Stefan Reh

A Stochastic Reliability Model for Application in a Multidisciplinary Optimization of a Low Pressure Turbine Blade Made of Titanium Aluminide

CURRENTLY, THERE ARE A LOT OF RESEARCH ACTIVITIES DEALING WITH TITANI-UM ALUMINIDE (TIAL) AS A NEW MATERIAL FOR LOW PRESSURE TURBINE (LPT) BLADES. EVEN THOUGH THE SCATTER IN MECHANICAL PROPERTIES OF SUCH AN INTERMETALLIC ALLOY IS MORE DISTINCTIVE AS IN CONVENTIONAL METALLIC ALLOYS, STOCHASTIC INVESTIGATIONS ON TIAL ARE VERY RARE. FOR THIS REASON, WE ANALYZED THE SCATTER IN STATIC AND DYNAMIC ME-CHANICAL PROPERTIES OF THE CAST ALLOY TI-48AL-2CR-2NB. IT WAS FOUND THAT THIS ALLOY SHOWS A SIZE EFFECT IN STRENGTH WHICH IS LESS PRONOUNCED THAN THE SIZE EFFECT OF BRITTLE MATERIALS. A WEAKEST-LINK APPROACH IS ENHANCED FOR DESCRIBING A SCALABLE SIZE EFFECT UNDER MULTIAXIAL STRESS STATES AND IMPLEMENTED IN A POSTPROCESSING TOOL FOR RELIABILITY ANALYSIS OF REAL COMPONENTS. THE DEVELOPED RELIABIL-ITY TOOL WAS INTEGRATED INTO A MULTIDISCIPLINARY OPTIMIZATION OF THE GEOMETRY OF A LPT BLADE. SOME PROCESSES OF THE OPTIMIZATION WERE DISTRIBUTED IN A WIDE AREA NETWORK, SO THAT SPECIALIZED TOOLS FOR EACH DISCIPLINE COULD BE EMPLOYED. THE OPTIMIZATION RESULTS SHOW, THAT IT IS POSSIBLE TO INCREASE THE AERODYNAMIC EFFICIENCY AND THE STRUCTURAL MECHANICS RELIABILITY AT THE SAME TIME, WHILE ENSURING THE BLADE CAN BE MANUFACTURED IN AN INVESTMENT CASTING PROCESS.

Mechanismenorientierte bruchmechanische Charakterisierung von Luftfahrtlegierungen unter realitätsnahen Beanspruchungen

Im Rahmen dieser Arbeit wird eine Vorgehensweise aufgezeigt, wie sich Ermudungsrisse im Hautfeld eines Flugzeugrumpfs auf reprasentativen, biaxialen Kreuzproben abbilden und untersuchen lassen. Grundlage hierfur ist eine genaue Analyse der Rissbeanspruchungen und der Belastungsverlaufe des sogenannten IMA-Rumpfschalentests. Die Ubertragung der Lasten erfolgte mittels Finite Elemente Simulationen und einer Auswertung des vorgegebenen Last-Zeitverlaufs. Experimentell wurden dann exemplarisch Kreuzproben aus den Aluminiumlegierungen AA2024-T351 und AA5028-H116 fur das Szenario eines Umfangsrisses uber einem gebrochenen Stringer getestet. Begleitet wurden die Experimente mittels digitaler Bildkorrelation zur Untersuchung des Verschiebungsfeldes der Probe. Fur die Ermittlung der tatsachlichen Rissspitzenbeanspruchung K_I und K_II wurde auserdem ein Post-Prozessor basierend auf dem Wechselwirkungsintegral entwickelt. Weitere Tests und Simulationen untersuchten das Ablenkungsverhalten des Ermudungsrisses an Kreuzproben unter biaxialen Belastungen. Somit stehen Methodiken zur Verfugung, mit denen sich realitatsnah Ermudungsrisse des Flugzeugrumpfs auf kostengunstigen Kreuzproben mit einer hohen Aussagefahigkeit untersuchen lassen. Da bei duktilen Materialien der Ursprung aller relevanten Effekte der Ermudungsrissausbreitung in der plastischen Zone an der Rissspitze liegt, wurde diese zusatzlich auf mehreren Grosenskalen grundlegend untersucht und charakterisiert. Hierzu wurden Simulationen und Experimente unter zyklischen Mode I und Mixed-Mode Beanspruchungen exemplarisch an der Aluminiumlegierung AA2024-T3 durchgefuhrt. Wahrend der ersten Belastung bildet sich eine primare plastische Zone. Alle nachfolgenden Lastzyklen fuhren dann zur zyklischen plastischen Zone, die sich wiederum abhangig der Lastsituation in eine vorwarts und eine ruckwarts zyklische plastische Zone aufspalten lasst. Die Detailauswertung der experimentell ermittelten DIC-Verschiebungsfelder erfolgte hierbei durch deren Projektion auf ein FE-Netz. Zusammen mit einem Materialmodell konnten die Totaldehnungen in elastische und plastische Dehnungen unterteilt werden. Plastische Verformungen erzeugten dabei Versetzungen in duktilen Materialen, die im Fall von AA2024-T3 zu einer Verfestigung fuhren. Anhand von Mikrohartemessungen wurde dabei die Auspragung der plastischen Zone im Volumen schichtweise untersucht. Mittels Electron Channeling Contrast Imaging (ECCI) wurde auf mikrostruktureller Ebene schlieslich beobachtet, wie zunehmende plastische Verformungen zu steigenden Versetzungsdichten im Werkstoff fuhren. Diese experimentellen Beobachtungen erweiterten somit das Mechanismenverstandnis zum Verformungsverhalten von Al-Legierungen unter Ermudungsbeanspruchung.

