Investigating spall failure in shock compressed iron

The spall response of pure iron was studied using high power pulsed laser experiments at the LLNL Janus facility. Thin iron foils of varying initial microstructures were subjected to peak pressures of about 60 GPa and strain rates ranging from 10⁶ s^-1 – 10⁷ s^-1. Simultaneous time-resolved free surface velocity measurements and recovery techniques were used to investigate spall strength and failure mechanisms. These uniaxial strain experiments yielded strengths between 5 and 10 GPa for nanocrystalline and single crystal iron, respectively. Post-shock characterization and Molecular Dynamics simulations verify that this difference in strength is due to void initiation sites. Grain boundaries in nano and polycrystalline iron are favorable sites for voids to form and will consequently cause failure to occur along grain boundaries perpendicular to the shock direction. In contrast, the formation and interaction of twin boundaries in single crystal iron are the cause for void initiation, growth, and coalescence that ultimately cause ductile failure.