A Continuum Mechanics Approach for Crack Initiation and Crack Growth Predictions

Very often, different approaches are used for crack initiation and crack growth predictions. The current article introduces a recently developed approach that can be used for the predictions of both crack initiation and crack propagation. A basic assumption is that both crack nucleation and crack growth are governed by the same fatigue damage mechanisms and a single fatigue damage criterion can model both stages. A rule is that any material point fails to form a fresh crack if the total accumulated fatigue damage reaches a limit. For crack initiation predictions, the stresses and strains are obtained either directly from experiments or though a numerical analysis. For the prediction of crack growth, the approach consists of two steps. Elastic‐plastic stress analysis is conducted to obtain the detailed stress‐strain responses. A general fatigue criterion is used to predict fatigue crack growth. Compact specimens made of 1070 steel were experimentally tested under constant amplitude loading with different R‐ratios and the overloading influence. The capability of the approach to predict both crack initiation and the crack growth under these loading conditions was demonstrated by comparing the predictions with the experimental observations.     

Deformation and life assessment of high temperature materials under creep fatigue loading

The knowledge of mechanical long term behaviour under static and cyclic loading for high temperature components requires methodologies for life assessment in order to employ the full potential of materials. A phenomenological life time prediction concept which was developed for multi‐stage creep fatigue loading demonstrates the applicability of rules for synthesis of stress strain path and relaxation including an internal stress concept, as well as mean stress effects. Further, a creep fatigue interaction concept which was also developed covers a wide range of creep dominant loading as well as fatigue dominant loading. Service‐type experiments conducted at different strain rates and hold times for verification purposes demonstrate the acceptability of life prediction method for variation of conventional 1 %Cr‐steels as well as modern high chromium 9‐10 %Cr‐steels. Generally, the service life of components is influenced by multi‐axial behaviour. Multi‐axial experiments with e.g. notched specimens and with cruciform specimens accompanied by advanced methods for calculation of stress strain path and life time prediction stress conditions are of future interest.     

Application of continuum damage mechanics model in terms of fatigue life assessment for low cycle and quasi static loadings

The aim of this study is to test the classical Lemaitre model based on continuum damage mechanics (CDM) approach in the range of low cycle and quasi‐static fatigue life. Study is carried out with the use of results of experimental tests for C45 steel (according to AISI: 1045 steel) carried out under variable‐amplitude loading. Loading programs are of two‐step character and include blocks of cycles of different lengths and R = ‐1 coefficients. Fatigue lives are calculated according to Lemaitre model from experimentally obtained stress and strain histories recorded during fatigue tests. The results are compared with experimental tests results and with fatigue lives calculated with the use of by traditional fatigue approach based on Palmgren‐Miner damage summation hypothesis. Experimental test of fatigue life calculation results for C45 steel reveals that continuum damage method, using the recorded stresses and strains, predicts fatigue life better as compared to the remaining methods. The study also contains many detailed analyses of experimental results.     

