Fatigue Design Criterion for Welded Structures

Abstract

For continuously welded structures subjected to cyclic loading, the highly stressed zones where cracks initiate and lead to failure are usually located at weld toes. At these critical points, called hotspots, the very local stress states are difficult to determine so that standard fatigue criteria are very diflicult to apply for fatigue life prediction. This work presents a fatigue design criterion for continuously welded thin sheet structures, based on a unique S-N curve. The approach, which refers to the hot-spot stress concept, defines the design stress S as the geometrical stress amplitude at the hot-spot.In practice, the geometrical stress state is calculated by means of the finite element method (FEM) using thin shell theory. Meshing rules for the welded connection, which can be applied methodically to any welding situation, allow the hot-spot location, and therefore the design stress of any structure, to be determined.Experimental data and FEM calculations show that a unique S-N curve can be obtained whatever the geometry of the welded structure and the loading mode. NOMENCLATURE e e = thickness of the attachment, and thickness of the plate. e!$fi$ = weld leg length on the attachment side, and on the plate side. I$), E p ) = shell element number i, perpendicular to the intersection curve of the shell element mean surfaces, on the attachment side, and on the plate side. Fi = load in the Oi direction. F,,, F-= maximum load and minimum load. @), n? = node number i, on the attachment side and on the plate side. N = number of cycles to fatigue failure. R = load ratio. = F,,,,,,/F,, S = design stress. uHs = geometrical stress at the hot spot. crL = actual stress at the hot spot (or local stress). AuHs = uHs range.

Assessment of welded structures by a local multiaxial fatigue approach

“…In order to be as general as possible, it is necessary to be able to take account of multiaxial service loadings. A new method was thus developed by J. L. Fayard et al ., 4 –6 which has already proved its efficiency on structural applications. As in the previous approach proposed by the Institut de Soudure, this method is based on the use of a local equivalent stress derived from Dang Van’s multiaxial criterion.…”

“…Computational methods to predict fatigue resistance of such structures have been proposed by different researchers; in particular in France J. L. Fayard et al . have developed a new efficient tool for analysing automotive structures obtained by continuous welding 4 –6 …”

“…This method is based on a structural approach combining testing and shell finite element model calculations with defined meshing rules (detailed for instance in Ref. [5] that take into account the increase of local rigidity induced by the weld. In this way, no special consideration is given to the local geometry (in contrast to the previous section); the main assumption is that continuous welds are of the same type, and thus of the same quality, from one component to another and therefore induce comparable local effects.…”

“…The ‘good’ rule is the one that allows the interpretation of all the experimental results with the minimum scatter. On this basis, a design rule for welded components used in the automotive industry was established, see Fayard et al 4 , 5 . These components are generally made by joining metal sheets of approximately 2 mm thickness by automatic Metal Active Gaz (MAG) welding.…”

Preface

“…His contribution to fatigue theory which is based on a multiscale analysis of the materials at the grain size level and on the concept of shakedown. His what is broadly known as the Dang-Van criterion which is an efficient procedure to estimate the life-time of structures [Fayard et al 1996;Ballard et al 1995;Dang Van and Maitournam 1993]. The Dang Van criterion has since been adopted in a variety of companies in the automotive and the aerospace industries.…”