Page 370 - Handbook of Materials Failure Analysis

368 CHAPTER 14 Fatigue failure analysis of welded structures

less than the required value, cracking might occur, especially in the bottom sheet, due
to the severe plastic deformation.
Magnesium has hexagonal crystal structure which results in limited slip systems
and poor ductility and formability at room temperature; therefore, SPR generally
causes cracking in magnesium alloys [49]. The formability of magnesium improves
at higher temperature because of the additional slip systems activated [50,51]. Dur-
andet et al. [49] used a laser beam at the joint location and showed that it can provide
enough heat to prevent initiation of cracks in AZ31 magnesium alloy. This method is
called “laser-assisted self-pierce riveting” or LSPR, and it was employed for joining
the components of the Demo-structure.

3.3 MATERIAL PROPERTIES AND MODELING
3.3.1 Material properties
As stated in Section 3.1, the components of the Demo-structure are made of three
different magnesium alloys: AZ31B sheet, AM30 extrusion, and AM60B cast. In
order to simulate the Demo-structure, the mechanical properties for these alloys were
obtained from the literature [10,52,53]. The microstructure of the three alloys differ
as a result of the different manufacturing process. The rolling and extrusion pro-
cesses produce highly textured microstructures for AZ31B and AM30; therefore,
material properties for these alloys are different under tension and compression.
On the other hand, a randomly oriented crystal structure is observed for AM60B cast,
which results in symmetric tension-compression behavior.
Figure 14.8 shows the monotonic behavior of the three alloys, in which RD, TD,
and ED represent the rolling, transverse, and extrusion directions, respectively.
Table 14.1 summarizes the monotonic properties of the three magnesium alloys in
different directions.
Similar to monotonic loading, cyclic loading experimental results for the three
Mg alloys were obtained from the literature [10,54,55]. The stabilized hysteresis
loops for the three alloys at different strain amplitudes are shown in Figure 14.9.
The strain-life curves were obtained for the three alloys under fully reversed load-
ing [10,54,55] and are shown in Figure 14.10. In this figure, the fatigue properties
correspond to RD for AZ31B, and ED for AM30 magnesium alloys.
The monotonic and cyclic properties of the rivets are unknown, and therefore the
elastic steel properties, that is, E ¼ 210GPa and ν ¼ 0:3 were used in the simulation.

3.3.2 Material modeling
After material properties are obtained from experiments or from the literature, the
next step is to generate a FE model for the structure of interest and assign the cyclic
stress-strain curves to the corresponding components. However, for the case of the
Demo-structure the procedure is more complex than general problems. The wrought
Mg alloys involved in the Demo-structure behave asymmetrically under tension and
compression and this behavior could not be modeled by available material models in
the Abaqus FE package [56]. Therefore, an asymmetric constitutive model was