Issue 9

T. Marin et alii, Frattura ed Integrità Strutturale, 9 (2009) 76 - 84; DOI: 10.3221/IGF-ESIS.09.08 76 Fatigue design of welded joints using the finite element method and the 2007 ASME Div. 2 Master curve T. Marin, G. Nicoletto University of Parma, Department of Industrial Engineering, Italy, marin@ied.unipr.it R IASSUNTO . Lo studio della fatica nei giunti saldati è solitamente basata sugli approcci della tensione nominale, quella di hot spot o quella di intaglio, i quali però presentano svariate limitazioni quando sono associati ad analisi mediante il metodo degli elementi finiti. In questo lavoro viene presentata una definizione più recente di tensione strutturale e la sua implementazione in un postprocessore per codici FEM. Essa permette un efficace utilizzo dei risultati delle simulazioni per valutare la resistenza a fatica di strutture anche complesse. Le applicazioni presentate confermano le principali caratteristiche del metodo, ossia l’insensibilità alla mesh e stime accurate della vita a fatica e del punto di propagazione delle cricche. A BSTRACT . Fatigue design of welded structures is primarily based on a nominal stress; hot spot stress methods or local approaches each having several limitations when coupled with finite element modeling. An alternative recent structural stress definition is discussed and implemented in a post-processor. It provides an effective means for the direct coupling of finite element results to the fatigue assessment of welded joints in complex structures. The applications presented in this work confirm the main features of the method: mesh-insensitivity, accurate crack location and life to failure predictions. K EYWORDS . Welded joints; Structural stress; Fatigue assessment; Finite element analysis. I NTRODUCTION elding is the most widespread joining technique for metallic structures due to its applicability to many geometric configurations. The principal failure mode in welds is fatigue cracking, [1], therefore many efforts have been put in the study of the strength of welded joints and in the definition of design guidelines, [2]. As the finite element method has become the favorite tool in structural analysis, there is an unquestionable need for a direct connection of the fatigue assessment approaches to the simulations. In this work, after a brief review of some well established techniques in Section 2, a novel approach developed at Battelle Institute, and recently entered in the ASME standards, is described in Sections 3-4. Some applications to experimental tests are then presented in Section 5. This method allows a relaxation of some finite element modeling difficulties, mainly the mesh sensitivity, and grants the possibility of using alternatively solid and shell elements to model welded joints. The results of the FE analysis are then used in combination to a fatigue master curve that consolidates a large number of welded joint configurations. A PPROACHES TO FATIGUE ASSESSMENT OF WELDED JOINTS t is common practice to separate the approaches to fatigue assessment of welded joints into “global” and “local” families, [3-4] . The former consists of the long standing nominal stress method, which is still the most used for its simplicity and is the basis of all standards and design codes in use. With nominal stress approach no attempt is made W I