Rainer Masendorf

Zur Wiederverwendung von Durchläufern im Treppenstufenversuch

Kurzfassung Viele Bauteile sind während ihrer Nutzungsdauer schwingenden Beanspruchungen ausgesetzt. Ermüdungsfestigkeitsnachweise sollen den sicheren Betrieb schwingend beanspruchter Bauteile garantieren. Ein wichtiger Kennwert zur Führung von Ermüdungsfestigkeitsnachweisen ist die Bauteildauerfestigkeit. Ihre experimentelle Bestimmung ist zeit- und kostenintensiv. Häufig stehen nur wenige Prüfkörper zur Verfügung, sodass eine statistische Auswertung unmöglich wird. Um dennoch Aussagen über die Bauteildauerfestigkeit abzuleiten, können so genannte Durchläufer wiederholt eingesetzt werden. Im vorliegenden Beitrag werden die Auswirkungen des Wiedereinsatzes von Durchläufern diskutiert. Hierbei werden keine werkstoffmechanischen Betrachtungen zur Vorschädigung oder zur Trainierwirkung angestellt. Vielmehr werden Simulationsmethoden vorgestellt, mit denen sich maximale Fehler bei der 50%-Dauerfestigkeitsschätzung ableiten lassen. Durch nachträgliche Berücksichtigung des maximalen Fehlers kann ein Bemessungskennwert auch mit wenigen Proben abgeleitet werden.

On the accuracy of estimating fatigue notch factors

Abstract The fatigue notch factor is the quotient of the nominal stress endurance limits of unnotched and notched specimens. It represents the notch sensitivity of a material or a component under cyclic loading. For example, the fatigue notch factor is used in analytical strength assessments with nominal stress and is also suitable as a quality criterion for materials and entire components. On the one hand, an analytical approach to rate the reliability of the fatigue notch factor is derived assuming a log-normal distribution for the population of the endurance limits of the unnotched and notched specimens. The analytical approach provides the best achievable result and can be used to rate real experiments. On the other hand, the staircase method, which is often used to determine endurance limits, is simulated using Monte Carlo simulations. From the simulations, information about the reliability of the real test can be gathered, which can be compared to the analytical approach. In conclusion, in this paper the reliability of the fracture of two originally log-normally distributed random variables each used in an own staircase test is examined. The result is compared to an analytical approach. In former articles the scope has only been on single test series, the examination of error propagation, e.g, by using the staircase method and building the fracture of the two results afterwards, is missing in former examinations. The staircase method is a procedure to estimate fatigue notch factors with reliabilities comparable to those of the analytical approach. To increase the number of evaluable tests, runouts should be reused.

Execution and evaluation of cyclic tests at constant load amplitudes – DIN 50100:2016

Abstract Tests with constant load amplitudes are used to characterize the fatigue strength behavior of material specimens and components. The S-N curve derived from these test results describes the relationship between the load amplitude and the corresponding cycles to failure. Different concepts for carrying out and evaluating fatigue tests make it difficult to compare the results from different research institutes. The aim of the new version of the German standard DIN 50100:2016 is to define a procedure for determining an S-N curve for metallic alloys that does not allow any scope for interpretation. It is assumed that the test results are subject to logarithmic normal distributions in both load and cycle direction. It is further assumed that the S-N curve in the high-cycle fatigue regime and the long-life fatigue regime can be approximated by a bilinear function. For the determination of the straight line of finite life, the pearl string method and the load level method are available for determining the position parameter and the slope of the power function according to Basquin. Long-life fatigue strength is determined using the staircase method and forms the knee point of the S-N curve on average with the straight line of finite life. For the long-life fatigue regime, a horizontal course or a decrease with low inclination, depending on the material group examined, is assumed. In addition, DIN 50100:2016 contains information on the accuracy of the estimation of the mean values and the scatter of the characteristic values according to the sample size. The goal of achieving comparability of S-N curves is supported by extensive examples. An English translation of DIN 50100:2016 is available also.

Distribution functions for the linear region of the S-N curve

Abstract This study establishes a database containing the results of fatigue tests from the linear region of the S-N curve using sources from the literature. Each set of test results originates from testing metallic components on a single load level. Eighty-nine test series with sample sizes of 14 ≤ n ≤ 500 are included in the database, resulting in a sum of 6,086 individual test results. The test series are tested in terms of the type of distribution function (log-normal or 2-parameter Weibull) using the Shapiro-Wilk test, the Anderson-Darling test and probability plots. The majority of the tested individual test results follows a log-normal distribution.

