The automotive industry nowadays needs to reduce the weight of vehicles for environmental and economical purpose. One expected way to reach this goal is to optimize the mechanical structures design and the selection of employed material regarding to the fatigue lifetime point of view. An experimental investigation has been performed within the collaboration framework of Renault, ArcelorMittal and Blaise Pascal University in order to assess the fatigue behaviour of spot welded specimens under constant amplitude loading with or without overloads. About three thousands spot welds are used to assembly the so-called body-in-white automotive structure. The most frequent location of the fatigue failure, when it occurs, deals with spot welds. The present work aims thus to characterize and to analyse the resistance of spot welds under constant amplitude fatigue loading with periodical overloads or not. The study focuses on the influence of periodical overload cycles on the fatigue strength of shear lap spot welds. Specimens are taken from hot rolled and cold rolled steel grades. A specific device prevents any local bending rotation of the specimens during the test, especially when the fatigue crack has initiated and then propagates laterally. The overload frequency corresponds to one overloading cycle per 99 constant amplitude cycles. The overload ratio, i.e. the ratio between the excess load cycles over the constant amplitude cycles, are equal to 1.4 or to 2.3. These values were determined from finite element analysis for vehicle loading events. All the fatigue tests were performed under a loading force ratio equal to 0.1. The comparison of F-N curves with periodical overloads against classical constant amplitude F-N curves indicates that overloads may not always decrease the fatigue strength of spot welds. In addition, depending on the steel grades and on the force levels, two kinds of failure modes can be observed. One is the shear failure of the welded nugget. Such a failure occurs for high force level. The other failure mode is a transversal cracking of the metal sheets. This failure mode is predominant in the high cycle fatigue domain. It may be divided into several steps as the crack initiation, sheet thickness crack crossing and the lateral crack growth. The potential drop method has been used to distinguish these different fatigue damaging steps.
Influence of overloads on the fatigue behaviour of spot-welds
2009
8 Seiten, 10 Bilder, 2 Tabellen
Conference paper
English
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