Klemens Rother

Effektive Modellbildung, Analyse und Bewertung für die rechnerische Lebensdaueranalyse geschweißter Strukturen*

Kurzfassung Die Analyse geschweißter Strukturen ist eine Herausforderung für den Berechnungsingenieur. Verbesserte Soft- und Hardwareleistung ermöglicht heute die Berechnung ganzer Baugruppen in einer Analyse. Gerade für verschweißte Strukturen bieten sich durch diese Möglichkeiten besondere Vorteile, die zu einer genaueren Abbildung der Kraftflüsse in der Struktur beitragen und zudem dem Berechnungsingenieur die Arbeit erleichtern. Der vorliegende Beitrag behandelt die Vorgehensweise zur Modellbildung, Strukturanalyse und Auswertung von geschweißten Strukturen mit der Finite-Elemente-Methode. Es wird ein hierarchisches Konzept bestehend aus einer Globalanalyse eines Schweißteils zur Lokalisierung kritischer Positionen anhand von Strukturspannungen und lokalen Betrachtungen nach Kerbspannungskonzept beschrieben.

Fatigue behavior of spot-welded joints in air and under corrosive environments

A bstractThe main aim of the project was to evaluate the influence of combined effects of fatigue loading and exposure to cyclic corrosion testing on the corrosion and the fatigue resistances of coated steel-based materials joined by resistance spot welding. Seven steel-based materials including cold rolled mild steels, high-strength steels, and press-hardened steels (PHS) were selected and provided by steel suppliers with different surface coatings for resistance spot welding. Panels were joined using conventional resistance spot welding in both lap-shear and T-peel designs. Joined samples were painted by e-coating following the industrial process. Metallographic characterization of the steel materials revealed that microstructures and metallic coating composition and thickness were as expected. Cross section of spot welds showed good quality and typical evolutions of hardness. Tensile tests and fatigue tests were performed on reference samples (fatigue in “air,” i.e., nonexposed to corrosion) leading to typical SN-lines as described in part I of this paper. The results were used to evaluate the influence of corrosion on tensile strength of the joined samples. Fatigue tests performed on combined and alternating corrosion and fatigue revealed that fatigue life is affected by extent of corrosion near the spot weld, with a reduction of the fatigue life at higher applied load and slight increase for lower load, compared to tests “in air.” This phenomenon was not observed for lap-shear configuration in alternated fatigue-corrosion mode.

Performance Study of the Simplified Theory of Plastic Zones for the Fatigue Check

As elastic-plastic fatigue analyses are still time consuming the simplified elastic-plastic analysis (e.g. ASME Section III, NB 3228.5, the French RCC-M code, paragraphs B 3234.3, B 3234.5 and B3234.6 and the German KTA rule 3201.2, paragraph 7.8.4) is often applied. Besides linearly elastic analyses and factorial plasticity correction (Ke-factors) direct methods are an option. In fact, calculation effort and accuracy of results are growing in the following graded scheme: a) linearly elastic analysis along with Ke correction, b) direct methods for the determination of stabilized elastic-plastic strain ranges and c) incremental elastic-plastic methods for the determination of stabilized elastic-plastic strain ranges.The paper concentrates on option b) by substantiating the practical applicability of the simplified theory of plastic zones STPZ (based on Zarka’s method). Application relevant aspects are particularly addressed. Furthermore, the applicability of the STPZ for arbitrary load time histories in connection with an appropriate cycle counting method is discussed.Note, that the STPZ is applicable both for the determination of (fatigue relevant) elastic-plastic strain ranges and (ratcheting relevant) locally accumulated strains. This paper concentrates on the performance of the method in terms of the determination of elastic-plastic strain ranges and fatigue usage factors. The additional performance in terms of locally accumulated strains and ratcheting will be discussed in a future publication.Copyright

Fatigue damage of weld seams with and without postweld treatment under multiaxial loading

Abstract The service durability of weld seams is one of the most challenging problems of component fatigue. Hereby, the complexity is due to several peculiarities of welded structures: the detailed weld seam geometry with macroscopic and microscopic notch effects and/or defects, the properties of the welded material, residual stresses and multiaxial proportional or non-proportional loading conditions. Non-experimental approaches to design against fatigue have to account for the difference between weld seams with and without post-weld treatment as well as proportional and non-proportional loading. The application of fracture mechanics parameters allows for a more realistic description of damage progression. This paper is intended to explain possible design approaches based on numerical fracture mechanics and their combination with stress and strain based approaches using clear and classified damage criteria (engineering crack size, fracture). All numerical approaches are presented with special regard to complexity and modeling requirements from the point of view of the practicing design engineer. Non-proportional loading is considered by the identification of a damage critical plane which requires special post-processing routines within the numerical analysis. Again, the question of practicability is raised.