High-speed fragmentation of porous metal rings

In this paper, we have conducted dynamic ring expansion tests on 3D-printed AlSi10Mg porous samples utilizing both electromagnetic and mechanical testing techniques. The electromagnetic loading setup developed by Zhang and Ravi-Chandar (2006, 2008) is employed as a benchmark for evaluating and comparing the performance of the experimental configuration recently proposed by Nieto-Fuentes et al. (2023) to investigate the fragmentation of metallic rings using a pneumatic launcher. A total of 67 experiments have been carried out covering a wide range of strain rates from 2200 s − 1 to 16300 s − 1 . The tests performed with both experimental techniques were imaged using high-speed cameras to obtain time-resolved information on the mechanics of sample deformation and fragmentation. The recorded data allowed us to determine the number of fragments, the elongation of the specimens at the onset of fracture, and the fragmentation time. Moreover, the fragments ejected from the samples have been soft recovered, measured, and weighed. A good correlation is observed between the results obtained from electromagnetic and mechanical loading setups regarding the fragments size distribution and the evolution of the number of fragments with the loading rate. This agreement serves as a robust validation for the experimental configuration put forth by Nieto-Fuentes et al. (2023) , which allowed reaching higher strain rates than the setup of Zhang and Ravi-Chandar, 2006 , Zhang and Ravi-Chandar, 2008 , and it is notable for its simplicity, fast operation, and quick assembly. In addition, scanning electron microscopy and X-ray tomography analysis performed on recovered fragments from tests conducted at different expansion velocities with both testing techniques has provided indications on the evolution of the porous microstructure of the material at high strain rates, showing that the porosity of 3D-printed AlSi10Mg is instrumental for the propagation of cracks leading to the fragmentation of the rings. Moreover, fractography analysis of the crack surfaces revealed that while the fractures occurred without the preceding formation of necks, yet the fracture at the microscopic level was essentially ductile. The influence of the porous microstructure on the fragmentation mechanisms has been further investigated through finite element simulations that incorporate the voids’ size distribution of the specimens obtained from X-ray tomography analysis (Marvi-Mashhadi et al., 2021). The numerical results have demonstrated both quantitative and qualitative agreement with the experiments, showing that large pores and clusters favor stress concentration and subsequent fracture initiation and progression. Consistent with the statistical fragmentation theory of Mott (1947) for elastic-plastic materials that break without previous necking, the release waves emanating from the large pore defects and early fractures seem to play a critical role in determining the scale of the fragment size distribution in printed AlSi10Mg specimens.