Introduction
Recently, magnesium (Mg) has attracted wide interest in several
applications, from biomedical to automotive sectors due to their
attractive properties 1–5. Particularly, the
lightweight of magnesium makes this material extremely attractive for
transportation industries, such as automobile and aircraft industries,
due to the possibility to reduce the fuel consumption and the gas
emission 6. However, to fully exploit the lightweight
of Mg, researchers have started focusing on the addition of alloying
elements to increase the specific strength of pure Mg. In addition, if
passive alloying elements are added, it is also possible to increase the
poor corrosion of pure Mg 7,8. Among the different Mg
alloys, ZK alloys, and specifically ZK60, are particularly interesting
for structural applications since the addition of Zinc and Zirconium
increases the corrosion resistance of the base material and the
mechanical properties (strength and ductility) due to their grain
refinement effect 9. However, the diffusion of these
materials for structural applications is still limited due to the lack
of knowledge regarding their fracture behavior in presence of
geometrical discontinuities (notches). The presence of geometrical
discontinuities such as notches is in fact very common in structural
components, and they are known to be detrimental for static and fatigue
resistance 10–13. Some preliminary works on the
effect of notches on the fracture behavior of Mg alloys have been
carried out. Most of the works focuses on the effect of notches on AZ
alloys. Yan et al 14, for example, investigating the
deformation and failure behavior of AZ91 alloy found that the presence
of notches decreases the ductility, similarly to what is reported in
Ref. 15. Interestingly, despite decreasing the
ductility, the presence of notches was found to increase the strength of
AZ alloys 16,17. However, these works consider only
notch geometries where the notch acuity is low, while real components
are weakened also by sharper notches. Additionally, to the best of the
authors’ knowledge, no work considers the effect of notches on ZK60
alloys. Thurs, this work aims to fill this lack by investigating the
fracture behavior of ZK60-T5 extruded samples weakened by eleven
different notch geometries. The investigated notch geometries were such
that different notch acuities and sharpness are considered, in order to
cover a wider range of potential real applications. Namely, U notched
specimens with notch radii of 1.5, 3, 4, 5, and 6 mm and V notched
specimens with notch angles of \(35\), \(60\) and \(90\), and notch
radii of 0.4 and 0.8 mm were considered. The mechanical properties of
unnotched samples were also tested to evaluate the effects of notches on
the failure behavior. In addition, the fracture surfaces were
investigated by means of Field Emission Scanning Electron Microscope
(FE-SEM) to understand how the different notch geometries impact on the
failure mechanisms.
In addition, this work aims to overcome the other limitation hampering
the widespread diffusion of Mg and its alloys in structural
applications, i.e. the need for a robust design tool against failure in
the presence of notches. Several criteria have been developed in the
recent years to predict the fracture of different material classes
(metals, ceramics, polymers), from the notch stress intensity factors
(NSIFs)-based criterion 18 to the theory of critical
distance (TCD) approach 19,20. However, these criteria
suffer from different drawbacks. NSIFs-based criterion, for example, is
limited by its geometry dependency and by the need of evaluating
accurately the stress field ahead of the geometrical discontinuities to
correctly perform the fracture assessment 21. The TCD
approach, although overcoming the geometry limitation of the NSIFs-based
criterion, is still limited by the need to evaluate accurately the
stress field 22. In recent years, the strain energy
density (SED) approach has been reported to be a promising candidate for
an easy and accurate prediction of the fracture behavior of different
notched materials 23–27. The SED approach, that will
be described in detail in Section 3, does not suffer from the geometry
dependency and from the need of evaluating accurately the stress field
ahead of the geometrical discontinuities, and has therefore gained wide
interest among engineers and practitioners, especially for the
possibility offered to predict the failure loads of differently notched
components with acceptable engineering values range between -20% and
+20%. However, to the best of the authors’ knowledge, the SED approach
has never been applied to Mg and its alloys and we thus aim herein to
fill this lack. In particular, the experimental data of the eleven
differently notched ZK60-T5 specimens were compared with the predictions
provided by the SED approach, showing accurate predictions, with most of
the predictions characterized by a deviation of 10%.