Volume 46, Issue 4

As revealed by an inspection of hot gas path components, a dampening pin for the stage 4 moving blades of an industrial gas turbine for power plant applications brake in the summer of 2003. The failure mode of the fractured pin was high cycle fatigue. The root cause of the failure was probably a combination of the dynamic service load and an increased notch effect caused by an increased surface roughness of the pin. In the case of the notch-sensitive material Udimet 720, this was detrimental to the fatigue limit. The measured surface roughness of the failed part was out of specification and traceable to the use of non-OEM (Original Equipment Manufacturer) components. High temperature corrosion did not seem to have played a significant role in the failure mechanism.

This paper aims at a better understanding of the fatigue behaviour of highly loaded railway wheel materials under special consideration of the microstructural gradients in the wheels/tires due to the industrial heat treatments and elevated temperatures developing in the wheel/rail contact area during service loading. The experiments with specimens machined from original components were performed with servohydraulic testing systems. In addition to mechanical hysteresis measurements, the change of the specimen temperature and the electrical potential (voltage) due to plastic deformation processes were recorded in ambient temperature tests. Besides characteristic cyclic softening and hardening processes, dynamic strain ageing effects were observed in a defined temperature range in constant amplitude tests. The microstructural characterisation of the different material conditions was done by light (LM), scanning (SEM) and transmission (TEM) electron microscopy together with digital image processing.

Generally, most machine parts are loaded with a combination of different variable forces and moments. This often causes a state of multiaxial stress in the fatigue critical areas of the parts. In most cases, a nonproportional cyclic multiaxial state of stress occurs. Compared to the in-phase loading, a multiaxial loading with a phase difference of 90° between the stress components and a load ratio of γ a /∊ a = 1.33 (τ a /σ a ≈ 0.5) between tension/compression and torsion lead to a significant influence on the fatigue lifetime. The reason is the changing direction and ratio of the principal stresses during one cycle. The present research results for constant and variable amplitudes indicate that a phase difference of 90° causes a decreased lifetime compared with proportional loading under strain controlled tests with different ductile materials AlMgSi1 and SAE 1015, while the experimental analysis with magnesium die casting alloy AZ 91 reveals an increase of fatigue life compared with the in-phase loading. The lifetime prediction with a new concept on the basis of microcrack growth is compared with the experimental results.

Instrumented notched tensile-impact testing (NTI test) makes possible a comprehensive toughness characterization for very thin and/or flexible materials also by using fracture mechanical methods. Through instrumentation of the pendulum device, load–extension diagrams can be recorded which allow a description of deformation and fracture behaviour in a wide-ranging way. The aim of this contribution is to show details of this still relatively unusual test method “instrumented notched tensile-impact testing”. Therefore, information is given on suitable specimens, performance of the test and its analysis. Furthermore, some examples of application are shown.

Kurzfassung Im vorliegenden Beitrag wird der Einfluss mehrerer Parameter (Fahrer, Beanspruchung etc.) auf die Spannungen in der Fahrzeugkarosserie diskutiert. Darüber hinaus wird die Variation der Spannungen während der realen Testfahrt mit denen verglichen, die sich während des Gesamtfahrzeug-Nachfahrversuches ergeben.

Kurzfassung Das vorgestellte Messverfahren von Längenänderungen an Proben erfolgt laseroptisch. Dabei wird der Speckle-Effekt genutzt, eine Interferenzerscheinung von gestreutem kohärentem Licht. Die zu vermessende Probe bedarf keiner vorgängigen Präparierung wie beispielsweise dem Anbringen von Messmarken. Das Verfahren liefert bei hoher Messfrequenz präzise und normgerechte Messwerte, auch bei sprunghaft verlaufenden Probendeformationen. Es ist für Messungen an Proben bei unterschiedlichen Temperaturen geeignet.

Kurzfassung In diesem Beitrag werden einleitend die Schwerpunkte der weltweiten Normungsarbeit erläutert und ihre Ergebnisse in Zahlen angegeben. Die neuesten Entwicklungen in der Normung auf dem Gebiet der Werkstoffprüfung sind für folgende Gebiete vorgestellt: Mechanisch-technologische Prüfverfahren (Härteprüfung, Zugversuch, Kerbschlagbiegeversuch), Prüfung und Kalibrierung von Werkstoffprüfmaschinen und Prüfbescheinigungen nach DIN EN 10204. Des Weiteren sind die aktuellen Informationen über die Sitzungen der entsprechenden internationalen Normungsgremien enthalten.