Volume 50, Issue 5

A failure analysis was carried out on a riveted half pulley mounted on the CVT system of the new Piaggio 850cc TWIN engine. First of all, different design variants were analysed with simplified models in order to single out the one with the best performance. The best configuration was then re-analysed with a more detailed model including the stress state induced by the riveting process, which was simulated with a separate model. The working conditions corresponding to Piaggio standard endurance cycle were used to estimate the fatigue damage withstood by the component. Finally, the critical working condition was singled out and the corresponding stress state was compared with the one arising during Piaggio standard bench test for the examined component.

The aim of the paper is to analyze a very common orthopedic implant fractured during its use and removed from the human body. The implant was analyzed by using optical and electronical metallography. The results of this study revealed the existence of nonmetallic inclusions which could promote the fracture initiation. The microstructural aspect indicates significant plastic deformations near the holes and the presence in the same places of crevice corrosion. In order to develop new types of plate implants, a Finite Element Analysis was performed on a similar plate. The optimal conditions for fixture were established by studying the stresses and strains distributions in the modeled plate. The three dimensional design of the surgical implants and the mechanical behaviour simulation under the external load will be useful to avoid the implant fracture and to better solve any problem concerning long bones implantation.

In this study retrogression and aging treatment have been applied to the samples of an aluminum alloy AA 7075-T6. The retrogression treatment has been performed with different temperatures and times and then aging treatment has been applied at 130°C for 12 h. By determining the effects of the retrogression treatment temperatures and times on the samples, the microstructral and also mechanical changes in AA 7075-T6 alloy have been examined. All microstructures have been investigated by Scanning Electron Microscopy (SEM). Additionally, the mechanical properties have been determined by the notched impact test and the tensile test.

In this study, AA 6063 (AlMgSi0.5) Aluminum alloy plates were welded by a solid state welding procedure, friction stir welding (FSW). The influence of pin structure on the microstructure of the weld region and the mechanical properties of the joints were studied. It has been seen that the helical structure on pinscrew has an effective role on the formation of a zigzag line in the weld region. While rather long and uninterrupted zigzag lines in the weld region are obtained when right helical pinscrew is used, left helical pinscrew gave rise to shorter zigzag lines. While the zigzag line has no effect on the mechanical properties of the as-welded joint, heat treatment after the welding (PWHT) procedure seriously affects the strength of the joint due to the zigzag line formed in the weld region. The mechanical strength decreases with right helical pinscrew in PWHT, whereas, left helical pinscrew causes an increase in mechanical strength.

Austempered ductile iron (ADI) exhibits a wide range of mechanical properties depending on the austempering conditions. It is thus extremely important to evaluate the different characteristics of ADI welds where joining is inevitable. In this study, unalloyed ferritic-pearlitic ductile iron specimens which contain 3.69% C, 2.47% Si and 0.2% Mn were austempered in different temperatures to obtain an upper and lower bainitic microstructure and then welded using Gas Tungsten Arc Welding (GTAW) without filler. The results showed that the lower and upper bainitic ADIs can be welded successfully without any crack formation in the heat affected zone (HAZ). It was also observed that the HAZ of the specimen which have been austempered at 275°C for 60 min exhibit a higher hardness than that austempered at 375°C. As the critical austempering time for a homogeneous HAZ characteristic 100 min has been identified.

Kurzfassung Der vorliegende Beitrag befasst sich mit dem Aufbau und den Eigenschaften von Laserschweißverbindungen des Chromstahls X 20 Cr 13 mit der Nickellegierung NiCr 20 TiAl. Die Laserschweißungen wurden mit und ohne Zusatzwerkstoff durchgeführt. Zur Gefüge- und Eigenschaftsbeeinflussung der Schmelzzone wurde als Zusatzwerkstoff Titan eingesetzt. Die Eigenschaftscharakterisierung an den Versuchsschweißungen erfolgte mittels Kleinlasthärteprüfung nach Vickers. Im Bereich der Schmelzzone und der Wärmeeinflusszonen erfolgten die Gefügeuntersuchungen am Rasterelektronenmikroskop (REM). Die chemische Zusammensetzung des Schweißgutes wurde mittels EDX-Analyse bestimmt. Die Auswirkung der Elemente der miteinander verschweißten Werkstoffe auf die Schmelzzone wurde mithilfe thermodynamischer Berechnungen abgeschätzt.

Kurzfassung Das neuartige Rekonstruktionsverfahren DIRECTT (Direkte iterative Rekonstruktion computertomographischer Trajektorien) verarbeitet die Objektprojektionen im Unterschied zur bekannten gefilterten Rückprojektion als selektiv gefilterte inverse Radontransformation in einer iterativen Prozedur. Damit kann die Ortsauflösung des Detektors weit übertroffen werden, weil die Limitierungen des Abtasttheorems durch Überabtastung umgangen werden. Zweidimensionale Modellrechnungen demonstrieren auch die Vorzüge für lokale (ROI-) Rekonstruktionen und fehlende Projektionen (Limited-View-Problem) im Vergleich zur gefilterten Rückprojektion.

A preferential orientation of fibres is induced by the production process of fibre reinforced autoclaved aerated concrete. For a better understanding of the development of the fibre arrangement during the foaming process of the concrete, digital radiography was used to study the evolution of orientation in situ. The strength and the deformation behaviour of this building material are largely influenced by the fibre orientation. Therefore information on the fibre orientation is highly desirable. Computed tomography measurements allow for the non destructive investigation of the material. The contrast between fibres and concrete matrix is sufficiently high, even for glass fibres. Two different ways to study the fibre orientation are compared. The results correlate with the strength and the deformation behaviour of the samples. A high resolution study of the fibre environment shows that the fibres align the adjacent pores.

The application of the shakedown theory to the analysis of the fatigue problems is presented. This point of view on the fatigue strength and the fatigue damage allows interpreting all types of industrial problems of fatigue in an efficient manner. The description of the stabilized mechanical state after shakedown at all scale, gives access to the engineering values which will drive the fatigue damage of the structure. The evolution of stress and strain tensors thus obtained for a cycle of fatigue, allows predicting the fatigue strength of structures. At high cycle fatigue, when elastic shakedown takes place, the fatigue strength can be described by a simple combination of shear amplitude and hydrostatic pressure at any point of the calculated structure, irrespective of the complexity of the structure and of the loading. At low cycle fatigue, from the stabilized state of the stress and strain, the fatigue strength is derived from a combination of the plastic, or the inelastic strain energy and the hydrostatic pressure. Numerous examples of fatigue analysis for different industrial areas are presented.