Methoden zur Abschätzung zyklischer Werkstoffkennwerte

Zyklische Kennwerte konnen z.B. mithilfe des Uniform Material Law (UML) aus der Zugfestigkeit abgeschatzt werden. Das UML gilt fur einen eingeschrankten Bereich von un- und niedriglegierten Stahlen. Mithilfe einer grosen Datenbasis wird die Treffsicherheit des UML bewertet und eine neuen Abschatzmethode abgeleitet, die fur die gesamte Werkstoffgruppe der Stahle (ausgenommen Stahlguss) gilt. Fur die aus der abgeschatzten Dehnungswohlerlinie bestimmte PSWT-Schadigungsparameter-Wohlerlinie wird ein Konzept zur Umrechnung auf eine Ausfallwahrscheinlichkeit von 2,5 % angegeben.

Estimating the Lifetime of Fuel Cells by the Methods of Fatigue-Strength Analysis under Variable Stress Amplitude

Abstract Estimating the fatigue life of metallic components has become an essential requirement in the industry because of the need to use light-weight constructions in conformance with stringent safety-relevant standards. However, the application of such methods for predicting the lifetime of electrochemical devices such as fuel cells is a novel approach. In this case, pertinent events and parameters, such as normal operation, open-circuit voltage, as well as starting and stopping processes, were extracted from the data determined from the measured current and voltage-time curve. For this purpose the classic counting methods, level-distribution counting and frequency counting are applied. The result thus obtained is assessed with the use of a damage parameter which describes the voltage drop as a function of time or of the number of cycles counted. Finally, the calculative estimation of the lifetime is determined by linear damage accumulation.

Fatigue Life Calculation Concepts for Structures with Locally Modified Properties

The influence of forming on the cyclic behaviour of various thin sheet metals with austenitic phases is looked into, with the main focus lying on the variation of cyclic parameters and the progression of work hardening over the materials’ fatigue life. There is a marked interaction of hardening due to increased dislocation density resulting from forming and hardening due to cyclic straining under varying strain amplitudes of strain controlled LCF testing. This is strongly dependant on the degree of forming and the amplitude of the following cyclic loading. Also, the temperature influence during forming of addtional forming elements in a temperature controlled die on the fatigue performance of austenitic and complex three phase steels was analysed, showing a marked influence of the forming temperature on the fatigue performance, enabling positive effects to be achieved by choosing suitable material specific forming temperatures.

Mittelspannungseinfluss bei Strukturen aus umgeformtem Blech: Herrn Prof. Dr.-Ing. H. Zenner zum 70. Geburtstag gewidmet

Kurzfassung Überlagerte Mittelspannungen können einen erheblichen Einfluss auf die Lebensdauer von Bauteilen unter schwingender Beanspruchung haben. Im Verlauf einer Lebensdauerberechnung kann ihr Einfluss auf die ertragbare Spannungsamplitude mithilfe der Mittelspannungsempfindlichkeit berücksichtigt werden. Nach verschiedenen Ansätzen wird die Mittelspannungsempfindlichkeit als konstant angenommen oder in Abhängigkeit von Zug- oder Wechselfestigkeit abgeschätzt. Die Treffsicherheit dieser Ansätze bei Feinblechen wird anhand von Wöhlerversuchen untersucht. Ein neuer Ansatz zur Abschätzung der Mittelspannungsempfindlichkeit mithilfe künstlicher neuronaler Netze zeigt, dass die Mittelspannungsempfindlichkeit neben der Zugfestigkeit auch von der Formzahl bzw. vom Spannungsgradienten abhängt. Die Übertragbarkeit der Mittelspannungsempfindlichkeit vom Probestab auf eine Struktur aus umgeformtem Feinblech wird abschließend durch den Vergleich von Berechnung und Versuch überprüft.

Low Heat Joining – Manufacture and Fatigue of Soldered Locally Strengthened Structures

The advantages of low heat joining techniques, such as low distortion and little influence on the local material properties due to the low introduced amount of heat, shall be made usable for the manufacture of high strength structures by increasing the process reliability. The dependency between the parameters of the joining process, the seam geometry, the type of solder, the load type und the fatigue life especially of soldered structure with local strengthening shall be examined to allow a calculative estimation of the part’s life.

Fatigue Life Calculation Concepts for Structures with Inhomogenous Strength Properties

A fast transfer of new manufacturing- and material technologies to actual design use is only to be expected if calculation concepts exist, which allow an easy estimation of the parts’ strength properties. This Collaborative Research Project (CRP) will develop new manufacturing and joining techniques to create high strength structures by adjusting the local strength properties of parts. These types of structures are usually loaded dynamically. To specifically optimise the fatigue behaviour, stiffness and weight of a structure, the influences of locally strengthening manufacturing processes must be considered in the fatigue life calculation concept. It is the goal of this project to include a simulation of the fatigue behaviour in the process simulation of manufacturing. Recently research on the influence of local strengthening by cold forming on fatigue life was undertaken in cooperation within another research project. The fatigue life calculation of sheet metal structures can be based on a calculation concept developed in CRP 362, subproject C5, which takes into account the influence of forming, [1,2,3,4]. This concept shall be extended to incorporate the effects of local martensite forming in the cold formed areas.