Volume 52, Issue 5

Kurzfassung Ermüdungsfestigkeitsnachweise von Bauteilen werden häufig nach dem Nennspannungskonzept bzw. dem örtlichen Spannungskonzept geführt. Einen zentralen Punkt dieser Konzepte bildet die lineare Schadensakkumulation nach Palmgren und Miner (Miner-Regel). Statistische Auswertungen zeigen, dass die Lebensdauerabschätzung nach der linearen Schadensakkumulation im Mittel auf der unsicheren Seite liegt und eine höhere Treffsicherheit wünschenswert ist. Durch die Anwendung effektiver Schadenssummen kann die mittlere Lebensdauerüberschätzung korrigiert werden. Im Einzelfall kann es dennoch zu größeren Abweichungen zwischen Rechnung und Versuch kommen. Zur Verbesserung der Lebensdauerabschätzung mittels linearer Schadensakkumulation werden zwei Lebensdauerkorrekturverfahren vorgestellt. Die Anpassung der rechnerischen Gaßnerlinie an Versuchsergebnisse erfolgt in Abhängigkeit von der Kollektivform und der Wöhlerlinienneigung durch einen Lebensdauerkorrekturexponenten anstatt einer (multiplikativen) konstanten effektiven Schadenssumme. Neben der höheren Treffsicherheit wird der Übergang der Gaßnerlinie in die Wöhlerlinie für Rechteckkollektive sichergestellt.

Kurzfassung Ein neues Sensorkonzept zur Überwachung von Ermüdungsrisswachstum in technischen Strukturen wird vorgestellt. Damit können unter Betriebsbelastungen fortlaufend Daten zur Position der Rissspitze und zu den bruchmechanischen Beanspruchungsgrößen gemessen werden. Die erforderlichen Informationen liefert eine piezoelektrische Polymerfolie, die mit der Oberfläche der zu überwachenden Struktur fest verbunden ist. Aus den an einem diskreten Sensorfeld abgegriffenen elektrischen Potenzialen werden in Verbindung mit der Lösung des nichtlinearen inversen Problems die K-Faktoren sowie Risskoordinaten berechnet.

Extent of experiment-related variability in fatigue crack propagation is essential to be known while comparing the performance with other materials, for assessment of welds or damage tolerance analysis. Since such a variability can be dependent on material or parameter investigated, and may originate from the size effect, thin sheets of the relatively new airframe alloy AA6056-T6 have been investigated for baseline data using C(T) and M(T) specimens. As a screening step, various parameters such as surface cladding, crack plane orientation, specimen width and thickness, and initial notch length have been varied on C(T) specimens. The material also stems from different heats. The maximum crack opening displacement (COD max ), obtained from a displacement gauge, is calibrated against the optical crack length for the indirect crack length measurement and its precision is increased by acquiring an individual calibration curve for each specimen. The variability in the mid-regime fatigue crack propagation range is found to be relatively low (± 35%). With this range as the tolerance, the alloy is found to exhibit mostly a parametric non-variance, since, except for the specimen type, other parameters investigated are found to be non-influential. M(T) specimens are found to yield conservative fatigue crack propagation data.

A number of common defects in stainless steel welding result from the presence of hydrogen in the weld. The service life of the stainless steel joints is further significantly dependent on the presence of hydrogen in the respective environment and the susceptibility of various weld microstructures to hydrogen degradation. As a relatively new materials generation, supermartensitic stainless steels (SMSS) are increasingly applied to substitute more expensive alloys, particularly in the oil and gas industries. As a result of their martensitic microstructure these alloys are prone to hydrogen assisted cracking (HAC). The resistance of SMSS to hydrogen assisted stress corrosion cracking (HASCC) during sour service has been extensively studied, predominantly for industrial purposes. Studies are primarily conducted with parent materials based on standard test procedures. The principal hydrogen behavior in welded SMSS microstructures has been less investigated. The central objectives of the study are to determine the hydrogen interactions with the microstructure of a Gas Tungsten Arc (GTA) welded SMSS and hydrogen trapping mechanisms. The interactions of hydrogen with various Tungsten Inert Gas (TIG) welded SMSS microstructures are investigated by means of X-ray diffraction (XRD) and optic (OM) and electronic microscopy (SEM). A number of methods have been employed for the estimation of the quantities of absorbed hydrogen. Hydrogen interaction with structural defects and the characteristics of hydrogen desorption have been studied by means of thermal desorption spectroscopy (TDS), and hydrogen content measurements (LECO analyses). The effects of the respective microstructure on hydrogen absorption and desorption behavior are discussed in detail.

