Page 362 - Handbook of Materials Failure Analysis

360 CHAPTER 14 Fatigue failure analysis of welded structures

spot-welds with dissimilar thicknesses. Therefore, Pan [27,29] proposed a local
strain model for such cases. Local stress and strain values at the spot-weld edge were
obtained from a three-dimensional elastic-plastic FE simulation. Nonlinear geometry
and nonlinear material properties were included in the FE analysis. Cyclic principal
strain range was considered as the fatigue damage parameter and was related to total
fatigue life.
This chapter briefly explains the procedure for fatigue failure analysis of weld-
ments, with a special focus on spot-welded structures. The components involved in
the fatigue modeling are introduced and described. Different approaches for fatigue
modeling of welded structures are presented and their advantages and drawbacks are
discussed. A real-life welded structure, which is essentially a subassembly of auto-
motive body parts, is studied for fatigue strength analysis. The fatigue modeling is
performed using a global and a local approach. The prediction results, in terms of the
fatigue failure location and the fatigue life, are compared to the experimental results
and the results are discussed.

2 FATIGUE MODELING OF WELDED STRUCTURES BY LOCAL
APPROACHES
Several fatigue models based on the local approaches have been proposed for spot-
welded structures in the past four decades. These models can be categorized into
three major groups: fracture mechanics, structural stress, and local notch stress/strain
approaches. General features of these approaches are explained in this section, and a
well-known fatigue model from each approach is explained.

2.1 FRACTURE-MECHANICS APPROACH
The fracture mechanics approach was one of the first solutions to avoid the problems
associated with the global approaches. This approach has been applied to spot-welds
since the 1970s. The spot-welding process produces a circular joint between two or
more sheets, with a notch at the spot-weld edge. Because the notch radius is small
compared to the sheet dimensions, the spot-weld in some studies is considered a
sharp notch [11–13,30]. Therefore, the spot-weld is treated as a circular region sur-
rounded by a pre-existing crack.
Furthermore, dependency of the fatigue strength to the material’s static properties
may help to identify an appropriate fatigue modeling approach. For the case of
smooth specimens and blunt notched components, where the crack initiation process
dominates, fatigue life is closely related to the material strength [31]. However,
fatigue life in sharp notched and cracked components is controlled by crack propa-
gation and is insensitive to material strength [32]. Experimental results for steel spot-
welds demonstrate that the base metal strength has an insignificant effect on the
fatigue strength [13,31]. These observations support the idea that spot-welds are
crack-like flaws, and fracture mechanics is the right approach.