Anwendung des Örtlichen Konzepts in der Turbinenschaufelauslegung

Die Anforderungen an große Kraftwerksturbinen (Gas‐ und Dampfturbinen) sind im Zusammenhang mit der Energiewende in den letzten Jahren signifikant gestiegen. Durch die wachsende Einspeisung von Strom aus erneuerbaren Energien wie Wind und Sonne gewinnt die Erhöhung der Flexibilität im Kraftwerksbetrieb immer mehr an Bedeutung. Dafür müssen zuverlässige und robuste Turbinen entwickelt werden. Turbinenschaufeln sind eine der Schlüsselkomponenten einer Turbine und ein entscheidender Erfolgsfaktor, um die aktuellen und zukünftigen Markt‐ und Kundenanforderungen zu erfüllen und die damit verbundenen Herausforderungen zu meistern. Die Auslegung von Turbinenschaufeln ist komplex. Turbinenschaufeln sind im Betrieb häufig extremen, hohen sowie komplexen mechanischen und thermischen Belastungen ausgesetzt. Diese mechanischen und thermischen Betriebsbelastungen führen zu hohen Bauteilbeanspruchungen und häufig zu einem Ermüdungs‐ und/oder Kriechlebensdauerverbrauch. Hersteller großer Kraftwerksturbinen können ihre Turbinen und deren Komponenten in den allermeisten Fällen nicht im Vorfeld auf Prüfständen testen. Außerdem können Fehler in der Auslegung der Komponenten zu deren Versagen im Betrieb und damit zumindest zu erheblichen wirtschaftlichen Schäden führen. Aus diesen Gründen und vor dem Hintergrund der zunehmend flexibleren Fahrweise von Kraftwerken und der damit verbundenen höheren Anzahl an Lastwechseln sind für die Entwicklung und Auslegung von Turbinenschaufeln sowie deren Lebensdauervorhersage geeignete, zuverlässige und abgesicherte Berechnungs‐ und Bewertungskonzepte sowie Regeln unerlässlich, um den sicheren Betrieb der Turbinenschaufeln zu ermöglichen. Dies gilt insbesondere für die genaue Beschreibung des Ermüdungsverhaltens unter Betriebsbelastung. Der vorliegende Artikel beschäftigt sich mit den hier verwendeten Konzepten zur Lebensdauervorhersage und zum Betriebsfestigkeitsnachweis von Gas‐ und Dampfturbinenschaufeln.     

A Walker exponent corrected model for estimating fatigue life of metallic materials in loading with mean stress

Mean stress significantly influence the fatigue life predictions of metallic materials. The Walker mean stress equation with its additional material parameter w provides good predictions for a wide range of materials. Unfortunately, additional tests are necessary to determine the Walker exponent w. In order to overcome this shortcoming, for aluminum alloys, the Walker exponent w was correlated linearly with the sum of ultimate tensile strength and true fracture strength. Then, a Walker exponent corrected effective strain energy density criterion was developed by incorporating the Walker mean stress equation into the strain life curve. The capability of fatigue life prediction for the developed model was checked against the tested data of 304 L stainless steel, SAE 1045 steel, 7075‐T651 aluminum alloy, and Incoloy 901 superalloy, and comparisons were also performed by using the Lv's Walker exponent corrected model. The developed model provides more satisfactory results, especially for the considered materials in loading with mean stress.     

Vorhersage der Lebensdauer und Dauerfestigkeit von hochfesten,durchhärtenden Stählen

Fatigue Lifetime and Endurance Limit Prediction for High‐Strength Steels Smooth and notched specimens of the bearing steel 100Cr6 (SAE 52100) in a bainitic condition were used to determine the S‐N curves under tensile, torsional and combined in‐ and out‐of‐phase loading. In the area of high‐cycle fatigue, crack initiation was most likely caused by inclusions like Titanium Carbonnitrides or Aluminium Oxides. A micro mechanical model for the crack initiation by inclusions was developed. Another model was developed to describe the influence of these inclusions on the lifetime. A weakest‐link model, using the statistical distribution of inclusions and surface flaws, was used to describe the endurance limit.     

Verhalten gekerbter Biegeproben aus der Aluminiumlegierung AA7075 im Kurzzeitermüdungsbereich

Prediction on Fatigue Life of Notched Specimens under Cyclic Bending Loading Pulsating 3P‐bending fatigue tests are conducted on edge‐notched specimens of AA7075. Measurements of electrical potential drop across notches were used to determine the number of cycles up to crack initiation. Cyclic material data determined from strain–controlled constant amplitude loading are use in FE‐analyses to the determination time functions of the local stresses and strains at the notch root using non‐linear material model according to Chaboche and Lemaitre. Using these FE computations, the fatigue life is predicted by the equivalent strain approach of the “ASME Boiler and Pressure Vessel Code” and compared with the results of the plastic strain energy approach. It is found that both approaches lead to relatively good predictions.