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Journal ArticleDOI

Overload effects on a ferritic-baintic steel and a cast aluminium alloy: two very different behaviours

01 Oct 2011-Materialwissenschaft Und Werkstofftechnik (Wiley)-Vol. 42, Iss: 10, pp 845-854

AbstractLoad controlled fatigue tests were performed up to 107 cycles on flat notched specimens (Kt = 2.5) under constant amplitude and variable amplitude loadings with and without periodical overloads. Two materials are studied: a ferritic-bainitic steel and a cast aluminium alloy. These materials have a very different cyclic behaviour: the steel exhibits cyclic strain softening whereas the Al alloy shows cyclic strain hardening. The fatigue tests show that, for the steel, periodical overload applications reduce significantly the fatigue life for fully reversed load ratio (Rσ = –1), while they have no influence under pulsating loading (Rσ = 0). For the Al alloy overloads have an effect (fatigue life decreasing) only for variable amplitude loadings. The detrimental effect of overloads on the steel is due to ratcheting at the notch root which evolution is overload's dependent. An gekerbten Flachproben (Kt = 2,5) wurden Schwingfestigkeitsversuche bis 107 Schwingspiele unter konstanten und variablen Amplituden mit und ohne periodische Uberlasten durchgefuhrt. Die beiden untersuchten Werkstoffe waren ein zyklisch entfestigender ferritisch-bainitischer Stahl und eine zyklische verfestigende Aluminiumgusslegierung. Die Schwingfestigkeitsversuche zeigten fur den Stahl, dass die periodischen Uberlasten die Lebensdauer des Stahls unter Wechselbelastung (Rσ = –1) signifikant verkurzten, wahrend unter Schwellbelastung (Rσ = 0) kein Einfluss zu beobachten war. Fur die Aluminiumgusslegierung war nur unter variablen Amplituden ein lebensdauerverkurzender Effekt zu beobachten. Der nachteilige Einfluss der Uberlasten auf die Lebensdauer des Stahls ist auf uberlastenabhangiges, zyklische Kriechen im Kerbgrund zuruckzufuhren.

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Journal ArticleDOI
Abstract: This paper displays, through the example of a cast aluminium wheel (G-AlSi7Mg0.3 T6, i. e. EN AC-42100 T6 or A 356 T6), the evaluation of critical areas through the method of Required Fatigue Strength (RFS). The RFS-values are derived from the local design spectra of the most critical areas of the component by searching the position of local particular Woehler-lines resulting in the prescribed allowable damage sum Dal within the cumulative damage calculation by the modified Palmgren-Miner hypothesis according to Haibach. The RFS-value is nothing more than the knee point of the calculated Woehler-line with defined slopes and cycle position of the knee point. As long as the production process delivers a material quality, i.e. a component related fatigue strength exceeding the calculated RFS-values, the required service durability is fulfilled. The paper demonstrates, through the example of a cast aluminium passenger car wheel, the evaluation of critical areas through RFS-values derived from the local design spectra of the most critical areas of the component and the experimental verification of the required service durability for 300,000 km and load occurrence probability of Po,Load ≤ 1%.

2 citations


Dissertation
05 Apr 2019
Abstract: Dans l’industrie automobile, les aciers lamines sont largement utilises pour realiser les pieces constitutives d’un vehicule. Les proprietes en fatigue a grand nombre de cycles de ces produits plats sont generalement determinees sur le materiau dans son etat tel que sortie d’usine (i.e., vierge de toute sollicitation) sous chargements cycliques uniaxiaux a amplitude constante. Cependant, la fabrication de piece structurelle fait generalement intervenir des etapes de mise en forme qui ont pour consequence de modifier l’etat du materiau et donc d’impacter les proprietes en fatigue. D’autre part, les chargements en service reellement subi par une telle piece sont souvent multiaxiaux et d’amplitude variable dans le temps. Cette etude a pour but de prendre compte les notions de multiaxialite et d’effets d’histoire de chargement lors de la caracterisation en fatigue par le biais de mesures d’auto-echauffement sous chargements cycliques. La caracterisation en fatigue multiaxiale fait generalement intervenir un critere de fatigue necessitant la connaissance de la limite d’endurance du materiau sous sollicitation de cisaillement. Comme cette grandeur est difficile a obtenir pour le cas des produits plats, un nouvel essai de cisaillement et deux geometries d’eprouvette en forme de disque sont proposes, permettant d’utiliser des mesures d’auto-echauffement. Ainsi, la limite d’endurance sous sollicitation de cisaillement a pu etre determinee. Dans le but d’etudier l’impact des effets d’histoire sur les proprietes en fatigue, differents essais d’auto-echauffement ont ete realises en considerant differents niveaux de pre-deformation plastique initial, differents pre-chargements cycliques et differents rapports de charge. Les resultats obtenus ont permis de proposer un modele probabiliste a deux echelles permettant d’une part de decrire l’evolution des courbes d’auto-echauffement quel que soit l’effet d’histoire etudie et, d’autre part, de donner une bonne estimation de l’influence de ces effets sur les courbes S-N-P.

