Femtosecond two-pulse laser approach for spall failure in thin foils

Assessing a material's spall resistance requires a mechanism capable of generating shockwaves with sufficient pressure to replicate the high strain rates of spallation. In this study, we introduce a novel two-pulse laser approach that generates high hydrostatic tensile stress to induce spall failure by simultaneously illuminating both free surfaces of micro- and nanoscale-thick Ni foils with femtosecond laser pulses. By comparing this method with the conventional single-pulse approach, we demonstrate the unique effects of introducing a secondary laser pulse. Experimental observations utilizing electron microscopy and focused ion beam milling reveal that the proposed approach effectively induces failure at the center of the sample, resulting from the interaction of unloading tensile waves in the bulk. Notably, the spall threshold for nanoscale Ni foils is significantly reduced in the two-pulse approach compared to the single-pulse method. While the single-pulse method requires a threshold fluence of 2,500 J⋅m−2, the two-pulse approach lowers this to 1,750 J⋅m−2, highlighting its potential to generate higher tensile stress with less laser pulse energy. Both experimental results and molecular dynamics simulations indicate ductile failure mechanisms involving nucleation, growth, and coalescence of voids, preceded by the emission of stacking faults and Shockley partial dislocations. Overall, the two-pulse approach offers an alternative method for investigating the spall behavior of metals under high strain-rate deformation in a small-scale laboratory setup, with potential for high-throughput experiments.