Efficient Lifetime Prediction of High Pressure Turbine Blades in Real Life Conditions

Jet engines of airplanes are designed such that in some components damage occurs and grows in service without being critical up to a certain level. Since maintenance, repair and component exchange are cost-intensive and limit the operating life of the engine, it is necessary to predict the component lifetime using an acceptable computational effort. To efficiently calculate the lifetime consumption of turbine components with sufficient accuracy under operational conditions, we developed a hybrid approach, which is based on the following three steps: First, the possible operation space is analyzed and reduced to define a manageable Design-of-Experiments (DoE) space. Subsequently, precise aerodynamic and structural mechanic simulations of the component are performed at each DoE support point and the results are stored in a database. Next, the lifetime consumption of the component for the operation profile of interest is calculated based on interpolated stress and temperature fields using suitable lifetime prediction models. The implemented lifetime models are based on accepted lifetime prediction models for creep, fatigue and combined loading, which were extended to incorporate the loading situation on a high pressure turbine (HPT) blade. Due to efficient data management, the computational time for calculating the lifetime consumption of a whole HPT blade is approximately four seconds for one take-off. Consequently, a full three dimensional lifetime consumption analysis of the lifespan of a HPT blade is possible within a few hours. Using the developed approach, it is now possible to predict the lifetime of a HPT blade for different operators with the necessary precision in an acceptable time. To demonstrate the developed approach, a HPT blade of an exemplarily chosen jet engine with known flight history and documented borescope inspections will be used. Comparing the calculated lifetime of the HPT blade with the documented findings from shop visits reveals that the simulation is in good agreement for the investigated flight mission of the chosen engine.

Adaption of a material model and development of a stochastic failure criterion for ceramic matrix composite structures

Ceramic matrix composites (CMCs) show high thermal resistance combined with damage tolerance and non-brittle failure. Therefore, they are suitable for mechanically loaded components at high temperatures. For dimensioning of components, an adaption of existing simulation and calculation methods for CMC materials is required. The material investigated in this work is a wound oxide CMC with a defined fiber orientation and a highly porous matrix. Due to manufacturing conditions, randomly distributed macroscopic pores are present in the composite. Since failure is initiated by these pores, a statistical investigation of strength is performed based on the weakest link theory. Several series of tensile tests were performed to reveal the relation between stress and strain and obtain their ultimate values. The application of two statistical criteria showed that fracture strains are well represented by Weibull distributions. The tensile tests were compared with results of finite element simulations, using the simulation software ANSYS®. An anisotropic Weibull criterion was set up and adapted to the results of the tensile tests. Hereby, the anisotropic nonlinear deformation behavior as well as the scatter of failure strain could be reproduced by numerical simulations. The adapted material model and failure criterion were validated by further experiments and exemplified by application in a reliability analysis of a notional flame tube.