Influence of Material Ductility on Fatigue Life under Multiaxial Proportional and Non-Proportional Normal and Shear Stresses

Current experiences show that a non-proportional loading of ductile materials such as wrought steels, wrought aluminium or magnesium alloys, not welded or welded, causes a significant fatigue life reduction under an out-of-phase shear strain or shear stress superimposed on a normal strain or normal stress compared with proportional in-phase loading. However, when ductility, here characterised by tensile elongation, is reduced by a heat treatment or by another manufacturing technology such as casting or sintering, the afore-mentioned life reduction is compensated or even inversed, i. e. longer fatigue life results compared with proportional loading. Some actual results, determined with additive manufactured titanium, suggest that microstructural features such as manufacturing-dependent internal defects like microporosities should be considered in addition to the ductility level. This complex life behaviour under non-proportional loading cannot always be estimated. Therefore, in experimental proofs of multiaxial loaded parts, especially safety-critical components or structures, with real or service-like signals, emphasis must be placed on retaining non-proportionalities between loads and stresses/strains, respectively

Multiaxial fatigue life response depending on proportionality grade between normal and shear strains/stresses and material ductility

Current experiences show that a non-proportional loading of ductile materials such as wrought steels, wrought aluminium or wrought magnesium alloys, not welded or welded, causes a significant fatigue life reduction under an out-of-phase shear strain or shear stress superimposed on a normal strain or normal stress compared with proportional in-phase loading. But when ductility, here characterised by tensile elongation, is reduced by a heat treatment or by another manufacturing technology such as casting or sintering, the afore-mentioned life reduction is compensated or even inversed, i.e. longer fatigue life results compared with proportional loading. However, some materials reveal even despite good elongation values a neutral behaviour or a prolongation of life. Responsible for these observations are not only the overall material ductilities or matrix-ductilities, but also the interaction between matrix and microstructural features such as manufacturing-dependent nodules, pores, internal defects like microporosities. They cause by local constraints a hindrance of deformations despite good overall ductilities. This complex life behaviour under non-proportional loading cannot always be estimated. Therefore, not only in strength hypotheses, but also in experimental proofs of multiaxial loaded parts, especially safety-critical components or structures, with real or service-like signals, emphasis must be placed on retaining non-proportionalities between loads and stresses/strains, respectively

Einfluß von Kaltverformungen bis 5% auf das Kurzzeitschwingfestigkeitsverhalten metallischer Werkstoffe

Bei einer zutreffenden Bemessung von oertlich elasto-plastisch beanspruchten Bauteilen fuer eine kleine Anzahl von Lastspielen muessen Eigenschaftsaenderungen des Werkstoffes infolge von fertigungsbedingten Kaltverformungen oder betrieblichen Ueberlastungen beruecksichtigt werden. Dehnungs- und lastgesteuerte Schwingfestigkeitsversuche mit dem zyklisch entfestigenden Stahl STE 47 und der zyklisch verfestigenden Knetlegierung AlCuMg2 zeigen, dass im Bereich der Kurzzeitschwingfestigkeit eine mit steigender Kaltverformung zunehmende Minderung der Anrisslebensdauer zu verzeichnen ist. Die Lebensdauerabnahme ist auf den Bauschinger-Effekt zurueckzufuehren. Durch die Beruecksichtigung des Bauschinger-Effektes mit Hilfe von Erstbelastungskurven in Zug- und Druckrichtung fuer unterschiedliche Kaltverformungsgrade sowie seiner Einbeziehung in einen Schaedigungsparameter kann der lebensdauermindernde Einfluss von Kaltverformungen beschrieben werden

Betriebsfestigkeit und Zuverlässigkeit von komplexen und intelligenten Strukturen

Structural durability of a product is achieved only by careful design, taking into account the different aspects of the required mission profile with regard to component strength. In order to include all effects on component strength (like material, manufacturing, local geometry, scatter) on one hand, and to meet competition requirements on the other hand, the design engineer has to apply efficient methods for guaranteeing service durability. A brief overview over these methods is presented. Today's developments are focusing on the reduction of time-to-market periods. This fact is putting more emphasis on numerical methods (simulations of dynamic system behaviour, of component properties and local stresses) in order to reduce experimental procedures of optimizing and proof. The mechanical components of future mechanical engineering constructions are increasingly being replaced by electric and even multifunctional components. Electronics are gaining influence which is reflected by current keywords like brake or steer-by-wire technology. Knowledge of the system behaviour as well as all interactions between electronical control and mechanical behaviour are absolutely imperative to assess the reliability of such approaches

Beurteilung von mehrachsig schwingbeanspruchten Schweißverbindungen in den IIW-Empfehlungen

Evaluation of Multiaxial Loaded Welded Joints in the 11W-Recommendations for Fatigue Design. As the evaluation of multiaxial stress state is a very complex issue, in design codes simple but reliable calculation methods are preferred. In this context the Gough-Pollard equation found access to the 11W-Recommendations requiring a ductility dependent modification of the multiaxial damage parameter D-MA. Welded steel joints show a decrease of fatigue life under non-proportional (out-of-phase) multiaxial loading (D-MA = 0.5) in comparison to proportional (in-phase) loading (D-MA = 1.0). In contrast, welded aluminium joints with lower ductility (semi-ductility) display no difference between proportional or non-proportional multiaxial loading (D-MA, = 1.0), but welded ductile aluminium joints reveal a behaviour like steels. From the background of these results, the conservative multiaxial damage parameter D-MA = 0.5 is recommended not only for the assessment of steel but also of aluminium joints submitted to multiaxial non-proportional loading. In case of semi-ductile material states the value D-MA = 1.0 can be applied

Fatigue assessment of laserbeam welded PM steel components by the notch stress approach

In the presented investigation of a laserbeam welded engine part the fatigue design methodology is shown. The fatigue critical area of the joint component is the hidden weld root which can be evaluated only by a local assessment concept. The local fatigue strength of a laserbeam weld of a complex engine component, which joins a PM with a formed sheet component, was assessed by the notch stress concept with the fictitious reference radius of rref = 0.05 mm. First, simplified specimens, following the main geometric dimensions of the parts, were manufactured. On these specimens the fatigue strength was identified by tests and the notch stresses calculated by finite element analysis. Based on these results a design SN-curve was derived to assess the fatigue strength of the engine component. The numerical assessment of the welded joint was verified by proof tests with the component. The assessment could be improved by considering statistical and stress gradient dependent siz e effects according to the concept of the highly stressed volume

Fatigue assessment of laserbeam welded PM steel components by the notch stress approach

In the presented investigation of a laserbeam welded engine part the fatigue design methodology is shown. The fatigue critical area of the joint component is the hidden weld root which can be evaluated only by a local assessment concept. The local fatigue strength of a laserbeam weld of a complex engine component, which joins a PM with a formed sheet component, was assessed by the notch stress concept with the fictitious reference radius of rref = 0.05 mm. First, simplified specimens, following the main geometric dimensions of the parts, were manufactured. On these specimens the fatigue strength was identified by tests and the notch stresses calculated by finite element analysis. Based on these results a design SN-curve was derived to assess the fatigue strength of the engine component. The numerical assessment of the welded joint was verified by proof tests with the component. The assessment could be improved by considering statistical and stress gradient dependent siz e effects according to the concept of the highly stressed volume