High Strain-Rate Material Behavior by Short Pulse Laser-Generated Stress Waves
Miller, Phillip Isaac
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The response of materials when subjected to high strain-rate, high pressure loading can vary drastically from static conditions. Dynamic material behavior arises whenever loading is suddenly applied to a material or structure and it is essential that this behavior is well understood for qualitative and quantitative predictability. Stress waves generated from tabletop short pulse lasers allows the study of material behavior in the pressure range of 10's GPa and strain-rates up to 10<super>8</super> s<super>-1</super>. In this dissertation, a parametric study was first carried out to understand the effect of various parameters on the stress wave generation by laser pulse absorption. The technique was then applied to several material systems ranging from conventional solids such as Si and aluminum, to biological cells and further to nanoporous oxides, to understand their unique response under high strain-rate loading. To better understand the applicability of the technique and the role of the loading parameters on stress generation, a parametric study of the laser fluence, pulse duration, and confinement of the laser absorption volume was performed. Similar stress profiles are observed under high fluence, confined conditions for lasers pulse durations spanning picoseconds to nanoseconds. Low fluence and unconfined absorption with picosecond laser pulses allows an order of magnitude enhancement in strain-rate, while still generating peak stresses relevant to dynamic material behavior. Using the knowledge gained from the parametric study, high strain-rate compressive yielding of aluminum alloys subjected to picosecond and nanosecond pulsed laser loading reveals an elastic-plastic response consistent with observations from other experimental techniques at lower strain-rates. Stress wave evolution as a function of propagation was studied, and no additional rate dependence from picosecond to nanosecond loading was observed. Previous application of the laser spallation technique was applied to measure the adhesion strength of cells on inorganic substrates. Due to the time scale of the interaction, post mortem observations were used to determine the loading required to remove the cells. Transient finite element analysis was carried out to investigate the detachment mechanism responsible for cell decohesion. Failure is driven by large interfacial strains experienced due to the very rapid acceleration of the substrate into the cell. Stress, strain, and interface failure evolution exhibit no rate or cell geometry dependence, indicating the laser spallation techniques ability to measure an intrinsic, undisturbed, and short time scale adhesion strength. The final study conducted was the determination of the dynamic behavior of a novel material system, synthetic nanoporous zeolite, measured by laser-induced loading. High strain-rate loading prompts a ductile to brittle transition for zeolite. Compression experiments demonstrate brittle fracture under moderate loading of several hundred MPa. Spallation measurements were performed with peak stresses up to 3 GPa. A spallation strength of 215 MPa for bulk zeolite was measured, with no dependency on peak stress. The low spallation strength, as well as scanning electron microscope observations of the spallation morphology, confirms that bulk zeolite behaves as the brittle solid at high strain-rates.
- Mechanical engineering