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%.