(Abstract) It is well known that plastic deformation behavior of metallic material depends on the length-scale and time-scale due to dislocation motions in material. The plasticity affected by scale down and higher strain rate is still open to discuss. In other words, both size effect and strain rate effect are critical issue for plasticity. Currently, macro-scale experiment of bulk materials for higher strain rate have been established. On the other hand, experiment of the micro and nano scales with higher strain rate has not been established. This study developed the experimental method to conduct microparticle collisions, causing high strain rate deformation on the target materials surface. This is called laser-induced projectile impact test (LIPIT), which is indentation impact test. We could successfully project micro particles, resulting in particle collision with the material surface at higher strain rate. By changing the laser energy, the indentation crater depth can be significantly changed. We thus established a testing method in the high strain rate on a micro scale. In addition, finite element method (FEM) was carried out to simulate the plastic deformation behavior due to particle collision at higher strain rate, so that we investigate plastic deformation at the microscale under higher strain rate.(Method) To investigate the plastic deformation behavior of the material in the high strain rate, laser ablation is employed for microparticles collision on the target material. In this study, the laser ablation source consists of three layers: constrained layer, energy absorption layer, and launcher plate. Before laser ablation, micro-particles are distributed on the surface of launcher plate. When the laser is irradiated through the constrained layer, the energy absorption layer undergoes rapid volume expansion (due to laser ablation), causing the launcher to deform significantly. This results in particles projection toward the target specimen. When the particles collision the target, they deform plastically and produce indentation crater. In this study, five different laser energy patterns were tested to investigate how the indentation crater changes. Zirconia particles were used as the projected particles. The target materials are annealed pure iron, untreated pure iron, and annealed pure copper.(Results and discussions) Using a scanning laser microscope, we obtained the relationship between laser energy and the indentation crater depth on the target. We confirmed that the indentation crater depth increased with increasing in laser energy. It is found that the indentation crater depths of treated pure iron becomes deeper than that of untreated iron. This is because the work hardening of target surface is reduced by the annealing treatment. Subsequently, FEM computation was performed to represent a high velocity collision test of micro-particles. In actual phenomena, strain rate dependent hardening and thermal softening due to creep are expected to occur near the indentation crater during high velocity particle impact. However, our FEM computation reveals that the effect of thermal softening was negligible in this study. This is because the heat source generated by the particle impact is very small and the heat diffuses quickly, so only the strain rate becomes dominant for the indentation crater depth. FEM computation investigates deformation behavior at high strain rates during particle impact. As in the experiment, the indentation crater depth increased with the particle velocity.(Conclusion) In this study, a high-speed particle impact test method using laser ablation was developed to investigate the micro-scale plastic deformation behavior at high strain rate. From the indentation crater observation, it was found that the crater depth increased with the increase of laser energy. The FEM model for this experiment was constructed to clarify the plastic deformation behavior of the target material.

Plastic Deformation Behavior at High Strain Rate by Using High Velocity Micro-Particle Collisions

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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