In this study, a boron-alloyed experimental steel has been rolled to form a sheet product, and an austenization treatment has been performed, followed by rapid quenching and then tempering. By austenization, a homogeneous austenite phase is obtained and a martensitic microstructure is formed after rapid quenching. This kind of microstructure exhibits high hardness, but low toughness. Thus, a tempering heat treatment is applied at a proper temperature (200 °C and 600 °C) and various times, in order to provide optimum toughness and to develop an ideal combination of strength and toughness. After this process, at first the phases formed during the equilibrium state have been studied for the alloy system using ThermoCalc data, and also their amounts have been evaluated. The microstructures acquired after the heat treatments have been examined by light microscopy. The effects of the formed microstructures on mechanical properties are comparatively given by their strength, toughness, and elongation values. Furthermore, the failure mechanism with fractografical examinations has been determined by using a Scanning Electron Microscope (SEM).

In this study, the hydro-abrasive erosion behaviour of a C 45 steel in different heat-treated states, which are normalizing, oil-quenching, tempering after oil-quenching and boronizing thermochemical surface treatment, was investigated on a designed and manufactured hydro-abrasive wear test apparatus. Boronizing was performed in a solid medium consisting of B 4 C, SiC, and KBF 4 at a temperature of 900 °C for two hours. The microstructure and the wear regions of the specimens were characterized using an optical and a scanning electron microscope. In the tribogical system, the specimens were rotated in a system instead subjecting them to a fluid and moving abrasive particles. The wear tests were performed under a fixed rotational speed of 53 rpm for each specimen in the respective heat-treated state. The weight loss at the end of each wear period of 10 hours was measured using a precision electronic balance with an accuracy of 1 mg. It was concluded from the investigation that oil-quenching and tempering treatments led to significant improvement of the hydro-abrasive erosion resistance, while as-boronized and as-normalized specimens exhibited the lowest erosion resistance. In spite of its high surface hardness in the boronized state, the C45 steel did not affect positively its hydro-abrasive erosion resistance. In the as-normalized specimen, the low hydro-abrasive erosion resistance corresponded with a decrease of hardness. An increasing amount of ferrite and pearlite in the C45 steel led to the reduction of the hydro-abrasive erosion resistance.

This work is the continuation of a study which is concerned with the kinematics of an universal joint and its use in shaft connections. In the present work, case studies are carried out by numerical implementation of the kinematic formulas illustrated in a previous paper in which two different systems for transmission of rotational motions are kinematically analysed. The first of these systems consists of two shafts (an input and an output shaft) connected via an universal joint, while the other comprises three shafts (an input, an intermediate, and an output shaft) and two universal joints connecting them. Furthermore, it is investigated how the angles between shaft axes, the arm lengths of the cross pieces, the angles between cross arms, and the angle between yoke planes of the intermediate shaft affect the characteristics of output motion. For both systems, the relationships between rotation angles of shafts are derivated by applying the rigidity condition on the cross arms. The case studies show that the arm lengths of cross pieces have no effect on the angular relations, independently of the angles between arms. However, in a coplanar system of the second type, either parallel or crossed assembled, it is concluded that an input rotation cannot be transmitted onto the output shaft without losing its kinematic features as far as the angles between the cross arms in each joint deviate from 90°, and also the angle between the yoke planes from 0°, respectively. The interference of the working planes of the joint is not included in the study.