2 citations


References
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Journal ArticleDOI
Abstract: This paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast‐iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context.

115 citations


Journal ArticleDOI
Abstract: In design codes (Eurocode, British Standard and others) for the dimensioning of welded joints, no distinction is made between low-, mediumand high-strength steels. Because of a lack of general knowledge about the benefits of high-strength steels and also because of missing information in design codes, in many cases design engineers still use lowor medium-strength steels (Rp0.2 < 400 MPa) and compensate for high loads under constant or variable amplitude loading or overloads by increasing dimensions. Given this situation, it was deemed necessary to establish criteria for the design of light-weight welded constructions under high and variable stress peaks using new classes of high-strength steels, such as S355N (normalized), S355M (thermomechanically treated), S690Q (water quenched) and S960Q (water quenched), and to perform more reliable evaluations of the fatigue performance of high-strength steel structures subjected to complex loading with regard to light-weight design and economics. For the comparison of the fatigue strengths of the investigated steels the notch factors present were taken into account. Additionally, the real damage sums were determined in order to give recommendations for the fatigue life estimation, i.e. Dal = 0.5. Under constant amplitude loading, no significant difference in the bearable local stress amplitudes for the butt welds can be detected for the four investigated steels. Under variable amplitude loading, the butt welded (lower notch factor) high strength steel S960Q has advantages in the case of the normal Gaussian spectrum and in the case of overloads, especially under pulsating loading. For the transverse stiffeners (high notch factor), slight advantages for the high strength steel S960Q exist, only in the case of pulsating overloads. However, the advantages of high strength steels in case of static loading are indisputable. In most of the investigated cases, overloads lead to a benefit in fatigue life.

29 citations


Journal ArticleDOI
Abstract: Experimental results of overload effects on the fatigue life and the fatigue crack initiation life of low- carbon steels are summarized and a life prediction model under variable-amplitude loading is put forward in the present paper. Test results show that overload has no marked effect on the fatigue crack initiation (FCI) life of notched elements and butt welds in low-carbon steels with non-continuous strain-hardening characteristics, but decreases the fatigue life of smooth specimens of low-carbon steels. This means that the overloading effect factor z = 0 for notched elements and butt welds in low-carbon steels and z < 0 for smooth specimens. Consequently, the interaction between loads in the load spectrum or/and the load sequence effect should be taken into account in the life prediction of smooth specimens under variable-amplitude loading. For notched elements and welded elements, the original expression of the FCI life and Miner's rule can be used directly to predict the FCI life under variable-amplitude loading without the need to take into account the interaction between loads in the load spectrum, because z = 0. When the FCI life occupies the major part of the fatigue life of the notched elements, the original fatigue life expression and Miner's rule can be approximately used to predict the fatigue life of notched elements under variable-amplitude loading. The above-mentioned life prediction model has been checked by the test results.

23 citations


Journal ArticleDOI
Abstract: Effects of prior low cycle fatigue (LCF) cycling on the subsequent high cycle fatigue (HCF) limit stress corresponding to a life of 10 7 cycles are investigated for Ti-6Al-4V at room temperature. Tests are conducted at 420 Hz on an electrodynamic shaker-based system at several different LCF maximum loads and under subsequent HCF at R =0.1, 0.5 and 0.8 using a step loading procedure. Under these load combinations, which include the possibility of overload or underload effects if cracks form, there is no statistically significant effect of the prior LCF on the subsequent HCF limit stress. While LCF loading at a high stress of 900 MPa is seen to result in strain ratcheting, no distinct features on the fracture surface and different mechanisms of crack propagation from those obtained at lower maximum loads were observed. LCF loading up to 50% of expected life did not produce any indications of crack formation from either the stress limit data or the fracture surfaces.

23 citations


Journal ArticleDOI
Abstract: Fatigue tests were performed on ferritic bainitic steel notched specimens (Kt = 2.5) under load controlled constant amplitude loading. These tests show that under constant amplitude tension compression loading, periodical overloads application have a detrimental effect on the fatigue crack initiation strength for fully reversed load ratio (Rσ = −1), while they have no influence under pulsating loading (Rσ = 0). A finite element analysis shows that in the fully reversed tension (Rσ = −1), the stabilized cyclic behaviour at the notch root is an elastic-plastic shakedown while elastic shakedown is obtained under pulsated regime (Rσ = 0), so that we can consider that the local cyclic behaviour has an influence on the overload effect. However, the overload application does not imply a remarkable modification of the stress and strain field under the subsequent constant amplitude loading and can not explain such a fatigue strength decrease in fully reversed tension.

2 citations