Michael Vormwald

Hot-spot stress evaluation of fatigue in welded structural connections supported by finite element analysis

Various welded joints from the floor structure of city buses have been examined numerically and experimentally under bending and tensional cyclic constant-amplitude loading. Hot-spot stresses at failure-critical locations were calculated by means of finite element analysis. The corresponding fatigue lives were determined experimentally. There was good overall agreement between calculated and experimentally determined critical locations. The recommendations within the IIW guideline concerning the Hot Spot Stress Approach were found to give accurate approximations for the probability of survival of fatigue-loaded welded joints. The restriction regarding the applicability of the Hot Spot Stress Approach to welds failing only from their transition zones could be relaxed, at least at least for welds similar to those investigated here.

Residual stress fields and fatigue analysis of autofrettaged parts

Abstract Compressive residual stresses improve the fatigue life of cyclically loaded components. Therefore, they are often introduced intentionally, e.g. by rolling, shot peening, proof load tests, or autofrettage. This paper addresses the calculation of residual stresses due to autofrettage and the resulting increase of the endurance limit. The deformation behaviour of materials unloaded from an elastic–plastic state is dependent on the maximum strain. Unloading curves can be determined experimentally using smooth specimens. In inhomogeneously stressed components each material element is subjected to a different maximum strain value. Thus, the unloading stress–strain behaviour is also inhomogeneous. In order to model this effect in finite element (FE) stress analyses a different unloading elastic–plastic material behaviour is attached to each FE depending on its maximum equivalent strain value reached in loading. Residual stress distributions using this procedure are compared to results from application of different methods proposed in the literature. The calculated residual stress field serves as input for calculating the endurance limit of the component. A fatigue crack growth arrest criterion is used for its prediction. The necessary stress intensity factors due to loading and residual stresses are calculated using the weight function method. Experimentally determined and predicted endurance limits are compared.

Notch stress and strain approximation procedures for application with multiaxial nonproportional loading

Abstract An extension of several algorithms is presented for estimating elastic-plastic notch stresses and strains for metallic materials under cyclic multiaxial nonproportional loading. The extension allows for using advanced models of incremental plasticity. The accuracy of the algorithms, i.e. the structural yield surface approach according to Köttgen et al. and the incremental Neuber approach according to Glinka et al., is shown by comparisons with finite element calculations and notch strain measurements. The accuracy of both approaches is comparable. The differences in predicted stress-strain paths are in the same order as those to measured and FE-calculated paths. These differences lead to a factor of up to two in calculated fatigue lives. Glinkas approach can be applied much more easily; Köttgens approach offers the opportunity for further improvements in accuracy.

Spectrum fatigue life assessment of notched specimens using a fracture mechanics based approach

Local strain based approaches have been used frequently for the prediction of crack initiation life of notched components. Limitations of the transferability of smooth specimen fatigue data to the notch root as well as cumulative damage problems have been handled by use of fatigue notch factors and consideration of some damage contribution below the fatigue limit. This type of a conventional approach has been applied as a reference case for the present study. Based on the mechanics of local strain approaches, a model for crack initiation life prediction is outlined. It takes explicitly into account the phenomena related to transferability (size, multiaxiality) as well as a load-sequence dependent damage accumulation. It is based on the closure behavior of short cracks that gives rise to load sequence dependent effective load ranges and continuously decreasing fatigue limits under spectrum loading. Prediction results are compared to experimental data from notched aluminum 7075-T7351 specimens of various shapes and sizes under both Gaussian and MINITWIST loading. The conventional approach provides unconservative estimates with a broad scatter of life ratios. The fracture mechanics based model gives a relatively low scatter of life ratios around a mean value of unity.

Improvement of Fatigue Life Prediction Accuracy for Various Realistic Loading Spectra by Use of Correction Factors

Abstract Some examples of using the correction factor method for improving the fatigue life prediction accuracy using the nominal-stress and local-strain approaches are considered. The ratio Nexp/Ncalc for a relevant loading spectrum was used to predict the fatigue life of a component loaded under a second spectrum with a similar loading frequency distribution. It is shown that the application area of the relative method is similar both for the nominal-stress and local-strain approaches. In the second case, correction is especially needed so that more reliable estimates are obtained. For both approaches, predictions are much improved when the two compared loading cases differ only in the reference stress level, or when the reference stress level and some other important loading spectrum parameters are identical but the loading frequency distribution differs somewhat.

Elastic-Plastic Fatigue Crack Growth

Conventional fatigue crack propagation approaches rely on similitude arguments and relationships between the stress intensity factor range and crack growth rate. The application limit of this approach is specified by small-scale yielding conditions. Still within these limits, an explanation and the straightforward modelling of the mean stress influence and the influence of variable amplitudes requires consideration of cyclic plasticity. Plasticity-induced crack closure greatly influences the crack growth rate. Modelling tools and algorithms are presented. Outside the small-scale yielding limits, the stress intensity factor range must be substituted by a crack driving force parameter of elastic-plastic fracture mechanics. Various proposals are presented and discussed with a focus on the ΔJ-integral. Together with an adequate consideration of crack closure, advances in simulating fatigue crack growth in this regime more realistically are presented. Multiaxial and mixed mode loading are a continuing challenge for actual research. These topics are discussed against the background of current expertise and available computational resources.

