Magnesium alloys are the lightest structural metal and recently
attention has been focused on using them for structural automotive
components. Fatigue and durability studies are essential in the
design of these load-bearing components.
In 2006, a large multinational research effort, Magnesium Front
End Research & Development (MFERD), was launched involving
researchers from Canada, China and the US. The MFERD project is
intended to investigate the applicability of Mg alloys as
lightweight materials for automotive body structures. The
participating institutions in fatigue and durability studies were
the University of Waterloo and Ryerson University from Canada,
Institute of Metal Research (IMR) from China, and Mississippi State
University, Westmorland, General Motors Corporation, Ford Motor
Company and Chrysler Group LLC from the United States. This paper
presents the results of benchmark coupon testing that were obtained
for monotonic and cyclic conditions on extruded AM30 alloy samples.
Tests were performed independently in Canada, China, and the US. In
general, the results reported by different institutions were in
good agreement.
Microstructure analyses revealed strong material texture with a
unique orientation of extension twinning with respect to the
initial basal plane. The cyclic deformation, therefore, was seen to
be dominated by twinning and detwinning. The unusual asymmetric
hysteresis of AM30 observed for fully reversed cyclic loading is
attributed to twinning under compression in the extrusion
direction, detwinning upon unloading from compression and
dislocation slip in tension.
The monotonic tests were performed under different strain rates
and at room temperature or 125°C. Cyclic tests were performed under
strain controlled conditions. Two strain amplitudes were
considered, 0.3% and 0.6% and all fatigue tests were performed
under standard laboratory conditions.
Raising the temperature from standard laboratory conditions to
125°C had a significant effect under monotonic loading: both the
yield and tensile strength dropped by about 25%, while ductility
increased by 300%. Under fatigue loading at room temperature,
extruded AM30 exhibits asymmetrical cyclic behavior at a strain
amplitude of 0.6%, whereas the cyclic behavior at 0.3% was
symmetric. The material showed significant plastic strain recovery,
cyclic hardening, and a clear endurance limit.