Page 58 - Biaxial Multiaxial Fatigue and Fracture

BiaxiaUMultiaxial Fatigue and Fracture
Andrea Carpinteri, Manuel de &itas and Andrea Spagnoli @Is.)
0 Elsevier Science Ltd. and ESIS. All rights reserved. 43

MULTIAXIAL FATIGUE ASSESSMENT
OF WELDED STRUCTURES BY LOCAL APPROACH

Florence LABESSE-JIED', Bruno LEBR Up, Eric PETITPAY and Jean-Louis ROBERT'
LEWES, Blaise Pascal University, IUT. B.P. 2235, 03101 Montluqon Cedex - France
GIA T Industries, CRET IMOD, 7, Route de Guerry, I8023 Bourges - France

ABSTRACT

The moving of vehicles on chaotic ground induces dynamic multiaxial loading on structures
and mechanical components. As a consequence, early fatigue damage occurs especially in
structural details such as notched areas and welded parts. A multiscale approach has been
developed to design the structures against fatigue, starting from the dynamics of the vehicle
and ending with the calculation of structural details using a local approach to assess the fatigue
life. The methodology of the local approach developed is introduced. The evaluation of the
prediction capability of this local approach is described. Finally, the application to the fatigue
life assessment of welded elements is presented and compared to experimental results. The
major parameters of the weld geometry that govern the material resistance against fatigue are
pointed out. They concern geometrical features depending on the quality of the weld. Their
influence on the weld durability is outlined and the way the proposed assessment method
accounts for them in a quantitative manner is detailed.

KEYWORDS
Welding, fatigue life, local approach, multiaxial loading, damage cumulation

INTRODUCTION

Welding is a technology commonly used nowadays in many applications of the mechanical
industry because it allows the making of complex structures from simple components as bars
or plates. Engineering designers and researchers have thus developed different evaluation
techniques for ensuring fatigue resistance of such welded structures [ 11. Structural details and
welded joints are assessed from the fatigue point of view in design codes principally on the
basis of the nominal stress range. Practically they are classified into many different classes
depending on the geometry and the applied loading. A fatigue strength curve is attributed to
each class, the denomination of which corresponds to the characteristic fatigue strength at 5.106
cycles [2]. Most design codes allocated to fatigue assessment of welded structures refer to this
principle. Figure 1 depicts the conventional Wiihler S-N curves generally consulted as
referenced standard quality fatigue properties.
In the case where the nominal stress can not be easily defined within the fatigue-prone
welded structure, the proof against fatigue uses either hot-spot or structural stresses [1,3]. The