Ermüdungslebensdauer von Baustahl unter komplexen Beanspruchungsabläufen am Beispiel des Stahles S460

Kurzfassung Der Baustahl S460 gewinnt im Stahlbau zunehmend an Bedeutung, birgt er doch ein Potenzial für ökonomische Einsparungen. Allerdings ist das Einsatzgebiet zu berücksichtigen. So kann der Werkstoff z.B. in schlanken druckbeanspruchten und damit stabilitätsgefährdeten Konstruktionen sowie bei Schwingbeanspruchung von Schweißverbindungen seine höhere Festigkeit gegenüber dem S355 nicht zum Tragen bringen. Der Baustahl S460 besitzt im ungeschweißten Zustand ein gutes bis sehr gutes (entsprechend seiner statischen Festigkeit) Widerstandsvermögen gegenüber einer Schwingbeanspruchung. Er ist in den vergangenen zwei Jahrzehnten intensiv experimentell untersucht worden und diente dabei als Modellwerkstoff bei der Entwicklung von Simulationsverfahren. Der vorliegende Beitrag stellt sowohl das zyklische Verformungsverhalten als auch das Versagensverhalten bei Schwingbeanspruchung ausführlich dar. Den experimentellen Befunden wird konsequent das Berechnungsergebnis unter Verwendung der aktuellen Modelle gegenübergestellt. Dabei wird der Bogen gespannt von der einfachen einachsigen Beanspruchung bis zur komplexen, mehrachsig nichtproportionalen Beanspruchung.

Risswachstumsverhalten der Aluminiumlegierung AlMg4.5Mn unter proportionaler und nichtproportionaler Schwingbelastung

Kurzfassung Im vorliegenden Beitrag wurde der Ermüdungsrissfortschritt unter proportionaler und nichtproportionaler Belastung experimentell untersucht. Hierzu wurden dünnwandige Rohrproben, die mit einem Schlitz versehen wurden, getestet. Die Proben wurden aus der Aluminiumlegierung AlMg4.5Mn gefertigt. Neben reiner Zug-Druck- und reiner Torsionsbelastung wurden sowohl die proportionale als auch die nichtproportionale Kombination dieser beiden Belastungstypen betrachtet. In den Versuchen wurden die Rissfortschrittslebensdauern, die Rissfortschrittskurven und die Risspfade identifiziert. Abhängig vom Belastungstyp initiierten 2 bis 4 Risse an unterschiedlichen Positionen innerhalb des Schlitzes. Wurde die Nennspannung als Lastparameter gewählt, so waren die Lebensdauerunterschiede zwischen proportionaler und nichtproportionaler Belastung sehr klein. Wurde jedoch die elastische Kerbspannung als Lastparameter gewählt, so ergaben sich deutliche Unterschiede in den Lebensdauern für die unterschiedlichen Belastungstypen.

Schwingfestigkeit von Schweißnahtenden und Übertragbarkeit von Schweißverbindungswöhlerlinien

Abstract Um die Geometrieeinflüsse von Schweißnahtenden genau erfassen zu können, wurden repräsentative Nähte mit einem hochauflösenden dreidimensionalen (3D) Scanner aufgenommen. Aus den gescannten Oberflächen konnten Volumenmodelle erstellt und anhand der Methode der finiten Elemente (FEM) elastizitätstheoretische Kerbspannungen berechnet werden. Aus diesen Ergebnissen wurde ein idealisiertes Nahtendmodell abstrahiert, welches in Bezug auf Größe und Verteilung nahezu die gleichen Kerbspannungen liefert wie die reale Geometrie. Zur Übertragbarkeit der Schwingfestigkeitsdaten wurde der spannungsmechanische und statistische Größeneinfluss in neuer Form kombiniert. Zur Lösung dieses Ansatzes wurde das numerische Verfahren NuMeSiS erarbeitet.

Damage mechanisms in PBT-GF30 under thermo-mechanical cyclic loading

The scope of this paper is the investigation of damage mechanisms at microscopic scale on a short glass fiber reinforced polybutylene terephthalate (PBT-GF30) under thermo-mechanical cyclic loading. In addition the principal mechanisms are verified through micro mechanical FE models. In order to investigate the fatigue behavior of the material both isothermal strain controlled fatigue (ISCF) tests at three different temperatures and thermo-mechanical fatigue (TMF) tests were conducted on plain and notched specimens, manufactured by injection molding. The goal of the work is to determine the damage mechanisms occurring under TMF conditions and to compare them with the mechanisms occurring under ISCF. For this reason fracture surfaces of TMF and ISCF samples loaded at different temperature levels were analyzed using scanning electron microscopy. Furthermore, specimens that failed under TMF were examined on microsections revealing insight into both crack initiation and crack propagation. The findings of this...