Cumulative fatigue damage analysis plays a key role in life prediction of elements and structures subjected to field loading histories. A general methodology for fatigue life prediction of metallic materials under variable amplitude loading is proposed in this paper. A novel theory of cumulative fatigue damage is presented. It is based on Continuum Damage Mechanics approach which relates the damage rate to that of cyclic loading through the damage function. A new damage function related to isodamage lines and remaining life aspects is analyzed. The final result is a nonlinear fatigue damage rule. The nonlinear damage accumulation rule proposed in this paper improves the deficiencies inherent in other rules and still maintains its simplicity in the application. The relationship with other formulations is pointed out together with the advantages of the proposed model. A number of fatigue damage rules can be derived as a special case of the model developed herein. A wide range of fatigue data available in the literature as well as from the laboratory are used to validate the proposed rule. The agreement between prediction and experiment is found to be excellent. The prediction results are also compared with existing fatigue rules.
A method is presented for the prediction of metallic materials creep behaviour at time‐variable temperatures, exclusively with creep input data under constant loading. The method is derived from a more general physical‐phenomenological model and, additionally, incorporates the prediction under time‐variable stresses. To test the method's predicting capability, a series of experiments was carried out for the creep strain at time‐variable temperatures and stresses for austenitic steel X8CrNiMoNb 16‐16. The test data were predicted reasonably well using the method in question.
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A creep function using only three fitting parameters is presented hereafter. The function is derived from a more general physical -phenomenological model representing metallic materials creep behaviour and describes all three stages of creep. The dependence of the fitting parameters upon the temperature are investigated and simple analytical relations are provided for that purpose. Those relations already incorporate the effect of stress. At the outcome, ten material constants are derived which are neither dependent on the stress nor on the temperature. On the basis of the derived constitutive relations a prediction is carried out for creep strain of three metallic materials and particularly steels, at different temperatures and under constant stress. The derived curves agree with the experiments, not only the ones carried out in the laboratory but also the ones taken from literature. Neues Model! zur Beschreibung von Kriechvorgangen in metallischen Werkstoffen bei verschiedenenTemperaturen. Eine Kriechfunktion, die mit nur drei Ausgleichsparametern auskommt, konnte aus einem eher allgemeinen physikalisch-phanomenologischen Model!, das das Kriechverhalten metallischer Werkstoffe darstellt, hergeleitet werden. Es beschreibt alle drei Stufen des Kriechens. Einfache analytische Beziehungen reichten aus, um die Temperaturabhangigkeit der Ausgleichsparameter wiederzugeben. Dabei ist der SpannungseinfluB bereits mil berucksichtigt. Zehn Werkstoffkonstanten wurden ermittelt, die weder von der Spannung noch von der Temperatur abhangen. Basierend auf den konstituierenden Gleichungen wird die Kriechdehnung dreier verschiedener Stahlsorten bei verschiedenen Temperaturen und unter konstanter Belastung berechnet. Die entwickelten Kurven stimmen mit experimentellen Ergebnissen aus Laborversuchen wie aus dem Schrifttum uberein.
The creep exposure of metallic parts usually occurs under time-variable loading conditions. For the prediction of creep behaviour under step loading a simple mechanical model is employed. The model is based on the time dependence of the material structure during creep. Therefore, the characteristic parameters of the model are considered to be functions of time and are correlated with the time dependence of the material structure parameters. For the definition of these functions a structure mechanical model has been employed. The mean internal material resistance has been considered as an appropriate structure parameter. The characteristic parameters of the model are defined in each step loading on the basis of the change of the internal stress derivative. For the verification of the model prediction the numerical results are compared with experimental results of the austenitic steel X 8 CrNiMoNb 1616 for the orthogonal cyclic stress function. Vorausbestimmung des Kriechverhaltens metallischer Werkstoffe bei Stufenbelastung anhand eines einfachen mechanischen Modells. Die Kriechbeanspruchung metallischer Teile findet in der Regel unter zeitveranderlichen Bedingungen statt. Zur Vorausbestimmung des Kriechverhaltens bei Stufenbeanspruchung wird ein einfaches mechanisches Modell herangezogen. Das Modell basiert auf der Zeltabhanqiqkelt der Werkstoftstruktur wah rend des Kriechens. Deshalb werden die charakteristischen Modellparameter als Zeitfunktionen betrachtet und zur Zeitabhanqiqkeit der Werkstoftstruktur korreliert. Zur Definition dieser Funktionen wurde ein strukturmechanisches Modell herangezogen. Der mittlere innere Werkstoffwiderstand wurde als ein geeigneter Strukturparameter betrachtet. Die charakteristischen Modellparameter werden in jeder Belastungsstufe auf der Basis der inneren Ruckspannungsableitung definiert. Zur Verifizierung der Voraussagen des Modells wurden die Rechenergebnisse mit experimentellen Ergebnissen verglichen, die am austenitischen Stahl X 8 CrNiMoNb 1616 fUr eine rechteckige Spannungsfunktion ermittelt wurden.Engineering parts are usually subjected to complex loading conditions. The need for reliable prediction of the material behaviour under such conditions led to the development of a variety of structural-mechanical models (e.g. lr 12 »combining both an understanding of the physical processes controlling non-elastic deformation and a correlation of those processes to phenomenologically derived expressions.In these models the time and history-dependent evolution of the structure during deformation is considered by using either one or more appropriate structural parameters. An increase of the number of structure parameters affects significantly the number of the constitutive equations required for the prediction of the material behaviour. On the other hand, models using only one structural parameter are easily applicable but, due to their wide scope, can lead to significant discrepancies from the material behaviour.To overcome these shortcomings, during recent years several efforts ha...
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