Effect of residual stresses on the fatigue behaviour of welded joints depending on loading conditions and weld geometry

An assessment of thin welded structures with actual IIW design lines results in an overestimation of fatigue lives (strengths) at high load levels and a conservative estimation at low load levels, in many cases, independently of the applied fatigue assessment approach (nominal, structural or notch stress). This is mainly due to the slopes of the design S-N line k = 3.0 for normal and k = 5.0 for shear stresses, which are valid for thick and stiff structures. To overcome this inconsistency for welded thin and flexible structures, the slopes k = 5.0 for normal and k = 7.0 for shear stresses are suggested, keeping the already known FAT values derived for the notch stress concept variants with rref = 1.0, 0.3 or 0.05 mm, respectively. However, the slope is not only determined by plate thickness; it is the result of an interaction between thickness, local geometry (stress concentration), structural stiffness, loading mode and last but not least residual stresses. This complexity makes it difficult to identify the driving parameters and to predict the slope of the S-N line in many cases.

S-N Lines for Welded Thin Joints — Suggested Slopes and FAT Values for Applying the Notch Stress Concept with Various Reference Radii

In order to make effective use of high-strength steels for designing light-weight structures, it is necessary to consider the interaction between the service loading, material, geometry and manufacturing technology. This interaction determines durability. The fatigue performance of high-strength steels can be better exploited if high stress concentrations are reduced through geometry improvement and manufacturing process control. With regard to their capability for high energy absorption, high-strength steels offer the best solutions for designing against impact loads and crash. These advantages of high-strength steels are realised in modern vehicle structures through the multi-material concept, where materials selection is made according to expected service loads and locally required strengths.



Leichtbaumoglichkeiten durch den Einsatz von hochfesten Stahlen



Fur einen effizienten Einsatz von hochfesten Stahlen fur den Bau von Leichtbaustrukturen ist es erforderlich, die Interaktion zwischen Betriebsbelastung, Werkstoff, Geometrie und Fertigung zu berucksichtigen. Diese Interaktion bestimmt die Betriebsfestigkeit. Die Ermudungsfestigkeit von hochfesten Stahlen kann optimiert werden, wenn hohe Spannungskonzentrationen durch eine bessere Gestaltung und durch gezielt gesteuerte Fertigungsprozesse abgebaut werden. Hoherfeste Stahle zeichnen sich durch eine hohe Energieabsorption aus und ermoglichen dadurch gute konstruktive Losungsmoglichkeiten gegen Crash bzw. schlagartige Belastungen. Diese Vorteile von hochfesten Stahlen werden bei modernen Fahrzeugstrukturen durch das Multimaterialkonzept realisiert. Die Werkstoffauswahl erfolgt nach den erwarteten Betriebsbeanspruchungen und erforderlichen ortlichen Festigkeitseigenschaften.

Light‐weight design chances using high‐strength steels

Lightweight design with welded high-frequency mechanical impact (HFMI) treated high-strength steel joints from S700 under constant and variable amplitude loadings

Application of the weakest link analysis to the area of fatigue design of steel welded joints

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.

G. Demofonti

Papers

High‐Strength Steels in Welded State for Light‐Weight Constructions under High and Variable Stress Peaks

High-strength steels in welded state for lightweight constructions under high and variable stress peaks : final report