material inhomogeneities become decisive for the fatigue behavior at elastic‐plastic strain amplitudes and the geometrical notch is no longer the failure location – as observed in high‐cycle fatigue (HCF) regime. Digital image correlation techniques enable the measurement of local strains at the surface of the welded region. A new method is proposed that is able to calculate related local stresses and identifies local cyclic material properties in structures of inhomogeneous material and geometry. This method combines strain measurements and Finite Element simulations. It is a fast iteration procedure that is able to perform the identification within less than ten iterative steps at several hundred local points. Transient material behavior is easy to capture and an application at high temperatures is anticipated due to the method is based on materials mechanics.     

Untersuchung des Einflusses der Beanspruchungsgeschwindigkeit auf das Deformations‐ und Bruchverhalten von Weizenkörnern

Compression tests of deformation and fracture behavior of wheat grains were carried out at different loading rates that varied from 0.02 to 0.11 mm s^–1. The contact model originally developed for the spherical particles was adapted to describe the deformation and fracture behavior of elliptical wheat grains. The distributions of basic fracture parameters of wheat grains such as fracture force, fracture strength, fracture energy, and fracture displacement were fitted with normal distribution function.     

Hochleistungsleichtbeton unter mehraxialer Druckbeanspruchung

An vielen Stellen im Tragwerk herrschen mehraxiale Spannungszustände. Ob sich die Tragfähigkeit von Hochleistungsleichtbetonen unter mehraxialer Druckbeanspruchung ebenso erhöht wie bei Normalbetonen, wurde in zwei‐ und dreiaxialen Tests untersucht. Es wurde ein Bruchkriterium für Leichtbetone abgeleitet und Parallelen und Unterschiede zu Normalbeton aufgezeigt. High Performance Lightweight Aggregate Concrete under Multiaxial Compression There are multiaxial stress conditions at many points in a structure. Whether the strength of high performance lightweight concrete under multiaxial compression increases as well as the strength of normal concrete, was verified in bi‐ and triaxial tests. A failure criterion for lightweight concrete was derived and analogies and differences to normal oncrete were pointed out.     

Microstructural evolution and mechanical properties of CoCrFeNiMnTix high-entropy alloys

This study was conducted to investigate the effect of titanium addition on the microstructure and properties of an equitaomic CoCrFeNiMn high-entropy alloy. Homogenized microstructures of CoCrFeNiMnTi_x (x = 0.1 and 0.3) alloys consist of face-centered cubic phase; however, addition of more titanium led to formation of a (chromium, titanium)-rich σ phase in CoCrFeNiMnTi_0.4 alloy. The average electron hole number calculations indicate the higher possibility of σ phase formation by adding more titanium. Furthermore, addition of an atom like titanium with a larger atomic radius in comparison with other elements can affect stability of face-centered cubic structure. Chromium and manganese has a destabilizing influence on the single face-centered cubic phase and manganese may reject chromium to facilitate the formation of a (chromium, titanium)-rich phase in alloys containing more than 5.5 at.% titanium (x>0.3). The mechanical properties revealed an improvement in strength without losing the ductility drastically by adding titanium up to 5.5 at.% (x = 0.3). Nevertheless, the strength remarkably increased and ductility significantly decreased in CoCrFeNiMnTi_0.4 alloy due to formation of brittle σ phase in the microstructure.     

Näherung für elastisch‐plastische Kerbbeanspruchungen einschließend thermisch induzierte Beanspruchungen

Strukturmechanische Belastungen unterscheiden sich maßgeblich von thermischen und somit verformungskontrollierten Belastungen, was sich insbesondere bei der Anwendung von herkömmlichen Kerbnäherungsverfahren zeigt. Im Rahmen dieser Ausarbeitung wird eine Methodik präsentiert, die es erlaubt, elastisch‐plastische Kerbbeanspruchungen infolge thermischer Belastung auf Grundlage fiktiv‐elastischer Berechnungen im Bauteil approximativ zu berechnen. Weiterhin wird gezeigt, wie die erzielten lokalen elastisch‐plastischen Teillösungen für die mechanische und die thermische Belastung miteinander kombiniert werden können und welchen Einfluss das auf die Güte der Ergebnisse hat.